Dynamics of soil organic carbon in density fractions during post-agricultural succession over two lithology types, southwest China

Novel perspective on qualitative assessment of swine manure compost maturity using organic carbon density fractions

Mature compost is safe and stable, yet quality assessments are challenging owing to current maturity indicators' limitations. This study employed density fractionation to separate organic carbon into light and heavy fractions, offering a new perspective for assessing maturity. Results showed that light fraction organic carbon progressively transitioned into heavy fraction during composting, reducing the proportion of total organic carbon from 82.82% to 44.03%, while heavy fraction organic carbon increased to 48.58%. During the first seven days, the reduction rate of light fraction organic carbon decreased slowly, while the increase rate of heavy fraction declined sharply, levelling off thereafter. Light/heavy fraction organic carbon ratio was significantly correlated with existing maturity indicators (carbon/nitrogen ratio, humic acid/fulvic acid ratio, biological growth-related indicators), with the ratio below 1.33 serving as a potential compost maturity marker. Thus, given its simplicity and reliability, organic carbon density fractions is an innovative indicator for compost maturity assessments.

Relationships between arsenic biotransformation genes, antibiotic resistance genes, and microbial function under different arsenic stresses during composting

Although the arsenic contamination and antibiotic resistance genes (ARGs) during composting have been studied separately, there is limited information on their interactions, particularly, the relationship between arsenic biotransformation genes (ABGs) and ARGs. Therefore, the present study used different forms of arsenic stress (organic and inorganic arsenic at 10 and 50 mg/kg) in pig manure and straw co-composting, to evaluate the effects of arsenic stress on microbial community structures, metabolic function, ABGs, and ARGs. The results showed that arsenic stress had different effects on different parameters and promoted the microbial formation of humic acid and the biodegradation of fulvic acid. Inorganic arsenic showed more rapid effects on microbial community structure, visible within about 20 days, while the effects of organic arsenic were later (about 45 days) due to the necessity of transformation. Moreover, the addition of organic roxarsone and inorganic arsenic resulted in higher expression of ABGs and ARGs, respectively. Arsenic addition also caused increased expression of genes associated with replication and repair. A significant relationship was observed between ABG and ARG expression, for instance, genes involved in arsenic reduction and oxidation were influenced by genes involved in aminoglycoside and chloramphenicol resistance genes (p < 0.05). These complex interactions among microorganisms, functional genes, and external parameters contribute to the understanding of the mechanisms underlying cross-contamination.

Impacts of Lithology and Slope Position on Arbuscular Mycorrhizal Fungi Communities in a Karst Forest Soil

The influence of lithology and slope position on arbuscular mycorrhizal fungi (AMF) communities has been explored in various ecosystems, but there is a limited understanding of these mechanisms in karst regions. This study focused on typical karst hills with contrasting lithologies, specifically dolomite and limestone. Additionally, three slope positions (upper, middle, and lower) were investigated within each hill in karst forest ecosystems. Total phosphorus (TP) content in the soil was higher in dolomite compared to limestone. Conversely, exchangeable calcium (Ca) was lower in dolomite than in limestone. Notably, the lithology, rather than the slope position, exerted a significant impact on AMF diversity and abundance and the presence of specific AMF taxa. Dolomite exhibited greater AMF richness and a higher Shannon index in comparison to limestone when not accounting for slope position. The AMF community composition differed between dolomite and limestone. For instance, without considering slope position, the relative abundance of Acaulospora, Diversispora, and Paraglomus was higher in dolomite than in limestone, while the relative abundance of Claroideoglomus displayed an opposing trend. Furthermore, a more complex interaction among AMF taxa was observed in dolomite as compared to limestone, as evidenced by an increase in the number of nodes and edges in the co-occurrence networks within the dolomite. The genera Glomus, Claroideoglomus, and Diversispora exhibited a higher number of links with each other and with other AMF taxa. The study identified TP and Ca as the primary factors determining variations in AMF diversity between dolomite and limestone. Consequently, it is imperative to consider the underlying lithology and soil conditions when addressing the restoration of degraded karst hilly areas.

