Bulk Soil Organic Carbon Research Articles

Particulate organic carbon is more sensitive to nitrogen addition than mineral-associated organic carbon: A meta-analysis

Soil organic carbon (SOC) is heterogeneous and consists of multiple pools differing in physical and chemical properties. SOC stocks are expected to increase in response to nitrogen (N) enrichment. However, the comprehensive understanding and general conclusions about the responses of different SOC pools such as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to N addition are still lacking, in spite of the fact that this knowledge is of valuable for the accuracy of predictions of the vulnerability of SOC to N deposition. Here, we conducted a meta-analysis to explore the effects of N addition on POC and MAOC, as well as their relative proportions in bulk SOC. We found that N addition increased POC (20.6%) to a greater extent than MAOC (4.5%). Consequently, N addition significantly increased the proportion of POC in bulk SOC by 10.4%, but significantly decreased the proportion of MAOC in bulk SOC by 2.6%. The response of POC to N addition was negatively correlated with the response of pH and microbial biomass carbon (MBC), while the response of MAOC to N addition was positively correlated with the response of pH and MBC. Notably, the positive effect of N addition on the proportion of POC in bulk SOC decreased as the initial proportion of POC increased. Inversely, the negative effect of N addition on the proportion of MAOC in bulk SOC increased with the initial proportion of MAOC. Overall, our findings suggest that although N could enhance SOC stocks, the larger increment of POC would make the SOC pools more susceptible to future global changes. Our results also highlight the need to explicitly incorporate the differential responses of POC and MAOC into ecosystem models to improve predictions of SOC stock responses to global change.

Interactive Effects Determine Radiocarbon Abundance in Soil Fractions of Global Biomes

Soil organic carbon (SOC) is heterogeneous, consisting of fractions with differing turnover rates. Climate, vegetation, and soil properties can all affect the characteristics of these different soil carbon fractions. However, there has been little investigation into the interactive effects of biotic and abiotic drivers on a large spatial scale. In this study, we utilized data from the international soil radiocarbon database (ISRaD) to investigate the radiocarbon abundance (an indicator of carbon persistence) in soil fractions from several different biomes. Bulk SOC was categorized into three fractions according to the density fractionation method: a free light fraction (fLF), an occluded light fraction (oLF) and a heavy fraction (HF). In addition to the impacts of significant factors such as depth and climate, interactive effects between soil fractions and environmental factors on radiocarbon abundance were prevalent. Specifically, there were significant interactions between climate, vegetation types, soil properties, and soil fractions affecting Δ14C levels. The difference in Δ14C of the shallow depth fractions was significant in the temperate forest, and was not significant in the boreal and tropical forests. The interactive effect between mean annual temperature (MAT) and mean annual precipitation (MAP) on Δ14C was significant in the shallower depth (i.e., 0–30 cm and 30–60 cm) of the oLF and in the deeper soils (i.e., 30–60 cm and 60–100 cm) of the HF. Soil properties also interact with soil fractions in determining Δ14C. After accounting for depth effect, oxalate-extractable aluminum (Alo) accounted for 63.5% of the remaining Δ14C variation in the fLF and accounted for 35.9% of the remaining Δ14C variation in the oLF. Rather than Alo, cation exchange capacity (CEC) accounted for 46.1% of the remaining Δ14C variation in the HF. These findings suggest that the way the interactions between climate, vegetation, and soil properties affect soil carbon persistence at various fractional depths is critical for the accurate prediction of soil carbon dynamics.

Decadal application of mineral fertilizers alters the molecular composition and origins of organic matter in particulate and mineral-associated fractions

