Microbial Biomass C Contents Research Articles

Soil organic carbon pools as influenced by 21 years of conservation agriculture management practices in Saskatchewan

Soil organic carbon (SOC) content is a key metric of soil quality and limited work has been done to examine the effect of long-term conservation agriculture management practices (CAMP) on SOC pools within western Canadian soils. We assessed the nature and permanence of sequestered SOC within 90 diverse Saskatchewan surface (0–10 cm) agricultural soils before and after 21 years of CAMP. Comparisons were made of total SOC, labile and dynamic SOC fractions (light fraction, water-extractable, and microbial biomass), respirable CO2–C during a 6-week incubation, along with spectroscopic characterization using 13C/12C stable isotope ratio and ATR-FTIR. Among soil climatic zones, the SOC content increased in the semi-arid Brown and Dark Brown soils, ranging from 2.4 to 3.7 Mg C ha−1 (111.4–187.7 kg C ha−1 year−1), but did not change in the subhumid Black, Dark Gray, and Gray soils. Overall, soils having the smallest initial SOC level were most responsive to CAMP and accumulated more SOC. According to the δ13C data, CAMP appeared to reduce annual crop moisture stress, especially within the Brown soil zone. Decreased light fraction and water-extractable SOC contents in Black, Dark Gray, and Gray soils could reflect more intense decomposition and greater surface stratification of crop residues. Brown soils experienced the largest increase in microbial biomass-C content. The CO2–C emissions from the Brown, Dark Brown, and Gray soils under CAMP suggest greater SOC stability in 2018 compared with 1996. The ATR-FTIR data pointed to enhanced SOC persistence, via more stabilized SOC forms and mineral-associated organic C fractions.

Sugarcane/Soybean Intercropping with Reduced Nitrogen Application Synergistically Increases Plant Carbon Fixation and Soil Organic Carbon Sequestration.

Sugarcane/soybean intercropping and reduced nitrogen (N) application as an important sustainable agricultural pattern can increase crop primary productivity and improve soil ecological functions, thereby affecting soil organic carbon (SOC) input and turnover. To explore the potential mechanism of sugarcane/soybean intercropping affecting SOC sequestration, a two-factor long-term field experiment was carried out, which included planting pattern (sugarcane monocropping (MS), sugarcane/soybean 1:1 intercropping (SB1), and sugarcane/soybean 1:2 intercropping (SB2)) and nitrogen addition levels (reduced N application (N1: 300 kg·hm-2) and conventional N application (N2: 525 kg·hm-2)). The results showed that the shoot and root C fixation in the sugarcane/soybean intercropping system were significantly higher than those in the sugarcane monocropping system during the whole growth period of sugarcane, and the N application level had no significant effect on the C fixation of plants in the intercropping system. Sugarcane/soybean intercropping also increased the contents of total organic C (TOC), labile organic C fraction [microbial biomass C (MBC) and dissolved organic C (DOC)] in the soil during the growth period of sugarcane, and this effect was more obvious at the N1 level. We further analyzed the relationship between plant C sequestration and SOC fraction content using regression equations and found that both plant shoot and root C sequestration were significantly correlated with TOC, MBC, and DOC content. This suggests that sugarcane/soybean intercropping increases the amount of C input to the soil by improving crop shoot and root C sequestration, which then promotes the content of each SOC fraction. The results of this study indicate that sugarcane/soybean intercropping and reduced N application patterns can synergistically improve plant and soil C fixation, which is of great significance for improving crop yields, increasing soil fertility, and reducing greenhouse gas emissions from agricultural fields.

Inorganic carbon accumulation in saline soils via modification effects of organic amendments on dissolved ions and enzymes activities

Organic amendments are effective in promoting salt leaching, improving availability of nutrients and increasing crop yield in saline soils. However, the effects of the combined application of organic amendments on soil inorganic carbon (SIC) content and its driving mechanisms are unknown. We examined how changes in soil biochemical properties including moisture content, pH value, dissolved ions, soil organic C (SOC), dissolved organic C (DOC), microbial biomass C (MBC), and activities of C- and N-related enzyme due to applying (i) only mineral fertilizers (control; Ctrl) or mineral fertilization in combination with (ii) manure (M); (iii) biofertilizer plus manure (B+M); and (iv) humic acid plus manure (HA+M) modify SIC pool over three years. Organic amendments increased the content of Ca2+ and HCO3– ions, but significantly decreased Na+, Cl−, and SO42− at 0–60 cm soil layers compared to Ctrl. Furthermore, the activities of β-glucosidase (BG), Xylanase (BX), Cellobiosidase (CE), and β-1,4-N-Acetyl-glucosaminidase (NAG) increased at 0–40 cm soil. Partial least squares path model manifested that organic amendment directly accumulated SIC content by increasing Ca2+ content and the activities of BX and NAG. Additionally, the decrease of Cl− contents under organic amendments favored the increase of MBC and SOC content, which further increased SIC content. Generally, the effects of HA+M were greater than other fertilization managements, which increased the stocks of SIC and total C at 0–60 cm soils by 27 Mg ha−1 and 47 Mg ha−1, respectively. Furthermore, the effect of organic amendments on SIC content is more pronounced below the first 20 cm. Thus, the application of organic amendments is recommended as an appropriate measure to not only improve soil condition for plant growth and SOC accumulation but also to store C as SIC at deeper depths.