Effect of different vegetation restoration on soil organic carbon dynamics and fractions in the Rainy Zone of Western China

Vegetation restoration on purple soil (Eutric Leptic Regosols) slopes aiming at reducing soil erosion in the Rainy Zone of Western China has significantly altered soil organic carbon (SOC) storage and distribution. A better understanding of the effects of different vegetation restoration types on SOC dynamics and fractions is critical in devising better policy to protect or enhance SOC stocks to improve soil quality and ecosystem function. In the present study, total, labile, and non-labile organic carbon (TOC, LC, and NLC), and carbon management index (CMI) of Cryptomeria fortunei (CF), mixed C. fortunei and Betula luminifera (MF), Neosinocalamus affinis (NA), and Camellia sinensis (CS) were compared with those of Zea mays field (ZM) on purple soil slopes in the Rainy Zone of Western China in order to develop more effective ways to implement vegetation restoration in the future. Different vegetation restoration types (CF, MF, NA and CS) increased TOC stock by 47.79%-118.31% and NLC stock by 56.61%-129.52% in the 0-50 cm soil layer compared with that of ZM. The direction and magnitude of changes in LC stock and CMI, however, depended strongly on the vegetation restoration type. Compared with ZM, CF had the largest increase of LC stock and CMI, whereas NA had the largest decrease of LC stock and CMI in the 0-50 cm soil layer. The LC:TOC ratio in four reforested species all declined significantly compared with that of ZM (p < 0.01), indicating decreased SOC activity after afforestation. The vegetation type and soil depth together explained more than 90% of the changes of TOC and its fractions in the plantations on purple soil slopes. Our study demonstrates that transforming the ZM into the CS is optimal to achieve the sustainable development goal, whereas transforming the ZM into the NA reduces the SOC activity and availability.

Difference in total N and its aggregate-associated N following cropland restoration in a karst region, Southwest China

Cropland conversion has been cited as one of the most effective measures for increasing soil nitrogen pool in karst degraded regions. However, it is still unclear how N associated with aggregate patterns and their contribution to net soil N accumulation after cropland conversion. The experiment included four treatments with one control and three restoration strategies, that is, maize-soybean rotation cultivation (the control), sugarcane, mulberry, and forage grass cultivation. Soil samples were selected to determine the soil aggregate amount and its associated N content and stock across 0-30 cm soil layer. Macro-aggregate (> 2 mm) was the predominant aggregate fraction in all cropland use types and had the largest N stock. Forage grass cultivation substantially increased N stocks in bulk soil and aggregate fractions. The N contents and stocks associated with aggregate were shown to be positively correlated with bulk soil N stocks. Furthermore, the increase in N stock in forage grass soil was largely caused by an increase in N stock within macro-aggregates (> 2 mm), which is further attributed to the increased N content within macro-aggregates. Overall, forage grass cultivation replaced maize-soybean cultivation which was proposed as an ecological restoration model to improve soil N sequestration capacity due to its function in increasing the N stock of aggregate in the karst degraded region of Southwest China.

Dynamics of aggregate-associated organic carbon after long-term cropland conversion in a karst region, southwest China

Cropland conversion has a major impact on soil C sequestration. However, it remains unclear about the changes in soil aggregate and their contribution to C accumulation following cropland conversion in a karst region, southwest China. In this study, three different cropland use types (sugarcane, mulberry and forage grass cultivation) were selected to replace maize-soybean cultivation. The soil was collected at a depth of 0 to 30 cm for analysis of soil aggregates and their OC content. Results showed that macro-aggregate was the predominant component underlying four cropland use types. Forage grass cultivation remarkably increased the OC stock and aggregate stability (MWD and GMD). OC content and stock associated with aggregate varied with cropland use types and soil depth, but were typically highest in forage grass fields. Macro-aggregates contained higher OC content and stock than other aggregate fractions, along with soil depth underlying four cropland use types. The increases in OC stock in forage grass field was mainly due to increased OC stocks within macro-aggregates, which is further attributed to the increase in OC content within macro-aggregates. Overall, forage grass cultivation replaced maize-soybean cultivation was suggested as an ecological restoration model to enhance soil C sequestration potential, owing to its role in increasing OC stock of aggregation and aggregate stability, in the karst region of southwest China.

