Labile Soil Organic Carbon Fractions Research Articles

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.

Dynamics Variation of Soil Labile Organic Carbon Fractions in Different Wetland Types of Dongting Lake under Seasonal Water Level Fluctuation

Soil labile organic carbon (LOC) fractions are very sensitive to environmental change and closely related to soil quality. They play an important role in the study of terrestrial carbon cycles. This study aimed to explore the sensitivity of soil LOC fractions to environmental changes and analyze their main influencing factors during three seasonal water level periods for scientific management of Dongting Lake wetlands. Soil under three typical wetland types (Carextristachya wetland (CTW), Phragmites australis wetland (PAW) and Salix babylonica (SBW)) in East Dongting Lake in China were collected during the normal season (May), rainy season (August) and dry season (December). Seasonal dynamics of soil LOC fractions (i.e., dissolved organic carbon (DOC), microbial biomass carbon (MBC) and easily oxidized carbon (EOC)) within these wetlands and their relationship to soil nutrients and carbon-cycle enzyme activity were analyzed. The results showed that the soil DOC contents of the three wetlands first increased and then decreased, with the exception of CTW from the normal season to the dry season, while the seasonal changes of soil MBC and EOC for all wetlands followed an opposite pattern. CTW had the largest DOC concentration (228.29 mg·kg−1) during dry season, while the highest contents of soil DOC, MBC and EOC were found in PAW during the three observed seasons, which ranged from 82.05 to 203.60 mg·kg−1, 262.54 to 325.74 mg·kg−1 and 3.30 to 4.61 g·kg−1, respectively. However, the contents of soil DOC and their proportions to soil organic carbon (SOC) of all wetlands during the normal season were 56.58~82.05 mg·kg−1 and 0.41~0.47%, respectively, which were the lowest among the three seasons. Nevertheless, the contents of both MBC and EOC as well as their ratios to SOC in these wetlands showed similar seasonal dynamics, with the lowest values recorded in the rainy season. From the normal season to the dry season, invertase activity in all wetlands increased, while cellulase activity decreased by 12.5–31.3%. The seasonal variation of catalase activity for all wetlands was less distinctive, and the highest enzyme activity was during the rainy season. Correlation analysis revealed that soil LOC fractions for all wetlands were closely related to SOC, TN, TP and invertase for the three seasons, especially during the rainy season, but were negatively correlated with TK, cellulase and catalase activity. Generally, soil LOC fractions of the three wetlands were affected by the seasonal fluctuations of water levels and presented different distribution characteristics.

Combined application of animal manure and stover enhance labile organic carbon and benefit the microbial community in dark brown soil

ABSTRACT Returning organic materials is recognized as an effective practice to alter soil organic carbon (SOC) content, in which microorganisms play a critical role. The objective of this study was to evaluate the effects of different kinds of animal manure combined with stover (AM-S treatments) on labile organic C fractions and soil microbial community. The two-year experiment involved four treatments: maize stover only (S), maize stover plus ox manure (SO), maize stover plus chicken manure (SC), and maize stover plus pig manure (SP). In 2018 and 2019, AM-S treatments increased the levels of SOC, dissolved organic carbon (DOC), easily oxidized carbon (EOC), microbial biomass carbon (MBC), and maize yields. Maize stover plus ox manure treatment could provide better accumulation of MBC, and enhance bacterial and fungal richness and diversity. Maize stover plus pig manure treatment enhanced the contents of DOC and maize yield and also recorded the highest percentage of Proteobacteria (57.6%). Also, the EOC contents were 13.0% and 21.5% higher than SO and SC respectively. In all, our findings suggested that the application of maize stover plus pig manure was the most effective treatment for enhancing labile SOC fractions and improving the microbial community.

