Soil Organic Carbon Stability Research Articles

Responses of soil organic carbon compounds to phosphorus addition between tropical monoculture and multispecies forests

Tropical forests are sensitive to nitrogen (N) and phosphorus (P) availability, and under nutrient application the variation of soil organic carbon (SOC) preserving mechanism remains to be explored. To reveal the forest-specific SOC preservation via biochemical selection in response to nutrient application, we investigated a monoculture (Acacia plantation) and a multispecies forest both with chronic fertilization in subtropical regions, and measured specific fingerprints of plant- and microbial-derived C compounds. In addition, to quantify the effect of P application on SOC content among tropical forests, we conducted a meta-analysis by compiling 125 paired measurements in field experiments from 62 studies. In our field experiment, microbial community composition and activity mediated forest-specific responses of SOC compounds to P addition. The shift of community composition from fungi towards Gram-positive bacteria in the Acacia plantation by P addition led to the consumption of microbial residual C (MRC) as C source; in comparison, P addition increased plant species with less complex lignin substrates and induced microbial acquisition for N sources, thus stimulated the decomposition of both plant- and microbial-derived C. Same with our field experiment, bulk SOC content had neutral response to P addition among tropical forests in the meta-analysis, although divergences could happen among experimental durations and secondary tree species. Close associations among SOC compounds with biotic origins and mineral associated organic C (MAOC) in the multispecies forest suggested contributions of both plant- and microbial-derive C to SOC stability. Regarding that fungal MRC closely associated with MAOC and consisted of soil N pool which tightly coupled to SOC pool, the reduce of fungal MRC by chronic P addition was detrimental to SOC accumulation and stability in tropical forests.

Iron mineral type controls organic matter stability and priming in paddy soil under anaerobic conditions

Associations of iron (hydr)oxides (FeOx) with organic carbon are vital in regulating the stability of soil organic carbon (SOC). Like SOC, FeOx is chemically dynamic in soils, particularly under anaerobic conditions. However, previous research has not clarified how the stability of FeOx (goethite versus ferrihydrite) and the formation pathway of FeOx-OC associations (adsorption versus coprecipitation) affect the stability of FeOx-bound OC and, subsequently, the priming effect (PE) under anaerobic conditions. With an aim to bridge this gap, we incubated paddy soils for 80 d under anaerobic conditions after adding free 13C-glucose, ferrihydrite- or goethite-bound 13C-glucose formed by either adsorption or coprecipitation. Compared with the free glucose addition, the FeOx-bound glucose addition increased 13CO2 production by 8%–21% but reduced 13C–CH4 production by 7%–10%. Ferrihydrite-bound glucose was mineralised more than goethite-bound glucose; this is consistent with its lower crystallinity facilitating reduction and, thus, higher OC bioavailability. Glucose induced a negative priming effect (PE) for CO2 but a positive PE for CH4, whereas FeOx-bound glucose showed the opposite trend. This may be because FeOx-bound glucose provides an energy source and electron acceptor for Fe-reducing bacteria; this promotes the dissimilating reduction of iron and combines with an aggravated microbial P limitation resulting from the FeOx input. The crystallinity of FeOx affected the amount of primed CH4 rather than its formation pathway. In conclusion, the crystallinity of FeOx controls the stability of FeOx-OC associations and the PE of SOC decomposition under anaerobic conditions.

Differential carbon accumulation of microbial necromass and plant lignin by pollution of polyethylene and polylactic acid microplastics in soil

