Partial replacement of mineral fertilisers with animal manures in an apple orchard: Effects on GHG emission

Partial replacement of mineral fertilisers (MF) with animal manures is a good alternative to reduce MF use and increase both nutrient cycling in agriculture and soil organic matter. However, the adoption of this practice must not lead to increased environmental impacts. In this two-year study conducted in an apple orchard, MF were partially replaced with various animal manures, including cattle slurry (CS), acidified cattle slurry (ACS), solid cattle manure (CsM), or poultry manure (PM), and their impacts on greenhouse gas emission (GHG: CO2, N2O and CH4) were examined. A control (CTRL) receiving only MF served as the baseline, representing the conventional scenario in orchard fertilisation. Overall, replacing MF with manures increased GHG emissions, with the magnitude of the impacts depending on the specific characteristics of the manures and the amount of nutrients and organic matter applied. Comparing to the CTRL, application of ACS and CS led to higher CH4 and N2O emissions, while PM application increased both N2O and CO2 emissions. In contrast, replacement with PM and CsM decreased CH4 emissions. Nevertheless, results varied between the two years, influenced by several factors, including soil conditions. While acidification showed potential to mitigate CH4 emissions, it also led to increased N2O emissions compared to CS, particularly in 2022, suggesting the need for further investigation to avoid emission trade-offs. Replacement with CS (20.49 t CO2-eq ha-1) and CsM (20.30 t CO2-eq ha-1) showed comparable global warming potential (GWP) to the conventional scenario (CTRL, 19.49 t CO2-eq ha-1), highlighting their potential as viable MF substitutes.

Effects of forest fire smoke deposition on soil physico-chemical properties and bacterial community

The number of forest fires has increased globally, together with considerable smoke emission that significantly impacts the atmospheric environment and associated ecosystems. Most current studies have focused on the in situ effects of fire on the forest ecosystem. However, the mechanisms by which smoke particles affect adjacent ecosystems are largely unexplored. In this study, a simulated forest fire combustion system was developed to evaluate the effect of different smoke concentrations (control, low and high) on soil physico-chemical properties of adjacent farmland at two soil depths. The abundance and diversity of bacterial community were also determined. The results showed that smoke deposition increased the contents of total carbon (TC), total nitrogen (TN), and total phosphorus (TP) in the 0-10 cm soil layer; however, no significant changes in soil water content (SWC) and pH values was observed. The ACE(Abundance Coverage-based Fastimator) and Chao1 diversity indices of bacterial community generally showed a downward trend whereas the PD_whole_ tree diversity index increased after 180 d of smoke deposition. The relative abundance of Proteobacteria remained stable, while abundance of Firmicutes in soil decreased after 180 d of smoke deposition. Smoke deposition slightly affected the physical and chemical properties of the 10-20 cm soil, but the range of variation of the relative abundance and diversity dominant bacteria exceeded that of the 0-10 cm soil. A significant positive correlation was found between the soil properties and the alpha diversity indices during the first 30 d after smoke deposition; the correlation then decreased gradually. Redundancy analysis revealed that Proteobacteria, Firmicutes, and Actinobacteria were generally positively correlated with TC, TN, and SWC. As a whole, the study reveals that the effects of smoke deposition on soil physico-chemical properties and bacterial community depends on smoke concentration where relatively low concentration appears to be beneficial to soil bacterial community.

Soil δ15N spatial distribution is primarily shaped by climatic patterns in the semiarid Caatinga, Northeast Brazil

Soil nitrogen isotopic composition (δ15Nsoil) is an invaluable tool as it integrates nitrogen (N) transformations in soils. In addition to serving as a baseline to understand the N cycle, spatial representations of δ15Nsoil across landscapes (or isoscapes) is a multi-purpose tool useful to investigate, for example, plant-microbe interactions, animal migration and forensics. We investigate the climatic and edaphic controls of δ15Nsoil utilising data from 29 geographical locations sampled across the semiarid Brazilian Caatinga biome. The sampling covered a mean annual precipitation (PA) gradient ranging from 0.51 to 1.36 m a-1 and eight soil types originating from three different geological origins. Our data show that the combination of higher aridity and lower seasonality (ψ) leads to higher values of δ15Nsoil. Moreover, soil total carbon had a positive relationship with δ15Nsoil, appearing within the best-supported models according to the information-theoretic approach undertaken here. The contribution to the plant communities by the Fabaceae trees expressed as their basal area was not related to δ15Nsoil values, suggesting that the magnitude of biological N fixation in the Caatinga is not large enough to be reflected in the soil. In addition, considering PA in a categorical fashion, i.e., 'high' (> 0.8 m a-1) and 'low' PA (< 0.8 m a-1), we found that, within the wetter category, δ15Nsoil was positively related to several soil properties (i.e., clay content, effective cation exchange capacity, exchangeable calcium, silt content, pHH2O, total phosphorus and sum of bases) and negatively related to sand content. Our study provides new insights into the functioning of semiarid ecosystems from a pedo-isotopic perspective and contributes to the overall understanding of the N cycle in the Caatinga region, with the potential to support the development of new conceptualisation of biogeochemical process and testing of global models that simulate N and C cycles.

Soil microbial residue characteristics in Pinus massoniana lamb. Plantations

A large amount of stable soil organic matter (SOM) is derived from microbial necromass, which can be assessed by quantifying amino sugar biomarkers. Pinus massoniana Lamb. Plantations are widely distributed in China and play a vital role in forest carbon sequestration. However, the patterns of soil microbial residue remain poorly understood. In this study, amino sugars were used to characterize patterns of soil microbial residues at three soil depths (0-10, 10-20, and 20-30 cm) in P. massoniana plantations of different ages (young, middle-aged, near-mature, mature, and over-mature; denoted as YG, MD, NM, MT, and OM, respectively). In the topsoil (0-10 cm), the total nitrogen (TN) content of the OM forest was the highest, whereas the soil organic carbon (SOC) content of the MT forest was the highest. Consistent with changes in SOC and TN, total microbial residue content decreased with increasing soil depth. However, the total microbial residues C to SOC contribution increased considerably with increasing depth, suggesting that more SOC was derived from microbial residues in the subsoil than that from the topsoil. The fungal residue C to SOC contribution was higher than that of bacterial residue C. Total amino sugar content in the topsoil increased with increasing age, and MT and OM had a significantly higher content than that of other forests. At all soil depths, SOC and TN content predominantly determined microbial necromass, whereas soil microbial biomass content predominantly determined microbial necromass in the topsoil; soil pH predominantly determined microbial necromass in the 10-20 cm soil layer; and soil pH and Ca2+ content were the primary factors in the soil layer below 20 cm. The study provides valuable insights into controls of microbial-derived organic C could be applied in Earth system studies for predicting SOC dynamics in forests.

Field data on pre-season rice straw degradation using a microbial substrate and the effects on methane emissions during rice cultivation

Rice straw is one of the most abundant biomass wastes derived from rice cultivation activities. The current rice straw management practice during the wet (rainy) season in Malaysia involves the integration of straw into the soil. This practice offers both advantages and disadvantages to rice farmers and the environment. Straw integration may improve nutrient availability while concurrently causing high greenhouse gas (GHG) emissions due to the increase in soil carbon activity. In this work, the use of microbial substrate to enhance the degradation of straw was compared to an existing technique that used no additional inputs during soil integration. The data collected consisted of overall microbial enzyme production, soil organic carbon, soil nitrogen content, seasonal greenhouse gas emissions, plant characteristics, and crop yield. In brief, these data can be used as means of demonstrating the effects of improved straw degradation during the pre-season on the overall GHG emissions during the planting season.

