Soil Organic Carbon Pool Research Articles

Contrasting multiple deterministic interpolation responses to different spatial scale in prediction soil organic carbon: A case study in Mollisols regions

Accurate estimation of SOM content of Mollisols influences fundamental sub-micron to global-scale biogeochemical processes and carbon-climate feedbacks. Comparative analysis of the responses of Multiple deterministic Interpolation models to spatial scale variations is the basis for multiscale soil organic carbon (SOC) pools simulation and evaluation. This study mainly manifested as spatial variation at different analytical levels of spatial correlation and the change of characteristic data attributes that characterize this change. Three spatial scales were selected (HQ, endemic area; XF, local area; JX, most area). A set of 164 topsoils (0–20 cm, sampling density of 0.1 points / km2) samples were taken, and 14 environmental variables and 14 soil characters variables were employed to contrast prediction accuracy of ordinary kriging (OK), inverse distance weighting (IDW), radial basis function (RBF), global polynomial (GPI), local polynomial (LPI), regression kriging interpolation methods such as (RK) and geographically weighted regression kriging (GWRK) responses to different spatial scale. Using a cross-validation procedure to evaluate the performance of the models. Overall, RK and GWRK, due to the introduction of the auxiliary variables, effectively predict SOM content and the improvement of prediction accuracy depends on the spatial scale, compared with IDW, RBF, GPI, and LPI.Furthermore, soil characters variables (TN, pH) as covariates have a more significant impact on the final prediction than environmental variables (Elevation, Slope, etc.). Therefore, the mutations and trends associated with the spatial resolution are analyzed in different spaces. The root means square error (RMSE) of RK and GWRK are reduced by 46.66 % and 64.26 % compared with OK. RK with fixed the regression coefficient obtained by OLS reveals that the uniformity of the object distribution in HQ and XF have Adjusted R2 0.955 and 0.915. Otherwise, the RI of GWRK with 0.792 adjusted R2 in JX is relatively 17.6 % higher than RK. The results from our case study highlight a key role in selecting the optimal spatial interpolation method for a given dataset associated with spatial scales and analyzing their regularity and applicability on different spatial scales.

Amendment of crop residue in different forms shifted micro-pore system structure and potential functionality of macroaggregates while changed their mass proportion and carbon storage of paddy topsoil

Soil organic carbon (SOC) sequestration in croplands was well known through physical protection of particulae organic carbon (POC) in macroaggregates, particularly with crop residue amendment. How different forms of crop residue could affect development of macroaggregates and their micro-pore system remained unclear. In a field experiment, a rice paddy was amended respectively with fresh (CS), manured (CM) and pyrolyzed (CB) maize residue at 10 t ha−1 in carbon equivalent, in comparison to no residue amendment (CK). In 3 years following amendment, undisturbed topsoil (0–15 cm) cores were collected under the treatments respectively. Following size fractionation of water-stable aggregates and SOC pool analysis, macroaggregate (MAC) samples were characterized with SR-µCT tomography. Compared to CK, bulk SOC storage increased by 13.9 % and 31.3 %, POC storage by 32.6 % and 63.3 % while MAC mass proportion by 7.4% and 24.5%, under CM and CB, respectively. Coincidently, total micro-porosity of MAC was increased by ca 30 % but porosity ratio of connected pores to isolated pores increased almost 1-fold though fractal dimension (FD) increased slightly, both under CM and CB. However, indices of pore throats were seen higher under CB than other treatments though micro-pore networking promoted under both CM and CB. In addition, micro-porosity and pore structure as well as POC were all observed similar between fresh residue and no residue treatments. Thus, crop residue amendment in different forms shifted micro-pore structure and potential functionality of MAC while changed their mass proportion and carbon storage of the paddy topsoil. Overall, crop residue recycled as biochar to paddy could ensure SOC sequestration through enhanced physical protection of POC in MAC, which could further potentially contribute to soil function improvement. How the short-term changes in macroaggregation and the pore system could contribute to the soil fertility and functioning deserve further studies.

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

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

Do long-term N additions affect the soil organic carbon pool in temperate grasslands?