The extent to which long-term application of mineral fertilizers regulates the quantity, quality, and stability of soil organic matter (SOM) in soil matrices remains unclear. By combining four biomarkers, i.e., free and bound lipids, lignin phenols and amino sugars, we characterized the molecular composition, decomposition and origins of SOM in response to 10-year fertilization (400 kg N ha−1 yr−1, 120 kg P ha−1 yr−1 and 50 kg K ha−1 yr−1) in a cropland in North China. We focused on two contrasting fractions: particulate organic matter (POM), and mineral-associated organic matter (MAOM). Fertilization increased soil organic carbon (SOC) by 23% in MAOM, and altered its composition and origins, despite having a limited effect on bulk SOC levels. Fertilization increased plant-derived terpenoids by 46% in POM and long-chain lipids (≥C20) by 116% in MAOM but decreased short-chain lipids (<C20) by 54% in the former fraction. Fertilization reduced suberin-derived lipids by 56% in POM and 30% in MAOM but increased lignin-derived phenols by 74% in POM and 31% in MAOM, implying that crop residues were preferentially stored via the POM form. Fertilization decreased the contribution of microbial residues to SOC in both the fractions. Overall, mineral fertilizers tended to reduce certain labile components within POM (e.g., short-chain lipids), leading to the accrual of recalcitrant molecules (e.g., long-chain lipids, cutin-derived lipids, and lignin-derived phenols) in the MAOM fraction. Collectively, our study suggests that mineral fertilizers can increase SOM stability and persistence by modifying their molecular composition and preservation in the mineral-organic associations in a temperate agroecosystem.

Soil Aggregates and Aggregate-Associated Carbon and Nitrogen in Farmland in Relation to Long-Term Fertilization on the Loess Plateau, China

Soil aggregation plays a critical role in the maintenance of soil structure and crop productivity. Fertilization influences soil aggregation, especially by regulating soil organic carbon (SOC) and total nitrogen (TN) contents in aggregate fractions. Here, we conducted a fixed-site field experiment to quantify the effect of five N application rates: 0, 75, 150, 225, and 300 kg·N·ha−1, denoted as N0, N75, N150, N225, and N300, respectively, on soil aggregate stability, aggregate-associated SOC and TN sequestration and crop productivity. Soil aggregates were divided into &gt;0.25 (&gt;5, 5–2, 2–1, 1–0.5, 0.5–0.25) and &lt;0.25 mm through wet and dry sieving methods. The results showed that long-term fertilization increased the proportion of macro-aggregates (&gt;0.25 mm), decreased the proportion of micro-aggregates (&lt;0.25 mm), and improved the aggregates stability. Compared with N0, the proportion of micro-aggregates in N225 was significantly decreased by 66.45% under wet sieving, while the proportion of &gt;5 mm macro-aggregates in N225 was significantly increased by 19.24% under dry sieving (p &lt; 0.05). With the increase in N application rate, the bulk SOC and TN contents first increased and then decreased, and the SOC and TN of N225 were significantly increased by 17.75% and 72.33% compared with N0 (p &lt; 0.05). More specifically, fertilization promoted the distribution and enrichment of SOC and TN in macro-aggregates and reduced the C/N of the micro-aggregates and the contribution of SOC and TN in the micro-aggregates. Compared with N0, the contribution rate of macro-aggregates to SOC and TN of N225 under wet sieving was significantly increased by 84.13 and 17.18%, respectively, while the C/N of micro-aggregates of N225 under wet and dry sieving methods was significantly decreased by 45.95 and 31.74%, respectively (p &lt; 0.05). Moreover, fertilization improved the yield, and N225 significantly increased the total yield by 80.68% compared with N0 (p &lt; 0.05). In conclusion, N225 was the suitable N application for improving soil aggregate stability, carbon and nitrogen sequestration, and crop productivity on the Loess Plateau, China.

Post-farming land restoration schemes exhibit higher soil aggregate stability and organic carbon: Evidence in the Three Gorges Reservoir Area, China

The Three Gorges Reservoir Area was dominated by a farming landscape but has been the target of restorative land management schemes over recent decades, wherein croplands have been converted to forests or citrus woodlands run by commercial enterprises or, alternatively, abandoned for natural vegetation succession. How these ongoing restorative land management schemes imprint soil systems remains poorly understood. Accordingly, in this study, a space-for-time substitution approach was applied to compare soil aggregate stability and organic carbon stocks between lands used for ongoing arable production and areas subjected to restorative practices. Composite soil samples from reforestation woodlands, abandoned croplands, citrus woodlands, and present-day croplands were collected, and a variety of soil metrics (i.e., water-stable aggregate percentage, mean weight diameter, geometric mean diameter, erosion resistance of the aggregates, bulk soil organic carbon and aggregate-specific soil organic carbon) were used as a basis for comparison. Reforestation woodlands and abandoned croplands exhibited a higher mass percentage of macro-aggregates, higher values of mean weight diameter and geometric mean diameter, and higher organic carbon content within the sampled 0–40 cm depth soil profiles. Reforestation and cropland abandonment significantly increased the stability of soil aggregates and organic carbon stocks. Moreover, abandoned cropland samples exhibited higher soil aggregate stability and organic carbon stocks than reforestation woodlands. In contrast, soil aggregate stability for all sampled soil depths and organic carbon content except for the topsoil (0–10 cm) of citrus woodlands exhibited lower values than those for present day croplands. Exclusion of crop farming and reestablishment of restorative vegetation have therefore contributed to changes in soil physical structure and organic carbon stocks, with reforestation and cropland abandonment being more beneficial for soil aggregate stability and soil organic carbon stocks in the Three Gorges Reservoir Area.