Tropical altitudinal gradient soil organic carbon and nitrogen estimation using Specim IQ portable imaging spectrometer

The largest actively cycling terrestrial carbon pool, soil, has been disturbed during latest centuries by human actions through reduction of woody land cover. Soil organic carbon (SOC) content can reliably be estimated in laboratory conditions, but more cost-efficient and mobile techniques are needed for large-scale monitoring of SOC e.g. in remote areas. We demonstrate the capability of a mobile hyperspectral camera operating in the visible-near infrared wavelength range for practical estimation of soil organic carbon (SOC) and nitrogen content, to support efficient monitoring of soil properties. The 191 soil samples were collected in Taita Taveta County, Kenya representing an altitudinal gradient comprising five typical land use types: agroforestry, cropland, forest, shrubland and sisal estate. The soil samples were imaged using a Specim IQ hyperspectral camera under controlled laboratory conditions, and their carbon and nitrogen content was determined with a combustion analyzer. We use machine learning for estimating SOC and N content based on the spectral images, studying also automatic selection of informative wavelengths and quantification of prediction uncertainty. Five alternative methods were all found to perform well with a cross-validated R² of approximately 0.8 and an RMSE of one percentage point, demonstrating feasibility of the proposed imaging setup and computational pipeline.

Effects of vegetation patterns on soil nitrogen and phosphorus losses on the slope-gully system of the Loess Plateau

The slope-gully system, the erosion unit on the Loess Plateau, suffers from severe soil erosion and loss of soil nutrients. Restoring vegetation can effectively reduce soil erosion, thereby reducing the loss of nitrogen and phosphorus. In the Loess Plateau, owing to the shortage of water resources and the adverse effects of over-revegetation, the restoration of vegetation in large areas is limited. To efficiently prevent the loss of soil nutrients and reduce non-point source pollution, vegetation patterns need to be reasonably restored. However, it is currently not clear as to how this can be achieved. Different slope-gully systems were established in this study, including pattern A (no vegetation), pattern B (up-slope vegetation), pattern C (middle-slope vegetation), and pattern D (down-slope vegetation). Then, the effects of vegetation patterns on soil total nitrogen (TN) and soil total phosphorus (TP) losses associated with runoff and sediment processes was quantitatively evaluated through the simulated rainfall. The results showed that (1) vegetation pattern markedly affected the yields of runoff, sediment, soil nitrogen, and soil phosphorus, resulting in the following order: pattern A > pattern B > pattern C > pattern D. (2) The correlation between TN and runoff was higher than that between TN and sediment; conversely, TP was more strongly correlated with sediment than with runoff. (3) Nitrogen loss with runoff was the main source of TN (58.76-90.74%), while phosphorus loss with sediment was the main source of TP (48.51-89.30%). Compared with other vegetation patterns, the down-slope can more effectively reduce the yields of runoff and sediment, thereby reducing the loss of TN and TP. Therefore, it was suggested that the lower part of the slope should be considered when revegetating.

Impacts of the components of conservation agriculture on soil organic carbon and total nitrogen storage: A global meta-analysis

Conservation agriculture (CA) can be an important strategy for improving soil organic carbon (SOC) and total nitrogen (TN). Numerous studies have examined SOC and TN dynamics in different cropping systems. However, there is some uncertainty regarding the relative impacts of some CA practices, and it is not always clear how other agricultural management, particularly nitrogen addition, interacts with these practices to influence SOC and TN sequestration. Thus, we conducted a global meta-analysis of 752 comparisons from 97 papers to analyze the impacts of nitrogen fertilizers and CA practices (namely crop diversification, minimal soil disturbance (no-tillage) and permanent soil cover), on SOC and TN content worldwide. Overall, our study showed the most significant increase of SOC [21.39 % (CI = 15.16 to 28.64)] and TN [54.34 % (CI = 26.19 to 96.69)] stock with CA practices compared to conventional practices in the 0-15 cm soil depth. It also showed a significant increase in SOC and TN stock with all the individual components of CA compared to conventional practices in the 0-15 cm soil depth. However, the impact of CA on SOC and TN is reduced in 0-60 cm depths compared to surface soil depths due to the limited input of crop residue deeper in the soil profile. Manure and manure mixed with mineral-N led to greater SOC sequestration [20.67 % (CI = 15.23 to 27.10) and 41.67 % (CI = 31.03 to 52.79), respectively] than mineral-N alone [9.08 % (CI = 6.44 to 11.83)]. Cropping systems that included legume residue decreased the C/N ratio. This highlights that adequate mineral-N fertilizer addition may also be required in conjunction with residue retention practices to improve SOC and TN content. Overall, these results show that CA systems that include legume residue and manure mixed with mineral-N have great potential to increase SOC and TN, particularly at 0-15 cm and 0-30 cm soil depth.

Continuous no-till decreased soil nitrous oxide emissions during corn years after 48 and 50 years in a poorly-drained Alfisol

The soil quality benefits from switching from chisel-disk (CD) operations to continuous no-till (NT) in corn (Zea mays L.) and soybean (Glycine max L.) rotations have been proven over time; but to mitigate climate change, effects of continuous NT on nitrous oxide (N2O) emissions must be evaluated. The objectives of this study were to determine the influence of contrasting tillage practices (CD vs. NT) on soil N2O emissions, soil nitrogen (N) dynamics, corn grain yields, N removals and partial N balances, soil volumetric water content (VWC) and soil temperature following 48 and 50 years of tillage implementation in a long-term corn-soybean rotation experiment in a poorly-drained Alfisol. A four-time replicated randomized complete block design was conducted with tillage treatments [CD (grower's current practice) and NT] as main plots and fertility [a no-fertilizer control (CTR) and fertilizing corn N, P, and K (NPK)] as subplots. Corn grain yield, N removal, and partial N balances were greater in CD than NT in 2018 but not in 2020. Soil N2O-N was similar among tillage treatments in 2018 (3.2 kg N2O-N ha-1) but higher in CD (8.5 kg N2O-N ha-1) than in NT (6.2 kg N2O-N ha-1) in 2020. The CD treatment had higher two-yr cumulative N2O-N emissions (11.9 kg N2O-N ha-1) than NT (9.1 kg N2O-N ha-1), indicating that NT has a potential for reducing N2O-N in poorly-drained Alfisols. Grain yield-scaled N2O-N was lower in NT than CD in 2020 but not in 2018. Soil N2O emissions were positively associated with soil NO3-N, partial N balances, and corn grain yield and negatively associated with soil bulk density and temperature. We concluded that after 48 and 50 years, continuous NT can maintain corn grain yield and mitigate N2O-N emissions indicating to mitigate climate change and increase multi-sphere benefits, continuous NT practices should be implemented.