While the organic carbon stored in soil is a sizeable proportion of the total carbon stored in terrestrial ecosystems, it is also a considerable source of greenhouse gas emissions. In this study, we examined how the carbon pool had changed in an area of temperate grassland in Inner Mongolia, China, over a period of 14 years. A field experiment was set up in 2003 and was treated with 6 nitrogen (N) treatments:0, 2, 4, 8, 16, and 32 g·N·m−2·yr−1 as dry urea (CO(NH2)2). We collected soil samples in 2017, and divided them into three size aggregates: silt-clay fraction, microaggregate, and small macroaggregate. We determined various soil extracellular enzyme activities of these three categories, namely β-glucosidase (BG), N-acetyl-β-D-glucosaminidase (NAG), acid phosphatase (AP), peroxidase (PER), and phenol oxidase (POX). We found that the soil enzyme activities increased under N additions. In addition, the BG activity was higher, but the PER activity was lower, in the small macroaggregates than the silt-clay fraction. Furthermore, we found that the soil extracellular enzyme activities and soil physico-chemical properties in the small macroaggregates were not correlated. Rather, we found that some of the soil extracellular enzyme activities were negatively correlated with the pH value, microbial biomass carbon (MBC), total organic carbon (TOC), and positively correlated with the inorganic nitrogen content (IN), in the microaggregates and the silt-clay fraction. A key discovery was that the N additions had no effect on the 0–10 cm soil layer (fractions <2000 um) organic carbon pool or the distribution of different-sized aggregates, probably because they were regulated through the changes of plant, soil, and microbial interactions. The results will contribute to improve our understanding of how N additions affect TOC and different aggregate size classes in soil, and will support better predictions of how N deposition might contribute to future climate change.

Increasing soil organic carbon pools and wheat yields by optimising tillage and fertilisation on the Loess Plateau in China

AbstractThe effect of conservation tillage on soil organic carbon (SOC) sequestration and crop production is controversial in semi‐arid areas. In addition, fertilisation is another essential factor affecting soil carbon pool and crop yields. We sought to explore the changes of SOC pool and crop productivity after optimising fertilisation and tillage. An 11‐year field experiment on the Loess Plateau in China was conducted to evaluate the effects of (i) fertilisation (balanced fertilisation, low fertilisation, conventional fertilisation) and (ii) tillage (no‐tillage, subsoiling, ploughing) on input‐C, SOC stocks, labile C fractions, C pool management index (CMI) and wheat yield. SOC stock accumulation at 0–35 cm depth under balanced fertilisation increased by 59% compared to conventional fertilisation due to the larger amount and stabilisation efficiency of input‐C. Simultaneously, balanced fertilisation increased labile C contents and CMI at 0–10 cm depth. For tillage, no‐tillage and subsoiling improved input‐C and its stabilisation efficiency, and increased SOC stocks compared to ploughing. Balanced fertilisation combined with no‐tillage or subsoiling produced greater SOC stocks at 0–35 cm depth compared to other treatments. No‐tillage increased labile C contents and CMI at 0–10 cm depth, while subsoiling increased labile C contents and CMI at 0–10 and 35–50 cm depths. Wheat yield and sustainable yield index (SYI) were positively correlated with SOC, and the effect of SOC contributed to SYI largely through indirect effects of dissolved organic C and particulate organic C. Wheat yield and SYI were increased by optimising fertilisation and tillage, and the greatest yield was under balanced fertilisation combined with the subsoiling treatment. Our findings suggested that balanced fertilisation combined with subsoiling was an effective practice for increasing SOC sequestration, soil quality, wheat yields and had great potential application on the Loess Plateau in China.Highlights C sequestration under conservation tillage and fertilisation are controversial in semi‐arid areas. Balanced fertilisation promoted SOC sequestration in no‐tillage and subsoiling treatments. Subsoiling improved soil quality at 0–10 and 35–50 cm depths due to the greater SOC and labile C. Subsoiling was superior to no‐tillage and ploughing in maintaining crop productivity.