Long‐term organic amendments improved soil carbon sequestration to support crop production

AbstractBackgroundCurrently, soil productivity is severely restricted by the degradation of soil quality in China. Crop straw and livestock manure, as the main by‐products in agricultural production, are widely promoted to replace chemical fertilizer and improve soil quality to support agricultural sustainability.AimsWe investigated the long‐term effects of organic amendments and nitrogen (N) fertilization on soil quality, including the changes in soil organic carbon (SOC) content and soil aggregates distribution, and the dynamics of soil labile organic carbon (C) fractions and soil respiration, and crop yield.MethodsA field experiment (started in 2009) with a winter‐wheat and summer‐maize rotation system was designed with four treatments: without fertilization (CK), with manure (M), with straw (S), and with straw plus N fertilizer (SN). The relevant indicators were measured after 10 years of annual fertilization in 2018.ResultsCompared to the CK treatment, M, S, and SN significantly enhanced soil organic carbon (SOC) by 190, 67, and 113%, respectively. M and SN, but not S, significantly improved the dynamics of soil labile organic C fractions, soil aggregates distribution, mean weight diameter (MWD), and geometric mean diameter (GMD). Simultaneously, soil respiration of M and SN were significantly increased by 21% and 15% compared to CK. Based on the improvement of soil quality, the wheat and maize yield in a rotation of different treatments were as follows: M (20.33 Mg ha–1) > SN (14.09 Mg ha–1) > S (6.45 Mg ha–1) > CK (4.98 Mg ha–1).ConclusionOverall, long‐term organic amendments and N fertilization improved soil chemical and physical quality to support high‐yielding crop production.

The interplay of labile organic carbon, enzyme activities and microbial communities of two forest soils across seasons

Soil labile organic carbon (LOC) responds rapidly to environmental changes and plays an important role in carbon cycle. In this study, the seasonal fluctuations in LOC, the activities of carbon-cycle related enzymes, and the bacterial and fungal communities were analyzed for soils collected from two forests, namely Betula albosinensis (Ba) and Picea asperata Mast. (Pa), in the Qinling Mountains of China. Results revealed that the seasonal average contents of microbial biomass carbon (MBC), easily oxidized organic carbon (EOC), and dissolved organic carbon (DOC) of Pa forest soil were 13.5%, 30.0% and 15.7% less than those in Ba soil. The seasonal average enzyme activities of β-1,4-glucosidase (βG), and β-1,4-xylosidase (βX) of Ba forest soils were 30.0% and 32.3% higher than those of Pa soil while the enzyme activity of cellobiohydrolase (CBH) was 19.7% lower. Furthermore, the relative abundance of Acidobacteria was significantly higher in summer than in winter, whereas the relative abundance of Bacteroidetes was higher in winter. Regarding the fungal communities, the relative abundance of Basidiomycota was lowest in winter, whereas Ascomycota predominated in the same season. In addition, the soil LOC was significantly positively correlated with the CBH, βG and βX activities. Changes in LOC were significantly correlated with Acidobacteria, Bacteroidetes and Basidiomycota. We conclude that the seasonal fluctuations in forest soil LOC fractions relied on carbon cycle-associated enzymatic activities and microorganisms, which in turn were affected by climatic conditions.

Effects of long-term fertilizer management on soil labile organic carbon fractions and hydrolytic enzyme activity under a double-cropping rice system of southern China.