The carbon sequestration response of aboveground biomass and soils to nutrient enrichment in boreal forests depends on baseline site productivity

Nutrient enrichment can alleviate productivity limitations and thus substantially increase carbon (C) uptake in northern coniferous forests. Yet, factors controlling stand-to-stand variation of forest ecosystem responses to nutrient enrichment remain unclear. We used five long-term (13 years) nutrient-enrichment experiments across Sweden, where nitrogen (N), phosphorus, and potassium were applied annually to young Norway spruce forests that varied in their baseline ecosystem properties. We measured tree biomass and soil C and N stocks, litterfall C inputs, soil CO₂ efflux, and shifts in composition and biomass of soil microbial communities to understand the links between above and belowground responses to nutrient enrichment. We found that the strongest responses in tree biomass occurred when baseline site productivity was lowest. High increases in tree biomass C stocks were generally balanced by weaker responses in organic soil C stocks. The average ecosystem C-N response rate was 35 kg C kg^-1 N added, with a nearly five-fold greater response rate in tree biomass than in soil. The positive nutrient enrichment effects on ecosystem C sinks were driven by a 95% increase in tree biomass C stocks, 150% increase in litter production, 67% increase in organic layer C stocks, and a 46% reduction in soil CO₂ efflux accompanied by compositional changes in soil microbial communities. Our results show that ecosystem C uptake in spruce forests in northern Europe can be substantially enhanced by nutrient enrichment; however, the strength of the responses and whether the enhancement occurs mainly in tree biomass or soils are dependent on baseline forest productivity.

Digital soil mapping of soil total nitrogen based on Landsat 8, Sentinel 2, and WorldView-2 images in smallholder farms in Yellow River Basin, China

Predicting spatial explicit information of soil nutrients is critical for sustainable soil management and food security under climate change and human disturbance in agricultural land. Digital soil mapping (DSM) techniques can utilize soil-landscape information from remote sensing data to predict the spatial pattern of soil nutrients, and it is important to explore the effects of remote sensing data types on DSM. This research utilized Landsat 8 (LT), Sentinel 2 (ST), and WorldView-2 (WV) remote sensing data and employed partial least squares regression (PLSR), random forest (RF), and support vector machine (SVM) algorithms to characterize the spatial pattern of soil total nitrogen (TN) in smallholder farm settings in Yellow River Basin, China. The overall relationships between TN and spectral indices from LT and ST were stronger than those from WV. Multiple red edge band-based spectral indices from ST and WV were relevant variables for TN, while there were no red band-based spectral indices from ST and WV identified as relevant variables for TN. Soil moisture and vegetation were major driving forces of soil TN spatial distribution in this area. The research also concluded that farmlands of crop rotation had relatively higher TN concentration compared with farmlands of monoculture. The soil prediction models based on WV achieved relatively lower model performance compared with those based on ST and LT. The effects of remote sensing data spectral resolution and spectral range on enhancing soil prediction model performance are higher than the effects of remote sensing data spatial resolution. Soil prediction models based on ST can provide location-specific soil maps, achieve fair model performance, and have low cost. This research suggests DSM research utilizing ST has relatively high prediction accuracy, and can produce soil maps that are fit for the spatial explicit soil management for smallholder farms.

Effect of different polymers of microplastics on soil organic carbon and nitrogen - A mesocosm experiment

Agricultural microplastic pollution has become a growing concern. Unfortunately, the impacts of microplastics (MPs) on agricultural soil carbon and nitrogen dynamics have not been sufficiently reported. In an attempt to remedy this, we conducted a 105-day out-door mesocosm experiment in a soil-plant system using sandy soils amended with two types of MPs, low-density polyethylene (LDPE-MPs) and biodegradable (Bio-MPs), at concentrations of 0.0% (control), 0.5%, 1.0%, 1.5%, 2.0% and 2.5% (w/w, weight ratio of microplastics to air-dry soil). Soil organic matter (SOM), dissolved organic carbon (DOC), permanganate oxidizable carbon (POXC), available nitrogen (AN) of N-NH₄⁺ and N-NO₃^-, and dissolved organic nitrogen (DON) were measured on day 46 (D46) and 105 (D105) of the experiment. SOM was also measured after microplastics were mixed into soils (D0). For LDPE-MPs treatments, SOM on D0, D46 and D105 showed no significant differences, while for Bio-MPs treatments, SOM significantly (p < 0.05) decreased from D0 to D46. Compared to the control, soil POXC was significantly (p = 0.001) lowered by 0.5%, 1.0% and 2.5% LDPE-MPs and ≥ 1.0% Bio-MPs on D105. LDPE-MPs showed no significant effects on soil DOC and nitrogen cycling. 2.0% and 2.5% Bio-MPs showed significantly higher (p < 0.001) DOC and DON (on D46 and D105) and ≥1.5% Bio-MPs showed significantly lower (p = 0.02) AN (on D46). Overall, Bio-MPs exerted stronger effects on the dynamics of soil carbon and nitrogen cycling. In conclusion, microplastics might pose serious threats to agroecosystems and further research is needed.

Adaptive multi-paddock grazing enhances soil carbon and nitrogen stocks and stabilization through mineral association in southeastern U.S. grazing lands

Grassland soils are a large reservoir of soil carbon (C) at risk of loss due to overgrazing in conventional grazing systems. By promoting regenerative grazing management practices that aim to increase soil C storage and soil health, grasslands have the potential to help alleviate rising atmospheric CO₂ as well as sustain grass productivity across a vast area of land. Previous research has shown that rotational grazing, specifically adaptive multi-paddock (AMP) grazing that utilizes short-duration rotational grazing at high stocking densities, can increase soil C stocks in grassland ecosystems, but the extent and mechanisms are unknown. We conducted a large-scale on-farm study on five "across the fence" pairs of AMP and conventional grazing (CG) grasslands covering a spectrum of southeast United States grazing lands. We quantified soil C and nitrogen (N) stocks, their isotopic and Fourier-transform infrared spectroscopy signatures as well as their distribution among soil organic matter (SOM) physical fractions characterized by contrasting mechanisms of formation and persistence in soils. Our findings show that the AMP grazing sites had on average 13% (i.e., 9 Mg C ha^-1) more soil C and 9% (i.e., 1 Mg N ha^-1) more soil N compared to the CG sites over a 1 m depth. Additionally, the stocks' difference was mostly in the mineral-associated organic matter fraction in the A-horizon, suggesting long-term persistence of soil C in AMP grazing farms. The higher N stocks and lower ¹⁵N abundance of AMP soils also point to higher N retention in these systems. These findings provide evidence that AMP grazing is a management strategy to sequester C in the soil and retain N in the system, thus contributing to climate change mitigation.

Biochar rhizosphere addition promoted Phragmites australis growth and changed soil properties in the Yellow River Delta

Biochar addition can enhance plant growth and change soil physicochemical properties in saline soil. However, it is unclear whether the positioning of biochar additions (e.g., rhizosphere addition and surface addition) alters such impacts and whether such positioning effects interact with salinity levels. In the Yellow River Delta, China, we carried out a field experiment in which biochar was not added (control) or was added to the soil surface (surface addition) or to the soil at the rhizosphere position (rhizosphere addition) of Phragmites australis in three sites with different salt levels (1‰ - low, 5‰ - medium and 10‰ - high). Rhizosphere addition of biochar significantly improved the growth of P. australis, especially its fine root mass. Both rhizosphere addition and surface addition of biochar significantly decreased nitrate nitrogen content and electrical conductivity, and the inhibitory effects were more effective at the sites with medium and high salt levels in 2018. Structural equation modeling showed that biochar addition could directly increase the fine root mass of P. australis by decreasing the soil electrical conductivity, further improving the total mass of P. australis. Overall, rhizosphere addition of biochar is a better choice for improving the productivity of P. australis in saline soil and is beneficial to P. australis wetland restoration in the Yellow River Delta. Long-term field research is needed to better understand the effect and mechanism of biochar application.

