Soil Organic C Content Research Articles

Carbon and nitrogen dynamics as affected by land-use and management change from original rice paddies to orchard, wetland, parking area and uplands in a mountain village located in Shonai region, Northeast Japan

ABSTRACT This study investigated the effect of land-use and management change (LUMC) on carbon (C) and nitrogen (N) dynamics after 15–40 years. LUMC constituted change from rice paddies to chestnut orchard, wetland, and buckwheat upland fields in Shonai region, Yamagata Prefecture, Northeast Japan. Soil samples were collected from the top – (0–15 cm) and sub-layers (15–30 cm) for analysis of soil organic C (SOC) and its δ13C value, total N (TN), and their stocks. C decomposition (Dec-C) and net N mineralization (Net Min-N) were determined according to the production of CO2 and NH4 + + NO3 – by aerobic incubation and CO2 + CH4 and NH4 + by anaerobic incubation, respectively. The results reveal that 40 years after change from rice paddy to orchard and wetland, the SOC and TN contents in the top-layer were not significantly altered. However, in buckwheat upland fields and in the parking area where vegetation was absent, the SOC content decreased significantly. Conversion of rice paddies to amur silver grass wetland altered the soil δ13C the most, leading to an increase of 5.1‰ and 2.9‰ for the top – and sub-layers, respectively. In general, the incubation experiment results revealed that a change to orchard and wetland did not significantly decrease the Dec-C and Net Min-N. Whereas, the change from rice paddies to parking area significantly decreased the Dec-C and Net Min-N in both aerobic and anaerobic conditions. We conclude that LUMC over decades had various effects on the SOC and TN contents and stocks as well as their mineralization potentials.

The microbial-driven C dynamics within soil aggregates in walnut orchards of different ages based on microbial biomarkers analysis

Soil aggregates and microbial attributes are of great importance to clarify soil C dynamics in agroecosystems. However, whether and how walnut plantation with different ages affects microbial-driven soil C dynamics, especially at the aggregate scales, remains unclear. In this study, the microbial community (as measured by phospholipid fatty acids analysis), exo-enzyme activities (micro-plate enzyme method), and microbial residues (amino sugar analysis) were determined in soil aggregates, which were collected from surface soils in walnut plantations of various ages (0-, 7-, 14-, and 21-year) in the areas of Lincheng county, China. According to the optimal-moisture sieving method, soil aggregates were divided into three parts, including > 2 mm, 2–0.25 mm, and < 0.25 mm aggregates. Results showed that long-term walnut cultivation (14- and 21-year) improved soil aggregate stability as well as its associated nutrient conditions, and these changes were mainly reflected in the larger (>2 mm and 2–0.25 mm) aggregates. Besides, long-term walnut cultivation was favorable to promoting microbial growth (as indicated by total PLFAs), functions (hydrolase activities), and metabolism (microbial residues C, MRC) within aggregates. Irrespective of the walnut plantation ages, the above-mentioned microbial attributes increased with increasing soil aggregate sizes, whereas oxidase activities decreased. We also found that the increased plantation ages and decreased aggregate sizes could enhance the microbial contributions to soil organic C (SOC) accumulation (as indicated by the MRC/SOC ratios), although higher SOC contents have been found in the larger aggregates. Notably, the partial least squares path model further emphasized that walnut plantation ages and aggregate sizes had strong effects on microbial attributes (e.g., microbial communities, exo-enzyme activities, and microbial residues), as well as their importance for soil C dynamics. Consequently, the mature walnut plantations were beneficial for improving soil structure, microbial attributes, and SOC sequestration in the East Taihang Mountains in Hebei, China.

Root‐zone soil organic carbon enrichment is sensitive to land management across soil types and regions