The impacts of nitrogen addition on the active carbon pools in forest soils along an urban–rural gradient

The increased nitrogen (N) deposition accompanying urbanization alters the dynamics of soil carbon (C) in urban and peri-urban forests. However, our understanding of the magnitude and mechanism of this impact is still minimal. In this study, experimental plots were established in urban, suburban, and rural forests to investigate the effects of three levels of N addition: no addition (0), low addition (50 kg N·ha−1·yr−1), and high addition (100 kg N·ha−1·yr−1). The responses of the active components in the soil C pool, including dissolved organic C (DOC) and microbial biomass C (MBC), were monitored after simulated N deposition treatments in forests along an urban–rural gradient. The results showed that the concentrations of DOC and MBC in forest soils along the urban–rural gradient exhibited significant seasonal and site variation. Soil DOC and MBC concentrations in the rural forest were significantly affected by N addition, increasing by 13.39 % and 45.90 % under the low N treatment, respectively, and MBC concentration increased by 28.40 % under the high N treatment. On the contrary, soil DOC and MBC concentrations in urban and suburban forests did not change significantly. Among studied influencing factors, the total dissolved N provided the crucial nutrient sources for microbial growth and activity and thus became the primary factor affecting the active components in the soil C pool. The insensitivity of urban and suburban forests' active soil C pools to added N indicates that the N fertilization effect brought by urban expansion may not have a lasting impact on soil C pools of rural forests.

Exploring the Role of Stumps in Soil Ecology: A Study of Microsite Organic Carbon and Enzyme Activities in a Larix olgensis Henry Plantation

Stumps are a significant component of coarse woody debris in plantations, but their effect on microsite soil organic carbon (C) and enzyme activities remains understudied. Soil (Alfisol) samples were collected at varying distances from larch (Larix olgensis Henry) stumps and at different soil depths (0–20 cm and 20–40 cm) to analyze soil total organic C (TOC), particulate organic C (POC), easily oxidizable C (EOC), microbial biomass C (MBC), and enzyme activities. Results indicated that stumps significantly affected TOC and POC contents, with the greatest horizontal range of impact reaching up to 15 cm in both the topsoil and subsoil layers. Stumps also significantly affected MBC content, with the greatest horizontal range of impact reaching up to 55 cm in the subsoil layer. EOC content was the most affected, with the stumps’ impact extending to 55 cm in both soil layers. Additionally, the study showed that stumps had a significant impact on the activities of β-glucosidase and β-cellobiohydrolase, with the greatest horizontal range of impact reaching up to 15 cm for glucosidase and 35 cm for cellobiohydrolase in the topsoil layer. Stumps also significantly affected the activities of phenol oxidase and peroxidase, with the maximum horizontal range of stump impact extending up to 35 cm for phenol oxidase and 55 cm for peroxidase in the topsoil layer. This study enhances our understanding of the role of stumps in plantation ecosystems and offers valuable insights for future management strategies to maintain soil fertility and improve site productivity.

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.

Effects of enhanced-efficiency nitrogen fertilizers on CH4 and CO2 emissions in a global perspective

It is widely recommended that enhanced efficiency nitrogen fertilizers (EENFs; urease inhibitors, nitrification inhibitors, urease and nitrification inhibitors combined, coated controlled-release urea) be applied to croplands to improve N use efficiency and crop yield via regulating N transformations. However, EENFs may inevitably affect soil C dynamics for the coupled relationship between soil carbon (C) and N biogeochemical cycles. Yet, a comprehensive assessment of the effects of EENFs on soil C dynamics is lacking. Here, we conducted a global meta-analysis using 67 publications to assess the overall effects of EENFs on soil CH 4 production, CO 2 emission, organic C (SOC) content, dissolved organic C (DOC) content, microbial biomass C (MBC) content under different environmental and management conditions (climate conditions, soil properties and fertilizer management practices). Our results showed that on average, compared to conventional N fertilizer, EENFs with the same amount of N fertilizer have a non-significant impact on CH 4 emission, which further depended on environmental and management conditions. Best scenarios for CH 4 reduction included: paddy field (25.8%), urease inhibitor (26.9%) and medium N application rate (150–300 kgN ha −1 ; 23.6%), mainly due to the decreased stimulation of NH 4 + on CH 4 production; acid soil (33.7%), which was attributed to the enhanced methanotrophic communities' activities. The positive effect was also amplified by the increased mean annual precipitation and soil clay content that facilitated CH 4 production. Contrarily, EENFs significantly reduced CO 2 emission by 9.3%. The greater reduction was observed for the conditions producing more CO 2 after fertilization, i.e. field experiment, alkaline soils, low soil inorganic N content and high N application rate (≥300 kgN ha −1 ). Nitrification inhibitors and coated N fertilizer were the best available options as they can reduce inorganic C dissolution in calcareous soil and SOC mineralization, respectively. Besides, EENFs application did not significantly alter SOC, DOC and MBC contents. Our findings highlighted the role of EENFs played in decreasing soil C emission in agroecosystems. • Global use of EENFs did not significantly affect SOC, DOC, MBC contents. • EENFs succeed in reducing CO 2 emission (9.3%) rather than CH 4 emission. • EENFs provide a viable solution for decreasing soil C emissions in global agroecosystems.