Moderately prolonged dry intervals between precipitation events promote production in Leymus chinensis in a semi-arid grassland of Northeast China

BACKGROUND

Climate change is predicted to lead to changes in the amount and distribution of precipitation during the growing seasonal. This "repackaging" of rainfall could be particularly important for grassland productivity. Here, we designed a two-factor full factorial experiment (three levels of precipitation amount and six levels of dry intervals) to investigate the effect of precipitation patterns on biomass production in Leymus chinensis (Trin.) Tzvel. (a dominant species in the Eastern Eurasian Steppe).

RESULTS

Our results showed that increased amounts of rainfall with prolonged dry intervals promoted biomass production in L. chinensis by increasing soil moisture, except for the longest dry interval (21 days). However, prolonged dry intervals with increased amount of precipitation per event decreased the available soil nitrogen content, especially the soil NO₃^--N content. For small with more frequent rainfall events pattern, L. chinensis biomass decreased due to smaller plant size (plant height) and fewer ramets. Under large quantities of rain falling during a few events, the reduction in biomass was not only affected by decreasing plant individual size and lower ramet number but also by withering of aboveground parts, which resulted from both lower soil water content and lower NO₃^--N content.

CONCLUSION

Our study suggests that prolonged dry intervals between rainfall combined with large precipitation events will dramatically change grassland productivity in the future. For certain combinations of prolonged dry intervals and increased amounts of intervening rainfall, semi-arid grassland productivity may improve. However, this rainfall pattern may accelerate the loss of available soil nitrogen. Under extremely prolonged dry intervals, the periods between precipitation events exceeded the soil moisture recharge interval, the available soil moisture became fully depleted, and plant growth ceased. This implies that changes in the seasonal distribution of rainfall due to climate change could have a major impact on grassland productivity.

A systematic analysis and review of the impacts of afforestation on soil quality indicators as modified by climate zone, forest type and age

This global systematic analysis and review investigate the impacts of previous land use system, climate zone, forest type and forest age on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) stock, soil bulk density (BD) and pH at soil layers 0-20, 20-60 and 60-100 cm, following afforestation. Data came from 91 publications on SOC, TN and TP stock changes, covering different countries and climate zones. Overall, afforestation significantly increased SOC by 46%, 52% and 20% at 0-20, 20-60 and 60-100 cm depths, respectively. It also significantly increased shallower TN stocks by 28% and 22% at 0-20 and 20-60 cm depths, respectively, but had no overall impacts on TP. Previous land use system had the largest influence on SOC, TN and TP stock changes, with greater accumulations on barren land compared to cropland and grassland. Climate zone influenced SOC, TN and TP stock changes, with greater accumulations for moist cool than other climate zones. Broadleaf forests were better than coniferous forests for increasing SOC, TN and TP stocks of the investigated soil profile (0-100 cm). Afforestation for <20 years accumulated SOC and TN stocks only at the soil surface (0-20 cm), whilst afforestation for >20 years accumulated SOC and TN stocks to 100 cm soil depth. Changes to SOC and TN were positively correlated at depths down to 100 cm under all age groups, demonstrating that an increase TN could offset progressive N limitation, and maintains SOC accumulation as forests age. TP stock decreased significantly in topsoil (0-20 cm) for <20-year-old forest and did not change for >20-year-old forest, suggesting that it may become a limiting factor for carbon sequestration as forests age. Following afforestation, soil BD decreased alongside significant increases in SOC and TN stocks to 100 cm depth, but had no relationship with TP.

Poly-γ-glutamic acid bioproduct improves the coastal saline soil mainly by assisting nitrogen conservation during salt-leaching process

Salt-leaching is considered to be a major method for soil desalting in agriculture. Therefore, conservation of soil nutrition is significant to soil fertility and environment protection during the salt-leaching process. The effect of poly-γ-glutamic acid bioproduct (PGAB), which was manufactured by solid-state fermentation with the bacteria producing glutamic acid (GA) and poly-γ-glutamic acid (γ-PGA) and organic waste, on keeping nitrogen (N) during salt-leaching was investigated in this study. The isolated bacteria producing GA and γ-PGA were identified as Brevibacterium flavum and Bacillus amyloliquefaciens, respectively. After the saline soil was leached for 90 days, compared to the control, soil salinity (0-30 cm) in the PGAB treatment was decreased by 39.9%, while soil total N was significantly (P < 0.05) higher than other treatments. Furthermore, the microbial biomass N (0-30 cm) in PGAB treatment was increased by 119.5%; populations of soil total bacteria, fungi, actinomyces, nitrogen-fixing bacteria, ammonifying bacteria, nitrifying bacteria, and denitrifying bacteria and soil algae biomass were also significantly (P < 0.05) increased. In terms of physical properties, the percentage of soil aggregates with diameter > 0.25 mm was increased by 293.5%, and the soil erosion-resistance coefficient was increased by 50.0%. In conclusion, the PGAB can effectively conserve soil N during the process of salt-leaching and therefore offer a sustainable way to improve coastal saline soil.

Integrating hyperspectral imaging with machine learning techniques for the high-resolution mapping of soil nitrogen fractions in soil profiles

Soil nitrogen (N) plays a central role in soil quality and biogeochemical cycles. However, little is known about the distribution and spatial variability of the different fractions of soil N within entire soil profiles. This study aimed to investigate the potential of laboratory-based hyperspectral imaging (HSI) spectroscopy to retrieve and map total N (TN), available N (AvailN), ammonium N (NH₄-N), nitrate N (NO₃-N), and microbial biomass N (MBN) in soil profiles at a high resolution. HSI images of eleven intact soil profiles of 100 ± 5 cm depth from three typical soil types were recorded. A variety of nonlinear machine learning techniques, such as artificial neural networks (ANN), cubist regression tree (Cubist), k-nearest neighbour (KNN), support vector machine regression (SVMR) and extreme gradient boosting (XGBoost), were compared with a partial least squares regression (PLSR) to determine the most suitable model for the prediction of the various soil N fractions. Overall, the results showed that nonlinear techniques performed better than PLSR in most cases, with a high coefficient of determination (R²) and low root mean square error (RMSE). Among the models, SVMR was found to be superior to the other tested models for TN (R²_P = 0.94, RMSE_P = 0.17 g kg^-1), AvailN (R²_P = 0.94, RMSE_P = 13.35 mg kg^-1), NO₃-N (R²_P = 0.82, RMSE_P = 7.31 mg kg^-1), and NH₄-N (R²_P = 0.70, RMSE_P = 1.51 mg kg^-1) based on independent validation, whereas MBN (R²_P = 0.63, RMSE_P = 6.62 mg kg^-1) was predicted best by KNN. In addition, SVMR required less computational time and was less sensitive to spectral noise. It can therefore be concluded that HSI spectroscopy combined with SVMR is suitable for the high-resolution mapping of various soil N fractions in soil profiles.

