Variations in the profile distribution and protection mechanisms of organic carbon under long-term fertilization in a Chinese Mollisol

Molecular insight into the vertical migration and degradation of dissolved organic matter in riparian soil profiles

Due to the molecular complexity of dissolving organic matter (DOM), the vertical molecular distribution of riparian soil DOM (especially dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP)) in different land use types and their relationship with the bacterial community is still unclear. This study analyzed the spectral characteristics of riparian soil DOM from 0 to 100 cm in wild grassland, agricultural land, and bare land. The molecular distribution of DOM was revealed through Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the specific relationship between DOM and bacterial community composition (BCC) was evaluated. The results showed that the DOM in the upper soil layer (0-40 cm) was mainly composed of recalcitrant macromolecular organics, while that in the lower layer (40-100 cm) was labile small molecular organics. In agricultural land, the total storage of DOM was lower than that in wild grassland, but with a higher abundance of recalcitrant organic carbon (lignin, etc.). At the same time, the bacterial community in agricultural land is shifting towards copiotrophs. In addition, the abundance of labile C degrading genes increases with nitrate as the main electron acceptor. However, sulfates are mainly used as electron acceptors in wild grasslands. Both DOP and DON were dominated by lignin and displayed higher chemical diversity in the upper soil. The bioavailability of DOP in three types of soil is higher than that of DON. DOM-BCC network analysis shows that the recalcitrant DON and DOP molecules in soil are positively correlated with phylum Actinobacteriota in agricultural land. These results emphasize that the DOM molecular characteristics were closely related to the function of the soil bacterial community.

Soil Carbon Stocks and Greenhouse Gas Mitigation of Agriculture in the Brazilian Cerrado-A Review

New agricultural practices and land-use intensification in the Cerrado biome have affected the soil carbon stocks. A major part of the native vegetation of the Brazilian Cerrado, a tropical savanna-like ecoregion, has been replaced by crops, which has caused changes in the soil carbon (C) stocks. To ensure the sustainability of this intensified agricultural production, actions have been taken to increase soil C stocks and mitigate greenhouse gas emissions. In the last two decades, new agricultural practices have been adopted in the Cerrado region, and their impact on C stocks needs to be better understood. This subject has been addressed in a systematic review of the existing data in the literature, consisting of 63 articles from the Scopus database. Our review showed that the replacement of Cerrado vegetation by crop species decreased the original soil C stocks (depth 0-30 cm) by 73%, with a peak loss of 61.14 Mg ha^-1. However, when analyzing the 0-100 cm layer, 52.4% of the C stock data were higher under cultivated areas than in native Cerrado soils, with a peak gain of 93.6 Mg ha^-1. The agricultural practices implemented in the Brazilian Cerrado make low-carbon agriculture in this biome possible.

Physically separated soil organic matter pools as indicators of carbon and nitrogen change under long-term fertilization in a Chinese Mollisol

Isolation and quantification of soil organic matter (SOM) pools under the influence of management practices is needed for assessing the changes in soil fertility. However, the knowledge on how the active, slow and passive pools of SOM respond to long-term fertilization is scarce. Therefore, the present study was designed to isolate the active, slow, and passive pools of soil organic matter through physical fractionation under long-term fertilization. The treatments included; inorganic fertilization (NPK) either alone or combined with a normal dose of manure (MNPK) or a high dose of manure (1.5MNPK) with an unfertilized control (CK) for comparison. The isolated pools were analyzed and compared for their sizes, SOC and TN storage and their contribution to total SOC and TN sequestration. The results revealed that the fertilization enhanced the active, slow and passive pools of SOC and TN and their storage under applied treatments was patterned as 1.5MNK > MNPK > NPK > CK. The highest SOC and TN storage was observed in the active pool, while, greater response to fertilization (in terms of response ratio) was associated with the slow pool. Results show that fertilization enhanced the proportion of SOC and TN stocks to bulk SOC and TN stocks in active and slow pools, while a diminishing trend was found for passive pools. Moreover, the highest response ratio was found for TN sequestration in each pool as compared to SOC, suggesting preferential accumulation of TN over SOC in the studied soil. Nevertheless, the highest SOC and TN storage took place in the active pool. The slow pool showed greater response to applied fertilizer, with the highest values being observed under 1.5MNPK. This study concluded that long-term manure + inorganic fertilization is crucial for enhancing C and N sequestration by altering the size and response of SOM pools.

