Labile Soil Organic Carbon Fractions Research Articles

Responses of SOC, labile SOC fractions, and amino sugars to different organic amendments in a coastal saline-alkali soil

Organic amendment combined with mineral fertilizer is known as an effective management practice to improve soil carbon (C) sequestration in cropland. We evaluated the effects of three organic amendments combined with mineral fertilizer on soil organic carbon (SOC), labile SOC fractions, amino sugars, C mineralization, and C-cycle enzyme activities in a coastal saline-alkali soil. The three amendments, including straw (C/N of 75.5:1), straw biochar (C/N of 43:1), and soybean cake (C/N of 7.4:1), were continuously added into the soil at an equal C amount of 2250 kg C ha−1 for four years. Compared to mineral fertilizer alone, the contents of SOC in plots receiving straw, straw biochar, and soybean cake were increased by 7.6%, 17.3%, and 12.7%, respectively. The amendment of straw biochar mainly increased easily oxidizable carbon (EOC), particulate organic carbon (POC), and fungal-derived glucosamine, whereas straw and soybean cake mainly increased dissolved organic carbon (DOC), microbial biomass carbon (MBC), and bacteria-derived muramic acid. Meanwhile, the activities of C-cycle enzymes and C mineralization were higher in straw and soybean cake treatments than in straw biochar treatment. EOC and POC, but not DOC and MBC, were strongly positively correlated with fungal-derived glucosamine and SOC, suggesting that EOC or POC could be used as appropriate indicators to predict SOC sequestration potential. In addition, the C/N ratio of organic material did not affect SOC and its components. In conclusion, straw biochar promotes the maximum accumulation of SOC through increasing EOC, POC, and fungal necromass.

Substitution of Chemical Fertilizer with Organic Fertilizer Can Affect Soil Labile Organic Carbon Fractions and Garlic Yield by Mediating Soil Aggregate-Associated Organic Carbon

This study aimed to explore the impact paths on soil organic carbon and crop yield of completely or partially substituting chemical N fertilizer with organic fertilizers. A four-year field experiment was conducted and included four treatments: (i) N0, no N fertilization application; (ii) NF, only synthetic N fertilizer application; (iii) 1/2OF, organic fertilizer substituted for 100% of the synthetic N fertilizer, with the total N application amount being equivalent to half that of NF; and (iv) 1/3OF + 2/3NF, organic fertilizer substituted for 1/3 of the synthetic N fertilizer with the total N application amount from organic and synthetic fertilizer being equivalent to that of NF. Soil total organic carbon (TOC), labile organic-carbon fractions (microbial biomass carbon (MBC), dissolved organic carbon (DOC), particulate organic carbon (POC), and easily oxidized organic carbon (EOC)), the carbon pool management index (CPMI), soil aggregated distribution, and water-stable aggregate-associated organic carbon were determined. Structural equation modeling (SEM) was used to clarify the impact paths of TOC and garlic yield changes under different N fertilizer treatments. Results showed that compared with N0 and NF, 1/2OF and 1/3OF + 2/3NF significantly increased TOC contents by 14.1–20.6%. Soil MBC, DOC, and EOC under 1/2OF were significantly higher than under N0, whereas the 1/3OF + 2/3NF treatment had significantly greater POC. The CPMI was improved by organic fertilizer treatment, with 1/2OF treatment being significantly higher than N0 and NF. The proportion of soil aggregate mass with particle sizes >2 mm was significantly greater under N0, while 1/3OF + 2/3NF significantly increased the proportion of particle sizes of 0.5–2 mm. Soil water-stable aggregate-associated organic carbon showed a trend of first increasing and then decreasing, with the largest particle sizes being 1–2 mm. Moreover, organic fertilizer significantly increased soil water-stable aggregate organic carbon compared with N0 and NF. Similarly, the garlic yield increased with organic fertilizer treatment, while 1/3OF + 2/3NF significantly increased the yield by 37.2% and 15.3%, respectively, compared with N0 and NF. Furthermore, SEM analysis indicated that fertilizer regimes could directly affect TOC and labile organic carbon components by affecting aggregate-associated organic carbon. In particular, aggregates with particle sizes of 0.5–2 mm played an important role, indirectly affecting garlic yield and CPMI. These results indicate that organic fertilizer application has the potential to improve soil organic-carbon content and garlic yield; moreover, fully applying organic fertilizer can reduce N fertilizer input while still maintaining an increase in soil organic carbon and crop yield in the short term. However, caution is still needed regarding of the type and quantity of organic fertilizer added in different cropping systems, and with different soil textures.