Plastic film mulching affects N2O emission and ammonia oxidizers in drip irrigated potato soil in northwest China

In soil, ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) are regarded as key factors mediating nitrous oxide (N₂O) production and emission. However, there are scarce reports about the effect of film mulching on ammonia oxidizers, and the biological nitrification process of N₂O emission is unclear. This study was based on potato field experiments under different mulching films, including polyethylene mulching film (PM), transparent degradable mulching film (TDM), black degradable mulching film (BDM), and bare land (CK). AOB and AOA abundances were estimated using real-time PCR, and their diversity and community structure were measured using high-throughput sequencing. Result revealed that the total N₂O emission from CK was 12.32%-41.03% higher than that from film mulching soil. Under film mulching, according to total N₂O emission from soil and N₂O concentration in soil treatments were ordered as PM > BDM > TDM, and N₂O production was closely correlated with copy numbers of the amoA gene. BDM significantly increased the number of amoA-AOB gene copies (P < 0.01), and PM significantly increased the those of amoA-AOA (P < 0.01). BDM and TDM increased AOB operational taxonomic units (OTUs), Chao1, Simpson, and Shannon indices, while PM increased the AOA OTUs and Chao1 index. Variations in AOA abundance and diversity were closely related to soil mineral N and temperature changes induced by polyethylene film mulching (P < 0.05), whereas AOB showed no significant association with soil properties. Meanwhile, we did not find a distinct treatment effect on AOB community structure. Our findings indicate that (i) degradable film mulching increased AOB abundance and diversity and N₂O concentration, but obviously reduced N₂O emissions, and (ii) AOA were more sensitive than AOB to polyethylene mulching film.

Effects of nitrogen addition on microbial residues and their contribution to soil organic carbon in China's forests from tropical to boreal zone

Atmospheric nitrogen (N) deposition has a significant influence on soil organic carbon (SOC) accumulation in forest ecosystems. Microbial residues, as by-products of microbial anabolism, account for a significant fraction of soil C pools. However, how N deposition affects the accumulation of soil microbial residues in different forest biomes remains unclear. Here, we investigated the effects of six/seven-year N additions on microbial residues (amino sugar biomarkers) in eight forests from tropical to boreal zone in eastern China. Our results showed a minor change in the soil microbial residue concentrations but a significant change in the contribution of microbial residue-C to SOC after N addition. The contribution of fungal residue-C to SOC decreased under low N addition (50 kg N ha^-1 yr^-1) in the tropical secondary forest (-19%), but increased under high N addition (100 kg N ha^-1 yr^-1) in the temperate Korean pine mixed forest (+21%). The contribution of bacterial residue-C to SOC increased under the high N addition in the subtropical Castanopsis carlesii forest (+26%) and under the low N addition in the temperate birch forest (+38%), respectively. The responses of microbial residue-C in SOC to N addition depended on the changes in soil total N concentration and fungi to bacteria ratio under N addition and climate. Taken together, these findings provide the experimental evidence that N addition diversely regulates the formation and composition of microbial-derived C in SOC in forest ecosystems.

Shrub Encroachment Following Wetland Creation in Mixedgrass Prairie Alters Grassland Vegetation and Soil

Wetland decline under post-European settlement and land use change across western Canada has led to mitigation strategies, including wetland creation. Created wetlands can trigger environmental change, including woody species encroachment, in turn altering vegetation and soil. We quantify changes in shrub abundance from prior to wetland creation (1949) until 60 years later (2012) within a Mixedgrass ecosystem of the Verger watershed in Alberta, Canada. In addition, we compare remaining grassland with areas colonized by shrubland on similar ecosites for differences in (1) plant composition, including native and introduced flora, (2) herbage yield and forage accessibility for livestock, and (3) soil properties (surface organic depth, bulk density, mineral nitrogen (N), and carbon (C) concentration). Repeat photos show Shepherdia argentea shrublands increased from 0 to 88 ha (to 1.15% of study area) following wetland creation, with the greatest increase in the last 20 years. Relative to grasslands, shrublands had lower total plant diversity but greater presence of introduced plant species. Shrub patches were 94% lower in herbaceous production, with 77% of shrublands non-utilized by cattle, collectively leading to reduced grazing capacity. Relative to grasslands, shrublands had a thicker soil surface mulch layer, and where cattle were present, had increased mineral soil N and C. Overall, shrub encroachment following wetland creation has markedly altered vegetation and soils in this once grassland landscape, with negative impacts on native plant diversity, herbage production and forage accessibility, and has implications for the management of shrub encroachment.

Effect of plant hedgerows on agricultural non-point source pollution: a meta-analysis

Eutrophication has been a critical environmental issue due to soil nitrogen (N) and phosphorus (P) loss in runoff from agricultural lands. Plant hedgerow is an important measure to prevent soil erosion and reduce agricultural non-point source pollution (NPSP). In the present study, we searched 3683 research papers on plant hedgerows published from 1980 to March 2020. After screening, we used 53 effective papers on plant hedgerows for the meta-analysis by using Stata 15.1. The results showed that plant hedgerows significantly increased soil organic matter (SOM) (standardized mean difference (SMD) = 1.46; 95% confidence interval (CI) = 1.12-1.80 > 0), total N (TN) (SMD = 1.33; 95% CI 0.98-1.68 > 0), total P (SMD = 0.73; 95% CI 0.26-1.20 > 0), alkali N (SMD = 0.86; 95% CI 0.52-1.21 > 0), available P (SMD = 1.28; 95% CI 0.75-1.81 > 0) and readily available potassium (K) (SMD = 1.20; 95% CI 0.75-1.65 > 0) concentrations but exhibited no significant effects on soil total K concentration (SMD = 0.17; 95% CI - 0.13-0.47 < 0). Plant hedgerows showed a greater effect on SOM increase than soil N, P, and K, and soil TN increase than the available state, but the opposite trend was observed for P and K. This meta-analysis can clarify the influence of plant hedgerows on soil nutrients and provide ideas for the prevention and control of agricultural NPSP.

Integrating cover crops with chicken grazing to improve soil nitrogen in rice fields and increase economic output

Winter fallow is important for renewing and improving soil fertility under double-cropping rice systems, such as those in southern China. Using a regenerative farming technology of integrating grass-chicken farming in a winter fallow field, we investigated soil nitrogen conversion and assessed the agricultural economic benefits of the whole farmland ecosystem. To test the effects of chicken grazing on the fallow system, we established field treatments involving adding chickens to a field planted with the cover crops, including cover milk vetch (Astragalus sinicus) with chicken grazing treatment (MC) and cover ryegrass (Lolium spp.) with chicken grazing (RC); cover crops only, including cover milk vetch (Astragalus sinicus) treatment (M) and cover ryegrass (Lolium spp.) (R); and a bare fallow field treatment (CK). We found that both cover crops (M and R) and cover crops with chicken grazing (MC and RC) increased nitrate, ammonium, dissolved organic nitrogen, and total nitrogen contents, and the increase was higher in MC and RC treatments. We also observed increased straw biomass and grain yield in the all four treatments, with more increases with chicken treatments as compared with CK. On the economic profits, MC increased by 101.72% and RC increased by 104.12% as compared with CK, while R increased by 5.19% and M reduced by 1.86% as compared with CK. The nitrogen transfer rate (the output/input ratio) of MC, RC, M, and R increased by 66.71%, 71.50%, 65.97%, and 59.97%, respectively, while the nitrogen accumulation rate (input-output) of MC, RC, M, and R increased by 480.56%, 612.98%, 356.74%, and 267.65%, respectively. Our study demonstrates that retaining nitrogen and gaining economic profit by integrating cover crops with chicken grazing is potentially more sustainable than adding cover crops alone. We further suggest that using the integrated grass-livestock farming technology can reduce environmental damage caused by commercial fertilizers.