Fallow Land Enhances Carbon Sequestration in Glomalin and Soil Aggregates Through Regulating Diversity and Network Complexity of Arbuscular Mycorrhizal Fungi Under Climate Change in Relatively High-Latitude Regions

Soil aggregation and aggregate-associated carbon (C) play an essential function in soil health and C sequestration. Arbuscular mycorrhizal fungi (AMF) are considered to be primary soil aggregators due to the combined effect of extraradical hyphae and glomalin-related soil proteins (GRSPs). However, the effects of diversity and network complexity of AMF community on stability of soil aggregates and their associated C under long-term climate change (CC) and land-use conversion (LUC) in relatively high-latitude regions are largely unexplored. Therefore, an 8-year soil plot (with a 30-year cropping history) transplantation experiment was conducted to simulate CC and LUC from cropland to fallow land. The results showed that Glomus, Paraglomus, and Archaeospora were the most abundant genera. The diversity of AMF community in fallow land was higher than cropland and increased with increasing of mean annual temperature (MAT) and mean annual precipitation (MAP). Fallow land enhanced the network complexity of AMF community. The abundance families (Glomeraceae and Paraglomeraceae) exhibited higher values of topological features and were more often located in central ecological positions. Long-term fallow land had a significantly higher hyphal length density, GRSP, mean weight diameter (MWD), geometric mean diameter (GMD), and C concentration of GRSP (C-GRSP) than the cropland. The soil aggregate associated soil organic carbon (SOC) was 16.8, 18.6, and 13.8% higher under fallow land compared to that under cropland at HLJ, JL, and LN study sites, respectively. The structural equation model and random forest regression revealed that AMF diversity, network complexity, and their secreted GRSP mediate the effects of CC and LUC on C-GRSP and aggregate-associated SOC. This study elucidates the climate sensitivity of C within GRSP and soil aggregates which response symmetry to LUC and highlights the potential importance of AMF in C sequestration and climate change mitigation.

Straw incorporation improved the adsorption of potassium by increasing the soil humic acid in macroaggregates

Straw incorporation has been broadly demonstrated to be effective for the maintenance of soil potassium (K) fertility in farmlands, which increases K and carbon (C) inputs and improves soil stability due to aggregate formation and physiochemical bonding. However, the response of K retention in aggregate fractions (AFs) to soil organic carbon (SOC) changes is poorly understood. Field trials under a completely random experimental design considering two factors, straw return and K fertilization, were conducted to study the comprehensive effects of SOC and various AFs on soil K adsorption. The results indicated that the soil exchangeable and nonexchangeable K pools (EKP and NKP) increased upon straw incorporation due to an increase in macroaggregates (>2 mm fraction). The synergistic increase in SOC and humic acid (HA) contents, which resulted in a complex molecular structure and improved soil aggregation, promoted K adsorption. Good linear relationships existed between the apparent K balance and the EKP and NKP values in the >2 mm fraction. Structural equation modeling (SEM) indicated that SOC and various AFs exerted positive and significant effects on soil EKP and NKP, and thus verified 96% of the total variation in K adsorption. Thus, combination of straw and K fertilization increased the aggregate-associated C and K, which were primarily correlated with the >2 mm fraction. These direct measurements and estimates provide insights into the aggregates associated with K, which enhances the understanding of the chemical behavior of soil K upon straw incorporation.