The wide microplastics (MPs) occurrence affects soil physicochemical and biological properties, thereby influencing its carbon cycling and storage. However, the regulation effect of MPs on soil organic carbon (SOC) formation and stabilization remains unclear, hindering the accurate prediction of carbon sequestration in future global changes under continuous MP pollution. Phospholipid fatty acids, amino sugars and lignin phenols were used in this study as biomarkers for microbial community composition, microbial necromass and plant lignin components, respectively, and their responses to conventional (polyethylene; PE) and biodegradable (polylactic acid; PLA) MPs were explored. Results showed PLA MPs had positive effects on soil microbial biomass, while the positive and negative effects of PE MPs on microbial biomass varied with MP concentration. PE and PLA MPs increased microbial necromass contents and their contribution to SOC, mainly due to the increase in fungal necromass. On the contrary, PE and PLA MPs reduced lignin phenols and their contribution to SOC, mainly owing to the reduction in vanillyl-type phenols. The response of microbial necromass to PLA MPs was higher than that to PE MPs, whereas the response of lignin phenols was the opposite. MPs increased SOC level, with 83%–200% and 50%–75% of additional SOC in PE and PLA treatments, respectively, originating from microbial necromass carbon. This finding indicates that the increase in SOC pool in the presence of MPs can be attributed to soil microbial necromass carbon, and MPs increased capacity and efficacy of microbial carbon pump by increasing microbial turnover and reducing microbial N limitation. Moreover, the increase in amino sugars to lignin phenols ratio in PE treatment was higher than that in PLA treatment, and the increase in SOC content in PLA treatment was higher than that in PE treatment, indicating a high possibility of SOC storage owing to PLA MPs.

Effects of vineyard inter‐row management on soil physical properties and organic carbon in Central European vineyards

AbstractThe intensity and frequency of inter‐row management in vineyards are highly diverse and depend on local environmental conditions and the wine grower's attitude and experience. Reasons for different management include water conservation, weed and pest control, biological activity promotion and soil fertility and biodiversity preservation. We studied different soil cover management in 16 paired vineyards located at eight sites in the Leithaberg and Carnuntum regions of eastern Austria. To this end, we compared inter‐rows with medium intensity (Periodically Mechanically Disturbed) and low intensity (Permanent Green Cover). We investigated the effects of these different management intensities on soil organic carbon, bulk density, saturated and unsaturated hydraulic conductivity, pore size distribution and percolation stability in the upper soil layer from 3 to 8 cm. Soil organic carbon and percolation stability were significantly higher and soil bulk density was significantly lower in vineyards with permanent green cover. No significant differences were observed for saturated hydraulic conductivity, pore size distribution and plant available water. This may be attributed to a minor effect as a result of the time lag of up to 2 years since the last tillage. Regression analysis to predict plant‐available water for local vineyard soils also showed that texture, total organic carbon and bulk density were suitable predictor variables. These results suggest that both investigated inter‐row management systems support a good soil structure for winegrowers. Organic carbon content and parameters interacting with organic carbon may still be improved with permanent vegetation cover systems; however, the positive effects on plant available water are limited.

Representing cropping systems with the MEMS 2 ecosystem model

AbstractCroplands have been the focus of substantial investigation due to their considerable potential for sequestering carbon. Understanding the potential for soil organic carbon (SOC) sequestration and necessary management strategies will be enabled with accurate process‐based models. Accurately representing crop growth and agricultural practices will be critical for realistic SOC modeling. The MEMS 2 model incorporates a current understanding of SOC formation and stabilization, measurable SOC pools, and deep SOC dynamics and is seen as a highly promising tool to inform management intervention for SOC sequestration. Thus far, MEMS 2 has been developed to represent grasslands. In this study, we further developed MEMS 2 to model annual grain crops and common agricultural practices, such as irrigation, fertilization, harvesting, and tillage. Using four Ameriflux sites, we demonstrated an accurate simulation of crop growth and development. Model performance was strong for simulating aboveground biomass (index of agreement [d] range of 0.89–0.98) and green leaf area index (d from 0.90 to 0.96) across corn, soybean, and winter wheat. Good agreement with observations was also achieved for net ecosystem CO2 exchange (d from 0.90 to 0.96), evapotranspiration (d from 0.91 to 0.94), and soil temperature (d of 0.96), while discrepancy with the available soil water content data remain (d from 0.14 to 0.81 at four depths to 100 cm). While we will continue model testing and improvement, MEMS 2 (version 2.14) has now demonstrated its ability to effectively simulate the growth of common grain crops and practices.