Assessment of changes in soil organic carbon fractions and enzyme activities under apple growing ecosystems in temperate North-Western Himalayas

For better understanding about the dynamics of soil organic carbon (SOC) and enzyme activities in soil health management under intense cropping and fluctuating climatic scenario, we examined quantitatively the spatial distribution and relationship pattern of SOC (potassium dichromate oxidizable C (KCr2O7 -C), total organic carbon (TOC or SOC stocks) and SOC pools of labile organic carbon (LBC), particulate organic carbon (POC) and microbial biomass carbon (MBC) as well as enzyme activities viz:- dehydrogenase (DHA), urease (URE) and phosphatase (PPTase) under apple growing ecosystems in north western temperate Himalayas, India. Descriptive statistics, Pearson’s correlation and spatial variability analysis of SOC attributes and enzyme activities were performed. Analysis of the results show consistent decline in SOC with depth and altitude exhibiting distinct spatial variability (CV = 15.7%–44.4%); representing significantly greater proportion in orchards of higher altitude by a margin of 346% and 295% and surface layers by 32% and 68% than sub-surface layers of those mid and lower altitudes. SOC stocks ranged from 32.1 to 94.6 Mg ha−1 with significant variation (CV = 18.0%–30.9%) between soil depths. Amongst labile carbon fractions, LBC observed significantly maximum spatial variability (CV = 15.0%–27.2%) followed by MBC (CV = 7.7%–20.6%) while as POC represents least non-significant variation (CV = 7.2%–15.5%). Although LBC (0.78 to 4.90 g kg−1) accounts for significant higher fraction (490%), MBC represents substantial reduction of 36% to 71% from surface to sub-surface layers. Decrease in microbial quotient (MQ) from an average value of 4.0% (surface) to 2.7% (sub-surface) indicates distinct spatial variability (CV = 7.5%–17.8%). The stratification ratio (SR) of SOC changed significantly amongst three altitudes varying from 2.75 to 3.96. For soil enzyme activities tested, DHA, URE and PPTase observed significant decline with depth showing maximum activity loss of 504% for URE followed by 470% for PPTase and 310% for DHA. Relationship studies recorded significantly positive correlation (p =0.05 and 0.01) for TOC with SOC and POC (0.752* and 0.814**) and between SOC and POC (0.675*), although SOC was best correlated with LBC (0.834**) and MBC (0.650**). All carbon fractions showed strong positive and significant (p = 0.05 and 0.01) relation with MBC and all enzymes (DHA, URE and PPTase) exhibited similar relationship with SOC (r = 0.638*), (r = 0.725*) and (r = 0.502**). Principal component analysis (PCA) summarized the spatial variation and relationship among SOC attributes and enzyme activities. These results of soil quality signature can be used to design and implement effective management measures in terms of addition of appropriate nutrients and effective carbon sequestration practices for soil health sustainability.

Loss of soil microbial residue carbon by converting a tropical forest to tea plantation

Land-use change can lead to profound changes in the storage of soil organic carbon (SOC) in the tropics. Soil microbial residues make up the majority of persistent SOC pools, yet the impact of land-use change on microbial residue C accumulation in the tropics is not well understood. Here, we investigated how the conversion of tropical primary montane rainforest to secondary forest and the conversions of secondary forest to Prunus salicina plantation and tea plantation, influence the accumulation of soil microbial residue C (indicated by amino sugars). Our results showed that the secondary forest had a higher SOC than that of the primary forest (+63%), while they had no difference in microbial residue C concentration, indicating a relatively slow microbial-derived C accrual during secondary succession. Moreover, the P. salicina plantation and tea plantation had lower SOC than the secondary forest (−53% and −57%, respectively). A decrease in fungal biomass (−51%) resulted in less fungal and total residue C concentrations in the tea plantation than in the secondary forest (−38% and −35%, respectively), indicating microbial-derived C loss following the forest conversion. The change in microbial residue C depended on litter standing crop rather than soil nutrient and root biomass. Litter standing crop affected microbial residue C concentration by regulating fungal biomass and hydrolytic enzyme activities. Taken together, our results highlight that litter-microbe interactions drive microbial residue C accumulation following forest conversions in the tropics.

Warming has a minor effect on surface soil organic carbon in alpine meadow ecosystems on the Qinghai-Tibetan Plateau.

The alpine meadow ecosystem on the Qinghai-Tibetan Plateau (QTP) is very sensitive to warming and plays a key role in regulating global carbon (C) cycling. However, how warming affects the soil organic carbon (SOC) pool and related C inputs and outputs in alpine meadow ecosystems on the QTP remains unclear. Here, we combined two field experiments and a meta-analysis on field experiments to synthesize the responses of the SOC pool and related C cycling processes to warming in alpine meadow ecosystems on the QTP. We found that the SOC content of surface soil (0-10cm) showed a minor response to warming, but plant respiration was accelerated by warming. In addition, the warming effect on SOC was not correlated with experimental and environmental variables, such as the method, magnitude and duration of warming, initial SOC content, mean annual temperature, and mean annual precipitation. We conclude that the surface SOC content is resistant to climate warming in alpine meadow ecosystems on the QTP.