Strategic grazing management and nitrous oxide fluxes from pasture soils in tropical dairy systems

Greenhouse gases emissions are considered one of the most important environmental issues of dairy farming systems. Nitrous oxide (N₂O) has particular importance owing to its global warming potential and stratospheric ozone depletion. The objective of this study was to investigate the influence of two rotational grazing strategies characterized by two pre-grazing targets (95% and maximum canopy light interception; LI_95% and LI_Max, respectively) on milk production efficiency and N₂O fluxes from soil in a tropical dairy farming system based on elephant grass (Pennisetum purpureum Schum. cv. Cameroon). Results indicated that LI_95% pre-grazing target provided more frequent defoliations than LI_Max. Water-filled pore space, soil and chamber temperatures were affected by sampling periods (P₁ and P₂). There was a significant pre-grazing target treatment × sampling period interaction effect on soil NH₄⁺ concentration, which was most likely associated with urinary-N discharge. During P₁, there was a greater urinary-N discharge for LI_95% than LI_Max (26.3 vs. 20.9 kg of urinary-N/paddock) caused by higher stocking rate, which resulted in greater N₂O fluxes for LI_95%. Inversely, during P₂, the soil NH₄⁺ and N₂O fluxes were greater for LI_Max than LI_95%. During this period, the greater urinary-N discharge (46.8 vs. 44.8 kg of urinary-N/paddock) was likely associated with longer stocking period for LI_Max relative to LI_95%, since both treatments had similar stocking rate. Converting hourly N₂O fluxes to daily basis and relating to milk production efficiency, LI_95% was 40% more efficient than LI_Max (0.34 vs. 0.57 g N-N₂O/kg milk·ha). In addition, LI_95% pre-grazing target decreased urea-N loading per milk production by 34%. Strategic grazing management represented by the LI_95% pre-grazing target allows for intensification of tropical pasture-based dairy systems, enhanced milk production efficiency and decreased N-N₂O emission intensity.

Effects of grassland afforestation on structure and function of soil bacterial and fungal communities

Grassland afforestation strongly influences the structure and function of soil microorganisms. Yet the mechanisms of how afforestation could simultaneously alter both the soil fungal and bacterial communities and its implications for ecosystem management are poorly understood, especially in nitrogen-limited ecosystems. Using high-throughput sequencing of 16S rRNA and ITS rRNA genes, the present study investigated the changes in soil properties and soil microorganisms after afforestation of natural grasslands with Chinese pine (Pinus tabuliformis) on the Loess Plateau in China. Results showed that soil bacterial diversity had no significant differences among the grassland (GL), forest-grassland transition zone (TZ), and forestland (FL), while soil fungal diversity in the GL was significantly higher than that in the FL and TZ (P < 0.05). The proportion of shared OTUs in the soil bacterial community was higher than that in the soil fungal community among the three land use types. The dominant bacterial phylum shifted from Proteobacteria to Actinobacteria, while the dominant fungal phylum shifted from Ascomycota to Basidiomycota after the GL conversion to the FL. The functional groups of ECM fungi increased significantly while biotrophic fungi decreased significantly after grassland afforestation. Both the soil bacterial and fungal communities in the TZ showed great similarity with those in the FL. In addition, among all examined soil properties, soil nitrogen (N) showed a more significant effect on the soil microbial communities. The reduction of soil N after grassland afforestation resulted in both the structure and function changes in soil microbial communities. Our results demonstrated simultaneously differential changes in the composition and diversity of both soil bacterial and fungal communities after afforestation from grasslands to planted forests.

Effect of roxarsone metabolites in chicken manure on soil biological property

Roxarsone (ROX), an organoarsenic feed additive, occurs as itself and its metabolites including As(V), As(III), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in animal manure. Animal manure improves soil biological property, whereas As compounds impact microorganisms. The integral influence of animal manure bearing ROX metabolites on soil biological quality is not clear yet. Herein, the effect of four chicken manures excreted by chickens fed with four diets containing 0, 40, 80 and 120 mg ROX kg^-1, on soil biological attributes. ROX addition in chicken diets increased total As and ROX metabolites in manures, but decreased manure total N, ammonium and nitrate. The elevated ROX metabolites in manures increased soil total As, As species and total N, and increased first and then decreased soil nitrate and nitrite, but did not affect soil ammonium in manure-applied soils. The promoting role of both soil As(III) and ammonium on soil microbial biomass carbon and nitrogen, respiration and saccharase activity, were exceeded or balanced by the inhibiting effect of soil nitrate. The suppression of soil catalase activity by soil As(V) was surpassed by the enhancement caused by soil nitrate and nitrite. Soil urease, acid phosphatase and polyphenol oxidase activities were not suitable bioindicators in the four manure-amended soils. Soil DMA did not affect soil biological properties, and MMA was not detectable in all manure-amended soils. The above highlights the complexity of joint influence of soil As and N on biological attributes. Totally, when ROX is used at allowable dose in chicken diet, soil biological quality would be suppressed in manure-amended soil.

Spatio-temporal distribution of soil nitrogen in Poyang lake ecological economic zone (South-China)

Revealing the spatio-temporal distribution of soil nitrogen (N) contributes to N management and prevention of N pollution. The objective of this work is to study the spatio-temporal distribution of soil N and their driving factors in the topsoil (0-20 cm) of farmland in Yugan county, China in 1982 and 2012. Data were collected from 200 sampling sites of the second national soil survey in Yugan in 1982 and 423 sampling sites of the soil testing and formula fertilization project in 2012. On average total N (TN) and available N (AN) significantly increased from 1.50 g kg^-1 and 153.04 mg kg^-1 in 1982 to 1.58 g kg^-1 and 179.75 mg kg^-1 in 2012, respectively. The distance of spatial autocorrelation for TN increased from 2.79 to 6.18 km and from 2.97 to 18.00 km for AN from 1982 to 2012. The nugget/sill ratio for TN (0.472 in 1982 and 0.581 in 2012) indicated that soil TN driving by natural characteristics in 1982 to human activities in 2012. The nugget/sill ratio for soil AN (0.471 in 1982 and 0.688 in 2012) indicated that soil AN is more influenced by human activities. The major factors driving the spatio-temporal distribution of soil N was N application rate. To promote the sustainable development of agriculture and eco-environment, we should improve the awareness of farmers on chemical fertilizers (particularly N) and the level of N fertilizer management, increase the use of manure and organic fertilizer and facilitate rational fertilization by farmers.

Dynamics of soil carbon and nitrogen stocks after afforestation in arid and semi-arid regions: A meta-analysis

Arid and semi-arid regions store approximately 27% of global soil organic carbon (SOC), thus playing an important role in the global carbon cycle. Although afforestation is widely used to achieve the goals of landscape protection and ecological restoration in these regions, its effects on the dynamics of SOC and total nitrogen (TN) stocks as well as the related controlling factors are still poorly understood. In this study, a meta-analysis was conducted by compiling results from published studies to evaluate the influences of afforestation on dynamics of SOC and TN stocks in mineral soils (0-30cm) in arid and semi-arid regions, and to examine whether the changes in SOC and TN stocks were influenced by prior land-use, planted tree species and plantation age. The results showed that overall, afforestation significantly increased SOC stock by 131% and TN stock by 88%. Prior land-use was found to be the most important factor that influenced the dynamics of SOC and TN stocks following afforestation. Significantly larger increases in SOC and TN stocks were observed after barren land afforestation in comparison to cropland and grassland afforestation. The accumulations in SOC and TN stocks after afforestation decreased in the following order: broadleaf deciduous forests>coniferous forests>broadleaf evergreen forests. However, significant differences in SOC and TN accumulations were only detected between broadleaf deciduous forests and broadleaf evergreen forests. Plantation age did not affect the dynamics of SOC and TN stocks after afforestation, mainly due to the rapid accumulations in SOC and TN after barren land afforestation. The results indicate that afforestation, especially on barren land with broadleaf deciduous species, provides substantial opportunities for SOC and TN sequestrations in arid and semi-arid regions.