Effect of long-term land use change on soil organic carbon fractions and functional groups

Soil organic matter (SOM) fractions and functional groups can be used to study the short-term as well as the long-term effects of land use change on soil organic carbon (SOC) dynamics. In this study, soil samples of grassland (native vegetation), planted forest and cropland (both converted from grassland) were collected from three soil pedogenetic horizons of A-horizon, AB-horizon and Bk-horizon in a semiarid area of Northeast China. The amounts of SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MOC) were evaluated. After grassland conversion into planted forest and cropland, the amount of SOC decreased by 5.71 and 13.59%, POC decreased by 17.39 and 34.72%, and MOC decreased by 2.84 and 8.36%. Based on the analysis of MOC composition, the average content of humic extract-C decreased by 2.19% and humin-C decreased by 3.08% for planted forest, meanwhile humic extract-C decreased by 14.14% and humin-C decreased by 6.24% for cropland, compared to grassland. This result suggested that labile SOC fractions were more affected by land use change. Solid-state 13C nuclear magnetic resonance (NMR) spectra of bulk soil showed that native grassland SOC had a higher proportion of alkoxy C but a lower proportion of aromatic C, and the aromaticity was less than that of planted forest and cropland. Our results suggested that although the chemical structure of SOM tend to be stable, SOM fraction levels were not enough to recover. Reasonable grassland management measures should be formulated to prevent the continued conversion of native grassland.

Disentangle the drivers of soil organic carbon mineralization and their temperature sensitivity in both topsoil and subsoil: Implication of thermal stability and chemical composition

Information about the temperature dependence of soil organic carbon (SOC) mineralization is important to predict the possible carbon (C) release from the terrestrial ecosystem facing global warming. However, the way in which SOC quality and mineral protection of SOC affects mineralization rates and their response to increasing temperature remains poorly understood. Here, we focused on investigating thermal stability and chemical composition of soils which differed greatly with the type and extent of organo-mineral associations, and revealing their implications for SOC mineralization. Chemical composition and thermal stability of SOC were analyzed with 13C nuclear magnetic resonance spectroscopy (13C NMR) and simultaneous thermogravimeteric analysis and differential scanning calorimetry (TG-DSC), respectively. Soil mineralization and its temperature sensitivity (Q10) were assessed by incubating soils at 15 and 25 °C for 90 days.Our results showed that SOC of subsoil was more biologically stable, but exhibited higher temperature sensitivity than those of topsoil. Significant differences in chemical composition and thermal stability of SOC were detected across soils developed from four parent materials but not between two soil layers, and indicators obtained from the two techniques showed apparent associations with depth-specific SOC mineralization rates. Increase in SOC mineralization rates at two incubation temperatures and two soil layers were observed when the soils were characterized by higher percentage of labile SOC fractions (i.e., O-alkyl C and organic carbon oxidized during exothermic reactions). The Q10 values of topsoil were positively and mostly related to the percentage of Alkyl C. In addition to notable relationships with chemical composition, the Q10 values of subsoil were positively correlated with Exo1/Exo2 ratios and Energy density values, and negatively correlated with TG-50. It was interesting to note that thermal stability was possibly as a function of soil texture and total oxides. Results from VPA analysis further revealed that SOC mineralization rates at two soil layers and Q10 of topsoil were mainly driven by SOC quality, while it was related to both SOC quality and chemical protection for subsoil, implying that factors controlling the temperature sensitivity of SOC mineralization were depth-specific. Altogether, substrate accessibility as a function of chemical composition and chemical protection would play vital role in determining the feedbacks of the subsoil SOC pool to the future climate warming.