AbstractMeasuring and monitoring changes in soil organic C (SOC) concentrations and stocks are important for understanding ecosystem functions. Sequestration of SOC has traditionally been measured from resource‐intensive studies over extended periods of time, but an alternative approach of calculating root‐zone SOC enrichment is described here as a proxy for estimating SOC sequestration using nonlinear distribution of SOC concentration in the surface 30 cm. From literature data across 52 studies, estimated SOC concentration at 30‐cm depth was not different between pairs of conservation management and conventional‐till cropland, suggesting support for the concept of using SOC concentration at 30‐cm depth as a baseline concentration from which enrichment can be calculated. Root‐zone SOC enrichment across this (n = 420) and an earlier study (n = 171) was 12.0 ± 0.6 Mg C ha–1 (mean ± SE) under conventional‐till cropland (n = 209), 16.1 ± 0.6 Mg C ha–1 under no‐till cropland (n = 269), 20.6 ± 1.0 Mg C ha–1 under grassland (n = 90), and 25.8 ± 3.8 Mg C ha–1 under woodland (n = 23). Regional climatic conditions and soil texture were important in influencing baseline SOC storage and total SOC stock but were minor factors in affecting root‐zone SOC enrichment with conservation management. Root‐zone SOC enrichment is a relatively simple and straightforward calculation method to indicate SOC sequestration, and if deployed across a diversity of management and farm locations, could lead to better SOC stock inventories and analyses of how contemporary management influences SOC stock change in different ecoregions.

Priming effect and its regulating factors for fast and slow soil organic carbon pools: A meta-analysis

The priming effect (PE) plays a critical role in the control of soil carbon (C) cycling and influences the alteration of soil organic C (SOC) decomposition by fresh C input. However, drivers of PE for the fast and slow SOC pools remain unclear because of the varying results from individual studies. Using meta-analysis in combination with boosted regression tree (BRT) analysis, we evaluated the relative contribution of multiple drivers of PE with substrate and their patterns across each driver gradient. The results showed that the variability of PE was larger for the fast SOC pool than for the slow SOC pool. Based on the BRT analysis, 67% and 34% of the variation in PE were explained for the fast and slow SOC pools, respectively. There were seven determinants of PE for the fast SOC pool, with soil total nitrogen (N) content being the most important, followed by, in a descending order, substrate C:N ratio, soil moisture, soil clay content, soil pH, substrate addition rate, and SOC content. The directions of PE were negative when soil total N content and substrate C:N ratio were below 2 g kg -1 and 20, respectively, but the directions changed from negative to positive with increasing levels of this two factors. Soils with optimal water content (50%–70% of the water-holding capacity) or moderately low pH (5–6) were prone to producing a greater PE. For the slow SOC pool, soil pH and soil total N content substantially explained the variation in PE. The magnitude of PE was likely to decrease with increasing soil pH for the slow SOC pool. In addition, the magnitude of PE slightly fluctuated with soil N content for the slow SOC pool. Overall, this meta-analysis provided new insights into the distinctive PEs for different SOC pools and indicated knowledge gaps between PE and its regulating factors for the slow SOC pool.

Decrease in soil pH has greater effects than increase in above‐ground carbon inputs on soil organic carbon in terrestrial ecosystems of China under nitrogen enrichment

Abstract Impacts of nitrogen (N) enrichment on soil carbon (C) budgets in terrestrial ecosystems have been well documented by numerous field experiments and syntheses. Although previous studies have largely attributed this phenomenon to the increased organic C inputs, the potential mechanisms of how N enrichment increases soil organic C (SOC) remain contentious. In this study, we conducted a meta‐analysis comprising 234 published field N‐addition experiments from multiple terrestrial ecosystems across China to evaluate the impacts of N enrichment on the SOC budget. Although the meta‐analysis revealed that N‐addition significantly enhanced plant biomass and SOC concentration across the selected studies, we found that SOC concentration was independent of, or even decreased with, the enhanced plant biomass due to increased soil C loss as increasing organic C inputs. The negative correlation between plant C inputs and SOC under N enrichment, meanwhile, appeared reversible by the concomitant changes in soil pH, which depended on the magnitude of soil acidification. Increased SOC in terrestrial ecosystems was closely associated with decreased soil pH, which reduced soil C losses by limiting microbial degradation. Synthesis and applications. We suggest that soil carbon (C) budget is determined by the trade‐off between plant C inputs and nitrogen (N)‐induced soil acidification. In contrast to other studies, our findings demonstrate that N‐mediated soil acidification, rather than increased above‐ground C inputs, is the main driver increasing soil organic C (SOC) under elevated N inputs in terrestrial ecosystems of China.