How do land use and land cover changes after farmland abandonment affect soil properties and soil nutrients in Mediterranean mountain agroecosystems?

Mediterranean mountains are sensitive agroecosystems that have suffered intense land use and land cover changes (LULCC) during the last century. From the middle of the twentieth century, most of the cultivated lands in Mediterranean mountains were abandoned, allowing the recovery of vegetation (through natural revegetation and afforestation programmes). To examine the effects of farmland abandonment, secondary succession (natural revegetation) and afforestation, an intensive soil sampling was carried out in the Araguás catchment (Central Spanish Pyrenees) including sparsely vegetated areas (badlands), grasslands, shrublands and afforested sites. LULCC were mapped, and soil physico-chemical properties were analysed in reference sites (unaltered areas during the last centuries) and in the different land uses. Likewise, the soil organic carbon (SOC) content in the bulk soils and in the fractions separated by density fractionation have been studied. This study evidenced that farmland abandonment led to a mosaic landscape with different land use and land covers. Results show that LULCC significantly affect soil physico-chemical properties (soil texture, stoniness, pH, SOC, total carbon, CorgN ratio, bulk density and field capacity). Significant differences were observed between secondary and afforested sites following farmland abandonment. Afforestation triggered higher SOC than shrubland sites (natural revegetation) (1.4 and 1.1% respectively), suggesting a slower process of organic matter accumulation after farmland abandonment in the natural revegetation compared to afforestation. The significant role of grassland sites for enhancing the accumulation of SOC has been also confirmed. The results showed also significant differences in the relative contribution of each organic fraction to the bulk SOC: the amount of labile fraction (free and occluded labile fractions) is significantly higher in afforested and shrubland sites (58.1 and 51.2% respectively) than in grassland sites (36.8%). Understanding the effects of LULCC on soil properties and SOC dynamics is essential when planning post-land management practices after farmland abandonment.

Effect of cover cropping on soil organic matter characteristics: Insights from a five-year field experiment in Nebraska

Cover crops are increasingly incorporated into crop rotations to improve soil health including the postulated positive impact on soil organic carbon (SOC) storage. However, a consistent positive response of SOC to cover cropping was not observed in several studies, necessitating further clarification of mechanisms by which cover crops impact soil organic matter (SOM) dynamics. In this study, we evaluated the effect of five years of winter cover cropping within a corn-soybean rotation in Nebraska under no-till on SOC and nitrogen (N) storage in the bulk topsoil (0–10 cm depth) as well as in particulate (free and occluded), water-extractable, and mineral-associated organic matter fractions. Furthermore, cover crop effects on potential C and N losses from the topsoil in form of CO2 and N2O emissions and leached organic C and N were evaluated. The different SOM fractions were analyzed for their functional group composition using diffused reflectance infrared Fourier transform spectroscopy to account for potential changes in the SOM composition due to cover crops. Our results indicated no effect of cover cropping on C and N losses (gaseous and leaching) and on bulk SOC and total N storage five years after implementation. Differences were detected for the SOM fractions, with a positive effect of cover crops on the amount of water-extractable organic matter. The spectroscopic results suggested a significant cover crop effect on the composition of free particulate organic matter fraction. Both fractions seem to be suitable for detecting early changes in SOM characteristics due to cover cropping under the studied conditions. A significantly measurable impact on bulk SOC and N storage in 0–10 cm depth for the studied corn-soybean systems might require more than five years of cover cropping, or management changes that target an increase in cover crop biomass production and input into the soil.