Soil Microbial Community in Relation to Soil Organic Carbon and Labile Soil Organic Carbon Fractions under Detritus Treatments in a Subtropical Karst Region during the Rainy and Dry Seasons

Soil Microbial Community in Relation to Soil Organic Carbon and Labile Soil Organic Carbon Fractions under Detritus Treatments in a Subtropical Karst Region during the Rainy and Dry Seasons

The effects of straw returning and nitrogen fertilizer application on soil labile organic carbon fractions and carbon pool management index in a rice–wheat rotation system

Straw returning and nitrogen (N) fertilizer application are important agricultural measures to improve soil carbon pool. However, the combined effect of various straw-returning amounts and N-fertilizer application rates on soil carbon pool is still unclear. Therefore, a field experiment with three straw-returning amounts and three N-fertilizer application rates was conducted for 4 years in central China. The results revealed that the addition of N fertilizer clearly enhanced the contents of soil organic carbon (SOC) by 2.2%− 2.7%, dissolved organic carbon (DOC) by 20.1%− 39.2%, microbial biomass carbon (MBC) by 37.9%− 44.2%, and easily oxidizable carbon (EOC) by 18.4%− 23.6%. Moreover, it also enhanced the activities of invertin by 33.4%− 46.4%, cellulase by 23.5%− 32.4%, and polyphenol oxidase by 20.5%− 27.1%, as well as the carbon pool management index (CPMI) by 21.3%− 27.6%. The CPMI increased gradually as the N-fertilizer application rate increased. However, straw returning did not significantly change the contents of SOC, EOC, and MBC; invertin activity, or CPMI. Based on the structural equation model, it was observed that N fertilizer application enhances MBC and subsequently improves CPMI by increasing EOC. This highlights the vital role of soil microorganisms in the soil carbon cycle and the significance of supplementing N to improve and sustain agricultural soil carbon reservoirs. Additionally, this study indicates that the combined application of N fertilizer and straw returning has a more positive effect on improving the soil carbon pool.

Soil labile organic carbon fractions mediate microbial community assembly processes during long-term vegetation succession in a semiarid region.

Conceptual diagram for the labile organic carbon (OC) fractions mediating microbial assembly processes during long-term vegetation succession.

Responses of Soil Labile Organic Carbon on Aggregate Stability across Different Collapsing-Gully Erosion Positions from Acric Ferralsols of South China

Soil labile organic carbon (LOC) is a valuable and sensitive parameter of the changes in soil carbon (C) pools and further affects soil structural stability. However, the influences of soil-aggregate stability on LOC fractions under erosion conditions are still unclear, especially under the collapsing gully area of south China. Soils of five positions of collapsing gully erosion, including the upper catchment (UC), collapsing wall (CW), colluvial deposit (CD), scour channel (SC) and alluvial fan (AF) from Acric Ferralsols were investigated and sampled. Soil aggregate stability and LOC fractions were measured and analyzed. Soil water-stable aggregate and passive C (passive-C) contents significantly increased by 67–76% and 8.7–13.0% at the UC, CW, CD and SC positions, respectively, while soil labile C (labile-C) content was lower at these positions as compared to the AF position (p < 0.05). Moreover, the UC position’s soil C pool management index (CPMI) significantly increased by 37–40% compared to CW, CD, SC and AF soils, indicating that the soil of the UC position had a more stable C pool due to its stronger structural stability. SOC, silt, and amorphous iron oxide (Fea) contents significantly contributed to aggregate stability. We demonstrated that the depletion of soil aggregate stability could result in the decreases in soil LOC fractions, while soil properties of the OC but not the LOC pool regulated aggregate stability and thus affected soil structure across different collapsing gully erosion positions in the subtropical Acric Ferralsols region of south China. This study contributes to developing strategies to prevent soil erosion and improve global C cycle and soil quality, which could be beneficial to strengthen soil and water conservation, and improve soil fertility (e.g., SOC) and vegetation recovery, such as tea and tobacco.