Charred plant fragment–aluminium complexes in buried humic horizons of a cumulative volcanic soil profile: Formation, accumulation and potential role

AbstractThe objective of the present study was to investigate the formation, accumulation and potential role of charred plant fragment (CPF)–aluminium (Al) complexes in several buried A (Ab) horizons of a cumulative volcanic soil profile, focusing on CPFs, active Al components and soil age. Nineteen soil samples were collected from four buried humic horizons (2A–5A, from 7.3 cal ka BP to AD 1471), and their CPF–carbon (C), soil organic C (SOC), acid oxalate‐extractable Al (Alox), pyrophosphate‐extractable Al (Alpy, Al–humus complexes) and short‐range ordered Al minerals (Alox − Alpy) were quantified. The proportion of CPF–C content (up to 15.8 g kg−1) to SOC content (up to 117 g kg−1) ranged from 2.3% to 15.1% (8.4% on average). The contents of active Al components (6.71–77.1 g kg−1) increased: Alpy &lt; Alox − Alpy &lt; Alox. No consistent trends with age were found for the accumulation levels of CPF–C, SOC and active Al components. However, significant positive correlations (p &lt; 0.001) were found between the Alpy and CPF–C and SOC contents, suggesting that the accumulation and stabilisation of soil organic matter in these humic layers proceeded simultaneously with the formation of organo–Al complexes, including CPF–Al complexes. On the other hand, significant negative correlations (p &lt; 0.01) were obtained between the Alox − Alpy and Alpy, CPF–C or SOC contents. From these correlations, it was assumed that the complexation of organic constituents, including CPFs, with Al competed with the synthesis of short‐range ordered Al minerals (including allophane and imogolite), irrespective of soil age and climatic conditions with the range of more than 6000 years. This competitive relationship in the presence of CPFs was first described in the current study.Highlights Significance of charred plant fragment–aluminium complexes in humic horizons was discussed. There were no consistent trends with age for the accumulation levels of organic constituents. The accumulation of organic matter proceeded simultaneously with the formation of the complexes. The formation of the complexes competed (&gt;6000 years) with the synthesis of aluminium minerals.

Distribution and functional groups of soil aggregate-associated organic carbon along a marsh degradation gradient on the Zoige Plateau, China

Alpine marsh wetlands in the Qinghai-Tibetan Plateau are ecological hotspots for global carbon (C) cycling in Central Asia; however, the soil C sinks are increasingly limited by marsh degradation. To assess the physical mechanisms by which changes in the soil C sequestration occur, the distribution and functional groups of soil organic C (SOC) in aggregates along a marsh degradation gradient on the Zoige Plateau in China were investigated using soil samples collected from differently degraded marshes. Our results showed that, beyond the state of light degradation, the large aggregate content decreased significantly by 31.1–55.2%, while the micro-aggregate content increased significantly by 23.1–93.1%. The SOC content in the aggregates decreased significantly by 39.1–84.8% and the dominant aggregate size for SOC preservation decreased to small or micro-aggregate. Marsh degradation also decreased the relative ratio of aliphatic-C while increasing that of alcohol and phenolic-C, and polysaccharide-C. Index I (relative decomposition of SOC in aggregates) was 73.5–429.0% higher for degraded marshes than for relatively pristine marsh (RPM), while Index Ⅱ (relative recalcitrance of SOC in aggregates) was 13.8–79.5% lower in comparison. Therefore, we concluded that marsh degradation probably reduces the organic C sequestration capacity of soil by increasing the decomposition of SOC in aggregates after both the dominant aggregate size for SOC preservation and aggregate stability decreased. This result represents the physical mechanism by which the decrease in SOC content occurs in response to marsh degradation in alpine regions.

Soil aggregate‐associated organic carbon and nitrogen response to long‐term no‐till crop rotation, cover crop, and manure application