Effect of home construction on soil carbon storage-A chronosequence case study

Urbanization results in the rapid expansion of impervious surfaces, therefore a better understanding of biogeochemical consequences of soil sealing is crucial. Previous research documents a significant reduction in soil carbon and nitrogen content, however, it is unclear if this decrease is a result of top soil removal or long-term soil sealing. In this study, soil biogeochemical properties were quantified beneath homes built on a crawl space at two depths (0-10 cm, and 10-20 cm). All homes, 11-114 years in age, were sampled in the Piedmont region of Alabama and Georgia, USA. This age range enabled the use of a chronosequence approach to estimate carbon loss or gain under the sampled homes. The difference in soil carbon content beneath homes and adjoining urban lawns showed a quadratic relation with age. Maximum C loss occurred at approximately fifty years. The same pattern was observed for MBC: C ratio suggesting that the soil carbon content was decreasing beneath the homes for first fifty years, then increased afterward. The average soil C and N content in the top 10 cm were respectively 61.86% (±4.42%), and 65.77% (±5.65%) lower underneath the homes in comparison to urban lawns. Microbial biomass carbon (MBC), and nitrogen (MBN) were significantly lower below the homes compared to the urban lawns, while bulk density and phosphorus content were higher beneath the homes.

Low C/N ratio raw textile wastewater reduced labile C and enhanced organic-inorganic N and enzymatic activities in a semiarid alkaline soil

Application of raw and treated wastewater for irrigation is an extensive practice for agricultural production in arid and semiarid regions. Raw textile wastewater has been used for cultivation in urban and peri-urban areas in Pakistan without any systematic consideration to soil quality. We conducted a laboratory incubation study to investigate the effects of low C/N ratio raw textile wastewater on soil nitrogen (N) contents, labile carbon (C) as water-soluble C (WSC) contents, and activities of urease and dehydrogenase enzymes. The 60-day incubation study used an alkaline clay loam aridisol that received 0 (distilled water), 25, 50, and 100% wastewater concentrations, and microcosms were incubated aerobically under room temperature at 70% water holding capacity. Results revealed that raw wastewater significantly (p < 0.05) changed soil N pools and processes, WSC contents, and enzymatic activities. The organic and inorganic N species increased with increasing wastewater concentrations, whereas WSC contents followed an opposite trend. The highest NH₄⁺-N and NO₃^--N contents were observed in soil treated with 100% wastewater. The extractable organic N (EON) contents always represented >50% of the soil total Kjeldahl N (TKN) contents and served as the major N pool. However, nitrification index (NO₃^--N/NH₄⁺-N ratio) decreased at high wastewater concentrations. A significant negative correlation was observed between EON and WSC (p < 0.05) and between net nitrification and WSC/EON ratio (p < 0.01). In contrast, nitrification index and WSC contents were correlated, positively suggesting WSC potentially controlling N turnover in nutrient-poor aridisol. We found significant (p < 0.0001) positive correlations of soil urease and dehydrogenase enzymatic activities with soil-extractable mineral N contents indicating coupled N cycling and soil biological activity. Higher production and accumulation of soil NO₃^--N and EON contents in concentrated wastewater-treated soil could pose an ecological concern for soil fertility, biological health, and water quality. However, the EON could lead to mineral N pool but only if sufficient labile C source was present. The effects of wastewater irrigation on soil N cycling need to be assessed before it is recommended for crop production.

Enrichment of natural 15N abundance during soil N losses under 20years of continuous cereal cropping

It is generally accepted that the enrichment of natural ¹⁵N abundance in soil over time is reflective of historic N cycling and loss, but this process in cropping soils is not yet clear. In this study, we identified an enrichment gradient of natural ¹⁵N abundance during 20-year chronosequence of cereal cropping on Alfisols in southwest Queensland, Australia, that have no history of fertilisation. We demonstrate that the increase in soil ¹⁵N abundance is explained by isotopic fractionation of ¹⁵N during organic N mineralisation and nitrification, which lead to isotopically heavier ammonium retained in the soil and isotopically lighter soil nitrate taken up and removed by seasonal crops during harvest. Here we present a framework for natural ¹⁵N isotopic fractionation co-occurring with N losses during long-term cultivation.

Forest burning affects quality and quantity of soil organic matter

Fire alters ecosystem carbon cycling and generates pyrogenic matter such as charcoal, which can be incorporated into soils. The incorporation and cycling of charcoal in soils is a potential carbon sink, but studies investigating charcoal and carbon dynamics in soils are still lacking. We investigated soil carbon, charcoal and nitrogen dynamics in the top 20cm of a sandy soil within a eucalypt forest in eastern Australia at three sites representing a chronosequence from 3months to 14years post-fire. In the short-term, fire removed litter, but resulted in an increase in both the charcoal and non-charcoal SOC content of the soils, which we attribute to above-ground charcoal generation and its incorporation into the soil profile, as well as below-ground root mortality. On a decadal timeframe, charcoal was preferentially retained in the sandy soil, in which other stabilisation mechanisms are limited, so that the influx of dead root carbon had no remnant effects. The incorporation and retention of charcoal in the soil profile is highly important to carbon cycling in such sandy soils with high fire frequency. It is highly likely that these effects are not limited to the upper 20cm of soil and future studies should investigate deep soil charcoal cycling.

Assessment of carbon storage under rainforests in Humic Hapludox along a climosequence extending from the Atlantic coast to the highlands of northeastern Brazil

An understanding of the stock of soil organic carbon (SOC) in the umbric epipedon of Oxisols located in the tropical forests surrounded by a semi-arid region is limited but essential because of their importance in the global cycle of carbon (C). The purpose of this study was to assess the effects of climatic (temperature and rainfall), soil organic matter (SOM) composition and litter on the stability of C in surfaces and subsurfaces in five Humic Oxisols along a 475-km climosequence from 143 to 963ma.s.l. in a tropical environment in northeastern Brazil. We assessed vertical changes in SOC; soil total nitrogen (N); C from the microbial biomass; δ(13)C, δ(15)N and the humified composition of SOM; the composition of the humin (HUM) fraction by Fourier Transform Infrared (FTIR); and Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) at depth. The elemental and isotopic composition of the litter samples were analyzed in all areas studied. The results indicated that the current climate and recalcitrant organic compounds are not preponderant factors in the formation of the umbric epipedon, as suggested by the partial influence of temperature and rainfall on SOM. In addition, SOM was dominated by easily decomposable compounds, as indicated by the predominance of aliphatic C-H groups in the HUM fraction in the FTIR spectra; by the thermal oxidation through DSC-TG, which revealed that approximately 50% of the HUM was composed easily decomposable compounds; and by the high proportion of organic C present in the microbial biomass. Values of δ(13)C showed a predominance of C3 plant-C in SOM whereas δ(15)N patterns indicated that N dynamics differ among the profiles and drive the accumulation of C. These findings can help to characterize the susceptibility of these soils to changes in climate and land use and the implications for the sequestration of soil C.