Alterations in soil microbial phospholipid fatty acid profile with soil depth following cropland conversion in karst region, southwest China

The soil microbial community is a key indicator used to evaluate the effectiveness of land use changes in degraded ecosystems. However, there is currently limited information on the vertical variations of soil microbial communities under different cropland types in the karst region of southwest China. In this study, three different cropland types (sugarcane, mulberry, and forage grass) were selected to replace maize-soybean cultivation in a karst area of southwest China. Secondary forest was included for comparison. Soil was collected at soil depths of 0-10 cm, 10-20 cm, 20-30 cm, and 30-50 cm to conduct a phospholipid fatty acid (PLFA) analysis of the soil microbial communities. The results showed that the abundances of total PLFAs and PLFAs of bacteria, fungi, actinomycetes, and arbuscular mycorrhizal fungi were significantly higher in forage grass cultivation and secondary forest areas than those in other three cropland types. The abundance of total PLFA and its functional groups significantly decreased with increasing soil depth. Bacteria was the predominant community (> 52%) across all land use types and soil depths. A principal component analysis indicated that soil microbial PLFA composition in forage grass field and secondary forest areas differed significantly from that in mulberry, sugarcane, and maize-soybean fields across 0-50-cm soil layer. Soil organic carbon was identified as the primary factor affecting the abundance and structure of soil microbial communities in the four soil layers. These results suggest that greater improvements in soil quality and fertility could be made by replacing the maize-soybean rotation system with forage grass cultivation than by replacing it with mulberry or sugarcane cultivation in the karst region of southwest China.

Karst tiankeng create a unique habitat for the survival of soil microbes: Evidence from ecoenzymatic stoichiometry

Clarifying the soil microbial metabolism and resource limitations could help to understand the functions and processes of aboveground ecosystems, as well as to predict ecosystem stability under global climate change. Karst tiankeng is a kind of large-scale negative surface terrain on the surface which is similar to an oasis in degraded karst landscapes, but their soil microbial resource limitations still unclear. In this study, we evaluated and compared the soil microbial resource limitation in non-degraded tiankeng (NDT), moderately degraded tiankeng (MDT), heavily degraded tiankeng (HDT), and outside tiankeng (OT) by calculating soil ecoenzymatic stoichiometry. Overall, soil microbial communities were more limited by C and P in karst tiankeng ecosystem. The soil microbial C and P limitations significantly differed with the karst tiankeng degradation increased, and the lowest C and P limitations were observed in NDT. The higher microbial C and P limitations were observed in OT. Linear regression and redundancy analysis indicated that soil microbial C and P limitations were significantly influenced by soil nutrients. Karst tiankeng degradation influence the biogeochemical cycle and function of karst tiankeng systems. Our results highlight that karst tiankeng (especially the NDT) can provide a stable habitat for the survival of microorganisms in karst areas. Karst tiankeng is essential for regional ecological restoration and biodiversity conservation.

Stoichiometric Variation in Soil Carbon, Nitrogen, and Phosphorus Following Cropland Conversion to Forest in Southwest China

Soil organic carbon (SOC), nitrogen (N), and phosphorus (P) are three essential soil nutrients for plant growth, and their stoichiometric ratios are already important indices of elemental balance and the soil fertility status in soil ecosystems. The evolution mechanism of the SOC, Total Nitrogen (TN), Total Phosphorus (TP), and stoichiometry following the “conversion of cropland to forest program” (CCFP) in southwest China is not yet clear. Seven different CCFP restoration models, including Zenia insignis (RD), Toona sinensis (XC), Castanea mollissima (BL), Citrus reticulate (GJ), Zenia insignis and Guimu-1 elephant grass (RG), Guimu-1 elephant grass (GM), and abandoned cropland (LH), were chosen to explore changes in the concentration and stoichiometry of the SOC, TN, and TP, and their recovery times, at a depth of 0–100 cm. The results indicate that the SOC and TN concentrations in different restoration models all increased with restoration years in the topsoil, whereas the soil TP concentration remained relatively stable. The soil C:N and C:P ratios increased with increasing restoration years in the topsoil, whereas the N: P ratio was relatively stable over time. After ten years of reforestation, the SOC and TN concentrations decreased as the soil layer increased. The effects of the restoration model on the C: N ratios were greater in shallow soils. Our results suggest a complex reaction of SOC, soil TN, and soil TP concentrations and stoichiometry to the vegetation restoration mode, particularly in the topsoil. This research further improves the understanding of SOC, N, and P interactions and restricted nutrition, and provides relevant theoretical support for vegetation restoration in the southwest karst region.