Fertilization is an effective way to improve soil quality, increase soil fertility and soil microbial diversity in paddy soil. To explore the changes of soil labile organic carbon (C) fractions and hydrolytic enzyme activity after 34 years fertilization treatments in a field experiment in double-cropping rice system of southern China. There were four treatments, including chemical fertilizer alone (MF), rice residue and chemical fertilizer (RF), 30% organic matter and 70% chemical fertilizer (OM), and the control without fertilizer input (CK). We measured soil organic carbon (SOC) content, soil labile organic C fractions, SOC related hydrolytic enzyme activity, correlation coefficients of soil enzyme activity with SOC content and its labile organic C fractions. The results showed that MF, RF and OM increased SOC content by 4.5%, 22.4% and 53.5%, respectively. Compared with MF and CK, RF and OM increased soil labile organic C fractions [cumulative C mineralization (Cmin), permanganate oxidizable C (KMnO4-C), particulate organic C (POC), dissolved organic C (DOC), light fraction organic C (LFOC), microbial biomass C (MBC)] and the proportion of each labile organic C fractions to total organic C. The contents of Cmin, KMnO4-C, POC, DOC, LFOC and MBC under OM treatment were 3.5, 3.1, 3.7, 1.9, 1.2 and 1.9 times higher than CK treatment, respectively. The proportion of labile organic C fractions to total organic C of RF and OM treatments was significantly higher than that in CK. The order of soil hydrolytic enzyme activity [α-glucosidase (αG), β-glucosidase (βG), β-xylosidase (βX), cellobiohydrolase (GBH), and N-acetyl-β-glucosaminidase (NAG)] was OM>RF>MF>CK. The soil hydrolytic enzyme activity under OM treatment increased by 111.8%, 14.1%, 127.3%, 285.6% and 91.4% compared with CK, respectively. Furthermore, RF and OM treatments were beneficial to soil peroxidase (POD) activity. MF treatment was beneficial to soil polyphenol oxidase (PPO) activity. There was a significant positive correlation between soil hydrolytic enzyme activity and SOC content and its labile organic C fractions. In conclusion, the combined application of organic manure, rice straw returning and chemical fertilizer is an effective method to improve soil labile organic C fractions and hydrolytic enzyme activity in a double-cropping rice paddy field of southern China.

The effects of hummock-hollow microtopography on soil organic carbon stocks and soil labile organic carbon fractions in a sedge peatland in Changbai Mountain, China

Hummock-hollow microtopography is common in the northern peatlands of the world, but its effects on soil organic carbon (SOC) components are still poorly understood. In this study, we investigated effects of microtopography on SOC stocks and soil labile organic carbon (LOC) fractions in a sedge peatland in Changbai Mountain in northeast China. We found that SOC and soil LOC fractions had much heterogeneity in microtopography. SOC concentration in hummocks was significantly higher than under hummocks and in hollows. On average, the total SOC stock to a depth of 0.3 m below the ground surface was 19.00 kg C/m2. Fifty six percent of the total SOC stock was stored in soils in and under hummocks, despite the hummock only covering 30% of the total area. Light fraction organic carbon (LFOC), easily oxidizable organic carbon (EOC), microbial biomass carbon (MBC) and dissolved organic carbon (DOC) in hummocks were significantly higher than under hummocks and in hollows. Redundancy analysis (RDA) identified that SOC explained most variations in soil LOC fractions (59.6%), followed by soil total phosphorus (7.4%) and soil water content (6.6%). Our findings indicate that the hummock is an important carbon pool in the sedge peatland. Hummock-hollow microtopography creates heterogeneity in hydrological conditions and soil physicochemical properties, and thus influences SOC stocks and soil LOC fractions at a small scale. This study highlights the importance of microtopography in carbon storage and cycling and has direct implications for the assessment of the carbon sequestration function in northern peatlands.

Changes in soil organic carbon fractions and enzyme activities in response to tillage practices in the Loess Plateau of China