Tracer elements revealed the soil organic carbon sources in a dam-controlled watershed

The dynamics of the soil organic carbon (SOC) pool and the mechanisms that underpin those dynamics are at the frontier of terrestrial carbon (C) cycling research. Soil erosion seriously affects the migration and redistribution of SOC, driving a critical part of the C cycle. In this study, soil samples were collected from a 14.69 m sediment profile formed between 1960 and 2017, which was located in a small agricultural catchment on the Loess Plateau in northwest China. Nuclear tracer elements (137Cs, 210Pb) and stable C isotopes (δ13C) from organic matter were used to quantitatively identify the SOC sources in different siltation stages. The method used assumes that the contribution of a source is proportional to the elemental concentration in that source. SOC in soil samples from different erosion source areas (farmland, forest land, grassland, shrub land and ditch wall) were compared against the deposit in the check dam. The concentration of SOC in the sediments of the check dam was between 1.21 g kg−1 and 4.03 g kg−1. Nearly half of the SOC in the check dam sediments was traced back to the ditch wall. Farmland contributed 24.8 %, while shrub land contributed the least SOC at 0.4 %. SOC erosion attributed to farmland decreased gradually, while the relative contribution from the ditch wall increased. Within the agricultural catchment area, there was a net migration of SOC from the various land cover types (the source) to the sedimentary area behind the dam (the sink). These results indicate that a dynamic balance exists between redistribution and heterotopic storage of SOC during sedimentation. This study provides an opinion for sediment source identification and the soil C cycle in watersheds on the Loess Plateau.

Soil organic carbon stocks and stability of organic matter in permafrost-affected soils of Yamal region, Russian Arctic

Chemical composition of soil organic matter determines its decomposability and may affect soil microbial activity. This is very important for understanding variability in soil organic carbon pools in context of changes in plant cover or climate. This paper is aimed to characterize existing carbon pools and molecular organization of the humic acids, isolated from various permafrost-affected soils of Yamal region and to assess the potential vulnerability of soils organic matter in context of possible mineralization processes. Soils for most of studied sites were characterized predominantly by acidic to strong acidic pH. Profile distribution of carbon, nitrogen and C:N ratio showed non-gradual changing with the depth due to manifestation of cryopedogenesis in soil profiles which lead to cryogenic mass transfer. Mean soil organic carbon stocks for the study area were 7.9 ± 2.2 kg m−2 (for 0–10 cm layer), 15.0 ± 5.5 kg m−2 (for 0–30 cm), 24.0 ± 8.0 kg m−2 (for 0–100 cm). Analysis of soil organic carbon stocks among predominant soil types (Russian soil classification system) revealed relatively high variability of values in 0–10 cm (from 4.5 kg m−2 in Peaty Gleyzem underlain by permafrost to 9.6 kg m−2 in Peaty Podzol underlain by permafrost) and 0–30 cm layers (from 9.9 kg m-2 in Cryic Podbur underlain by permafrost to 24.1 kg m−2 in eutrophic peat soil underlain by permafrost). Results of solid-state 13C NMR spectrometry showed low amounts of aromatic fragments in studied soils. All studied soils are characterized by predominance of aliphatic structures, and also carbohydrates, polysaccharides, ethers and amino acids. High content of aliphatic fragments in studied humic acids shows their similarity fulvic acids. Low level of aromaticity reflects the accumulation in soil of poorly decomposed organic matter due to cold temperatures. Our results provide further evidence of high vulnerability and sensitivity of permafrost-affected soils organic matter to Arctic warming. Consequently, these soils may play a crucial role in global carbon balance under effects of climate warming.