AbstractSoil C and N sequestration using conservation practices to alleviate climate change are necessary; however, diverse crop management influences on aggregate‐associated C and N levels need consideration. Our objective was to compare bulk soil and aggregate‐associated C and N under cover crops hairy vetch (Vicia villosa Roth) and winter wheat (Triticum aestivum L.), poultry litter application, fallow control, and crop rotations of continuous cotton (Gossypium hirsutum L.), cotton–corn (Zea mays L.), continuous corn, corn–soybean [Glycine max (L.) Merr.], continuous soybean, and soybean–cotton in a long‐term (2002–2018) no‐tillage field experiment at Milan, TN. Soil organic C (SOC) and N were assessed at 0–15 and 15–30 cm bulk soil and in &gt;2, 0.25–2, 0.053–0.25, and &lt;0.053 mm aggregates at depths 0–5, 5–10, 10–15, and 15–30 cm. In 0–15 cm, poultry litter resulted in 4.14, 3.92, and 3.48 Mg ha–1 significantly (P ≤ .05) higher SOC, and 0.50, 0.52, and 0.44 Mg ha–1 significantly higher N than hairy vetch, wheat, and fallow, respectively. Continuous corn and corn–soybean showed significantly higher SOC content in &gt;2 and 0.25–2 mm at 0–5 cm, whereas in other crop rotations, 0.25–2 mm exhibited significantly higher SOC content than &gt;2 mm. Poultry litter had significantly higher soil N in &gt;2 and 0.25–2 mm at 0–5 cm than hairy vetch, wheat, and fallow. Inclusion of continuous corn or rotation of corn and soybean, integrated with poultry litter applied as manure, can serve as SOC and N sinks in macroaggregates enhancing C and N sequestration under no‐till management and humid subtropical climate.

Land management effects on wet aggregate stability and carbon content

AbstractLand management affects soil structure and many other soil properties and processes. Our objectives were to evaluate soil organic C (SOC), aggregate size distribution, aggregate‐associated C, and soil structure as affected by long‐term land management and slope. A chronosequence of 38 on‐farm sites with low to high (5–18%) slopes was selected to evaluate 5–40 yr of management. The sites were classified as business as usual (BAU) cropland (BAU‐Crop), BAU pasture (BAU‐Past), newly established conservation reserve program (CRP) areas (CRP‐New), and established CRP (CRP‐Old). Soil samples were collected from the 0‐to‐5‐ and 5‐to‐15‐cm depth increments and processed for soil property measurements including fractionation by wet sieving into five aggregate size classes (&gt;2,000, 1,000–2,000, 500–1,000, 250–500, and 53–250 μm). Within the surface 5 cm, mean weight diameter (MWD) and geometric mean diameter (GMD) were used to characterize soil structural stability. The BAU‐Past and CRP‐Old sites had 79% more macroaggregates (&gt;2,000, 1,000–2,000, and 500–1,000 μm), 123% higher MWD, 38% higher GMD, and 47% higher SOC than BAU‐Crop or CRP‐New sites. The 5‐to‐15‐cm depth increment showed a similar but lower magnitude response. Aggregate‐associated C was quantified using a constant soil mass that reflected aggregate size distribution to prevent overestimating C content. Lower‐slope locations had more SOC, more macroaggregates, more C associated with macroaggregates, and higher GMD and MWD compared with high‐slope locations across all management classifications and soil depths. The results support our hypothesis that the high‐slop soils may benefits from specific management decisions than the lower‐sloping soils as a function of landscape property. We recommend reestablishing grassland on sloping land that is susceptible to excessive soil erosion, although those practices will likely take a long time to restore soil structural stability and SOC content to precultivation levels.

Filling gaps in models simulating carbon storage in agricultural soils: the role of cereal stubbles

Carbon (C) input is a prerequisite for the formation of soil organic matter and thus for soil organic C (SOC) sequestration. Here we used the C-TOOL model to simulate SOC changes in a long-term field experiment (1932–2020) at Askov, Denmark, which involved four different levels of nutrients added in mineral fertilizer (0, 0.5, 1, 1.5 NPK) and a four-crop rotation. The C input into soils consists of belowground and aboveground plant biomass and was estimated using allometric functions. The simulation showed that modelled SOC based on standard allometric functions of C input from crop residues did not adequately matched measured SOC contents. However, applying modified allometric functions based on current and the previously measured results for aboveground and belowground C inputs in winter wheat and grass clover in rotations provided much better match between simulated and measured SOC contents for fertilized treatments at normal and high level of fertilization. This improved indicators of C-TOOL model performance (e.g. yielding RMSE of 2.24 t C ha−1 and model efficiency of 0.73 in 1.5 NPK treatment). The results highlight that standard allometric functions greatly overestimates the amount of C in winter wheat stubble left after harvest in treatments dressed with NPK compared with modified functions. The results also highlight further needs for improvement of allometric functions used in simulation models for C-accounting in agroecosystems.