Binding characteristics of humic substances with Cu and Zn in response to inorganic mineral additives during swine manure composting

Composting is suitable for recycling livestock manure into valuable organic fertilizer, which can improve soil quality while mitigating potential risk of heavy metal pollution. Humic substances (HS) in compost have been demonstrated to play a key role in regulating the redistribution of heavy metal fractions. However, limited direct information have been reported on how different components of HS complexes with heavy metals to affect their bioavailability during composting. In this study, sequential extraction procedures (H₂O, KCl, Na₄P₂O₇, NaOH and HNO₃) were used to assess the characteristics that HS bound with Cu and Zn during composting of swine manure and straw added either 5% boron waste (BW) or 5% phosphate rock (PR). Organically complexed fraction extracted by Na₄P₂O₇ contained only 33-41% of the Cu but most of the Zn (81-87%). During composting, initially mobile fractions of Cu and Zn (extracted by H₂O or KCl) changed into more stable fractions (extracted by NaOH and HNO₃), and both organic matter and fulvic acids (FA) were identified as critical factors to explain this redistribution based on redundancy analysis. Over 80% of Cu and Zn were complexed with FA of HS. However, exogenous additives (phosphate rock and boron waste) enhanced Cu conversion by promoting humification (Humic acid/Fulvic acids, HA/FA) whereas they had limited influence on Zn, due to the relatively weak binding relationship between Zn and HA.

Evidence linking calcium to increased organo-mineral association in soils

Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO₃-bearing) or without carbonates (CaCO₃-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ ¹³C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ ¹³C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO₃-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ ¹³C values. Bulk SOC content was twice as high in the CaCO₃-bearing profiles, which also had lower bulk δ ¹³C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δ ¹³C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ ¹³C values.

SUPPLEMENTARY INFORMATION

The online version of this article (10.1007/s10533-021-00779-7) contains supplementary material, which is available to authorized users.

Resource limitation of soil microbes in karst ecosystems

Knowledge about resource limitation to soil microbes is crucial for understanding ecosystem functions and processes, and for predicting ecosystem responses to global changes as well. Karst ecosystems are widespread in the world, and play a key role in regulating the global climate, however, the patterns of and mechanisms underlying microbial resource limitation in karst ecosystems remain poorly known. Here we investigated the microbial resource limitation in a karst region, by selecting four main land-use types, i.e. cropland, grassland, shrubland and secondary forest, in areas underlain by two lithology types, i.e. dolomite and limestone, in southwest China. Ecoenzymatic stoichiometry was used as an indicator of microbial resource limitation. Overall, soil microbes in karst ecosystems were more limited by carbon and phosphorus, rather than by nitrogen. Further analyses revealed that the patterns of carbon and phosphorus limitation were different among land-use or lithology types. Microbial carbon limitation was greatest in cropland and forest but lowest in grassland, and was greater under dolomite than under limestone. Microbial phosphorus limitation decreased from secondary forest to cropland under dolomite areas, but showed no difference among ecosystem types under limestone areas, indicating that lithology controls the pattern of microbial phosphorus limitation along the post-agriculture succession. Our study describes a general pattern of microbial resource limitation in karst ecosystems, and we suggest that lithology may provide a new mechanism for explaining the variations of microbial resource limitation along the post-agriculture succession in different regions.

Seasonal Changes and Vertical Distribution of Fine Root Biomass During Vegetation Restoration in a Karst Area, Southwest China

In karst ecosystems, plants absorbing smaller amounts of nutrients, owing to shallow soil, show limited growth. In addition, fine roots (diameter < 2 mm) contribute to the regulation of nutrient cycles in terrestrial ecosystems. However, the spatial and temporal variations of fine root biomass in different vegetation types of the karst region remains poorly understood. In this study, we investigated the seasonal and vertical variation in biomass, necromass, and total mass of fine roots using sequential soil coring under different stages of vegetation restoration (grassland, shrubland, secondary forest, and primary forest) in Southwest China. The results showed that the fine root biomass and necromass ranged from 136.99 to 216.18 g m^-2 and 47.34 to 86.94 g m^-2, respectively. The total mass of fine roots and their production ranged from 187.00 to 303.11 g m^-2 and 55.74 to 100.84 g m^-2 year^-1, respectively. They showed a single peak across the vegetation restoration gradient. The fine root biomass and total fine root mass also showed a single peak with seasonal change. In autumn, the fine root biomass was high, whereas the necromass was low. Most of the fine roots were concentrated in the surface soil layer (0-10 cm), which accounted more than 57% root biomass, and decreased with increasing soil depth. In addition, fine root production showed a similar vertical pattern of variation with biomass. Overall, our results suggested that fine roots show clear seasonal and vertical changes with vegetation succession. Moreover, there was a higher seasonal fluctuation and a greater vertical decreasing trend in late-successional stages than in the early-successional stages. The conversion of degraded land to forest could improve the productivity of underground ecosystems and vegetation restoration projects in the fragile karst region should, therefore, continue.