The responses of soil labile organic carbon fractions and enzyme activities to tillage practices are important to address the issues of agricultural sustainability. However, how these critical soil characteristics change throughout the various growth stages of winter wheat under different tillage practices are less clearly understood. A long-term field experiment (11 years) of six tillage practices (no tillage, NT; subsoiling tillage, ST; plow tillage, PT; NT with ST rotation, NT/ST; ST with PT rotation, ST/PT; and PT with NT rotation, PT/NT) was started in the Loess Plateau in 2007, and the changes in soil labile organic carbon fractions and enzyme activities were measured at the sowing, jointing, filling, and harvest stages in 2018. This experiment led to several conclusions: (1) NT/ST, NT, ST, ST/PT, and PT/NT treatments significantly increased soil organic carbon (SOC) by 2.05%-43.93%, microbial biomass carbon (MBC) by 9.13%-84.61%, permanganate oxidizable organic carbon (POXC) by 3.64%-89.62%, and particulate organic carbon (POC) by 6.41%-77.59% throughout all stages of growth when compared with PT; (2) The activities of cellobiohydrolase (CBH), β-glucosidase (BG), β-xylosidase (BXYL), β-N-acetylglucosamines (NAG) and alkaline phosphatase (AP) were 17.15%-169.16%, 2.85%-65.72%, 6.69%-96.99%, 22.76%-136.65% and 0.12%-29.11% higher after the NT/ST, NT, ST, ST/PT, and PT/NT treatments, respectively, than after PT throughout all the stages of growth; (3) These conservation tillage practices improved the soil ecoenzymatic stoichiometry by increasing the ratios of Ln(CBH + BG + BXYL): Ln(AP) and Ln(NAG):Ln(AP) and by reducing the ratio of Ln(CBH + BG + BXYL): Ln(NAG) compared with PT; (4) Soil enzyme activities were positively associated with the SOC fractions. Redundancy analyses (RDA) found that the key factors driving the changes in soil enzyme activities were POXC and SOC at the sowing stage, SOC and MBC at the jointing and harvest stages, and SOC and POC at the filling stage; (5) the NT/ST treatment was associated with the highest SOC fractions, soil enzyme activities and yield, and thus was a sustainable, efficient method to improve soil quality and crop yield in the Loess Plateau.

Higher soil organic carbon accumulation in the subsoil layer by 37 years combined application of inorganic fertilizers with manure than with rice straw in a double-rice paddy soil

ABSTRACT Organic incorporation is widely recommended to increase Soil Organic Carbon (SOC) content, but little was done to compare the effects of using manure and straw in different soil layers. The study was set up to assess the changes of SOC and oxidizable organic carbon fractions in three soil depths (0–15, 15–30 and 30–45 cm) in a double-rice paddy field subjected to 37 years fertilization schemes [control (CK), combined inorganic fertilization (NPK), inorganic fertilizer NK with pig manure (NKM), and inorganic fertilizer NP or NPK incorporated with rice straw (NPS and NPKS)]. The 37 years’ application of NPK, NKM, NPS and NPKS increased SOC content up to 16.5%, 38.7%, 24.4% and 24.2% across the three soil depths, respectively, compared with CK. Both NPS and NPKS treatment had 77% higher SOC accumulation by sequestering more labile SOC fraction (CHL) in topsoil but not the subsoil layers compared with NPK. Long-term NKM application performed best in SOC accumulation (threefold higher compared with NPK) in all studied soil depths, and in the content of CHL in topsoil. Overall, the integration of inorganic fertilizer with organic manure is more effective than straw incorporation on improving SOC accumulation and stability in not only the topsoil but also the subsoil layers.

Effect of land use and cover change on soil organic carbon fractions and enzymatic activities in lakeshore wetland of north shore of Lake Chaohu