Responses of soil labile organic carbon fractions and enzyme activities to long-term vegetation restorations in the karst ecosystems, Southwest China

To reveal the effects of karst long-term vegetation restorations on total soil organic carbon (TSOC), labile organic carbon fractions and enzymes, in the present study, the typical karst vegetation restoration region of Southwest China was selected as study area. Four types of long-term vegetation restoration engineering including seven kinds of planting measures were selected as study objects. Responses of TSOC and its labile fractions accumulation and distribution to the different vegetation restorations were studied. And the effects of these vegetation restorations on soil enzymes activities of urease, sucrase, amylase, and alkaline phosphatase were also researched. Results showed the vegetation restorations significantly improved TSOC accumulation and obviously changed microbial biomass carbon (MBC), easy oxidated organic carbon (EOC), and dissolved organic carbon (DOC) contents and their proportions to TSOC. And the effects of these different vegetation restorations on TSOC and its labile fractions were obviously different. TSOC content and reserves, MBC and EOC contents of Cupressus funebris and Lonicera japonica planting were high. While TSOC content and reserves of the two grassland restoration measures were the lowest. The vegetation restorations significantly increased soil enzyme activities of amylase (AMY), alkaline phosphatase (ALP), urease (URE) and sucrose (SUC). And there were significant differences in responses of different soil enzyme activities to vegetation restorations. The four soil enzyme activities were significantly correlated with TSOC and its labile fractions contents, especially soil AMY and ALP. Soil ALP and AMY were closely related to TSOC mineralization and accumulation and the formation and transformation of labile organic carbon fractions in Southwest China karst ecosystems. The present results provided scientific bases for the vegetation restoration of degraded karst ecosystems and were of great significance in carbon cycle intervention based on land use regulation in Southwest China.

Recovering of soil organic matter and associated C and N pools on regenerated forest ecosystems at different tree species influence on post-fire and reclaimed mine sites

Understanding the processes in soils after disturbances could be helpful in better designing restoration practices. The labile fraction of soil organic carbon (LSOC) plays a vital role in recovering soil properties and quality. This study compared labile C and N fractions, including microbial biomass C (MBC), cold-water-soluble C (WSC), water-soluble N (WSN), hot-water-extractable C (HWC), hot-water-extractable N (HWN), particulate organic carbon (POC), and particulate organic nitrogen (PON) in post-mining, post-fire, and undisturbed soils under different tree cover (Scots pine, common birch, and black alder). Nearly 30 years after the disturbance, post-mining soils were characterized by lower soil organic carbon (SOC) and total nitrogen (TN) as well as their labile fractions compared to those of post-fire and undisturbed soils. Disturbed soils had less stable SOC than undisturbed soils. Compared to the post-fire site, the higher proportion of WSC in SOC in post-mining soils suggests higher SOC turnover and leaching potential during primary succession compared to secondary succession. The post-fire soils had a similar SOC stock to undisturbed soils but differed in the C and N labile fractions stock. Post-fire soils were characterized by higher POC stocks due to charcoal admixture and lower WSN, HWC, and HWN fractions than undisturbed soils. The studied tree species differently affected the recovery of SOM properties after disturbances. Post-mining soils under alder had higher SOC, TN, and labile fractions (except MBC) than soils under birch and pine cover. However, this was not true for the post-fire site, where only WSN was higher under the alder. The soils under birch stands had the highest MBC stocks across all sites. The soils under pine had less stable SOM than those under birch and alder, which may accelerate podzolization processes. Results indicate a longer time for the recovery of C and N pools during primary succession than in secondary succession on sandy soils. Alder, as phytomelioration species that increases the C and N pools in post-fire sites, has limited usefulness.