Stability of soil organic carbon under long-term fertilization: Results from 13C NMR analysis and laboratory incubation

Long-term fertilization has shown a high relevance as regards soil organic carbon (SOC) sequestration, but the degree of stability of the sequestered SOC has not been widely studied up to now. Using physical fractionation combined with laboratory incubation and NMR spectroscopy, we evaluated the differences in SOC stability caused by long-term fertilization. Four SOC fractions were isolated and examined for contents and chemical composition and cumulative amount of CO₂-C respired from the fractions under six fertilization treatments: control (CK); balanced inorganic fertilization (NPK); NPK combined with pig manure (MNPK); NPK combined 1.5 times of pig manure (1.5MNPK); and NPK combined with high amount of manure (M2NPK). The highest contents of SOC were recorded for the coarse particulate organic carbon (cPOC) fraction, ranging from 17.25 to 30.47 g kg^-1 under CK and M2NPK. The highest cumulative amount of CO₂-C was released from the cPOC fraction under manure treatments (M2NPK and 1.5NPKM), which was 56 and 43% higher than that from CK, whereas the lowest amount of CO₂-C was released from the mineral associated-C (MOC) fraction under the same treatments, being 65 and 49% higher than that released from CK, suggesting low SOC stability in cPOC and high SOC stability in MOC fractions. However, manure treatments (M2NPK and 1.5NPKM) greatly lowered the specific amount of C-mineralized (C-mineralized per unit total SOC) in fractions and whole soil, suggesting the ability of manure to accumulate more SOC by reducing SOC losses. Moreover, carbonyl-C was found to be the form of SOC experiencing major degree of sequestration under current fertilization practices. The SOC stability indices; aromaticity index (AI), hydrophobicity index (HI) and alkyl-C/O-alkyl-C were found to be higher in manure treated plots further suggesting higher stability of SOC under manure addition. Thus, long-term manure combined with mineral fertilizers would enhance SOC stability through minimizing SOC losses and promoting accumulation of stable C forms in a Chinese Mollisol.

Optimizing residue and tillage management practices to improve soil carbon sequestration in a wheat-peanut rotation system

The sustainable development of agriculture has been challenged by the decline of soil quality and the change of climate. It is well known that soil carbon (C) sequestration plays crucial roles in improving soil structural stability, mitigating greenhouse emissions, and promoting plant nutrient supply. Therefore, a 3-year field experiment was conducted to evaluate the effects of different residue and tillage management practices on soil C sequestration in a wheat-peanut rotation system. Four treatments were studied: moldboard plow tillage with wheat residue returning (PTS), rotary tillage with wheat residue returning (RTS), no tillage with wheat residue mulching (NTS), and no tillage with wheat residue removal (NT). Our results indicated that residue return favored the improvement of soil C sequestration capacity relative to residue removal. In addition, NTS improved soil C sequestration in the surface soil layer (0-5 cm), but markedly reduced soil C sequestration in the deeper soil layers (5-30 cm). NTS thus caused a more obvious soil stratification phenomenon, which was not conducive to improving soil quality. At the 5-30 cm soil depths, the soil labile organic C fractions concentrations, carbon pool management index (CPMI), macroaggregates-associated C storage, intra-aggregate C fractions concentrations, and soil total organic carbon (TOC) storage under PTS were all higher than those under other treatments. Overall, a peanut strategic cultivation management mode that combines moldboard plow tillage and wheat residue return may be used as a reference for optimizing agricultural soil management to achieve the improvement of soil C sequestration capacity in a wheat-peanut rotation system.

Soil Compressibility and Resilience Based on Uniaxial Compression Loading Test in Response to Soil Water Suction and Soil Organic Matter Content in Northeast China

Due to the widespread use of heavy machinery, improper soil tillage practices, and insufficient soil organic materials input, soil compaction has become a major issue affecting soil function in modern agriculture and the sustainability of the environment. The aim of the present study was to evaluate the responses of soil mechanical parameters to soil water content and soil organic matter content (SOM), and to investigate the physical properties of nine disturbed soils in a black soil region in Northeast China. The soil samples were capillary saturated and subjected to 6, 10, 100, 600, and 800 kPa soil water suction (SWS), and pre-compression stress (σp), compression index (Cc), and decompression index (Dc) were measured. SWS and SOM, and their interaction, significantly influenced the mechanical parameters. σp increased with an increase in SWS until 600 kPa, while Dc exhibited an opposite trend with an increase in SWS. Cc had a peak value at SWS of 100 kPa. All mechanical parameter values were higher under high SOM than under low SOM. σp, Cc, and Dc were influenced variably by different soil physicochemical factors. Structural equation modeling results revealed that soil mechanical parameters were directly and indirectly influenced by soil texture and mean weight diameter of aggregates, in addition to SOM and SWS. According to the results of the present study, based on soil mechanical and physical properties, increasing SOM and ensuring suitable soil water content during tillage could be applied as management strategies to minimize further soil compaction and improve soil resilience, and thus promote the sustainable development of agriculture in Northeast China.