Optimization of tillage rotation and fertilization increased the soil organic carbon pool and crop yield in a semiarid region

AbstractTillage and mineral fertilization are important strategies for maintaining crop production, but their effects on soil organic carbon (SOC) and labile fractions in semiarid regions are controversial. A 10‐year field experiment was conducted to evaluate the effects of fertilization and tillage on SOC stock, labile fractions, soil quality and crop yields in a wheat–maize cropping system at the Loess Plateau. Results showed that balanced fertilization could provide comprehensive nutrition for crops and increase the crop residue input carbon, thereby increasing the SOC, readily oxidizable carbon, particulate organic carbon and dissolved organic carbon contents. In 0–50 cm layers, the SOC stock was increased by 3% under balanced fertilization compared to conventional fertilization. For tillage, no‐tillage (zero tillage) and subsoiling rotation with less soil disturbance and higher stabilization rate of input carbon was the most effective tillage practice for increasing SOC stock. This tillage technique increased the labile carbon contents by 41%–63%, 5%–19% and 26%–67% compared to ploughing in 0–10 cm, 20–35 cm and 35–50 cm layers, respectively. Balanced fertilization combined with no‐tillage and subsoiling rotation (BF + NS) increased carbon pool management index due to higher SOC and labile carbon contents, which contributed to the improvement of soil quality and the promotion of crop growth. Among all treatments, BF + NS treatment produced the highest SOC stock as well as wheat and maize yields. Therefore, BF + NS is the best strategy to increase SOC sequestration and to improve soil quality and productivity in semiarid regions.

Storage or loss of soil active carbon in cropland soils: The effect of agricultural practices and hydrology

Few studies have simultaneously addressed the issue of the short- and long-term hydrological control of organic carbon (OC) export from soils and the role of the leaching process in the long-term dynamics of the soil OC pool. We combined short- and long-term approaches by investigating dissolved organic carbon (DOC) at the outlet of a small drainage catchment and establishing a relationship between DOC concentrations (3.5 ± 1.8 mgC L-1 on average) and subsurface runoff (175 mm yr−1 on average). We then calculated the annual DOC export as a function of average annual water runoff for a 9-year period. We assumed that the annual flux of leaching is proportional to the active soil OC stock, which we compared with data from the literature. We added a leaching function to the AMG two-compartment model of soil carbon dynamics. The innovative use of the Rock–Eval method for agricultural soils made it possible to determine the stable and active carbon fractions (OCp and OCA, respectively), necessary to characterize the system in the model, for 52 plots in organic and conventional agricultural farms in the Seine Basin. No significant difference was found in OC for the two agricultural systems (11.4 ± 2.5 gC kg−1 vs. 12.3 ± 4.2 gC kg−1, respectively, for the 0 to 30 cm layer).Using the AMG model with its leaching function, we calculated the equilibrium value of OCA, representing the size of the OCA pool that would be reached in the long term under constant farming practices and hydrological conditions in a given plot. Deviation from this equilibrium indicates whether carbon storage or loss occurs. Overall, for the plots sampled in the Seine Basin, an annual carbon loss of ∼−0.24 % yr−1 of the total OC pool was found. This may increase by 15% (i.e., to ∼−0.28% yr−1) under higher subsurface runoff, which is plausible under ongoing climate change (e.g., 600 mm yr−1 vs. 175 mm yr−1 currently observed).

Particle size primarily shifts chemical composition of organic matter under long‐term fertilization in paddy soil

AbstractThe chemistry of soil organic matter (SOM) is critical for soil functions. However, the chemical composition and oxidation state of physical SOM fractions under long‐term mineral and organic amendments in paddy soils are still unclear. Here, we investigated this issue using three particle size classes of SOM sampled from five long‐term (37‐years) fertilisation regimes in a subtropical paddy soil which were analysed using pyrolysis–gas chromatography–mass spectrometry (Py–GC/MS). Coarse particulate organic matter (cPOM, &gt;250 μm), fine particulate organic matter (fPOM, 53–250 μm) and mineral‐associated organic matter (MOM, &lt;53 μm) were isolated. The five fertilisation regimes were (1) unfertilised (control), (2) nitrogen (N, urea), (3) nitrogen + phosphorus + potassium (NPK, urea + Ca(H2PO4)2 + KCl), (4) NPK + straw (NPKS) and (5) NPK + Astragalus sinicus L. + pig manure (NPKM). After 37‐years, the SOM content of the control, N, NPK, NPKS and NPKM treatments had increased by 16.3%, 12.3%, 31.6%, 41.7% and 59.8% compared with the initial level, respectively. The pools of soil organic carbon (SOC) and total N (TN), as well as the C/N ratio in the soil matrix, were localised by particle size, with the majority in the mineral‐associated organic matter (MOM). Particle size, not fertilisation regime, primarily associated with shifts in the chemical composition of SOM, with selective preservation of lignin and polysaccharides in the particulate organic matter (POM) and enrichments of lipids and N‐containing compounds in the MOM. The origin (plant‐ and microbial‐derived) and oxidation state of SOM were also regulated by particle size (especially the coarse particles) but not fertilisation treatment. Despite long‐term persistence, the MOM had a lower C oxidation state (implying higher vulnerability to microbial utilisation) than the coarse POM (cPOM) and fine POM (fPOM). In conclusion, fertilisation regime regulated the bulk SOM (SOC and TN) pool, whereas physical particle size generated a divergence trend in the chemical composition and oxidation state of SOM in the subropical paddy soil, indicating the greater role of physical arrangement in SOM chemistry than the quality of amendments.Highlights Particle size, not fertilisation regime, regulated chemical composition and oxidation state of SOM. POM is enriched in lignin and polysaccharides while MOM is enriched in lipids and N‐containing compounds. Physical arrangement plays a greater role in SOM chemistry than the quality of amendments.