Improvement and Stability of Soil Organic Carbon: The Effect of Earthworm Mucus Organo-Mineral Associations with Montmorillonite and Hematite

Improvement and Stability of Soil Organic Carbon: The Effect of Earthworm Mucus Organo-Mineral Associations with Montmorillonite and Hematite

Conservation tillage enhances the sequestration and iron-mediated stabilization of aggregate-associated organic carbon in Mollisols

Conservation tillage practices, which involve minimal or no soil disturbance and crop residue retention, are known to help preserve soil organic carbon (SOC). However, the mechanisms underlying the minerals-mediated chemical and physical stabilization of SOC remain unclear. Here, a long-term field experiment was initiated in 2012 to investigate the effects of tillage managements on the contents and chemical composition of SOC density fractions, iron oxides transformation and aggregate stability in Mollisols. We utilized three treatments: conventional tillage (CT) without crop residue, reduced tillage (RT) and no tillage (NT) with straw mulching. Compared to CT, RT and NT significantly increased SOC content by 13.6 % and 17.9 % in the 0–20 cm soil layer due to an increase in the aromatic compound contents. Furthermore, NT and RT increased the mean weight diameter by 17.7 % and 10.7 %, respectively, indicating increased aggregate stability compared to CT. Additionally, the contents of amorphous iron oxides (Feo) and complex iron oxides (Fep) increased under NT and RT by 10.6–14.4 % and 12.7–41.1 %, respectively, in bulk soil and silt + clay fractions within macroaggregates (>0.25 mm). The contents of Feo and Fep were strongly positively correlated with aggregate stability (p < 0.001, r2 = 0.64), and promote the physical protection of SOC. Both NT and RT enhanced the aromatic-C content and aromatic-C/aliphatic-C ratio by 13.6 %–24.6 % and 16.5 %–38.9 % in macroaggregates compared with CT. Moreover, the aromatic-C/aliphatic-C ratio increased with increasing Feo plus Fep contents (p < 0.001, r2 = 0.73), which led to an increased in the recalcitrant-C proportion, and this shift was benefit for the accumulation of mineral-associated OC. These results indicate that long-term conservation tillage can augment the accessibility of Fe for binding C, possibly by forming organo-Fe complexes, which subsequently improve soil aggregation, and thus promoted the chemical stability and long-term sequestration of SOC in Mollisols of Northeast China.

Effect of Nitrogen and Phosphorus on Soil Enzyme Activities and Organic Carbon Stability in Qinghai–Tibet Plateau

Alpine grassland ecosystems are the most important ecosystem type, exhibiting a high sensitivity to anthropogenic nitrogen (N) and phosphorus (P) inputs into terrestrial ecosystems and significantly affecting the carbon (C) cycle within these ecosystems. However, the effects of N and P additions on soil C stability and the processes of organic C transformation remain unclear. This study measured the soil enzyme activities and oxidizable C fractions over a 9-year period following N and P additions to an alpine meadow in the Qinghai–Tibet Plateau. It included four treatments: control (CK), N addition, P addition, and combined N and P addition (NP), utilizing statistical methods such as analysis of variance (ANOVA), correlation analysis, and redundancy analysis. The findings indicated that NP addition significantly increased the non-labile soil oxidizable C fraction in the topsoil layer. Redundancy and correlation analyses revealed strong associations between the vegetation characteristics, C-cycling enzyme activities, soil-oxidized C fractions, and SOC stability index. These results underscore the role of NP addition in enhancing SOC accumulation and stability in grassland ecosystems, with the soil vegetation properties and C source enzyme activities serving as key regulators of the SOC stability. This study offers valuable insights into predicting the SOC dynamics amid rising N and P availability, thereby elucidating the effect of nutrient addition on soil C-cycling mechanisms.

Soil organic carbon stocks and fractions under integrated systems and pasture in the Cerrado of Northeast Brazil

The adoption of integrated systems can reduce the pressure for the conversion of areas under native Cerrado to agricultural lands. However, little is known about how these systems affect total organic carbon pools and stocks in the Cerrado of Northeast Brazil. We aimed to evaluate the dynamics of soil organic carbon (SOC) and C and N stocks in a Cerrado under different cropping systems, in the agricultural frontier of Matopiba, Brazil. The systems were (i) Native Cerrado vegetation (NCF), (ii) integrated crop-livestock-forest system (ICLF), (iii) integrated crop-livestock system under no-tillage (ICL1), (iv) integrated crop-livestock with recent soil tillage (ICL2), and (v) pasture (PA). Soil samples were collected at 0.0–0.10, 0.10–0.20, 0.20–0.30, and 0.30–0.50 m depths to determine the soil C and N stocks, SOC fractions, and the carbon management index (CMI). Total C stocks in the 0.0–0.50 m soil profile were highest in ICL2 (127 Mg ha−1), ICL1 (112 Mg ha−1), and PA (101 Mg ha−1) and lowest in NCF (61 Mg ha−1) and ICLF (67 Mg ha−1). Similarly, total N stocks were highest in ICL1 (6.7 Mg ha−1), PA (6.7 Mg ha−1) and ICL2 (6.5 Mg ha−1), and lowest in NCF (4.7 Mg ha−1) and ICLF (4.8 Mg ha−1). ICL2 and ICL1 presented the greatest amount of humic fractions among the systems. The adoption of conservationist management systems that provide a permanent input of organic residues in the long-term is crucial for SOC stabilization and increases in soil C and N stocks. Integrated crop-livestock systems are the most suitable alternative for sequestering C and N in grain production systems in the Cerrado of Matopiba region.

Exploring the Influence of Natural and Agricultural Land Use Systems on the Different Lability Organic Carbon Compounds in Eutric Endocalcaric Arenosol

It is important to ensure the ratio of stable and labile soil organic carbon (SOC) compounds in the soil as this influences ecosystem functions and the sustainability of soil management. The aim of this investigation was to determine the changes in SOC compounds and soil quality improvement in Arenosol soil after the conversion of arable land to natural and agricultural land use. The land use types included pine afforestation (PA), uncultivated abandoned land (UAL), unfertilised and fertilised cropland (CLunf, CLf), and unfertilised and fertilised grassland (GRunf, GRf). To assess the lability of organic carbon (OC) compounds, levels of mobile humic substances (MHSs), mobile humic acids (MHAs), mobile fulvic acids (MFAs), active C pool (POXC), and water-soluble C (WEOC) compounds were determined. It was found that faster OC accumulation occurs in PA soil than in CLf, and is somewhat slower in grassland uses (GRf and UAL). As the amount of SOC increased, more MHS formed. A significant increase in their quantity was found in PA (+92.2%) and CRf and UAL (+51.5–52.7%). The application of mineral fertilisers promoted the formation of MHSs in CLf and GRf. PA, GRunf, and GRf soils had more suitable conditions for MHA formation (MHA/MFA &gt; 1.3), whereas CLunf soil contained more MFAs. The POXC pool was insensitive to land-use changes in the Arenosol. After land-use conversion, POXC amounts were significantly (p &lt; 0.05) higher in natural ecosystems (UAL and PA) and fertiliser perennial grasses than in CL. The amount of WEOC increased the most in UAL, PA, and GRf (7.4–71.1%). The sequence of decrease in land use was GRf, UAL, and PA &gt; CLunf, CLf, and GRunf. The decreasing order of the carbon management index (CMI) of different land uses (PA &gt; UAL &gt; GRf &gt; GRunf &gt; Clunf) confirms that faster OC accumulation in Arenosol soil occurred in PA and grassland land uses (GRf and UAL). The values of the carbon lability index (CLI) variation (CLunf &gt; GRunf GRf &gt; UAL &gt; PA) show that in PA, UAL, and GRf land uses, mobile organic matter (OM) forms are relatively less formed, which stabilises OC accumulation in the soil. The CMI showed that UAL and GRf were the most suitable soil uses for Arenosol soils.

Phragmites australis straw and biochar additives regulate soil organic carbon fractions in a degraded coastal salt marsh

Severely degraded coastal salt marshes with impaired ecological functions urgently need to be restored to improve blue carbon sequestration. Although plant residues and biochar have been used to restore degraded wetlands, the dose-effects of these plant additives on soil organic carbon fractions in degraded coastal wetlands are still unclear. Here, we investigated the dynamic changes in DOC components and mineral-associated organic carbon, such as calcium-bound organic carbon (Ca-OC) and iron/aluminum-bound organic carbon (Fe/Al-OC), in a degraded coastal salt marsh in a 60-day incubation experiment with low (0.1%), medium (1%), and high (3%) doses of Phragmites australis straw (PS) and Phragmites australis-derived biochar (PB) addition treatments. The results showed that the medium and high doses of PB additives increased humus-like substances, with the maximum proportion occurring on day 60 of the incubation. The fluorescence index (FI) and biological index (BIX) were >1.9 and 0.8, respectively, and the humification index (HIX) was < 4 in most treatment groups. The mineral-associated organic carbon (Ca-OC and Fe/Al-OC) contents were higher in the 1% PS and 3% PS treatments (p < 0.05). The PS additives increased the soil Ca-OC/SOC and Fe/Al-OC/SOC ratios, whereas the PB additives decreased them. This study demonstrated that PS and PB are potential carbon-rich additives for enhancing soil organic carbon stability and carbon sequestration in coastal wetland restoration projects, contributing to regional carbon neutrality.

Tillage and mulching effects on carbon stabilization in physical and chemical pools of soil organic matter in a coarse textured soil

Characterization of soil organic carbon (SOC) in terms of size, storage inside aggregates and chemical recalcitrance is vital to understand mechanisms of its stabilization and to devise climate smart agricultural management practices. Tillage and residue retention are known to influence carbon (C) storage within soil aggregates and its accumulation as particulate organic matter as well as organomineral complexes. However, the effect of tillage and crop residue retention on C stabilization in coarse textured soils through various mechanisms is not well understood. We studied the effect of conservation agriculture involving no tillage with surface residue mulch (NTM) in maize-wheat sequence on particulate (POC) and mineral associated organic C (MinOC), C storage within aggregates and acid non-hydrolysable C (NHC) in a sandy loam soil. Compared to conventional tillage without residue retention (CTM0), the NTM improved SOC stocks by 23% in top 15-cm soil and significantly increased coarse POC (∼92 to 284%) and fine POC (67 to 123%) with relatively little effect on MinOC. This indicated that MinOC had relatively small contribution towards SOC stabilization in coarse textured sandy loam soils with limited potential to form organomineral complexes. The results further showed that the effects of NTM were brought about by improved aggregate stability and C preservation inside macroaggregates of size >1 mm. Furthermore, greater amount of SOC (2.64 g C kg−1) and macroaggregate associated C (0.35–0.59 g C kg−1) occurred in recalcitrant forms (NHC) under NTM compared to conventional (CT) and deep tillage (DT). The NTM also impacted the belowground C input through improved root length density (RLD) and development of fibrous roots expressed as specific root length (SRL), which influenced SOC build-up and stabilization. It is concluded that compared to the existing practice of CTM0, the conservation agriculture involving NT with residue retention leads to SOC sequestration in coarse textured soils. Its large scale adoption, besides helping in climate change mitigation will lead to soil health improvement.

Effects of ant mounds construction on the carbon cycle in a drained peatland, Northeast China

Acting as ecosystem engineers, ant activities are significantly crucial in altering carbon (C) cycles by modifying the energy flows and nutrient cycles in soils. However, how ant mound constructions reshape soil organic carbon (SOC) pool heterogeneity and stability remains elusive. In this study, we compared differences in soil properties, vegetation biomass, greenhouse gas (GHG) emission, and molecular composition of SOC between ant mounds and nearby soils from a drained peatland, intending to decipher factors that govern carbon dynamics during ant excavates. Results indicated that ant mound construction simultaneously caused a decrease in SOC contents and an increase in nutrient level (total nitrogen and phosphorus), dissolved organic carbon (DOC), and its proportion to SOC (DOC/SOC) dramatically. The proportions of pyrolytic products derived from lignin, phenol, and aromatics decreased significantly from 31.70 % in nearby soils to 27.43 % in ant mounds. However, no significant difference in iron-bound organic carbon (OC-Fe) was found between ant mounds and nearby soils (p > 0.05). Ant mound building greatly accelerated SOC mineralization, with average CO2 emission fluxes 1–2 times higher than that in nearby soils. Notably, CH4 emissions from ant mounds was negative, meaning ant mounds acted as CH4 sinks. Meanwhile, net CH4 flux emission into the atmosphere from nearby soils were positive. Both plant biomass and SOC density (SOCd) in ant mounds were considerably lower than those observed in nearby soils. The structural equation model indicated that soil nutrient enrichment in ant mounds indirectly stimulated SOC decomposition by stimulating microbial abundance. Still, ant mounds construction had direct positive effects on labile carbon components accumulation and posed negative effects on resistant SOC compounds. Moreover, redundancy analysis and variation partitioning analysis confirmed that SOCd decline in ant mounds was primarily attributed to reduced soil moisture and elevated microbial abundance. The conceptual model implied that ants could effectively strengthen SOC content heterogeneity at the microscales, characterized by lower SOC storage and stability, and higher CO2 emission than those in nearby soils.

Relationship between calcium forms and organic carbon content in aggregates of calcareous soils in northern China

The agricultural soil organic carbon (SOC) pool is one of the most active C pools in terrestrial ecosystems, and soil aggregates are the main site of SOC fixation. Mineral sorption plays a crucial role in SOC accumulation. Calcium (Ca) is a typical soil mineral element that plays a crucial role in SOC accumulation and is fundamental for the formation of soil aggregates. Although Ca has been shown to regulate the stability of SOC, the relationship between different forms of Ca and SOC within aggregates in northern China remains unclear. In this study, we selected five soils (black soil, light chernozem soil, fluvo-aquic soil, sierozem soil, and loess soil) with different calcium carbonate (CaCO3) contents as the target materials. Water-stable aggregates were separated using the wet-sieving method. Soil respiration was measured using the static alkali absorption method (carbon dioxide [CO2] absorption by NaOH solution). The CaCO3 content was quantified using the CO2 gas volume method, and the continuous leaching method was used to analyze the different forms of Ca (water soluble-Ca, exchangeable-Ca, acid soluble-Ca, and bound to organic-Ca). In-situ field sampling was used to reveal the associations between different forms of Ca and SOC within aggregates. The results showed that besides the CaCO3 form, exchangeable-Ca (20.06 %–57.86 %) and bound to organic-Ca (26.69 %–73.91 %) were major Ca forms in calcareous soils. There was a nonsignificant negative correlation between CaCO3, acid soluble-Ca, and exchangeable-Ca and SOC in the bulk soil. However, there was a significant negative correlation among CaCO3, acid soluble-Ca, and exchangeable-Ca with SOC in macro-aggregates (> 0.25 mm) and micro-aggregates (0.25–0.053 mm). A 90-day incubation experiment was conducted to verify the previous speculations, which revealed that CaCO3 increased SOC mineralization and reduced the content of SOC, while enhancing the acid soluble-Ca and bound to organic-Ca content. In the treatment with added CaCO3, there was a negative correlation between acid soluble-Ca and bound to organic-Ca and SOC in bulk soil and macro-aggregates (> 0.25 mm). The effect of Ca2+ on SOC should not be ignored. Excessive Ca2+ may be converted into different forms of Ca to occupy the bonding sites for SOC on the surface of soil particles with silt and clay particles, forming a pseudo-aggregate structure, and the SOC traveling between soil particles, which is not conducive to SOC stabilization and accumulation.

Aridity affects soil organic carbon concentration and chemical stability by different forest types and soil processes across Chinese natural forests

Forest soils play a critical role in carbon (C) reservoirs and climate change mitigation globally. Exploring the driving factors of soil organic carbon (SOC) concentration and stability in forests on a large spatial scale can help us evaluate the role of forest soils in regulating C sequestration. Based on SOC quantification and solid-state 13C nuclear magnetic resonance spectroscopy, we investigated the SOC concentration and SOC chemical stability (indicated by alkyl-to-O-alkyl ratio and hydrophobic-to-hydrophilic ratio) in top 0–5 and 5–10 cm soils from 65 Chinese natural forest sites and explored their driving factors. Results showed that SOC concentration in 0–5 cm soils were highest in mixed forests but SOC chemical stability in 0–5 cm soils were highest in coniferous forests, while SOC concentration and chemical stability in 5–10 cm soil layers did not differ across forest types. SOC concentration in 0–5 cm was directly related to soil pH and soil bacterial diversity. Structural equation models showed that aridity indirectly affected SOC concentration in 0–5 cm by directly affecting soil pH. While SOC chemical stability in 0–5 cm soils was higher with increased aridity. According to the correlations, the potential mechanisms could be attributed to higher proportion of coniferous forests in more arid forest sites, lower relative abundance of O-alkyl C, higher MgO and CaO contents, and higher bacterial diversity in soils from more arid forest sites. Our study reveals the important role of aridity in mediating SOC concentration and chemical stability in top 0–5 cm soils in Chinese natural forests on a large-scale field investigation. These results will help us better understand the different mechanisms underlying SOC concentration and stability in forests and assess the feedback of forest SOC to future climate change.

Edaphic regulation of soil organic carbon fractions in the mattic layer across the Qinghai-Tibetan Plateau

The mattic layer is a main ecological function bearer of alpine meadow soils in the Qinghai-Tibet Plateau. It has high soil organic carbon (SOC) content with a variety of SOC fractions, which are thought to have different sensitivities to climate change. The effects of soil properties and climate on the SOC fractions in the mattic layer are not well understood. To address this, we analyzed the effects of environmental factors on two SOC fractions: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). A random forest model (RFM), partial correlation analysis, and structural equation model (SEM) were used to quantify the relative effects of soil and climatic factors on SOC fractions. We found that SOC and its fractions are primarily regulated by soil properties rather than climate. Partial correlation analysis and SEM revealed that climate indirectly affects SOC by influencing soil properties. Silt+Clay and exchangeable calcium (Caex) were found to be the strongest contributing factors of MAOC and POC, respectively. A distinct shift occurs in the mechanism underlying SOC stabilization with varying soil pH. In acidic and neutral environments, amorphous Al/Fe-(hydr) oxides contribute to the stability of MAOC, whereas free Al/Fe-(hydr) oxides promote SOC mineralization. Conversely, Caex positively influences the stabilization of both POC and MAOC throughout the pH range. These results can be extrapolated to predict SOC dynamics in future soil conditions affected by environmental change, especially for use in Earth system models.

Global pattern of organic carbon pools in forest soils.

Understanding the mechanisms of soil organic carbon (SOC) sequestration in forests is vital to ecosystem carbon budgeting and helps gain insight in the functioning and sustainable management of world forests. An explicit knowledge of the mechanisms driving global SOC sequestration in forests is still lacking because of the complex interplays between climate, soil, and forest type in influencing SOC pool size and stability. Based on a synthesis of 1179 observations from 292 studies across global forests, we quantified the relative importance of climate, soil property, and forest type on total SOC content and the specific contents of physical (particulate vs. mineral-associated SOC) and chemical (labile vs. recalcitrant SOC) pools in upper 10 cm mineral soils, as well as SOC stock in the O horizons. The variability in the total SOC content of the mineral soils was better explained by climate (47%-60%) and soil factors (26%-50%) than by NPP (10%-20%). The total SOC content and contents of particulate (POC) and recalcitrant SOC (ROC) of the mineral soils all decreased with increasing mean annual temperature because SOC decomposition overrides the C replenishment under warmer climate. The content of mineral-associated organic carbon (MAOC) was influenced by temperature, which directly affected microbial activity. Additionally, the presence of clay and iron oxides physically protected SOC by forming MAOC. The SOC stock in the O horizons was larger in the temperate zone and Mediterranean regions than in the boreal and sub/tropical zones. Mixed forests had 64% larger SOC pools than either broadleaf or coniferous forests, because of (i) higher productivity and (ii) litter input from different tree species resulting in diversification of molecular composition of SOC and microbial community. While climate, soil, and forest type jointly determine the formation and stability of SOC, climate predominantly controls the global patterns of SOC pools in forest ecosystems.

Trade-off between Pore-Throat Structure and Mineral Composition in Modulating the Stability of Soil Organic Carbon.

The preservation of soil organic carbon (OC) is an effective way to decelerate the emission of CO2 emission. However, the coregulation of pore structure and mineral composition in OC stabilization remains elusive. We employed the in situ nondestructive oxidation of OC by low-temperature ashing (LTA) combined with near edge X-ray absorption fine structure (NEXAFS), high-resolution microtomography (μ-CT), field emission electron probe microanalysis (FE-EPMA) with C-free embedding, and novel Cosine similarity measurement to investigate the C retention in different aggregate fractions of contrasting soils. Pore structure and minerals contributed equally (ca. 50%) to OC accumulation in macroaggregates, while chemical protection played a leading role in C retention with 53.4%-59.2% of residual C associated with minerals in microaggregates. Phyllosilicates were discovered to be more prominent than Fe (hydr)oxides in C stabilization. The proportion of phyllosilicates-associated C (52.0%-61.9%) was higher than that bound with Fe (hydr)oxides (45.6%-55.3%) in all aggregate fractions tested. This study disentangled quantitatively for the first time a trade-off between physical and chemical protection of OC varying with aggregate size and the different contributions of minerals to OC preservation. Incorporating pore structure and mineral composition into C modeling would optimize the C models and improve the soil C content prediction.

Variation of root traits and its influences on soil organic carbon stability in response to altered precipitation in an alpine meadow

Soil organic carbon (SOC) dynamics are strongly controlled by plant roots. Yet, how variation of root traits under precipitation change influences SOC stability remains unclear. As part of a 5-year field experiment manipulating precipitation including 90 % (0.1P), 50 % (0.5P), 30 % (0.7P) decrease, and 50 % increase (1.5P), this study was designed to assess the effects of changing precipitation on root traits and production dynamics by minirhizotron and examine how such influences regulate SOC stability in an alpine meadow on the Qinghai-Tibetan Plateau. We found that root length density (RLD), specific root length (SRL), root branching intensity (RBI), and root residue carbon input (RC input) exhibited no significant response, whereas root turnover (RT), root carbon (C), nitrogen (N) concentrations and C/N ratio were altered by precipitation change with nonlinear trends. Absorptive root RT positively correlated to manipulated precipitation within the interannual precipitation range in topsoil, but it showed no significant change under extreme drought treatment. Alpine meadows can maintain the SOC content and density under varied precipitation. However, it showed significant variation in aggregate stability and organic carbon (OC) distribution in aggregates in topsoil, which were mainly due to the strong direct effects of soil moisture and partly related to RLD and RC input of transport roots. Although subsurface soil aggregate stability and OC associated with aggregates were not modified, our results indicated a risk of SOC stability variation in subsurface soil if absorptive root RT and SRL changed. These findings provide vital information to predict responses of SOC dynamics of alpine meadow to future climate change.

Development of unique soil organic carbon stability index under influence of integrated nutrient management in four major soil orders of India

Stability of soil organic carbon (SOC) is pre-requisite for stabilization of C leading to long-term C sequestration. However, development of a comprehensive metric of SOC stability is a major challenge. The objectives for the study were to develop novel SOC stability indices by encompassing physical, chemical, and biochemical SOC stability parameters and identifying the most important indicators from a Mollisol, an Inceptisol, a Vertisol, and an Alfisol under long-term manuring and fertilization. The treatments were control, 100%NPK, 50% NPK+ 50% N through either farmyard manure, cereal residue, or green manure. SOC stability indicators were selected, transformed and integrated into unique SOC stability indices via conceptual framework and principal component analysis. Principal component analysis identified Al-macroaggregate, humic acid C-microaggregate, microaggregate-C, particulate organic matter-C-macroaggregate and polyphenol-microaggregate as the important SOC stability indicators. The principal component analysis -based SOC stability index varied from 0.2 to 0.9, 0.1 to 0.5, 0.2 to 0.6, 0.1 to 0.5 for Mollisol, Inceptisol, Vertisol and Alfisol, respectively. The SOC-stability index derived from conceptual framework and principal component analysis significantly lined up well with one another, although NaOCl-Res-C showed a high correlation with both conceptual framework (r = 0.8) and principal component analysis-based (r = 0.7) SOC stability indexes, suggesting that both methods might be used to quickly assess SOC stability in four soil orders. Overall, 50%NPK+50%N by farmyard manure or green manure emerged as the most effective management practices for enhancing stability of SOC in Mollisol, Inceptisol, Vertisol, and Alfisol of India which might act as major C sink in rice-wheat and maize-wheat cropping systems. The other aspect of C sequestration is to enhance agricultural productivity without depending much on expensive chemical fertilizers. The model yardstick thus developed for assessing SOC stability might be useful to other systems as well.