Landscape Change Affects Soil Organic Carbon Mineralization and Greenhouse Gas Production in Coastal Wetlands

AbstractPlant invasion and aquaculture activities have drastically modified the landscape of coastal wetlands in many countries, but their impacts on soil organic carbon (SOC) mineralization and greenhouse gas production remain poorly understood. We measured SOC mineralization rate and soil CO2 and CH4 production rates in three habitat types from 21 coastal sites across the tropical and subtropical zones in China: native mudflats (MFs), Spartina alterniflora marshes (SAs), and aquaculture ponds (APs). Landscape change from MFs to SAs or APs increased total and labile fraction of SOC, as well as carbon mineralization rate and greenhouse gas production, but there were no discernible differences in SOC source‐sink dynamics between SAs and APs. SOC mineralization rate was highest in SAs (20.4 μg g−1 d−1), followed by APs (16.9 μg g−1 d−1) and MFs (11.9 μg g−1 d−1), with CO2 as the dominant by‐product. Bioavailable SOC was less than 2% and was turned over within 60 days in all three habitat types. Proliferation of S. alterniflora marshes and expansion of AP construction had resulted in a net increase in soil CO2‐eq production of 0.4–4.3 Tg yr−1 in the last three decades. Future studies will benefit from better census and monitoring of coastal habitats in China, complementary in situ measurements of greenhouse gas emissions, and more sampling in the southern provinces to improve spatial resolution.

Soil Labile Organic Carbon Fractions and Carbon Management Index in Response to Different Fertilization under Organic Maize Farming System

Organic farming has been known to improve soil quality by enhancing soil organic carbon (SOC) contents. The labile organic carbon (LOC) pools and carbon management index (CMI) are commonly used as very sensitive indicators of changes in SOC and assessment of soil quality. This study was conducted to investigate the effect of organic farming practices on soil quality by LOC fractions and CMI analysis in a 6-year field experiment. Four treatments were included: compost (COM), green manure (GM), inorganic fertilizer (NPK), and no fertilization (NF). This study was designed to explore changes in SOC concentrations, soil labile organic C fractions (microbial biomass C (MBC), water and hot water-extractable C (WEC, HWEC), particulate organic C (POC), light fraction organic C (LFOC) and permanganate oxidizable C (POXC)) and CMI within the bulk soil under organic corn cultivation condition. Organic fertilization significantly increased SOC concentrations and stocks by 10 - 55% compared to NPK and NF, especially, compost treatment. All labile carbon fractions were higher in COM and GM compared to NPK and NP, except MBC content. Among the LOC fractions, POC showed the highest proportion (32 - 87%) on total SOC. The CMI varied from 0.87 to 2.77, organic fertilized treatments increased by 1.7 - 3.2 times over NPK. These results showed that POC and POXC could be used as a rapid and informative indicator to assess soil quality and SOC changes. Hence, organic farming management could therefore contribute to improved nutrient cycling services and higher soil quality.Effect of different fertilization on labile organic C fractions; water extractable C (WEC), hot-water extractable C (HWEC), microbial biomass C (MBC), permanganate oxidizable C (POXC), particulate organic C (POC) and light fraction organic C (LFOC) in soil.

Importance of soil labile organic carbon fractions in shaping microbial community after vegetation restoration