Alleviating Soil Acidification and Increasing the Organic Carbon Pool by Long-Term Organic Fertilizer on Tobacco Planting Soil

Long term tobacco planting leads to soil acidification. A ten-year experiment with various fertilization treatments (no fertilization (CK), chemical fertilizer (CF), organic-inorganic compound fertilizer (OCF), and organic fertilizer (OF)) was carried out between 2010 and 2020 in a continuous cropping system of Nicotiana tabacum in the brown soil of eastern China, to assess the effects of organic fertilizer on the improvement of tobacco planting soil acidification. The results indicated that treatments OCF and OF reduced the soil exchangeable acid content, of which the exchangeable aluminum showed the largest reduction by 51.28% with the OF treatment. In contrast, treatment CF showed more significant increases in exchangeable aluminum (Al) and Al saturation, and also apparently increased soil NO3−-N, NH4+-N and nitrification potential (NP) than other treatments. Treatments of OCF and OF significantly increased the total amount of exchangeable base (EBC) by 37.19% and 42.00% compared with CF, respectively. Redundancy analysis (RDA) showed that NP, NH4+-N, and NO3−-N were the important factors indicating soil acidification, while EBC and exchangeable K were the significant factors restricting soil acidification. Inevitably, OCF could improve the soil organic carbon pool and labile organic carbon pool. The structural equation model (SEM) showed that OCF treatment increased the soil organic carbon pool mainly by inhibiting soil nitrification and reducing the content of exchangeable Al. In conclusion, both OF and OCF treatments were effective methods to alleviate tobacco planting soil acidification, however OCF had more advantages in improving soil organic carbon pool.

The Contribution of Microorganisms to Soil Organic Carbon Accumulation under Fertilization Varies among Aggregate Size Classes

Long-term fertilization alters soil microbiological properties and then affects the soil organic carbon (SOC) pool. However, the interrelations of SOC with biological drivers and their relative importance are rarely analyzed quantitatively at aggregate scale. We investigated the contribution of soil microbial biomass, diversity, and enzyme activity to C pool in soil aggregate fractions (&gt;5 mm, 2–5 mm, 1–2 mm, 0.25–1 mm, and &lt;0.25 mm) at topsoil (0–15 cm) from a 27-year long-term fertilization regime. Compared to CK (no fertilization management), NP (inorganic fertilization alone) decreased all of the microbial groups’ biomass, while NPS and NPM (inorganic fertilization plus the incorporation of maize straw or composted cow manure) significantly reduced this negative effect of NP on microbial biomass and increased the microbial contribution to C pool. The results show that microbial variables were significantly correlated with SOC content in &gt;0.25 mm aggregates rather than in &lt;0.25 mm aggregates. Fungal variables (fungal, AM biomass, and F/B ratio) and enzyme activities (BXYL and LAP) in &gt;0.25 mm aggregates explained 21% and 2% of C, respectively. Overall, organic matter addition could contribute to higher C storage by boosting fungal community and enzyme activity rather than by changing microbial community diversity in macro-aggregates.