Nitrogen addition effects on soil mineral-associated carbon differ between the valley and slope in a subtropical karst forest

While the responses of soil organic carbon (SOC) dynamics to atmospheric nitrogen (N) deposition have been widely investigated, little is known in terms of if, and how the responses are modulated by topography. Here, a N addition experiment with three N addition levels, i.e., 0, 50 and 100 kg N ha−1 yr−1, was carried out in the valley and on the slope of a subtropical karst forest. Soil type was Cambisols on the slope but Luvisols in the valley developed on a mixture of limestone and dolomite. Bulk SOC content in the valley was marginally (p < 0.1) enhanced attributed to the stimulation of both particulate organic C (POC) and mineral-associated organic C (MAOC), but that on the slope was not significantly altered owing to enhanced POC but suppressed MAOC under N addition. High N addition (100 kg N ha−1 yr−1) significantly increased POC through accelerating aboveground litter input, and increased MAOC via increasing carbon use efficiency (CUE) and necromass formation in the valley. On the slope, high N addition significantly stimulated POC owing to increase of fine root biomass, but inhibited MAOC due to decrease in microbial necromass accumulation. Our results suggest that the impacts of N addition on SOC formation and hence persistence likely modulated by topography, so that provide a novel view in underlying the differential responses of SOC pool to N addition.

Long-term plastic mulching decreases rhizoplane soil carbon sequestration by decreasing microbial anabolism

Ridge-furrow with plastic mulching (RFPM) is a widely used agricultural practice in rain-fed farmlands. However, the impact of microbial related metabolism on soil organic carbon (SOC) is not fully understood. Amino sugar analysis, high-throughput sequencing, and high-throughput qPCR approaches are combined to investigate this topic, based on a long-term experiment. Treatments include flat planting without mulching (FP), ridge-furrow without mulching (RF), and RFPM. RFPM significantly decreases rhizoplane SOC contents, while bulk SOC contents change insignificantly across treatments. In terms of microbial metabolic pathways, RFPM decreases indicators of the in vivo metabolic pathway, whereas those of the ex vivo pathway are increased. In terms of microbial community features, core taxa module #1 is dominated by Sphingomonadaceae. These are putative high yield (Y) strategists, according to the microbial life-history strategy framework. They are closely related to the in vivo pathway and are most predictive for SOC; their abundance is highest under FP and lowest under RFPM. Core taxa module #2 is dominated by Chitinophagaceae, putative resource acquisition (A) strategists, that are closely related to the ex vivo pathway. Their abundance in the rhizoplane is highest under RFPM and lowest under FP. The RFPM-induced decline in SOC occurs simultaneously with the abundance of A-strategists with in vivo pathway but not the Y-strategists with ex vivo pathway. Overall, the result of this study shows a trade-off. In RFPM practice, the ex vivo microbial pathway is enhanced along with the abundance of A-strategists. This is not the case for the in vivo pathway and associated abundance of Y-strategists, which are closely associated with SOC. Our findings underlined the impact of rhizoplane microbial metabolic pathways on SOC status is key to agricultural practices in drylands such as RFPM, and advanced our understanding of how microbes affect the carbon cycling in dryland farming.

Soil organic carbon accumulation along a chronosequence of vegetation colonization on debris flow fans in Southwest China

Soil is one of the largest terrestrial carbon (C) storages, which offers great potential to mitigate climate change through C sequestration. In this study, we focused on soil organic carbon (SOC) accumulation along a chronosequence of vegetation colonization on inactive debris flow fans by investigating the temporal variation of SOC concentrations and properties such as C:N ratios, δ13C values and chemical compositions of soil fractions in both topsoils (0–10 cm) and subsoils (40–50 cm) with vegetation colonization. We found that SOC concentration increased with soil property improvement (such as increase of soil aggregation and cation exchange capacity). Petrogenic OC from parent materials and biospheric OC from vegetation are the two main sources of SOC, and vegetation colonization increased the fraction of biospheric OC in bulk SOC. Fine soil fractions were found to have higher C concentrations, i.e. higher C adsorption efficiency per unit soil mass. As a result, fine soil particles were more enriched in alkyl and O-alkyl C that are the main composition of biospheric OC added to soils after vegetation colonization. Our results show that vegetation colonization on debris flow fans not only increases SOC concentrations, but also makes the bulk SOC become more labile.