Human disturbance drives loss of soil organic matter and changes its stability and sources in mangroves

Mangrove soils with high organic carbon (C_org) content are likely to contain C_org that is vulnerable to remineralization during land use changes. Mangrove conversion to different land uses might deplete soil C_org stocks causing variable carbon dioxide emissions, but the extent of these emissions and the fraction of soil C_org (i.e., labile or stable/recalcitrant) that is mostly lost is poorly understood. Here, we study mangrove soil C_org degradability and its susceptibility to mineralization after mangrove disturbance. We measured changes in soil properties, organic matter (OM) stability and C_org pools and sources across a mangrove disturbance gradient (i.e., pristine forests, degraded mangroves receiving domestic sewage and shrimp farm effluents, and shrimp ponds). Results showed that the conversion of mangroves to shrimp ponds caused the most severe changes in soil properties, OM and C_org characteristics. Shrimp pond soils contained the lowest OM-C_org pools, consisted mostly of stable OM (i.e., recalcitrant and refractory; 36.0 ± 5.7% of the total OM) and enriched δ¹³C_org (-22.6 ± 2.7‰). Conversely, control mangrove soils had the largest OM-C_org pools consisting of a large unstable OM fraction (i.e., labile; 46.4 ± 4.2%) and lighter δ¹³C_org (-26.8 ± 0.4‰) being characteristic of C_org from a mangrove origin. Conversion of mangroves to shrimp ponds and its degradation by shrimp farm and domestic sewage effluents caused a loss of 97%, 61%, and 35% of soil C_org stocks in the upper meter, representing potential emissions of ~1200, 800, and 400 Mg CO₂ ha^-1, respectively. These losses were explained by enhanced OM mineralization of unstable fractions driven by the loss of the physico-chemical protection provided by fine-grained soils and vegetation cover. The differences in C_org stability among sites can be used to predict potential carbon dioxide produced during mineralization, hence aid at prioritizing areas for conservation, restoration or management.

Effects of Atrazine on Chernozem Microbial Communities Evaluated by Traditional Detection and Modern Sequencing Technology

Atrazine is a long residual herbicide commonly used in maize fields. Although atrazine can effectively control weeds and improve crop yield, long-term application leads to continuous pollution in the agricultural ecological environment, especially in the soil ecosystem, and its impact on soil microorganisms is still not clear. Four methods were used in the experiment to clarify the effect of atrazine on the bacterial populations of cultivated soil layers of chernozem in a cold region in different periods: high-performance liquid chromatography (HPLC), colorimetry, microplate, and high-throughput sequencing. The level of residual atrazine in cold chernozem decreased from 4.645 to 0.077 mg/kg soil over time, and the residue gradually leached into deep soil and then decreased after accumulating to a maximum value. Atrazine significantly affected the activities of urease and polyphenol oxidase activity in the soil layers at different periods but had no significant effect on sucrase and phosphatase activity. Atrazine significantly reduced the diversity of microbial carbon source utilization and total activity in soil layers of 0-10 and 20-30 cm but only reduced the diversity of microbial carbon source utilization in the 10-20 cm layer. Atrazine had no significant effect on bacterial populations (10-12 phyla, 29-34 genera), but had a slight effect on the relative abundance of various groups. Atrazine significantly reduced the diversity of bacterial populations in cultivated soil layers of chernozem in a cold region, and the diversity of bacterial populations decreased with decreased residue. This lays a foundation for guiding the safe use of herbicides on farmland in Northeast China.