Divergent contributions of living roots to turnover of different soil organic carbon pools and their links to plant traits

Abstract Rhizodeposits and root litter contribute critically to soil organic carbon (SOC) formation and decomposition. This root‐induced SOC turnover shows great interspecific variations. Bulk SOC consists of diverse functional pools differing in formation and stabilization. Yet, it remains unclear which plant traits regulate the effects of living roots on the turnover of different SOC pools across species. By performing 13CO2 continuous labelling of six grassland species for a growing season in a climate‐controlled chamber, we quantified the contributions of living roots to the dynamics of the fast‐cycling particulate organic C and the slow‐cycling mineral‐associated organic C, and explored their relations to plant traits. The results showed that new root‐derived SOC varied more than threefold among the six species. The variation in new root‐derived SOC was best explained by the ratio of shoot to root biomass. Plant species with higher shoot:root ratio formed more new root‐derived SOC. Most of the root‐derived C (72%) was incorporated into the particulate organic C pool. All species caused positive rhizosphere priming effects (RPE), which varied sevenfold across species. Among plant traits, specific root length was the best predictor of interspecific variations in the RPE, with greater RPE associated with higher specific root length. Most of the RPE (70%) occurred in the mineral‐associated organic C pool. Our results also showed that most plant species caused more old SOC decomposition via the RPE than new SOC formation, leading to net SOC losses, especially for the mineral‐associated organic C pool. Overall, we provide novel insights into the effects of plant traits on root‐induced turnover of particulate and mineral‐associated organic C. Our findings should be valuable for understanding how specific plant traits regulate SOC accumulation and stabilization. A free Plain Language Summary can be found within the Supporting Information of this article.

Long‐term (64 years) annual burning lessened soil organic carbon and nitrogen content in a humid subtropical grassland

Burning has commonly been used to increase forage production and nutrients cycling in grasslands. However, its long-term effects on soil organic carbon (SOC) and nitrogen (N) pools within the aggregates and the relation between aggregates-associated SOC and soil CO2 emissions need further appraisal. This study evaluated the effects of 64years of annual burning on SOC and N dynamics compared to annual mowing and undisturbed treatments in a grassland experiment established in 1950. Soils were sampled from four depths representing the upper 30cm layer and fractionated into macroaggregates, microaggregates and silt+clay fractions. The macroaggregates were further fractionated into three occluded fractions. The SOC in the bulk soil and aggregates were correlated to soil CO2 effluxes measured under field conditions. Compared to the undisturbed treatment, annual burning decreased aggregates stability, SOC and N in the upper 30cm layer by 8%, 5% and 12%, respectively. Grassland mowing induced greater aggregates stability than burning only in the upper 5cm. Burning also decreased SOC in the large macroaggregates (e.g., 0-5cm) compared to mowing and the undisturbed grasslands but proportionally increased the microaggregates and their associated SOC. Soil N associated with aggregates decreased largely following grassland burning, for example, by 8.8-fold in the microaggregates within the large macroaggregates at 20-30cm compared to the undisturbed grassland. Burning also increased soil CO2 emissions by 33 and 16% compared to undisturbed and mowing, respectively. The combustion of fresh C and soil organic matter by fire is likely responsible for the low soil aggregation, high SOC and N losses under burned grassland. These results suggested a direct link between grass burning and SOC losses, a key component for escalating climate change severity. Therefore, less frequent burning or a rotation of burning and mowing should be investigated for sustainable grasslands management.

A Simple Approach to Isolate Slow and Fast Cycling Organic Carbon Fractions in Central European Soils—Importance of Dispersion Method

Numerous approaches have been developed to isolate fast and slow cycling soil organic carbon (SOC) pools using physical and chemical fractionation. Most of these methods are complex, expensive, and time consuming and unsuited for high-throughput application, such as for regional scale assessments. For simpler and faster fractionation via particle size the key issue is the dispersion of soil. It is unclear how the initial dispersion of soil affects the turnover rates of isolated fractions. We investigated five commonly used dispersion methods using different intensities: shaking in water, shaking in water with glass beads, ultrasonication at 100 and 450 J ml−1 and sodium hexametaphosphate (Na-HMP). We used soils from long-term field experiments that included a change from C3 to C4 vegetation and adjacent control sites using δ13C isotope ratio mass spectrometry. We evaluated the degree of C3/C4 moieties of the fractions, mass and carbon recovery and reproducibility as well as the time expenditures of the dispersions, sieving and drying techniques to develop an efficient and cheap fractionation method. Our results indicate that ultrasonication as well as H2O treatment with and without glass beads resulted in fractions with different turnover. Moreover, isolation performances depended on soil texture. While the isolation of the fractions using water with and without glass beads was equivalent to ultrasonication in soils with low clay contents, these methods had limited potential for soils with high clay contents. Furthermore, treatment with water alone had less reproducible results than other tested methods. The SOC recovery was comparable and satisfactory amongst non-chemical dispersion methods and reached over 95% for each of these methods. The use of Na-HMP was unsuccessful due to high time expenditures and strong SOC leaching. We propose particle size fractionation combined with ultrasonic dispersion as a fast and highly reliable method to quantify slow and fast cycling SOC pools for a wide range of soil types and textures from agricultural sites in central Europe.