Preferential Erosion of Soil Organic Carbon and Fine-Grained Soil Particles—An Analysis of 82 Rainfall Simulations

Soil erosion by water causes the loss of soil mineral particles and soil organic carbon (SOC). For determining the effectiveness of soil conservation measures on arable land, rainfall simulations are regularly carried out in field trials in the Czech Republic. The objective of this study was to analyse a dataset from 82 rainfall simulations on bare fallow soils, containing information on slope inclination, soil texture, soil bulk density, SOC, and soil loss with respect to the preferential erosion of fine-grained soil particles and the enrichment of SOC in the eroded soil. Each rainfall simulation comprised a first rainfall period of 30 min and a second one of 15 min in duration. The rainfall intensity was 1 mm min−1 and the kinetic energy of the raindrops accounted for 8.78 J m−2 mm−1. Runoff samples were taken to determine the soil loss and SOC enrichment in the eroded material. Regression analyses revealed that on sites with &lt;14% slope inclination, SOC mitigated soil loss in the first rainfall period. On sites with &gt;14% slope inclination, soil loss was driven by preferential erosion of fine-grained particles in the first rainfall period. Low soil loss was generally coupled with high SOC enrichment and vice versa, indicating that preferential erosion of SOC occurred mainly in soils with low erosion susceptibility. In order to prevent erosion of SOC and maintain soil quality, soil conservation measures are important in all soil types.

Effects of intercropping and drought on soil aggregation and associated organic carbon and nitrogen

AbstractDrought has significant effects on soil physicochemical properties and thereby crop productivity. Intercropping is a traditional agricultural practice with a good ecological effect and can improve soil structure. However, the effects of intercropping on soil aggregate distribution and associated organic carbon (OC) and nitrogen (N) compared with monoculture under drought remains unclear. Therefore, a two‐year controlled rainfall field (i.e., 30–40 days of 100% rainfall reduction during crop growth) was carried out to examine the effects of intercropping and short‐term drought on soil aggregate stability and associated OC and N in Northeast China. Results showed that soil OC and N were predominant in macroaggregates (2–0.25 mm), accounting for 39%–51% of SOC, 36%–51% of N at 0–10 cm depth, and 44%–67% of SOC, 43%–66% of N at 10–20 cm depth. After 2 years of a rainfall reduction treatment, large macroaggregates (&gt;2 mm) increased by 84.7% at 0–10 cm and the aggregate stability improved. There was no difference in the mass of aggregates between monoculture and intercropping, but short‐term drought increased &gt;0.25 mm aggregates and stability in intercropping soil at 0–10 cm depth. Moreover, drought significantly increased bulk soil OC in 10–20 cm but decreased the OC and N concentrations in large macroaggregates and silt + clay fractions (&lt;0.053 mm) in 0–10 cm. Compared with the monoculture, intercropping enhanced OC and N concentrations in aggregates, and decreased soil C/N ratio. Therefore, these findings provide insights into how intercropping and short‐term drought interactively influence soil aggregation, and C and N processes.

Long-term warming reduces surface soil organic carbon by reducing mineral-associated carbon rather than “free” particulate carbon

Soils in alpine ecosystems store a large amount of carbon. However, the responses of soil organic carbon (SOC) to long-term warming and the microbial mechanisms underlying SOC dynamics remain poorly understood. Here, we investigated the effects of 20-year warming (1997–2017) on SOC pools in an alpine shrubland on the Qinghai-Tibetan Plateau. We analyzed the plant and microbial communities, as well as soil chemistry, particularly the SOC of bulk soil and its two physical fractions. Warming did not alter plant biomass (either above- or below-ground). However, warming decreased overall microbial biomass carbon and nitrogen, the biomass of fungi (indicated by phospholipid fatty acids, amino sugars, and OTU richness), and potential activities of hydrolytic enzymes (especially β-1,4-glucosidase), in the surface 0–10 cm soil. Moreover, warming reduced the SOC in bulk soil (by 21%) and the heavy fraction (by 25%), but did not change the SOC in the light fraction. Importantly, there were strong positive correlations between the SOC in bulk soil and in the heavy fraction with the total microbial and fungal necromass, suggesting that the warming-induced loss of soil carbon was mainly due to the suppression of the microbial pathway of soil carbon formation. In deeper layers (10–50 cm) of the shrubland, however, warming (to a much lower degree than that in the surface soil) had only a minor effect on soil carbon pools and the microbial community. A global meta-analysis further showed that warming on average did not significantly change the SOC in different fractions (bulk SOC, active C, and stable C) in the surface soil. Taken together, these results show that long-term warming leads to significant loss of surface soil carbon (mostly in the mineral-associated heavy fraction), likely due to suppressing the microbial (particularly fungal) pathway to soil carbon formation and/or stimulating the utilization of stable soil carbon (i.e. heavy fraction) by microbes in the alpine shrubland.