为了揭示湖滨带土地利用与覆被改变对土壤有机碳库及生态功能的影响，本文选取了巢湖湖滨带（北岸）9个典型样方，分析和比较了表层（0~30 cm）土壤有机碳组分特征以及相关酶的活性.结果表明，巢湖湖滨带不同采样点土壤总有机碳（TOC）含量变化范围为2.88~11.2 g/kg，平均含量为9.12 g/kg，其中原生芦苇（Phragmites australis）湿地土壤TOC含量最高（11.2 g/kg），而芦苇群落消失后形成的荒滩土壤TOC含量最低，仅为2.88 g/kg.表征湖滨带湿地缓冲性能的土壤阳离子交换量（CEC）也以原生芦苇湿地土壤为最高，并与TOC含量呈现明显正相关.湖滨带表层土壤溶解性有机碳（DOC）和易氧化有机碳（EOC）含量变化范围分别为150~370 mg/kg和1.7~5.2 g/kg，其变化幅度明显高于TOC，其中DOC含量各采样点差异最为显著.除多酚氧化酶外，次生水柳林（Homonoia riparia Lour.）表层土壤几种酶的活性较原生芦苇湿地土壤皆有所上升，特别是蔗糖酶活性增加幅度最为明显.受人类活动干扰较大的湖滨绿地公园和人工草滩土壤过氧化氢酶、脲酶和蔗糖酶酶活性普遍显著低于原生芦苇湿地.除多酚氧化酶外，土壤中几种酶活性与土壤有机碳组分EOC和DOC含量均呈现显著的正相关，其中蔗糖酶活性与DOC含量之间相关系数最高（r=0.907），其相关性均达到极显著水平.土壤溶解性有机碳和蔗糖酶可以作为表征因土地利用与覆被变化导致湖滨带湿地退化以及生态恢复效果的敏感性指标.;In order to explore the effect of land use and cover change on lakeshore soil organic carbon sequestration and it ecological function, nine typical sample plots with different plantations along the north shore of Lake Chaohu were investigated, and the surface soils (0-30 cm) organic carbon fractions and the enzymes activities were analyzed and compared. The results showed that soil total organic carbon (TOC) was 2.88-11.2 g/kg, with an average of 9.12 g/kg. The soil under native Phragmites australis reed wetland showed the highest TOC content, and the TOC content in the wasteland formed after the disappearance of P. australis reed community was the lowest with the value of only 2.88 g/kg. The cation exchange capacity (CEC), which can be used to characterize the buffer performance of lakeside zone, was highest in the native P.australis reed wetland soil and positively correlated with TOC. Soil labile organic carbon fractions content, including dissolved organic carbon (DOC) and easily oxidized organic carbon (EOC) from the lakeshore wetland, ranged from 150 to 370 mg/kg and from 1.7 to 5.2 g/kg, respectively. The range of change in soil DOC and EOC between different sampling sites was larger than that in TOC. Especially, the difference of DOC content between different vegetations was the most significant. The activities of several determined enzymes in the surface soil of secondary Homonoia riparia Lour. Forest, except polyphenol oxidase, were higher than those in native P. australis reed wetland soil. Especially, the invertase activity increased most obviously. However, lakeshore urban green park and artificial grassland showed significantly lower activities of catalase, urease and invertaseas compared with native P. australis wetland. Correlation analysis showed that there existed significant relationships between soil enzyme activities except polyphenol oxidase and soil DOC and EOC. Especially, the correlations of invertase activity to the content of DOC reached remarkably significant level with the highest coefficient (r=0.907). Based on the above results, it is concluded that the DOC and invertase as the sensitive indicators, could well characterize the degradation process and ecological restoration effect on lakeshore wetland buffering function due to land use and cover change.

Alterations of soil labile organic carbon fractions and biological properties under different residue-management methods with equivalent carbon input

Abstract Crop residue return to soil can greatly affect soil carbon (C) cycling. However, the differences in soil organic C (SOC) pool among different residue-management methods, including residue-biochar, are still unclear. We investigated the changes in SOC content, SOC stock, labile organic C fractions, C pool management index (CPMI), microbial abundance (bacteria, actinomycetes, and fungi), and C-cycle enzyme activities (invertase, cellulose, and β-glucosidase) after four years of applying residue and residue-biochar in a barley-cotton cropping system in Yancheng, China. Five treatments were developed based on equivalent C input: residue-biochar (RB, 3.5 Mg ha−1), residue plus microbial inoculant (RI, 5.0 Mg ha−1), residue (R, 5.0 Mg ha−1), residue mulching (RM, 5.0 Mg ha−1), and no residue or residue-biochar (CK). The results showed that SOC content, SOC stock, and the CPMI in RB were the highest among all treatments with residue and residue-biochar, being 16.3%, 8.7%, and 66.2% higher than that in CK, respectively. Regarding the labile organic C fractions, RB primarily increased the contents of particulate organic C (POC), light fraction organic C (LFOC), and permanganate oxidizable C (KMnO4-C), RI and R primarily increased the water-extractable organic C (WEOC) and microbial biomass C (MBC), whereas RM had a relatively small effect on all fractions. Soil microbial abundance and C-cycle enzyme activities were greatly enhanced by RI and R, particularly by RI. The principal component analysis revealed that POC, LFOC, and KMnO4-C were the major factors causing the changes in SOC, SOC stock, and CPMI. Soil microbial abundance and C-cycle enzyme activities were significantly correlated with WEOC and MBC. In conclusion, applying residue-biochar could effectively increase soil C sequestration by enhancing the POC, LFOC, and KMnO4-C contents in comparison with residue return.

Corn straw return can increase labile soil organic carbon fractions and improve water-stable aggregates in Haplic Cambisol

Corn straw return to the field is a vital agronomic practice for increasing soil organic carbon (SOC) and its labile fractions, as well as soil aggregates and organic carbon (OC) associated with water-stable aggregates (WSA). Moreover, the labile SOC fractions play an important role in OC turnover and sequestration. The aims of this study were to determine how different corn straw returning modes affect the contents of labile SOC fractions and OC associated with WSA. Corn straw was returned in the following depths: (1) on undisturbed soil surface (NTS), (2) in the 0–10 cm soil depth (MTS), (3) in the 0–20 cm soil depth (CTS), and (4) no corn straw applied (CK). After five years (2014–2018), soil was sampled in the 0–20 and 20–40 cm depths to measure the water-extractable organic C (WEOC), permanganate oxidizable C (KMnO4-C), light fraction organic C (LFOC), and WSA fractions. The results showed that compared with CK, corn straw amended soils (NTS, MTS and CTS) increased SOC content by 11.55%–16.58%, WEOC by 41.38%–51.42%, KMnO4-C and LFOC by 29.84%–34.09% and 56.68%–65.36% in the 0–40 cm soil depth. The LFOC and KMnO4-C were proved to be the most sensitive fractions to different corn straw returning modes. Compared with CK, soils amended with corn straw increased mean weight diameter by 24.24%–40.48% in the 0–20 cm soil depth. The NTS and MTS preserved more than 60.00% of OC in macro-aggregates compared with CK. No significant difference was found in corn yield across all corn straw returning modes throughout the study period, indicating that adoption of NTS and MTS would increase SOC content and improve soil structure, and would not decline crop production.

Available nutrients and labile carbon fractions in soil irrigated with sodic water

An ongoing long-term field experiment established in the year 1994 at Vegetable Science Research Farm, CCS HAU, Hisar, India under different vegetable cropping systems was selected to study the effects of sodic water irrigation, gypsum and farmyard manure on labile soil organic carbon fractions and available nutrients in soil. The present study was conducted after 20 years of experimentation during 2014-2015. The field was irrigated with high RSC (11.5 me/l) sodic water having three treatment levels each of gypsum, viz G0 control, G1=50 % neutralization of RSC, G2=100% neutralization of RSC, as well as FYM (F0 = control, F1= 10 tons/ha, F2 =20 tons/ha) in a sandy loam Typic Ustochrept. Results showed that the pH of soil was very high under F0G0, and it decreased with the use of FYM and gypsum as amendments, however a reverse trend was observed in electrical conductivity. There was an increase in different carbon fractions and available nutrients when FYM was applied over no FYM application. Application of gypsum increased microbial biomass carbon, light fraction carbon and available nutrients (N, P, S) content in the soil but dissolved organic carbon and available K content was found decreased. It can be concluded that application of FYM @20 t/ha along with gypsum @100% neutralization of RSC of irrigation water should be applied for maintaining soil quality and productivity.

Composition, predicted functions, and co-occurrence networks of fungal and bacterial communities_ Links to soil organic carbon under long-term fertilization in a rice-wheat cropping system

Soil microbial community compositions and soil labile organic carbon (LOC) fractions change rapidly in response to fertilizers and crop rotation, but our understanding of the associations between specific microbial taxa and C fractions is still limited. The objective of this study was to compare the composition and function of soil bacterial and fungal communities in response to fertilization and crop rotation and their correlations with LOC fractions. Communities were analyzed by PCR-amplification of bacterial 16S rRNA gene and fungal ITS sequences from soil DNA. Functions were predicted by in-silico tools. A long-term experimental rice-wheat cropping system was studied, and the soil collected from plots treated with different fertilizers, including a control without fertilizer (CK), inorganic nitrogen, phosphorous, and potassium (NPK), manure (M), and NPKM, was analyzed. Fertilizations, non-crop types, significantly modified the soil bacterial community composition and suspected functions, while both fertilization and crop types significantly affected the fungal community composition. Compared with CK, NPKM treatment significantly increased the relative bacterial abundances of Candidatus Solibacter, Nitrosporia, Geobacter and Bryobacter, but decreased the relative fungal abundances of Paraphaeosphaeria sporulosa, Emericellopsis, Cladosporium tennuissimum, Curvularia lunata, Stachybotrys chlorolonate and Moleospora. All responding OTUs correlated significantly with soil LOC fractions, especially particulate organic C. The suspected functions suggest that organic amendment provided larger quantities of organic C, then promoting the growth of microorganism involved in C degradation, which in turn contributing to C sequestration. Our results indicate that fertilizers and crop rotation alter the diversity and function of soil bacterial and fungal communities, thereby causing an impact on agroecosystems.

Effects of forest floor characteristics on soil labile carbon as varied by topography and vegetation type in the Chinese Loess Plateau

Vegetation type and topographic characteristics are important factors that affect the amount and spatial distribution of the forest floor and the soil carbon cycling. However, the relationships among vegetation type, topographic characteristics, forest floor properties, soil properties and soil labile organic carbon (LOC) fractions remain to be elucidated. Here, we conducted a field observational study to explore the relationships between the soil LOC fractions (microbial biomass carbon (MBC), extractable organic carbon (EOC) and permanganate oxidizable carbon (Pox-C)) and forest floor characteristics under various vegetation types (forest, forest-steppe and steppe) and topographic features (slope position and aspect). Our results showed that the vegetation type and topography are both important factors that significantly influence the forest floor properties (i.e., stock, thickness and coverage). The soil total nitrogen and phosphorus were significantly increased, and the soil pH and bulk density were decreased, with an increase in litter stock, coverage and thickness. Simultaneously, the SOC and soil LOC fractions were increased significantly with the increase in litter stock, coverage and thickness. Over 50% of the variation in the SOC and soil LOC fractions was jointly explained by environmental factors, litter properties and soil properties. Our results demonstrated that the SOC and soil LOC fractions are directly influenced by soil physical and chemical properties (e.g., pH, bulk density and soil total nitrogen) and indirectly affected by external environmental factors. Changes in vegetation and topography lead to increases in the litter stock, coverage, thickness and water content in the backslope position and northern slope, which could create favorable soil nutrient and environmental conditions (e.g., higher soil moisture and lower bulk density) to further enhance SOC and soil LOC fraction sequestration. The forest floor potentially links environmental factors and soil properties and serves as an important factor for soil C cycling.

Response of soil labile organic carbon fractions and carbon-cycle enzyme activities to vegetation degradation in a wet meadow on the Qinghai–Tibet Plateau

Vegetation degradation resulting from climate change and human activities in wet meadows is an important issue worldwide. This phenomenon is known to influence soil labile organic carbon (LOC) and enzyme activities due to changes in environmental conditions. However, little is known about the response of LOC and enzyme activities to vegetation degradation in high-altitude wet meadows. In this study, we examined the response of LOC and carbon-cycle enzyme activities to different intensities of vegetation degradation (i.e., non-degraded (ND), slightly degraded (SD) moderately degraded (MD), and heavily degraded (HD)) in a Tibetan wet meadow. The contents of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC) and light fraction organic carbon (LFOC) and the carbon-cycle enzyme activities (i.e., cellulase, amylase and β-glucosidase) were investigated in two growing seasons (2016 and 2017). We found that the content of soil SOC and LOC fractions declined with increasing soil depth in each degraded level except for HD. Vegetation degradation significantly decreased the amount of SOC at depths of 0–10 cm and 10–20 cm, and this decrease was attributed to the relative reduction of carbon source input and higher carbon decomposition. Vegetation degradation also significantly reduced the contents of SWC, DOC, MBC, LFOC, amylase and β-glucosidase in the topsoil layers (0–10 and 10–20 cm). However, the corresponding contents in deeper soil layers had no significant differences. In addition, the SWC, DOC, LFOC and the carbon-cycle enzyme activities were higher in 2016 than in 2017. Significant correlations were obtained between SWC, SOC, LOC fractions and enzyme activities. Soil moisture was found to be the main abiotic driver for variation of soil carbon and enzyme activities. Our results indicate that vegetation degradation in the Tibetan wet meadows decreased the quantity of topsoil labile carbon fractions and enzyme activities and heavily degraded vegetation may lead to a change of profile distribution in soil carbon pool.

Soil labile organic carbon fractions and soil enzyme activities after 10\xa0years of continuous fertilization and wheat residue incorporation

Labile organic carbon (LOC) fractions and related enzyme activities in soils are considered to be early and sensitive indicators of soil quality changes. We investigated the influences of fertilization and residue incorporation on LOC fractions, enzyme activities, and the carbon pool management index (CPMI) in a 10-year field experiment. The experiment was composed of three treatments: (1) no fertilization (control), (2) chemical fertilizer application alone (F), and (3) chemical fertilizer application combined with incorporation of wheat straw residues (F + R). Generally, the F + R treatment led to the highest concentrations of the LOC fractions. Compared to the control treatment, the F + R treatment markedly enhanced potential activities of cellulase (CL), β-glucosidase (BG), lignin peroxidase (LiP), and manganese peroxidase (MnP), but decreased laccase (LA) potential activity. Partial least squares regression analysis suggested that BG and MnP activities had a positive impact on the light-fraction organic carbon (LFOC), permanganate-oxidizable carbon (POXC), and dissolved organic carbon (DOC) fractions, whereas laccase activity had a negative correlation with those fractions. In addition, the F + R treatment significantly increased the CPMI compared to the F and control treatments. These results indicated that combining fertilization with crop residues stimulates production of LOC and could be a useful approach for maintaining sustainable production capacity in lime concretion black soils along the Huai River region of China.

Response of the Organic Carbon Fractions and Stability of Soil to Alpine Marsh Degradation in Zoige, East Qinghai-Tibet Plateau

Soil organic carbon (SOC) fractions play a crucial role in the carbon cycle of alpine wetland ecosystems and regional climate regulation. In the last decades, alpine marshes at Tibetan Plateau have undergone increased degradation. The effects of marsh degradation on soil labile and recalcitrant organic carbon fractions and on SOC stability remain unclear. In this study, we investigated changes in SOC fractions with different biochemical recalcitrance and stability in the marsh ecosystems with various levels of degradation in the Zoige Wetland, Eastern Qinghai-Tibet Plateau. Soil samples of relatively pristine marsh (RPM), lightly degraded marsh (LDM), moderately degraded marsh (MDM), heavily degraded marsh (HDM) and severely degraded marsh (SDM) were collected and analysed using the acid hydrolysis method. Compared with RPM, the concentrations of SOC labile fraction I (non-cellulosic polysaccharides), SOC labile fraction II (cellulose) and recalcitrant organic carbon (ROC) in degraded marshes decreased significantly: 29–97, 38–95 and 32–99%, respectively, except for the LDM. The SOC recalcitrance index (RIc) in the HDM and SDM was significantly lower than that in RPM by 11–35 and 15–47%, respectively; compared with RPM, the stability of organic carbon was significantly lower in the HDM and SDM. Additionally, the marsh degradation increased vertical variation in the concentrations of SOC fractions. The degradation of alpine marshes decreases the capacity for carbon (C) sequestration via decreasing the resistance of SOC to biodegradation, indicating that the alpine marsh degradation may weaken the carbon sink capacity of the Zoige Plateau, thus aggravating the global greenhouse effect.