Carbon stocks of homestead forests have a mitigation potential to climate change in Bangladesh

A total of 176 homestead forests at three altitudes in the Chittagong Hill Tracts, Bangladesh were randomly surveyed to estimate carbon (C) stocks and how stand structure affects the biomass C. All woody vegetations were measured, and litter and soil (0-30 cm depth) were sampled. The tree biomass C stock in the top two altitude forests was up to 37-48% higher than in low altitude, owing to significantly higher tree density and species diversity. An increase in species diversity index by one unit increased the biomass stock by 23 Mg C ha^-1. The C stock of litterfall in low altitude forests was 22-28% higher than in the top two altitude due to the deposition of litters downslope and deliberate use of mulch for soil improvement and conservation, resulting in up to 5% higher total soil C. The topsoil C was 10-25% higher than the deeper soil, depending on the altitude. The forest stored 89 Mg C ha^-1, indicating a potential for C sequestration in trees outside forest. This study would help policymakers to strengthen the recognition of small-scale forests for mitigation in REDD + (reducing emissions from deforestation and forest degradation, the role of conservation, sustainable management of forests, and enhancement of forest carbon stocks) and support owners through C credits from sustainably managed forests.

Long-term evaluation of the initiative 4‰ under different soil managements in Mediterranean olive groves

The 4‰ initiative implementation has increased the emphasis and interest in soil carbon and nitrogen storage in the last few years. This study evaluated the dynamics of soil organic carbon and total nitrogen under rain-fed olive groves over a long-term period (2004-2019). The management practices associated with achieving the 4‰ initiative objectives and the depth of analysis to measure the effectiveness of the initiative have generated uncertainties and wide debate in the scientific community. To contribute to this debate from a farm level, the objective of this study was to analyse the effects of conventional tillage and no-tillage with bare soil by using herbicides (after land management change from conventional tillage) on carbon and nitrogen stocks in complete soil profiles (depth > 100 cm) over 15 years in a Mediterranean olive grove. Soil samples were collected from each farm and analysed for carbon content and physical-chemical characteristics. This study indicates that management practices evaluated resulted in soil organic carbon and total nitrogen contents decreasing in soil, with a reduction >30% in all horizons. Results highlight a significant depletion of soil organic carbon stock with a significant decarbonisation process (-1.8 Mg C ha^-1 yr^-1) and total nitrogen stock (-0.57 and - 0.41 Mg N ha^-1 yr^-1) on average under both managements (no-tillage no tillage with herbicide and conventional tillage respectively) as compared to the initial situation. Furthermore, it was demonstrated that deep horizons are significant reservoirs of carbon (>50% in all cases) and in woody crops, its analysis within the dynamics of soil organic carbon stocks proposed by the 4‰ initiative was relevant. With these results, no-tillage with bare soil by using herbicides was demonstrated as an unsustainable agricultural practice and it is proposed to change the current soil management to sustainable management that increases the C inputs to achieve the 4‰ targets.

Long-term fertilization affects functional soil organic carbon protection mechanisms in a profile of Chinese loess plateau soil

Crop productivity and soil health are limited by organic carbon (OC), however, the variations in the mechanisms of SOC preservation in a complete soil profile subjected to long-term fertilization remains unclear. The objective of the study was to examined the content and profile distribution of the distinctive SOC protection mechanisms on a complete profile (0-100 cm) of Eumorthic Anthrosols in Northwest China after 23 years of chemical and manure fertilization. The soil was fractionated by combined physical-chemical and density floatation techniques. Throughout the profile, significant variations were observed among fractions. In the topsoil (0-20 and 20-40 cm), mineral coupling with the fertilization of manure (MNPK) enhanced total SOC content and recorded for 29% of SOC in the 0-20 and 20-40 cm layers. Moreover, MNPK increased the SOC content of the unprotected cPOC fraction by 60.9% and 61.5% in the 0-20 and 20-40 cm layer, while SOC content was low in the subsoil layers (40-60, 60-80 and 80-100 cm, respectively) compared with the control (C). The highest OC under MNPK in physically protected micro-aggregates (μagg) (6.36 and 6.06 g C kg^-1), and occluded particulate organic carbon (iPOC) (1.41 and 1.29 g C kg^-1) was found in the topsoil layers. The unprotected cPOC fraction was the greatest C accumulating fraction in the topsoil layers, followed by μagg and H-μSilt fractions in the soil profile, implying that these fractions were the most sensitive to the fertilization treatments. Overall, the unprotected, physically protected, and physico-chemically protected fractions were the dominant fractions for the sequestration of carbon across fertilization treatments and soil layers.