Studies have been increasingly focusing on soil carbon (C) pools to facilitate the sequestration and stabilisation of soil organic carbon (SOC). However, interactions between different SOC fractions and microbial communities have remained largely unexplored after vegetation restoration on the Loess Plateau. To address this gap, we analysed 16S and ITS rRNA gene sequencing, active SOC (Ca), slow SOC (Cs), passive SOC (Cp) and edaphic factors to decipher the importance of soil C fractions in shaping microbial communities after vegetation restoration. Compared to CK (Control, cropland), the average of SOC contents improved 70.52 % (p < 0.05), and the Ca, Cs and Cp improved 83.18 % (p < 0.05), 128.74 % (p < 0.05) and 18.69 % (p > 0.05). Under different land use types, the relative abundance of four bacterial phyla and two fungal phylum were > 80 % and 75 %, respectively. The Actinobacteria relative abundance in natural grassland and natural shrubland was higher than that in CK. The Shannon index, Chao index and OTUs of soil fungal and bacterial communities under CK were significantly lower than that of other land use types, except for artificial grassland (p < 0.05). The microbial communities transited from oligotrophic to copiotrophic groups according to the results of redundancy analysis (RDA), and the cumulative loads of two axes of RDA in bacterial and fungal communities approached 89.90 % and 88.19 %. The variation partitioning analysis showed that Ca and Cs exhibited large path coefficients in explaining the changes in soil microbial communities. And the microbial communities were mediated through change in labile organic C according to structural equation models, indicating that the structures and compositions of microbial communities were mainly under the control of labile SOC content after vegetation restoration. These results are referentially valuable for comprehending how to apply labile SOC in predicting variations of microbial communities and improving predictive ecosystem models for early-warning indicators of soil degradation.

Effect of biochar application rate on changes in soil labile organic carbon fractions and the association between bacterial community assembly and carbon metabolism with time

Biochar aging affects the stability of soil carbon. Analyzing the effect of biochar on soil organic carbon (SOC) forms and their relations with microbial community assembly and carbon metabolism with time is helpful for soil carbon sequestration (by adapting the farm management approach). Four treatments with no, low, medium, and high biochar application rates (0 %, 1 %, 2 %, and 4 % of the total dry weight of topsoil before winter wheat planting, abbreviated as control, LB, MB, and HB, respectively) were conducted in the field. The SOC and particulate organic carbon positively correlated with the biochar application rate. Biochar decreased readily oxidizable carbon (P < 0.05) after 8 months of application compared to the control; however, the difference disappeared with time. Biochar increased dissolved organic carbon (DOC) but had no effect on water- soluble organic carbon (WSOC); DOC and WSOC decreased with time. Furthermore, LB and HB stabilized the bacterial alpha diversities with time. Based on high-throughput sequencing, HB reduced the relative abundance of Actinobacteriota but increased that of Acidobacteria (P < 0.05) after 12 months of biochar application. Time-wise, the bacterial community assembly was determined by deterministic processes that were significantly affected by the available nitrogen, DOC, or WSOC. Compared with the control, biochar decreased bacterial links and improved bacterial metabolism of phenolic acids and polymers with time, as evidenced by Biolog EcoPlates. Structural equation modeling revealed that the contribution of bacterial assembly processes to carbon metabolism changed with time. Microbial carbon metabolism was most positively influenced by differences in the composition of bacterial specialists. These findings reinforced that changes in soil labile organic carbon were time-dependent but not necessarilty affected by the biochar application rate.

Correction: Zheng et al. Changes in the Soil Labile Organic Carbon Fractions following Bedrock Exposure Rate in a Karst Context. Forests 2022, 13, 516

The authors wish to make the following corrections to this paper [...]

14 year applications of chemical fertilizers and crop straw effects on soil labile organic carbon fractions, enzyme activities and microbial community in rice-wheat rotation of middle China