The size of clearings for charcoal production in miombo woodlands affects soil hydrological properties and soil organic carbon

Charcoal production is a major driver of forest degradation in miombo woodlands. Forests play a crucial role in regulating the hydrological cycle, so it is critical to understand how forest degradation and management practices impact water availability, particularly in drylands. Few studies have examined the effect of forest clearing size on the hydrological functioning of soil, particularly under real-world conditions where, following clearing, forests are subject to multiple and prolonged anthropogenic disturbances, as occurs in miombo woodlands which are cleared for charcoal production and commonly used for livestock grazing. The pilot project Transforming Tanzania's Charcoal Sector was established in 2012 with the aim of establishing a sustainable wood harvesting system for charcoal production based on rotational harvesting cycles that allow for natural forest regeneration. Two clearing sizes were established: large clearings (300 × 300 m) harvested by clear-felling, and small clearings (50 × 50 m) harvested in a checkerboard pattern. We examined the effect of these two clearing sizes on soil hydrological properties and soil organic carbon (SOC) in Kilosa district, Morogoro, Tanzania. Our analysis included four treatments: large clearings, small clearings, small intact plots (unharvested plots within the checkboard pattern), and village land forest reserve. For each treatment we assessed the tree cover and measured soil infiltration capacity, soil bulk density, SOC stock, and texture. We also examined the relationship between these variables and the distance to the closest road to better understand the impact of livestock and human disturbance. Our results show that large clearings had the lowest mean infiltration capacity (121 ± 3 mm h−1) and SOC stock content (12 ± 0.2 tonnes ha−1), and the highest bulk density (1.6 ± 0.005 g cm−3) of all the treatments. We found a positive relationship between infiltration capacity and basal area (R2 = 0.71) across all treatments. We also found that infiltration capacity, SOC stock and tree basal area increased with increasing distance from the closest road, while bulk density decreased. We conclude that, in terms of their impact on soil hydrological functioning and SOC stock, small clearings, while not completely unaffected, are better than larger ones. In small clearings, concurrent reductions in tree cover and a relatively low impact on soil hydrological properties could result in increased soil and groundwater recharge compared to unharvested forest areas. Controlling livestock grazing can further minimize soil degradation, producing additional gains.

Insights from size fractions to interpret the erosion-driven variations in soil organic carbon on black soil sloping farmland, Northeast China

Declines of soil organic carbon (SOC) and degradation of soil function are caused by long-term soil erosion and cultivation. Although the variations of bulk SOC have been extensively studied, the driving mechanisms of variations in SOC fractions, i.e., carbon in coarse particulate, fine particulate, and mineral-associated organic matter (CPOM-C, FPOM-C, and MAOM-C), and their relationship with soil erosion remain unclear on sloping farmland. In this study, two areas of farmland with different slope types (convex vs. straight) and reclamation periods (50 years vs. ≥100 years) in the black soil region of Northeastern China were chosen to explore the roles of soil erosion on the variations of SOC and their size fractions based on the 137Cs tracer technique. The results showed that the average net erosion rate of the straight slope was 2810.09 t km-2a-1 and the convex slope was 3801.71 t km-2a-1 with higher spatial variability. When compared with uncultivated land, SOC contents of convex and straight slopes have decreased by 13.58% and 35.18%, with a reduced rate of 0.089 g kg-1 a-1 and 0.116 g kg-1 a-1, respectively. Although the CPOM-C fraction showed the most significant relative decreases (over 73.24%), the absolute declines of MAOM-C were more significant, especially in the straight slope with a more extended cultivation period. The contributions of erosion to the SOC decreases on convex and straight slopes were 69.54% and 13.09%, respectively. Erosion exerted a significant influence on finer particles such as FPOM-C and MAOM-C. With increasing cumulative erosion, the absolute magnitude of each SOC fraction in soils decreased. But the proportion of MAOM-C increased, while FPOM-C and CPOM-C decreased. Overall, our findings demonstrated that soil erosion not only induced the loss of SOC fractions but also transformed the proportional distribution of their fractions, and then affected the functions of SOC. Moreover, the effects of tillage (i.e., an increase in microbial activity) on straight slopes are not negligible in farmland that has experienced long-term cultivation but relatively little erosion. Apart from erosion reduction in areas of severe erosion, appropriate fertilization for farmland with a long history of cultivation should also be adapted to achieve sustainable development of black soil resources.