Effects of forest conversion to rubber plantation and of replanting rubber trees on soil organic carbon pools in a tropical moist climate zone

In the past two decades, conversion of forests to rubber (Hevea brasiliensis) plantations has spread widely in continental Southeast Asia. In addition to the effect of the establishment of new rubber plantations, the impact of the replanting of rubber trees, i.e., second-rotation plantation establishment, on the soil organic carbon (SOC) pool is unknown because of the short history of land management at such sites. We examined whether the first-rotation rubber plantation (RUB_1st) soils have more of a legacy effect from previous natural forests than the second-rotation plantation soils in Cambodia, so as to improve SOC stock estimates for a series of land use changes in a tropical moist climate zone, e.g., the lowlands of continental Southeast Asia. The total SOC stock, estimated for soil masses equivalent to 0–30 cm depth, was larger in natural forest (FOR) plots (52.7 MgC ha−1) than in RUB_1st (38.4 MgC ha−1), early-stage second-rotation plantation (RUB_2nd_A1–6; tree age ≤6 years; 37.2 MgC ha−1), and middle-stage second-rotation plantation (tree age >6 years; 33.9 MgC ha−1) plots. Values for factors related to SOC dynamics, such as the mass and carbon/nitrogen (C/N) ratio of leaf litter, soil C/N ratio, soil aluminum content, and bulk density of coarse plant residue (>2 mm), were also higher in FORs than in the rubber plantations. The SOC pools in both high- and low-density fractions of topsoil were lost largely within the first 10 years of forest–rubber plantation conversion. The loss of SOC in the low-density fraction in the RUB_1st might have been compensated to some extent by the remaining plant residue in these former forests. Although weeding resulted in the mixing of temporarily growing C4 grasses with soil in RUB_2nd_A1–6 plots, these grasses might have contributed minimally to the SOC pool in the long term, as reflected by C stable isotope ratios. Accordingly, we presented a conversion factor of SOC stock as 0.66 ± 0.18 for the 9-year transition period from forests to rubber plantations on a flat topography in a tropical moist climate zone of continental Southeast Asia, and concluded that the replanting of old rubber plantations should take priority over the forest-rubber plantation conversion as possible to reduce the emission of greenhouse gas under the land use change.

Building Climate Change Adaptation and Resilience through Soil Organic Carbon Restoration in Sub-Saharan Rural Communities: Challenges and Opportunities

Soil organic carbon (SOC) is widely recognised as pivotal in soil function, exerting important controls on soil structure, moisture retention, nutrient cycling and biodiversity, which in turn underpins a range of provisioning, supporting and regulatory ecosystem services. SOC stocks in sub-Saharan Africa (SSA) are threatened by changes in land practice and climatic factors, which destabilises the soil system and resilience to continued climate change. Here, we provide a review of the role of SOC in overall soil health and the challenges and opportunities associated with maintaining and building SOC stocks in SSA. As an exemplar national case, we focus on Tanzania where we provide context under research for the “Jali Ardhi” (Care for the Land) Project. The review details (i) the role of SOC in soil systems; (ii) sustainable land management (SLM) techniques for maintaining and building SOC; (iii) barriers (environmental, economic and social) to SLM implementation; and (iv) opportunities for overcoming barriers to SLM adoption. We provide evidence for the importance of site-specific characterisation of the biophysicochemical and socio-economic context for effective climate adaptation. In particular, we highlight the importance of SOC pools for soil function and the need for practitioners to consider the type of biomass returns to the soil to achieve healthy, balanced systems. In line with the need for local-scale site characterisation we discuss the use of established survey protocols alongside opportunities to complement these with recent technologies, such as rapid in situ scanning tools and aerial surveys. We discuss how these tools can be used to improve soil health assessments and develop critical understanding of landscape connectivity and the management of shared resources under co-design strategies.