Traditional fertilization management can damage soil structure and lead to severe soil erosion. The practice of crop straw returning to the field reduces the negative impact of straw burning and improves soil quality. We investigated the effects of these agricultural practices on soil organic carbon components, enzyme activities, and soil microorganisms over 14 years of field experiments. Specifically, we studied four management strategies: no fertilizer or crop straw returning (CK), traditional chemical fertilization (NPK), crop straw returning (S), and crop straw returning with chemical fertilizer (NPKS). We found NPKS treatments significantly (P < 0.05) increased the dissolved organic carbon (DOC), microbial biomass carbon (MBC), particulate organic carbon (POC) and readily oxidized organic carbon (ROC) concentrations by 79.32 %, 82.16 %, 92.46 %, and 104.32 % relative to CK. Furthermore, under NPKS, the activities of soil enzymes related C, N, and P (α-glucosidase (αG), β-glucosidase (βG), cellulase (CBH), xylanase (βX), acetyl β-glucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphate (AP)) were increased by 54.66 %, 113.26 %, 76.73 %, 52.41 %, 45.74 %, 56.69 %, and 68.92 % relative to CK, respectively. Redundancy analysis and structural equation modelling showed that straw returning had positive effects on soil microbial community diversity and richness, and also improved microbial activity which is favorable in the degradation of soil carbon. Furthermore, we found that soil fungi were more sensitive than bacteria to changes in soil carbon composition and enzyme activities following straw returning. These results suggest that straw returning combined with chemical fertilizer can be an effective strategy to improve soil labile organic carbon components, enzyme activities, and ecological function of microorganisms.

Changes in the Soil Labile Organic Carbon Fractions following Bedrock Exposure Rate in a Karst Context

Soil labile organic carbon fractions (SLOCFs) mainly include microbial biomass carbon (MBC), dissolved organic carbon (DOC), easily oxidized organic carbon (EOC) and light fraction organic carbon (LFOC). The link between bedrock exposure rates with SLOCFs and the carbon pool management index under karst rocky desertification has not been well understood. We selected the bedrock exposure rate and vegetation coverage of 30–50% (light bedrock exposure, LBE), 50–70% (moderate bedrock exposure, MBE) and &gt;70% (intense bedrock exposure, IBE) as the experimental sample plots according to the classification standard of karst rocky desertification, and then selected a sample plot of 0–30% (secondary forest, SF) as the control. This study compared the concentrations and stocks of soil organic carbon (SOC) and SLOCFs and analyzed the relevant carbon pool management index on karst landforms at Anshun, S.W. China. The aims were to determine the relationship between bedrock exposure rates and SLOCFs and to identify the most limiting factors for SLOCFs in karst rocky desertification areas. We found that (1) the concentrations and stocks of SLOCFs declined with increasing soil depth. SOC, DOC and MBC showed IBE &gt; LBE &gt; MBE &gt; SF; LFOC decreased with increasing bedrock exposure rate, and EOC did not show obvious regularity. (2) The carbon pool management index and sensitivity index had significant differences under different bedrock exposure rates. Redundancy analysis and linear regression showed that the increase in bedrock exposure rate had a great impact on MBC, DOC, EOC and SOC. In conclusion, the increase of bedrock exposure rate has no side impact on the DOC, EOC and MBC of the soil, but side effects are exhibited by LFOC. Secondary forest improves the integrity of karst landscapes, and does not change the soil properties as well as the concentrations and stocks of SLOCFs in karst rocky desertification areas.

Changes in total organic carbon and organic carbon fractions of reclaimed minesoils in response to the filling of different substrates

Soil organic carbon (SOC) of reclaimed minesoil (RMS) is regarded as an important part of terrestrial SOC, and SOC losses of RMSs occur during the substantial soil compaction and the removal, storage and backfilling of the topsoil, which lead to poor soil structure. The filling of alternative soil substrates is considered to disturb the soil structure of RMSs, however, how SOC pool changes in the RMSs filled with various substrates and the mechanisms are less clearly understood. Therefore, a study on reclaimed area filled with three typical alternative soil substrates, including mining waste reclamation (MWR), river sand reclamation (RSR) and river mud reclamation (RMR), was started in mining area of Eastern China, where total SOC, labile SOC fractions, stable SOC fraction and soil physicochemical properties were measured. The results showed that (1) the total SOC, labile SOC fractions (microbial biomass carbon (MBC), readily oxidizable organic carbon (ROOC), light fraction organic carbon (LFOC) and particulate organic carbon (POC)) and stable SOC fraction (humic acids carbon (HAC)) contents of RMSs were lower than those of non-subsided cultivated land (NCL), the filling of alternative soil substrates had a significant effect on the SOC composition of RMSs, and the backfilling layer of RSR and the filling layer of MWR were more similar with NCL than other treatments in SOC composition; (2) In backfilling layer, bulk density (BD), connectivity index (τ), available phosphorus (AP), available potassium (AK) and soil urease (URE) were strong predictors for dissimilarities of SOC composition, and the difference in soil physics and soil fertility had more direct and indirect effects on the contents of SOC and SOC fractions; (3) In filling layer, water content (WC), macropore number (MN), microporosity (Φmac), available nitrogen (AN), electronic conductivity (EC), soil urease (URE) and sucrase (SUC) were strong predictors for dissimilarities of SOC composition, and the substrate texture and soil physics had more direct and indirect effects on the contents of SOC and SOC fractions. Easing the compactness and strengthening fertilizing management of backfilling layer, while improving the texture of filling substrates benefit the increasing of the SOC content of RMSs.