Patterns of organic carbon and nitrogen stocks in soil particle-size fractions along an aridity gradient in Northern China’s deserts

Clay and silt fractions (<53 μm in particle size) play a critical role in soil organic carbon (SOC) storage and are used to characterize SOC storage in terrestrial ecosystems. However, there is unclarity about SOC content and stock in soil particle-size fractions in deserts where the soil is immature. In this study, we investigated the composition of soil particles and the stocks of SOC and N in two particle-size fractions of 0–53 μm and 0–100 μm at the top (0–30 cm) and deeper (30–50 cm) layers along an aridity gradient. The very fine sand (53–100 μm in particle size) was the major component of bulk soil. The C and N stocks in the top 50 cm of soil decreased linearly with increasing aridity, from 63.5 MgC ha−1 to 4.1 MgC ha−1 and from 4.6 MgN ha−1 to 0.8 MgN ha−1, respectively. Organic C content in two soil particle-size fractions and their contribution to bulk SOC decreased linearly with increasing aridity. Organic C content in < 100 μm soil particle-size fraction accounted for 57.8 %-99.2 % of bulk SOC, and it was only 0.6 %-24.2 % in the < 53 μm fraction. SOC stock was estimated by < 100 μm soil particle-size fractions compared with clay and silt (<53 μm). Organic C content in two soil particle-size fractions was higher in the alpine and steppe deserts than in the Gobi and sandy deserts; it was significantly higher at 0–30 cm than at 30–50 cm for all deserts. Vegetation cover was the first predictor for the organic C content in two soil particle-size fractions and their contribution to bulk SOC, addressing the direct contribution of plant-derived C inputs to soil particle-size fraction. Our study provides strong empirical evidence that organic C and N in deserts are better estimated by soil particles < 100 μm rather than clay and silt-sized fraction (<53 μm in particle size).

Rapid improvement in soil health following the conversion of abandoned farm fields to annual or perennial agroecosystems

Incorporating perennial crops into agroecosystems has been shown to mitigate soil degradation and improve soil health by enhancing soil aggregation and soil organic carbon (SOC) accrual. However, our understanding of the ability and timeframe for perennial crop systems to build soil health within the context of conversion from abandoned crop land remains limited. Here, we examined changes in soil health in the first year following the conversion of an abandoned crop field into an agroecosystem planted with various treatments, including: novel perennial grain (intermediate wheatgrass, IWG; Thinopyrum intermedium), IWG/ alfalfa biculture, forage grass, tallgrass prairie, or annual wheat. We analyzed factors considered central to the concept of mitigating soil degradation to improve soil health (soil aggregation, aggregate organic carbon (OC), bulk SOC) and their soil biological and physicochemical correlates throughout the first growing season. Comparisons between treatments showed that both annual and perennial treatments rapidly and significantly improved soil health metrics including aggregation, aggregate stability, and OC levels compared to pre-conversion conditions. Such increases were positively correlated with the abundance of arbuscular mycorrhizal fungi (AMF hyphae, root colonization), labile SOC and microbial activity. Notably, IWG/ alfalfa biculture resulted in significantly higher levels of macroaggregate OC in comparison to other treatments, including tallgrass prairie, supporting the potential of perennial grasses to contribute to soil carbon gains. Overall, the conversion of this abandoned land to an agroecosystem produced rapid and substantial increases in soil health in the first year after planting.