Long‐term effect of tillage and straw retention in conservation agriculture systems on soil carbon storage

AbstractThe main principles of conservationagriculture (CA) are minimal soil disturbance, permanent soil cover, diversification of crop rotations, and retention of crop residues. Tillage has been implicated with increased soil organic C (SOC) mineralization, but recently this has been contested. Residue retention and cover crops result in higher soil C inputs and are therefore expected to increase SOC. This study aimed to evaluate the effect of 17 yr of residue retention (with and without) and tillage (plowing and no‐till) on soil C storage on two Danish sandy loam soils with different clay and SOC content under cool temperate humid climate conditions. Straw retention reduced bulk density at both sites. It also increased the C concentration with 0.47 mg C g–1 dry soil on average from 2002 to 2019, and SOC stocks in 2019 with 23 tonnes C ha–1, on average, at the site with higher initial SOC content. The effect of straw retention on soil C was higher when the soil was plowed, and no significant effect was observed in no‐till soils. Straw retention did not significantly affect C concentration at the site with low initial SOC. Tillage altered the C distribution in the soil profile, so that the C concentration in the upper layer (0–25 cm) was higher in the no‐till soils, but did not significantly increase total C concentration in deeper layers (0–50 cm). Use of no‐till and straw retention did not consistently affect crop yield across site‐years, but the interannual variability was large, obscuring possible differences in yields.

Soil salinity, fertility and carbon content, and rice yield of salt-affected paddy with different cultivation period in southwestern coastal area of South Korea

ABSTRACT Salt-affected reclaimed tidelands (RTLs) in coastal areas of South Korea are commonly used for rice (Oryza sativa L.) cultivation as excessive salts are removed through leaching by irrigation during rice growth. However, changes in soil properties and rice growth in RTLs by continuous rice cultivation are not well understood. In the present study, we investigated soil salinity indices (ECe, electrical conductivity of soil extracts; SAR, sodium adsorption ratio; ESP, exchangeable sodium percentage), fertility (mineral nitrogen (N), and available phosphorus (P) concentrations), and carbon (C) content as well as rice biomass and yield in 72 paddy fields within 10 RTL areas, which have been cultivated for different time periods (13–35 years). Continuous rice cultivation decreased ECe by 0.36 dS m−1 per year, indicating desalinization, but ESP and SAR were not changed. Available P concentration increased but mineral N concentration did not change with rice cultivation years. There was also an indication of increased soil organic C (SOC) concentration with rice cultivation. However, rice biomass and yield were not different among the RTLs. Instead, rice biomass and yield as well as SOC concentration were negatively correlated with ECe when all individual measurements from 72 sampling sites within the 10 RTLs were pooled, suggesting that salinity is a constraint for both rice production and SOC sequestration at field scales. Interestingly, soil mineral N concentration was positively correlated with ECe but negatively with rice biomass and yield. This result implies that heavy application of N fertilizer in highly saline soils may not benefit rice growth due to salinity stress. Our results show that long-term rice cultivation with fertilization and irrigation decreases salinity while increasing P availability. However, SOC content and rice biomass and yield were more responsive to the salinity status of each paddy field than during cultivation years, highlighting the need for field-specific soil management for improved rice production and enhanced SOC sequestration.

Tillage-residues affect mineral-associated organic matter on Vertisols in northern Mexico

Managing croplands for increased storage of soil organic matter (SOM) contributes to the development of resilient farming systems in a changing climate. We examined SOM dynamics in a wheat (Triticum durum L.) – maize (Zea mays L.) irrigated bed planting system established near Ciudad Obregón, Sonora, Mexico. Soil samples (0–15 cm) were collected from conventionally tilled raised beds (CTB) with all crop residues incorporated (CTB-I) and permanent raised beds (PB) with crop residues burned (PB-B), removed (PB-R), partly retained (PB-P) or fully retained (PB-K) receiving 0, 150 or 300 kg N ha−1, and analyzed for organic C (OC), total N (TN) and δ13C in whole-soil, light fraction (LF) and coarse- (sand) and fine- (silt and clay) mineral-associated organic matter (MAOM). Results indicated that PB-K and PB-B increased soil OC (p <0.05) in whole-soil relative to CTB-I, mainly through increases in sand- and silt-size MAOM, respectively. Similarly, N-fertilization increased soil OC and TN contents in whole-soil, coarse-MAOM and fine-MAOM, but not in the LF pool. Soil δ13C was higher (p <0.05) in PB-K (−20.18‰) relative to PB-B (−20.67‰), possibly due to the stabilization of partly decomposed maize-C in silt- and clay-size MAOM. Composition of SOM surveyed by CPMAS 13C NMR was not affected by tillage-residue management, and roughly consisted of 35% O-alkyl-C, 31% alkyl-C, 24% aromatic-C and 10% carboxyl-C. Our results indicate that long-term PB-K and PB-B adoption increased surface soil OC contents relative to CTB-I, even though pathways of SOM stabilization differed between systems. Under PB-K, accumulation of fine-MAOM was mostly related to straw-C inputs, whereas in PB-B it was closely associated to black-C precursors. Fine-MAOM appeared responsive to crop residue management, and should be therefore considered when analyzing mechanisms of SOM stabilization in irrigated croplands.