A Field Evidence of Cd, Zn and Cu Accumulation in Soil and Rice Grains after Long-Term (27 Years) Application of Swine and Green Manures in a Paddy Soil

Although inorganic and organic manures with high concentrations of heavy metals can lead to accumulation or contamination of heavy metals in soils, there are few reports on the effects of long-term application of swine and green manures on the accumulation of heavy metals in rice grains in paddy soils. A long-term field experiment, which was established in 1990 in paddy soil in Hangzhou, China, was used to investigate the effects of inorganic and organic manures on the availability and accumulation of heavy metals in soil and uptake by rice plant. The results showed that long-term application of nitrogen, phosphorus and potash (NPK) plus green manure or swine manure, and swine manure only increased 202%, 146%, and 100% for total Cd, and 5.5%, 7.6%, and 6.6% for total Cu in rice grains, respectively compared to the control without fertilization. Total Zn in rice grain was significantly increased by 13.9% for the treatment of NPK plus green manure. The accumulation of Cd, Zn, and Cu in rice grains after long-term application of swine and green manures is due to the combined effects of the increased concentrations of total and EDTA extractable Cd, Zn, and Cu in soil and the changes of soil properties. Furthermore, the highest bioconcentration factor for Cd was found in the treatment of NPK plus green manure while for Zn and Cu it was observed in NPK treatment. Thus, it may be concluded that green manure and manure with increased Cd, Zn, and Cu in rice grain results in a potential risk of metal accumulation in paddy soils.

Soil aggregation and soil aggregate stability regulate organic carbon and nitrogen storage in a red soil of southern China

Soil aggregation plays a critical role in the maintenance of soil structure, as well as in its productivity. Fertilization influences soil aggregation, especially by regulating soil organic carbon (SOC) and total nitrogen (TN) contents in aggregate fractions. The present study evaluated the influence of three contrasting fertilizer regimes (unfertilized control -CK-, mineral fertilization -NPK- and manure combined with NPK -NPKM) on soil aggregate stability, aggregate-associated organic carbon and total nitrogen sequestration and mineralization of SOC. Soil samples from (20 cm) depth were collected from a long-term fertilization experiment and analysed for size distribution ranging (>250 μm, 250-53 μm and <53 μm sizes), SOC and TN contents, as well as for mineralization of bulk and aggregate associated-SOC. Both NPK and NPKM fertilizations significantly enhanced SOC and TN contents in bulk soil and its constituent aggregates of >250 μm, 250-53 μm and <53 μm sizes, as compared to CK. Long-term NPK and NPKM increased SOC and TN stock in bulk soil by 45 and 98%, and by 70 and 144%, respectively, as compared to CK. Similarly, higher values of SOC and TN stock in all aggregate fractions was observed with the application of NPKM. Application of NPK and NPKM for 26 years significantly increased aggregate stability, which was positively correlated with total SOC contents in terms of mean weight diameter (MWD) (Adj. R² = 0.689, p < 0.03) and geometric mean diameter (GMD) (Adj. R² = 0.471, p < 0.24). Moreover, higher scores regarding cumulative mineralization for bulk soil and aggregate associated OC were observed with the application of NPK and NPKM. Irrespective of treatments, higher cumulative C-mineralization was observed for macro-aggregates (>250 μm size) followed by 250-53 μm and <53 μm size aggregates. Interestingly, a highly positive correlation was observed between aggregate stability and the cumulative amount of mineralization for bulk soil and aggregate fractions, with R² ranging from 0.84 to 0.99. This study evidenced that long-term fertilization of NPK and NPKM can improve soil aggregation, stability and associated OC and TN stock in aggregates, as well as aggregate-associated OC mineralization, which was further governed by aggregate size.