Fertilizer quality and labile soil organic matter fractions are vital for organic carbon sequestration in temperate arable soils within a long-term trial in Switzerland

Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned.

Cover crop functional types differentially alter the content and composition of soil organic carbon in particulate and mineral-associated fractions.

Cover crops (CCs) can increase soil organic carbon (SOC) sequestration by providing additional OC residues, recruiting beneficial soil microbiota, and improving soil aggregation and structure. The various CC species that belong to distinct plant functional types (PFTs) may differentially impact SOC formation and stabilization. Biogeochemical theory suggests that selection of PFTs with distinct litter quality (C:N ratio) should influence the pathways and magnitude of SOC sequestration. Yet, we lack knowledge on the effect of CCs from different PFTs on the quantity and composition of physiochemical pools of SOC. We sampled soils under monocultures of three CC PFTs (legume [crimson clover]; grass [triticale]; and brassica [canola]) and a mixture of these three species, from a long‐term CC experiment in Pennsylvania, USA. We measured C content in bulk soil and C content and composition in contrasting physical fractions: particulate organic matter, POM; and mineral‐associated organic matter, MAOM. The bulk SOC content was higher in all CC treatments compared to the fallow. Compared to the legume, monocultures of grass and brassica with lower litter quality (wider C:N) had higher proportion of plant‐derived C in POM, indicating selective preservation of complex structural plant compounds. In contrast, soils under legumes had greater accumulation of microbial‐derived C in MAOM. Our results for the first time, revealed that the mixture contributed to a higher concentration of plant‐derived compounds in POM relative to the legume, and a greater accumulation of microbial‐derived C in MAOM compared to monocultures of grass and brassica. Mixtures with all three PFTs can thus increase the short‐ and long‐term SOC persistence balancing the contrasting effects on the chemistries in POM and MAOM imposed by monoculture CC PFTs. Thus, despite different cumulative C inputs in CC treatments from different PFTs, the total SOC stocks did not vary between CC PFTs, rather PFTs impacted whether C accumulated in POM or MAOM fractions. This highlights that CCs of different PFTs may shift the dominant SOC formation pathways (POM vs. MAOM), subsequently impacting short‐ and long‐term SOC stabilization and stocks. Our work provides a strong applied field test of biogeochemical theory linking litter quality to pathways of C accrual in soil.

Long-term Conservation Agriculture increases sulfur pools in soils together with increased soil organic carbon compared to conventional practices

Long term depletion of sulfur (S) in soils is common in many cropping systems globally, and especially in intensive, rice-based rotations in Asia. We hypothesized that Conservation Agriculture (CA) practices (minimal soil disturbance and crop residue retention) will increase S in soils primarily through changes in soil organic carbon (SOC). A long term experiment was sampled after 24 crops of continuous practice of (A) contrasting soil disturbance (strip planting = SP and conventional tillage = CT) and (B) two levels of crop residue retention (low residue = LR and high residue = HR). Sulfur fractions and dynamics in soil at five depths (0–5, 5–15, 15–30, 30–45 and 45–60 cm) along with bulk density (BD), pH, SOC, total nitrogen (TN) and extractable nutrients were determined. Levels of total S (584 – 668 mg kg−1), organic S (50.0– 153 mg kg−1), available S (7.9 – 21.6 mg kg−1) and water soluble S (6.0 – 17.0 mg kg−1) were the highest in SP with HR (P < 0.05), while inorganic S (505 – 533 mg kg−1) and adsorbed S (2.3 – 4.4 mg kg−1) were significantly higher (P < 0.05) in HR plots irrespective of tillage systems at 0–30 cm soil depth. All the S fractions were positively and strongly correlated with SOC and except for inorganic and adsorbed S, all other S fractions were positively correlated with TN, extractable P, Zn, B and Fe. All S fractions were negatively correlated with BD. Inorganic S was the dominant S fraction but highly significant and positive correlations of available S with organic S (r = 0.92) and water–soluble S (r = 0.92) suggests these two fractions were the main sources of plant available S. This study suggests that both minimal soil disturbance and increased crop residue retention, core components of CA, increased S pools in soils primarily due to increased SOC sequestration.