Mid-infrared spectroscopy for rapid assessment of soil properties after land use change from pastures to Eucalyptus globulus plantations

There is an increasing demand for rapid and cost effective techniques to accurately measure the effects of land use change on soil properties. This study evaluated the ability of mid-infrared spectroscopy (MIRS) coupled with partial least squares regression (PLSR) to rapidly predict soil properties affected by land use change from agriculture (mainly pasture) to Eucalyptus globulus plantations in south-western Australia. We measured total organic carbon (TOC), total nitrogen (Total N), TOC/Total N (C/N ratio), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and total phosphorus (Total P). The PLSR calibration models were developed using mid-infrared (MIR) spectra (4000 to 450 cm(-1)) and square root transformed measured soil data (n = 180) from 23 paired pasture and E. globulus plantation sites representing the soils and climate of E. globulus plantation estates in south-western Australia. The calibration models for TOC, Total N, C/N ratio and Total P showed excellent correlations between measured and predicted data with coefficient of determination (R(2)) exceeding 0.91 and minimum root-mean-square error (RMSE) of calibration [TOC (R(2) = 0.95, RMSE = 0.36), Total N (R(2) = 0.96, RMSE = 0.10), C/N ratio (R(2) = 0.92, RMSE = 0.14) and Total P (R(2) = 0.91, RMSE = 0.06)]. The calibration models had reasonable predictions for MBC (R(2) = 0.66, RMSE = 0.07) and MBN (R(2) = 0.63, RMSE = 0.06). The calibrated models were validated using soils from 8 independent paired pasture and E. globulus sites (n = 64). The validated predictions were excellent for TOC (R(2) = 0.92, RMSE = 0.40) and Total N (R(2) = 0.91, RMSE = 0.12), but less so for C/N ratio (R(2) = 0.80, RMSE = 0.35), MBC (R(2) = 0.70, RMSE = 0.08) and Total P (R(2) = 0.75, RMSE = 0.12). The results demonstrate the potential of MIRS-PLSR to rapidly, accurately and simultaneously determine several properties in land use change affected soils.

Spatiotemporal variations and factors affecting soil nitrogen in the purple hilly area of Southwest China during the 1980s and the 2010s

Determination of soil nitrogen distributions and the factors affecting them is critical for nitrogen fertilizer management and prevention of nitrogen pollution. In this paper, the spatiotemporal variations of soil nitrogen and the relative importance of their affecting factors were analysed at a county scale in the purple hilly area of the mid-Sichuan Basin in Southwest China based on soil data collected in 1981 and 2012. Statistical results showed that soil total nitrogen (TN) increased from 0.88 g kg(-1) in 1981 to 1.12 g kg(-1) in 2012, whereas available nitrogen (AN) decreased from 84.22 mg kg(-1) to 74.35 mg kg(-1). In particular, AN showed a significant decrease in agricultural ecosystems but remained stable in woodland and grassland. Correspondingly, most of the study area exhibited increased TN content and decreased AN content in space. The nugget/sill ratios of TN and AN increased from 0.419 to 0.608 and from 0.733 to 0.790, whereas spatial correlation distances decreased from 12.00 km to 9.50 km and from 9.50 km to 9.00 km, respectively, suggesting that the spatial dependence of soil nitrogen became weaker and that the extrinsic factors played increasingly important roles in affecting the soil nitrogen distribution. Soil group and land use type were the two dominant factors in 1981, followed by topographic factors, vegetation coverage and parent material, whereas land use type became the most important factor in 2012, and the relative contribution of topographic factors declined markedly. The results suggested that land use related to cultivation management and fertilizer application was the decisive factor for soil nitrogen change. The increase in TN content and the decrease in AN content over the study period also suggested improper use of nitrogen fertilizer, which can result in nitrogen loss through increasing nitrification rates. Thus, effective measures should be taken to increase the uptake rate of nitrogen and prevent nitrogen pollution.

Contrasting effects of untreated textile wastewater onto the soil available nitrogen-phosphorus and enzymatic activities in aridisol

Water shortage and soil qualitative degradation are significant environmental problems in arid and semi-arid regions of the world. The increasing demand for water in agriculture and industry has resulted in the emergence of wastewater use as an alternative in these areas. Textile wastewater is produced in surplus amounts which poses threat to the environment as well as associated flora and fauna. A 60-day incubation study was performed to assess the effects of untreated textile wastewater at 0, 25, 50, 75, and 100% dilution levels on the physico-chemical and some microbial and enzymatic properties of an aridisol soil. The addition of textile wastewater provoked a significant change in soil pH and electrical conductivity and soil dehydrogenase and urease activities compared to the distilled-water treated control soil. Moreover, compared to the control treatment, soil phosphomonoesterase activity was significantly increased from 25 to 75% application rates, but decreased at 100% textile wastewater application rate. Total and available soil N contents increased significantly in response to application of textile wastewater. Despite significant increases in the soil total P contents after the addition of textile wastewater, soil available P content decreased with increasing concentration of wastewater. Changes in soil nutrient contents and related enzymatic activities suggested a dynamic match between substrate availability and soil N and P contents. Aridisols have high fixation and low P availability, application of textile wastewater to such soils should be considered only after careful assessment.

Turnover of organic carbon and nitrogen in soil assessed from δ13C and δ15N changes under pasture and cropping practices and estimates of greenhouse gas emissions

The continuing clearance of native vegetation for pasture, and especially cropping, is a concern due to declines in soil organic C (SOC) and N, deteriorating soil health, and adverse environment impact such as increased emissions of major greenhouse gases (CO2, N2O and CH4). There is a need to quantify the rates of SOC and N budget changes, and the impact on greenhouse gas emissions from land use change in semi-arid subtropical regions where such data are scarce, so as to assist in developing appropriate management practices. We quantified the turnover rate of SOC from changes in δ(13)C following the conversion of C3 native vegetation to C4 perennial pasture and mixed C3/C4 cereal cropping (wheat/sorghum), as well as δ(15)N changes following the conversion of legume native vegetation to non-legume systems over 23 years. Perennial pasture (Cenchrus ciliaris cv. Biloela) maintained SOC but lost total N by more than 20% in the top 0-0.3m depth of soil, resulting in reduced animal productivity from the grazed pasture. Annual cropping depleted both SOC and total soil N by 34% and 38%, respectively, and resulted in decreasing cereal crop yields. Most of these losses of SOC and total N occurred from the >250 μm fraction of soil. Moreover, this fraction had almost a magnitude higher turnover rates than the 250-53 μm and <53 μm fractions. Loss of SOC during the cropping period contributed two-orders of magnitude more CO2-e to the atmosphere than the pasture system. Even then, the pasture system is not considered as a benchmark of agricultural sustainability because of its decreasing productivity in this semi-arid subtropical environment. Introduction of legumes (for N2 fixation) into perennial pastures may arrest the productivity decline of this system. Restoration of SOC in the cropped system will require land use change to perennial ecosystems such as legume-grass pastures or native vegetation.

Contingent effects of plant species on soils along a regional moisture gradient in the Great Plains

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling.