Mineral-associated Organic C 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.

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.

Introducing Native Tree Species Alter the Soil Organic Carbon, Nitrogen, Phosphorus, and Fine Roots in Moso Bamboo Plantations

Bamboo and wood-mixed forests are management models that remarkably enhance the balance and productivity of bamboo ecosystems. However, the effects of this model on soil nutrients and enzyme activities remain largely unknown. This study compared the soil organic carbon, nitrogen, phosphorus, and enzyme activity, along with the characteristics of fine roots in pure Moso bamboo plantations (CK) and those mixed with Liriodendron chinense (ML), Sassafras tzumu (MS), Cunninghamia lanceolata (MC), and Pseudolarix amabilis (MP). The results showed that mixed forests improve carbon pools in 0–40 cm soil layers, increasing the total organic C(TOC), free particulate organic C (fPOC), occluded particulate organic C (oPOC), hot-water-extractable organic C (DOC), and mineral-associated organic C (MOC). They also increase soil total N, total P, available N, available P, NH4+-N, NO3−-N, inorganic P, organic P, and microbial biomass N. Bacterial and fungal abundances, along with enzyme activities (urease, acid phosphatase, polyphenol oxidase, peroxidase, and β-glucosidase), also improved. MP and MS were the most effective. Moreover, MS and MP supported a higher biomass and length of fine root and increased the nitrogen and phosphorus uptake of Moso bamboo. In conclusion, Sassafras tzumu and Pseudolarix amabilis are optimal for mixed planting, offering substantial benefits to soil nutrient dynamics and preventing soil quality decline in Moso bamboo forests, thereby supporting better nutrient cycling and carbon sequestration. This research offers insights into enhancing soil quality through diversified Moso bamboo forestry.

Saline-alkali land reclamation boosts topsoil carbon storage by preferentially accumulating plant-derived carbon

Saline–alkali land is an important cultivated land reserve resource for tackling global climate change and ensuring food security, partly because it can store large amounts of carbon (C). However, it is unclear how saline–alkali land reclamation (converting saline–alkali land into cultivated land) affects soil C storage. We collected 189 adjacent pairs of salt-affected and cultivated soil samples (0–30 cm deep) from the Songnen Plain, eastern coastal area, Hetao Plain, and northwestern arid area in China. Various soil properties, the soil inorganic C (SIC), organic C (SOC), particulate organic C (POC), and mineral-associated organic C (MAOC) densities, and plant- and microbial-derived C accumulation were determined. Saline–alkali land reclamation inconsistently affected the SIC density but significantly (P < 0.001) increased the SOC density. The SOC, POC, and MAOC densities were predicted well by the integrative soil amelioration index. Saline–alkali land reclamation significantly increased plant-derived C accumulation and the plant-derived C to microbial-derived C ratios in all saline–alkali areas, and less microbial transformation of plant-derived C (i.e., less lignin degradation or oxidation) occurred in cultivated soils than salt-affected soils. The results indicated that saline–alkali land reclamation leads to plant-derived C becoming the dominant contributor of SOC storage. POC storage and MAOC storage were strongly linked to plant- and microbial-derived C accumulation, respectively, caused by saline–alkali land reclamation. Our findings suggest that saline–alkali land reclamation increases C storage in topsoil by preferentially promoting plant-derived C accumulation.

Microbial transformation mechanisms of particulate organic carbon to mineral-associated organic carbon at the chemical molecular level: Highlighting the effects of ambient temperature and soil moisture

Soil dissolved organic matter (DOM), an important component of active soil organic carbon (C), plays a crucial role in controlling C transformation. However, the role of DOM in mediating the conversion of particulate organic C (POC) into mineral-associated organic C (MAOC) at the molecular level remains unclear. In this study, the microbial conversion of POC to MAOC at the molecular level was investigated at different ambient temperatures and soil moisture levels. An increase in ambient temperature led to the conversion of POC to labile C, whereas MAOC increased with soil moisture. CHO, CHNO, and CHOS increased with soil moisture and temperature. The highest temperature (35 °C) promoted conversion of lignin/cram (carboxyl-rich alicyclic molecules)-like molecules into aromatic components with the highest intragroup transformation of DOM (74%). The highest soil moisture (60%) favoured the conversion of refractory substances into aliphatic/peptide and lipid components with the highest intergroup transformation (28%). Compared to fungi, bacteria were the main driving force for DOM transformation. Therefore, changes in ambient temperature and soil moisture control the microbial formation and transformation of different C fractions (i.e. POC and MAOC) via DOM differently at the molecular level.

Microbial metabolic capacity regulates the accrual of mineral-associated organic carbon in subtropical paddy soils

In general, the accrual of mineral-associated organic C (MAOC) level is limited by the finite area of reactive minerals. However, microbial metabolism probably affects MAOC accumulation and saturation because microbial necromass C constitutes the primary component of MAOC. To address this gap in current understanding, we used 18O–H2O incubation and biomarker analyses to quantify microbial C metabolism and necromass accumulation respectively, and quantify their relationship with MAOC and saturation levels across gradients of soil organic C (SOC) that ranged from 4.0 to 40.1 g kg−1 in subtropical paddy soils. Results showed that particulate organic C (POC) increased linearly as SOC increased whilst MAOC tended to saturation. Notably, MAOC accumulation showed a significant positive relationship to increases in microbial necromass C indicating that MAOC accumulation prior to saturation was closely linked to the activity of microbial C metabolism. Compared with low C soils, lower microbial growth efficiencies (qgrowth) and slower biomass turnover rates were observed in high C soils. Moreover, the necromass accumulation coefficient (i.e. necromass produced per unit of microbial biomass) was also lower in high C soils than in low C soils. The lower microbial growth efficiency in the high C soils was attributed to greater C/P and higher clay proportion, which decreased microbial biosynthesis and substrate accessibility. Microbial communities showed adaptive shifts in composition to modulate bacterial metabolic activity, with increases in the relative abundance of r-strategists and decreases in the K-strategists. Management strategies for actively increasing the size of the MAOC pool, by bolstering microbial anabolism and necromass accumulation, are essential for increasing the MAOC content in paddy soils where the MAOC is unsaturated. For high C soils, where the MAOC is already saturated, the use of structural and recalcitrant organic materials to increase the size of the POC pool is needed to enhance carbon sequestration efforts.

Management and liming-induced changes in organo-Al/Fe complexes and amorphous mineral-associated organic carbon: Implications for carbon sequestration in volcanic soils

Amorphous mineral-associated organic carbon (AMAOC) and organo-Al/Fe complexes (Cp) play crucial roles in soil carbon (C) sequestration in volcanic soils. This study investigated the effects of land use management and liming on the C sequestration capacity of a young allophanic Andisol and old halloysitic-kaolinitic volcanic Ultisol sampled at 0–10 and 10–20 cm depth. This study was designed to test two hypotheses: (1) management and liming affect the C sequestration capacity of Andisol by reducing Cp and increasing AMAOC to offset Cp losses, and (2) they do not significantly affect the carbon sequestration capacity of Ultisol because of its more crystalline mineral forms and reduced amount of Cp. This study was conducted using a selective dissolution technique and density fractionation with agricultural soils and forests as control soils. After 43 years of intensive management and liming, the soil organic carbon (SOC) content decreased by 32% in the topsoil compared to the original native forest soil (178 ± 2 g kg−1 soil), mainly in the particulate organic matter (POM) fraction, which decreased by 74% compared to the native forest soil (61 ± 9 g kg−1 soil). However, Cp decreased only by 40% compared to the forest soil (62 ± 1 g kg−1 soil) from pHKCl 4.8–5.0, and AMAOC increased in 17% in the topsoil and 64% in the subsoil, supporting the first hypothesis of this study. In the cultivated Ultisol, SOC decreased by 35% due to the POM fraction, and mineral-associated organic C (MAOC) decreased by 37% compared to the forest soil (38 ± 7 g kg−1 soil). These results partially support the second hypothesis because the crystalline clay minerals from which MAOC originated remained unchanged and were strongly correlated with SOC. The mass balance of both soils indicated that the stable C losses from native forest soils were < 14%. These results highlight the importance of different C sequestration mechanisms to compensate for SOC losses in the young allophanic Andisol and old volcanic Ultisol. Our findings imply that both mechanisms can be used to estimate the C sequestration capcity in similar soil types.

Methodological choices in size and density fractionation of soil carbon reserves – A case study on wood fiber sludge amended soils

Soil organic carbon (SOC) is in the focus of research due to its central role in regulating climate and maintaining fertility and resilience of soils. Methodologically, shifting from whole soil C measurements to specific SOC fractions increases possibility to detect small changes in the vast SOC storage, and enhances estimation of SOC stability. However, SOC fractionation schemes are numerous and variable. In this study, deionized water and sodium hexametaphosphate (SHMP) were compared in soil dispersion by separating soils into coarse (0.25–2 mm), medium (0.063–0.25 mm) and fine (<0.063 mm) size fractions. The first two fractions were further separated by density (1.8 g cm−3) to obtain free particulate organic C (POC) and mineral associated organic C (MOC). The approach was applied to a clay and a silt loam soil with and without wood fiber sludge amendment to follow the added C. Aggregate disruption was enhanced with SHMP in comparison to water, but the effect was small and the use of SHMP decreased recovery of SOC, wherefore water was preferred. In both soils, 5–10 % of SOC occurred as coarse POC, 1–3% as coarse MOC, 5 % as medium POC, 10 % as medium MOC, and 70–85 % as fine MOC. The added C resided in the POC fractions with an indication of minor accumulation to the fine MOC in the clay soil. Longer time frame with repeated C additions would be needed to increase the stable MOC storages though saturation of the MOC reserve may hinder accumulation in the silt loam low in fines.

Increasing complexity of agroforestry systems benefits nutrient cycling and mineral-associated organic carbon storage, in south-eastern Brazil

Agroforestry systems are often promoted as solutions to address land degradation and climate change. However, agroforestry is an umbrella term for a large variety of systems and it is not clear how their degree of complexity influences their provision of soil-based ecosystem services, such as soil organic carbon (SOC) storage and nutrient cycling. Furthermore, a knowledge gap remains whether agroforestry systems perform equally well on all soil types. The objectives of this study were 1) to assess the links between agroforestry complexity, nutrient cycling and SOC fractions, and 2) to assess how soil texture influences these relationships in Brazilian agroforestry systems. We sampled 59 agroforestry plots across 30 sites in Sao Paulo state, Brazil, and 8 monocrop sites (6 pastures and 2 crop monocultures). The 38 sites represented a soil textural gradient, ranging from very sandy to very clayey (clay content range 25 – 620 g kg−1). An Agroforestry Complexity Index (ACI) was defined based on tree species richness, stem density and pruning management. Nutrient (N, P, K, Ca, Mg) and C contents were determined in litter and soil (0–30 cm depth) samples, and mineral-associated organic C (MAOC) and particulate organic C (POC) in soil samples were assessed as well. ACI was positively associated with C, N, P, Ca and Mg stocks in litter, and these litter nutrients were in turn positively associated with the corresponding soil nutrient stocks. Associations between soil nutrients and MAOC were stronger on sandy soils than on clayey soils, particularly for P, Ca and Cation Exchange Capacity (CEC). For POC, robust relationships with nutrients were only found on sandy soils. Structural Equation Models indicated causal relationships between agroforestry complexity, P and Ca cycling, and MAOC and POC stocks in topsoils. Our results indicate that nutrients effectively cycle from in situ mulch into plant-available soil pools and highlight the synergies between nutrient cycling and stable C stocks that can be achieved in complex agroforestry systems. These synergies seem to be particularly strong on sandy soils (<15 % clay).

Cover crop-mediated soil carbon storage and soil health in semi-arid irrigated cropping systems

Cover cropping is considered a climate-smart strategy to regenerate low-fertility soils in arid and semi-arid regions. However, the mechanisms of soil organic carbon (SOC) storage and soil health improvement in semi-arid irrigated cropping systems are poorly understood. The main goal of this study was to evaluate cover crop effects on various physical, chemical, and biological indicators of soil health in an irrigated silage corn (Zea mays L.)–sorghum [Sorghum bicolor (L.) Moench] rotation and understanding how changes in soil health contribute to SOC storage. We evaluated various SOC and nitrogen (N) pools, and microbial community structure under cover crop mixture of grasses, brassicas, and legumes (GBL), grasses and brassicas (GB), grasses and legumes (GL), and no cover crops (NCC) control for four years. Grasses included annual ryegrass (Lolium multiflorum Lam.) and winter triticale (× Triticosecale Wittmack), brassicas included turnip (Brassica rapa subsp. rapa L.) and daikon radish (Raphanus sativus var. Longipinnatus Bailey), and legumes included pea (Pisum sativum subsp. arvense L.) and berseem clover (Trifolium alexandrinum L.). Soil C pools: SOC, potentially mineralizable C, microbial biomass C, mineral-associated organic C (MAOC), and particulate organic C (POC) were greater with cover crops than without, but the effects were not consistent in all study years. Soil organic N was also greater under cover crops than under NCC. Soil C-to-N (C:N) ratio increased over the years but was similar among treatments. Cover crops used N and soil water to grow, significantly reducing soil inorganic N content and water-filled pore spaces in the first two years. However, fungal and bacterial communities grew better under cover crops with a high fungi-to-bacteria ratio than under NCC. The soil MAOC content was related to microbial community and enzyme activity while POC was affected by soil physicochemical conditions, specifically soil pH and mineral N availability, highlighting two major pathways of SOC sequestration in semi-arid cropping systems. This study showed how cover crops enhance SOC storage and soil health in semi-arid cropping systems.

The Different Roles of Mineralogy in Soil Organic Carbon Accumulation in Northern and Southern China

The sequestration of soil organic carbon (SOC) through mineral protection is an important approach to mitigating climate change. However, the effect of mineral composition on SOC stability is unclear at regional scales. In this study, we investigated the relationship between mineralogy and SOC in Alfisol and Mollisol from southern and northern regions of China. We analyzed soil at two layers for its SOC fractions, mineralogical characteristics and functional groups. It was found that the majority of SOC was stored as mineral-associated organic C (MAOC), which had higher δ13C values and narrower C/N ratios compared to particulate organic C. In Mollisol, the proportion of MAOC and the abundance of aromatic C were higher than that in Alfisol, while polysaccharide C was lower. Compared to Alfisol, Mollisol was dominated by illite, and had significantly (p &lt; 0.05) lower iron (Fe) and aluminum (Al) sesquioxides contents. The SOC content was positively correlated with illite in Mollisol, and with Fe and Al sesquioxides in Alfisol. The random forest model identified sesquioxides as the most important determinant of SOC accumulation (36%), followed by SOC fractions (18%) and functional groups (18%). In summary, our study suggests that SOC protection through mineralogy depends more on the composition of the host minerals, and not just on the clay content, and aromatic C is also important in the stabilization of SOC.

Cover cropping promotes soil carbon sequestration by enhancing microaggregate-protected and mineral-associated carbon

Cover cropping can improve soil C sequestration compared to no cover cropping, but the mechanism of C sequestration in soil aggregates and minerals needs more exploration. We explored C sequestration using C fractions in soil aggregates and minerals by cover crops in a five-year old summer cover crop – winter wheat (Triticum aestivum L.) rotation system at Changwu National Agroecology Experimental Station in the Chinese Loess Plateau. Three cover crops as soybean (Glycine max L., SB), sudangrass (Sorghum sudanense (Piper) Stapf, SG), soybean and sudangrass mixture (SS) were planted during summer fallow and incorporated into the soil two weeks prior to wheat planting each year. Soil samples at 0–10, 10–20, and 20–40 cm depths were collected at wheat harvest after 5-yr and analyzed for C fractions which were coarse particulate organic C (cPOC), intra-microaggregate fine particulate organic C (iPOC), free fine particulate organic C (fPOC), and mineral-associated organic C (MOC). The iPOC and MOC are considered as protected C against mineralization. Compared to no cover crop (CK), cover crops increased large macroaggregate proportion at 0–10 cm by 18–22 %, with SS having a greater mean-weight diameter (MWD) than other treatments. Cover crops had greater MOC, iPOC, and fPOC fractions than CK in most aggregate-size classes and the bulk soil at all depths, with SS having the greatest C fractions. The MOC was greater than any other C fractions, regardless of cover crop species. Cover cropping can enhance soil aggregation and C sequestration by increasing microaggregate-protected and mineral-associated C compared to no cover cropping, resulting in improved soil C stabilization. Cover crop mixture was more effective in promoting soil aggregate stability and C fractions than single cover crop species.

Profile and nano-scale distribution of soil organic carbon for upland and paddy soils from an alluvial plain in South China

Understanding the heterogeneous distribution of soil organic carbon (C) (SOC) in different soil environments is essential for predicting SOC preservation. However, SOC distribution differs in profile scales and molecular levels in various farmland soils, making it challenging to elucidate the environmental behaviors of SOC. In the present study, upland and paddy soil samples were both collected at different depths (0–100 cm) from an alluvial plain in the Pearl River Delta of South China. Wet chemistry experiments coupled with 13C-nuclear magnetic resonance (NMR) and spherical aberration correction scanning transmission electron microscopy (Cs-STEM) were used to determine the vertical distribution, chemical composition, and nano-scale sequestration mechanisms of SOC in upland and paddy soils. SOC content for upland soil, which was highest at the depth of 0–20 cm (6.98 ± 0.34 g·kg−1), decreased with increasing soil depth, while that of paddy soil had the highest content at a depth of 80–100 cm (86.66 ± 5.26 g·kg−1). Statistical analysis revealed that mineral-associated organic C (MAOC) in the upland soil and particulate organic C (POC) in the paddy soil were the major C fractions that were related to SOC content. Further analysis of NMR spectra showed that the molecular composition of upland SOC differs vertically, while the molecular composition and proportion of organic C in paddy soil varied with soil depth. Cs-STEM results revealed that the relative percentage of carboxyl C decreased from 0 to 20 cm (42.1%) to 80–100 cm (28.8%) with an increase in aromatic C from 7.7 to 27.2% for the upland soil, while the four C species showed similar abundance for paddy soil at different depths, consistent with the NMR results. Overall, the upland SOC was not as easily mineralized as that of paddy soil due to a lower percentage of easily oxidized carbon, higher percentage of MAOC, and higher decomposition degree. The results of this study would enrich our quantitative understanding of the profile distribution and chemical diversity of SOC from macro to micro scales in the alluvial plain from South China, and provide insights into C preservation in natural soils.

The Role of Bedrock Geochemistry and Climate in Soil Organic Matter Stability in Subtropical Karst Forests of Southwest China

The stability of soil organic matter (SOM) plays a critical role in soil carbon (C) dynamics under global warming. However, the factors influencing SOM stability, particularly the significance of bedrock geochemistry and its hierarchical relationship with climate and soil properties, remain poorly understood. To address this gap, we conducted a study along a large climatic gradient (Δtemperature &gt; 9 °C) in the subtropical karst forests of southwest China, quantifying SOM stability using thermal analysis and investigating the contributions of bedrock geochemistry, climate, and soil properties. Our results showed that SOM stability was positively correlated with mineral-associated organic C (MAOC) rather than particulate organic C. Hierarchical partitioning analysis further demonstrated that bedrock geochemistry was the predominant contributor to SOM stability variance, accounting for 23.7%. Following this, soil minerals contributed to 21.1%–22.6% of the variance, the mean annual temperature to 20.3%, and microbial biomass C to 17.2%. In particular, bedrock geochemistry—specifically the presence of calcium-rich bedrock—was found to enhance SOM stability by promoting the accumulation of exchangeable calcium and calcium carbonate in soils. Additionally, high temperature improved SOM stability by increasing the content and proportion of MAOC and soil pH. These results highlight the fundamental role of bedrock geochemistry in controlling SOM stability and emphasize the importance of considering hierarchical relationships among bedrock–soil–climate interactions for evaluating soil C dynamics.

Phosphorus Fertilization Boosts Mineral-Associated Soil Organic Carbon Formation Associated with Phagotrophic Protists.

Long-term fertilization affects soil organic C accumulation. A growing body of research has revealed critical roles of bacteria in soil organic C accumulation, particularly through mineral-associated organic C (MAOC) formation. Protists are essential components of soil microbiome, but the relationships between MAOC formation and protists under long-term fertilization remain unclear. Here, we used cropland soil from a long-term fertilization field trial and conducted two microcosm experiments with 13C-glucose addition to investigate the effects of N and P fertilizations on MAOC formation and the relationships with protists. The results showed that long-term fertilization (especially P fertilization) significantly (P < 0.05) increased 13C-MAOC content. Compared with P-deficient treatment, P replenishment enriched the number of protists (mainly Amoebozoa and Cercozoa) and bacteria (mainly Acidobacteriota, Bacteroidota, and Gammaproteobacteria), and significantly (P < 0.001) promoted the abundances of bacterial functional genes controlling C, N, P, and S metabolisms. The community composition of phagotrophic protists prominently (P < 0.001) correlated with the bacterial community composition, bacterial functional gene abundance, and 13C-MAOC content. Co-occurrence networks of phagotrophic protists and bacteria were more connected in soil with the N inoculum added than in soil with the NP inoculum added. P replenishment strengthened bacterial 13C assimilation (i.e., 13C-phospholipid fatty acid content), which negatively (P < 0.05) correlated with the number and relative abundance of phagotrophic Cercozoa. Together, these results suggested that P fertilization boosts MAOC formation associated with phagotrophic protists. Our study paves the way for future research to harness the potential of protists to promote belowground C accrual in agroecosystems.

Contribution of microbial necromass to soil organic carbon formation during litter decomposition under incubation conditions.

We conducted a 512-day incubation experiment to study the dynamics of microbial necromass and soil carbon fraction in the 'litter-soil' transformation interface soil layer (TIS) during litter decomposition, using a perennial C3 herb, Stipa bungeana, in the loess hills. The results showed that soil microbial necromass was dominated by fungi in the early and middle stages, and by bacteria in the late stage. The contribution of fungal necromass C to mineral-associated organic C (MAOC) was significantly higher (38.7%-75.8%) than that of bacteria (9.2%-22.5%) and 2-3 times more than the contribution rate of bacterial necromass. Soil organic C (SOC) content was decreasing during litter decomposition. The input of plant C resources stimulated microbial utilization of soil C fractions. The continuous decrease in particulate organic C during the early and late stages of decomposition was directly responsible for the decrease in SOC content. In contrast, the fluctuating changes in microbial necromass C and MAOC played an indirect role in the reduction of SOC. The increase in soil microbial necromass C caused by a single exogenous addition of litter did not directly contribute to SOC accumulation.

Divergent contribution of particulate and mineral-associated organic matter to soil carbon in grassland

Sequestration of soil organic carbon (SOC) is an effective means to draw atmospheric CO2. Grassland restoration is one of the fastest methods to increase soil C stocks, and particulate-associated C and mineral-associated C play critical roles in soil C stocks during restoration. Herein, we developed a conceptual mechanistic frame regarding the contributions made by mineral-associated organic matter to soil C during the restoration of temperate grasslands. Compared to 1-year grassland restoration, 30-year restoration increased mineral-associated organic C (MAOC) by 41% and particulate organic C (POC) by 47%. The SOC changed from microbial MAOC predominance to plant-derived POC predominance, as the POC was more sensitive to grassland restoration. The POC increased with plant biomass (mainly litter and root biomass), while the increase in MAOC was mainly caused by the combined effects of increasing microbial necromass and leaching of the base cations (Ca-bound C). Plant biomass accounted for 75% of the increase in POC, whereas bacterial and fungal necromass contributed to 58% of the variance in MAOC. POC and MAOC contributed to 54% and 46% of the increase in SOC, respectively. Consequently, the accumulation of the fast (POC) and slow (MAOC) pools of organic matter are important for the sequestration of SOC during grassland restoration. Overall, simultaneous tracing of POC and MAOC helps further understand the mechanisms and predict soil C dynamics combined with the input of plant C, microbial properties, and availability of soil nutrients during grassland restoration.

Effects of land use and site on organic carbon fractions in some humic soil profiles of KwaZulu-Natal, South Africa

Soil organic matter (SOM) and its different carbon (C) fractions play a vital role in storing C and are a source of plant nutrients in natural and managed ecosystems. This study was conducted at six sites in KwaZulu-Natal Province to determine the effect of land use on the distribution of C fractions in selected humic soil profiles from different sites, using chemical and physical fractionation. Land use at some sites had been converted from natural grassland to maize and cultivated pasture and at others from native forest/wattle plantation to sugarcane farming. Humic acids (HA), fulvic acids (FA), hot water extractable C (HWEC), particulate organic matter C (POM-C) and mineral-associated organic C (MAC) were analysed to 1 m depth. Cropping lowered POM-C and HWEC, in the top 100 mm, irrespective of site differences, under pasture, grassland, forest and wattle plantation. Amongst the most labile fractions, FA predominated, resulting in a HA:FA ratio < 1 for pastures. The MAC was not affected by land use, irrespective of site differences. These findings reveal that organic carbon in humic soils can be decreased by cultivation but does not inevitably indicate the loss of C storage in the stable pool.

Trait-based assembly of arbuscular mycorrhizal fungal communities determines soil carbon formation and retention.

Fungi are crucial for soil organic carbon (SOC) formation, especially for the more persistent mineral-associated organic C (MAOC) pool. Yet, evidence for this often overlooks arbuscular mycorrhizal fungi (AMF) communities and how their composition and traits impact SOC accumulation. We grew sudangrass with AMF communities representing different traits conserved at the family-level: 1) competitors, from the Gigasporaceae family; 2) ruderals, from the Glomeraceae family; or 3) both families combined. We labeled sudangrass with 13 C-CO2 to assess AMF contributions to SOC, impacts on SOC priming, and fungal biomass persistence in MAOC. Single-family AMF communities decreased total SOC by 13.8%, likely due to fungal priming. Despite net SOC losses, all AMF communities contributed fungal C to soil but only the Glomeraceae community initially contributed to MAOC. After a month of decomposition, both the Glomeraceae and mixed-family communities contributed to MAOC formation. Plant phosphorus uptake, but not hyphal chemistry, was positively related to AMF soil C and MAOC accumulation. AMF contribution to MAOC is dependent on the specific traits of the AMF community and related to phosphorus uptake. These findings provide insight into how variations in AMF community composition and traits, and thus processes like environmental filtering of AMF, may impact SOC accumulation.

The effect of thinning intensity on the soil carbon pool mediated by soil microbial communities and necromass carbon in coastal zone protected forests

Thinning is a common forest management measure that can effectively maintain the ecological service function of protected forests. However, the effect of thinning on the soil carbon (C) pool remains uncertain. In particular, we lack an understanding of the complete link between thinning and microbial communities, microbial necromass C, and consequently, soil C pools in coastal zone protected forests. In this study, three thinning intensities, i.e., a control treatment (CT, i.e., no thinning), light thinning (LT) and heavy thinning (HT), were established in three types of forests (Quercus acutissima Carruth, Pinus thunbergii Parl and mixed Quercus acutissima Carruth and Pinus thunbergii Parl, i.e., QAC, PTP and QP, respectively). Two years after the completion of thinning, we investigated the changes in the soil organic carbon (SOC) fractions, soil microbial community and soil microbial necromass C in the surface layer (0–20 cm) and thoroughly evaluated the relationship between the potential change in SOC and the microbial community. Compared with CT, there was no change in the SOC content under LT and HT, but thinning conducted in QAC increased the proportion of mineral-associated organic C (MAOC) in SOC. Moreover, both LT and HT reduced the soil carbon lability (CL) in the QAC and QP forests. Different thinning intensities changed the soil microbial community structure, and most of the variation was explained by thinning and the soil physicochemical properties. The proportion of soil bacterial and fungal necromass C to SOC increased with increasing thinning intensity. The content of soil bacterial and fungal necromass C was mainly controlled by the relative abundance of the core phylum (relative abundance>10 %). Thinning affected the soil C pool by affecting the content of soil bacterial and fungal necromass C, but their accumulation pathways was different. The results showed that thinning was beneficial to the stability of SOC. The microbial C pool, total organic C pool and even bacterial and fungal C pools should be distinguished when studying the soil C pool, which can effectively deepen our understanding of the mechanism by which soil microorganisms affect the soil C pool.