Absorptive and transport roots differ in terms of their impacts on rhizosphere soil carbon storage and stability in alpine forests

The fine roots of woody plants can be classified as absorptive roots and transport roots based on their distinct morphological, physiological, and functional traits. The potential ecological consequences of roots with different functional classifications on soil biogeochemical processes have been widely recognized. However, the magnitude of rhizosphere soil C stocks and the associated C stabilization mechanisms driven by these two root functional modules remain unknown. We quantified the soil organic C (SOC) contents and the C fractions in the rhizospheres of absorptive and transport roots in mineral soil in a spruce (Picea asperata Mast.) plantation and further estimated the rhizosphere SOC stocks of the two root functional modules by establishing a numerical model based on the extent of the rhizosphere. We also determined the characteristics of the SOC chemistry and metal-organic complexation in the rhizosphere to distinguish how the two root functional modules differentially impact rhizosphere SOC stability. The SOC content of the rhizosphere of absorptive roots was 15.7% higher than that of the rhizosphere of transport roots. This result can be mainly attributed to the higher stability of SOC (i.e., chemical recalcitrance and metal-organic bond) in the rhizosphere of absorptive roots. The numerical model analysis showed that the rhizosphere SOC pool of absorptive roots (0.27–2.7 kg C/m2) was twice as large as that of transport roots (0.18–1.36 kg C/m2). The contribution of the rhizosphere SOC stock of absorptive roots (63.5%) to the total rhizosphere SOC accrual was much higher than that of the rhizosphere SOC stock of transport roots (36.5%) in the scenario with a 1-mm extent. The rhizosphere soil C stock of absorptive roots plays a predominant role in the total rhizosphere soil C stock in alpine coniferous forests. Our findings highlight the importance of integrating function-based fine root classifications with rhizosphere soil C storage into land surface models of C cycling, which would be instrumental for accurately predicting soil C dynamics in alpine coniferous forest ecosystems.

Root‐zone enrichment of carbon, nitrogen, and soil‐test biological activity under cotton systems in North Carolina

AbstractSoils of the southeastern United States are generally low in organic matter. Root‐zone enrichment of organic carbon (C) and nitrogen (N) may be possible with conservation agricultural management, but little is known of how this might be distributed across private farms. A stratified random survey of 120 cotton (Gossypium hirsutum L.) fields from throughout North Carolina characterized root‐zone contents of soil organic C (SOC), total soil N (TSN), and soil‐test biological activity (STBA). Soils were mostly udults and aquults, but included some udepts, aquepts, aqualfs, and psamments. Depth distribution of soil properties at 0–10, 10–30, and 30–60 cm was fitted to a nonlinear regression model for each soil profile to calculate root‐zone enrichment over that of a baseline content estimated from concentration at 30‐cm depth. Continuous conservation tillage was practiced on 23% of fields. Root‐zone enrichment of TSN and STBA was greater with conservation than with disk tillage (1.30 ± 0.10 vs 0.82 ± 0.04 Mg N ha–1, respectively, p &lt; .001; 209 ± 16 vs 144 ± 6 kg CO2–C ha–1 3 d–1, respectively; p &lt; .001) (mean ± standard error). Difference in root‐zone enrichment of SOC between continuous conservation tillage and frequent disk tillage was 8.5 ± 2.5 Mg C ha–1 among eight subregions and was not affected by variations in baseline SOC content, which was greater in the Flatwoods than the Coastal Plain region due to inherent soil formation factors. These on‐farm results confirm that continuous conservation tillage is needed to enrich root‐zone SOC, TSN, and STBA under cotton production.

Soil organic C and total N as well as microbial biomass C and N affect aggregate stability in a chronosequence of Chinese fir plantations

Soil aggregation as one of the key processes can maintain soil quality. Revealing the relationships of soil aggregate stability with the relevant bio-chemical parameters could provide valuable guidance to maintain soil quality in the Chinese fir plantations. In this study, soil samples (0–20 cm and 20–40 cm depths) were collected from Chinese fir plantations with various stand ages (3 years, 9 years, 17 years, and 26 years) in Guangxi, China. Soil aggregates with four different sizes were obtained through the optimal moisture sieving method, including the >2 mm, 2-1 mm, 1–0.25 mm, and <0.25 mm fractions. For each fractions, the organic C (OC), total N (TN), microbial biomass C (MBC), microbial biomass N (MBN), and microbial respiration rate (MRR) were measured. Regardless of the Chinese fir plantation age, soil OC and TN contents were concentrated in the <0.25 mm fractions, while soil MBC, MBN, and MRR were relatively high in the 1–0.25 mm fractions compared with other fractions. Soil mean weight diameter (MWD) and geometric mean diameter (GMD) were higher in the 17-year Chinese fir plantations than other stand ages, which indicated stronger stability of soil aggregates in the 17-year Chinese fir plantations. In the process of Chinese fir planting, the peaks of soil OC, TN, MBC, MBN, and MRR were observed in the 17-year Chinese fir plantations. Based on the redundancy analysis (RDA) and Pearson's correlation analysis, soil aggregate stability was significantly positively correlated (P < 0.05) with the OC, TN, MBC, and MBN over time, particularly in the OC, which was the most important contributor of the chronological variation in soil aggregate stability. Overall, soil aggregate stability and aggregate-related OC, TN, MBC, and MBN are the highest in the 17-year Chinese fir plantations. With the growth of Chinese fir after 17 years, these factors decrease significantly. Therefore, it is necessary to adopt reasonable management methods (i.e., appropriate application of organic manure) to increase soil aggregate stability and maintain soil quality in Guangxi, China.

Grazing accelerates labile and recalcitrant soil carbon loss driving by rare microbial taxa in a desert steppe

AbstractLivestock grazing in grasslands regulates the multifunctionality of those ecosystems and has an adverse effect on soil carbon (C) dynamics and fractions. However, our understanding of grazing‐induced changes in soil microbial community and their links to soil organic C (SOC) fractions is limited. By using a long‐term grazing experiment in desert steppe, we examined impacts of different grazing intensity on the diversity and composition of soil microbial community and their potential contribution to SOC content and fractions. The results showed that 15‐year grazing reduced total SOC (TSOC), albeit not significant, but induced a notable decrease in both labile and recalcitrant SOC fractions, especially under high grazing intensity. Soil prokaryotic communities were not significantly impacted by grazing intensity. However, we observed that sensitive operational taxonomic units (OTUs), identified by indicator taxa analysis, grouped in distinct modules that reflected the different grazing intensities, suggesting that groups of microbes may cluster together and respond to grazing intensity. Association networks of prokaryotic OTUs and organic C fractions showed that SOC fractions were significantly related to rare taxa, indicating that grazing practice may act as a kind of environmental filtering and strongly affected soil microbial rare taxa and thereafter SOC fractions. Together, our findings suggest that grazing intensity did not change total microbial community and TSOC but affected SOC fractions by altering rare taxa. Overall, our results indicated that soil C fractions and rare microbial taxa were more sensitive to grazing intensity, which could act as indicator of variations in ecosystem functions in the semi‐arid desert steppe.

Long‐term organic management combined with conservation tillage enhanced soil organic carbon accumulation and aggregation

AbstractSuccessful synergy between organic management and conservation tillage could provide the best agroecological benefits by minimizing the negative effects of both synthetic chemicals and intensive tillage on soils. We studied the long‐term impacts of conventional and organic crop management under different tillage regimes on soil organic C (SOC) and soil aggregation, leveraging Rodale Institute's long‐term Farming Systems Trial. The experiment was initiated in 1981 with three cropping systems under intensive tillage management: (a) conventional (3‐yr rotation of corn–corn–soybean) with synthetic fertilizer application, (b) organic–legume (4‐yr rotation of corn–oats–soybean/wheat–wheat) organic system with legume as sole N source, and (c) organic–manure (8‐yr rotation of soybean/wheat–wheat/hay–hay–hay–corn silage/wheat–wheat–corn–oats) organic system with composted manure and legumes as N sources. There were eight replications for each of these cropping systems when the trial was initiated in 1981. In 2008, four replicates of each cropping system treatment were placed under conservation tillage management. Soils were collected from 0 to 10‐, 10 to 20‐, and 20 to 30‐cm depths in 2018 and measured for total SOC, active SOC fractions (microbial biomass C, water extractable C, permanganate‐oxidizable C), aggregate size distribution, and wet aggregate stability. Overall, total and active SOC increased by 16–132% under organic management compared with conventional management and the increase was more pronounced under organic–manure treatment than organic–legume treatment. Results also showed that 10 yr of tillage treatments did not influence active and total SOC concentrations, but influenced dry aggregate size distribution and wet aggregate stability. This study suggests that in long‐term organic cropping systems, incorporated composted manure and perennial hay vs. legumes alone accumulated the greatest amount of SOC compared with conventional systems. Conservation tillage in organic systems improved aggregation; however, beyond 10 yr, it is necessary to reflect tillage effects on SOC.

Impacts of land use and salinization on soil inorganic and organic carbon in the middle-lower Yellow River Delta

Soil inorganic carbon (SIC) is an important reservoir of carbon (C) in arid, semi-arid, and semi-humid regions. However, knowledge is incomplete on the dynamics of SIC and its relationship with soil organic C (SOC) under different land use types in the semi-humid region, particularly in coastal zones impacted by soil salinization. We collected 170 soil samples from 34 profiles across various land use types (maize-wheat, cotton, paddy, and reed) in the middle-lower Yellow River Delta (YRD), China. We measured soil pH, electrical conductivity (EC), water-soluble salts, and SOC and SIC contents. Our results showed significant differences in both SOC and SIC among land use types. The dry cropland (maize-wheat and cotton) soils had significantly higher SOC and SIC densities (4.71 and 15.46 kg C m–2, respectively) than the paddy soils (3.28 and 14.09 kg C m–2, respectively) in the 0–100 cm layer. Compared with paddy soils, reed soils contained significantly higher SOC (4.68 kg C m–2) and similar SIC (15.02 kg C m–2) densities. There was a significant positive correlation between SOC and SIC densities over a 0–100 cm soil depth in dry cropland soils, but a negative relationship in the paddy soils. On average, SOC and SIC densities under maize-wheat cropping were 15% and 4% lower, respectively, in the salt-affected soils in the middle-lower YRD than the upper YRD. This study indicated that land use types had great influences on both SOC and SIC and their relationship, and salinization had adverse effect on soil C storage in the YRD.

Long-term rice-oilseed rape rotation increases soil organic carbon by improving functional groups of soil organic matter

Aggregate carbon (C) pools, which are closely associated with soil organic C (SOC) stability and agricultural productivity, are greatly affected by the composition of soil organic matter (SOM) functional groups. However, the long-term effect of different rice-based rotations on the composition of SOM functional groups remains elusive. In this work, a 10-year field experiment was conducted to assess the effect of different rice-based rotations (rice-wheat [RW] and rice-oilseed rape rotation [RR]) on the composition of SOM functional groups and soil aggregate C pools. The composition of SOM functional groups was determined by the nuclear magnetic resonance spectroscopy method. Aggregate C pools were isolated through density gradient fractionation. After 10 years of rice-based rotation, compared with RW, RR significantly reduced the proportion of macromolecular functional groups (nuclear magnetic resonance zone (NMR) > 110 mg kg–1) in SOC, especially that of aromatic C (by 25.0%) and carboxyl C (by 26.8%), but significantly increased the contents of SOM compounds, particularly those of polysaccharides (by 30.7%), nitrogen carrier (by 28.4%) and lipids (by 34.7%). RR enhanced the proportions of large macroaggregate (by 56.0%) and intra-microaggregate particulate organic C (iPOC) (by 13.0%), as well as the SOC content (by 12.7%) relative to RW. Structural equation modeling revealed that RR could promote aggregate development and the stock of SOC as iPOC by enhancing the proportion of small functional groups in SOM (NMR < 110 mg kg–1) and promoting SOM compound formation, thereby improving soil C pool stability and content relative to RW in the long-term rotation.