Phosphorus fertiliser source determines the allocation of root-derived organic carbon to soil organic matter fractions

The efficiency of soil organic carbon (SOC) sequestration as a suitable negative emission technology depends mainly on plant-derived organic carbon input and its allocation to stabilised SOC pools. These processes may be affected by fertiliser use and soil type. The purpose of this study was to investigate the effect of the organic fertiliser (poultry manure compost), the mineral fertiliser (rock phosphate) and their mixture on organic carbon (OC) transfer from plant to soil. We studied OC allocation to protected SOC pools of a Luvisol and a Neoluvisol. We carried out a growth chamber experiment with 13C-enriched atmosphere, where ryegrass plants were grown for 7 wk in the two soil types which were amended with the three different fertiliser sources. We quantified root-derived OC input in three SOC density fractions and soil microbial biomass after 7 wk . We found that the addition of poultry manure compost and its mixture with rock phosphate led to more root biomass and more root-derived OC transfer to active pools compared to rock phosphate alone. Soil amended with poultry manure compost had higher microbial biomass contents than soil with mineral fertilisation due to higher available organic phosphorus. We also noticed variations in the dynamics of the stabilised OC pools amongst soils, which could be attributed to the impact of phosphorus fertiliser sources on SOC stabilisation processes. We concluded that organic and mineral phosphorus fertilisers may have a contrasting impact on OC flow from plant to soil and in particular on the allocation of root-derived OC to labile or stable SOC fractions.

“Fertile island” of Tamarix Chinensis accelerated the carbon decomposition in the coastal wetlands of the Yellow River Delta, China

Fertile island of shrub in the grass-dominated ecosystems would change the spatial distribution of soil organic carbon (SOC) and its fractions, adjusting the SOC dynamics and its response to global warming. However, the effects of fertile island on SOC decomposition and its temperature responses were still less known. An incubation experiment was conducted to investigate how the fertile island of Tamarix Chinensis affects SOC decomposition and its temperature sensitivity (Q10) in the coastal wetlands of the Yellow River Delta, China. Soil samples were collected from two areas, Tamarix Chinensis growing on bare land (SB) and the mixed-growing area of Tamarix Chinensis and herbage (SH). In each area, soil samples were collected from three sites, under the canopy of Tamarix Chinensis (CM), at the edge of the canopy (CP), and outside the canopy (OL). Results showed that the cumulative CO2 production increased from OL to CM due to the following reasons. Fertile island enriched SOC, labile fractions (i.e., O-alkyl and di- O-alkyl C), ammonium under the canopy in both sample areas and increased the relative abundance of labile SOC fractions (i.e., O-alkyl C) in SB, supplying more substrates for microbial decomposition. Subsequently, fertile island stimulated β-glucosidase and invertase activities, accelerating SOC decomposition. Moreover, fertile island decreased the Q10 value in the SB area because of the increased relative abundance of O-alkyl C. This study highlights the importance of O-alkyl C on SOC decomposition and its temperature sensitivity in the coastal wetlands of the Yellow River Delta.