Effects of Forest Vegetation Restoration on Soil Organic Carbon and Its Labile Fractions in the Danxia Landform of China

The Danxia landform is a unique red bed landform in China. The effects of vegetation restoration on soil organic carbon (SOC) components are still poorly understood in the Danxia landform region of southwest China. In this study, soil samples were collected from selected five different vegetation restoration types (shrub (SH), mixed conifer–broadleaf forest (MCBF), evergreen broad-leaved forest (EBF), Chinese fir forest (CFF), and bamboo forest (BF)) at 0–30 cm depth to discuss the concentrations and stocks of SOC and its labile organic carbon (LOC) fractions ((dissolved organic C (DOC), microbial biomass C (MBC), and easily oxidized organic C (EOC)) and their relationship with soil physicochemical properties and enzyme activities. The results indicated that the contents of SOC and LOC fractions as well as SOC stocks declined with increasing soil depth in five vegetation restoration types. At 0–30 cm depth, BF and CFF showed higher the average concentrations and total stocks of SOC and EOC compared with SH, EBF, and MCBF. The highest average DOC content was in BF, but no significant differences was observed in the total DOC stocks among five vegetation restoration types. BF and EBF showed significantly greater average MBC concentrations and total MBC stocks than other vegetation restoration types. SOC and its LOC fractions were positively correlated with soil moisture and three enzyme activities in different degrees under the five vegetation restoration types and closely related with total nitrogen (TN) and total phosphorus (TP) except for TP of CFF and BF and negatively affected by pH (except for CFF and the DOC and MBC of MCBF) and BD. Generally, soil TN, TP, and invertase were found to be the main driver factors for soil carbon accumulation. However, the overall levels of SOC and its labile fractions indicate that BF had the strongest carbon storage capacity, followed by CFF and EBF. This study can provide a good reference for ecosystem management and the selection of appropriate restoration strategies in Danxia landform regions.

Cover crops and N fertilization affect soil ammonia volatilization and N2O emission by regulating the soil labile carbon and nitrogen fractions

Introducing cover crops in monocropping systems to improve soil health has been broadly adopted worldwide. However, the impacts of different cover cropping systems on soil emissions and related soil fractions and their regulatory mechanism remain elusive. In this study, four cropping systems, i.e., relay intercropping with Orychophragmus violaceus L./cotton (OvC), Vicia villosa Roth/cotton (VvC), a cover crop mixture (Ov and Vv)/cotton (MSC) and a fallow field–cotton (FFC) system, were combined with four N application rates in a two-rotation cycle (2019–2021) to evaluate the effects on soil greenhouse gases (GHGs) emission, ammonia (NH3) volatilization, and labile carbon (C) and nitrogen (N) fractions and their relationships. Compared with the FFC system, the cumulative NH3 volatilization and nitrous oxide (N2O) emissions of OvC are noticeably reduced 13% and 58%, respectively, and those of VvC are comparable, while those of MSC are markedly increased by 15% and 24%, respectively. N fertilization significantly affects soil NH3 volatilization and N2O emissions, and a higher N rate results in more gaseous N emissions. In addition, NH3 volatilization and N2O emission are positively correlated with the soil microbial biomass N (MBN), dissolved organic N (DON), NH4+-N and NO3--N but negatively correlated with microbial biomass C (MBC) and dissolved organic C (DOC). OvC system decreased gaseous N emission by high soil MBC/MBN ratio, Vv as cover crop indirectly promote soil N2O emission through increased soil MBN, DON and MBC content, while cover crop mixtures increase soil labile C and N fractions and enhance gaseous N emission ultimately. Overall, application of N fertilizer and cover crop mixture both promoted soil NH3 and N2O emissions; the relay intercropping nonlegume cover crop Ov noticeably mitigates N2O and NH3 emissions through an increase in the soil MBC and reduce in the labile N, while introduction of the legume cover crop Vv increases N2O emissions but reduces soil NH3 volatilization due to a lower soil MBC/MBN ratio.

Short-term amendment of biosolid on agricultural soil: effects on C and N mineralization and microbial activity

Adding biosolids to agricultural soils can improve its quality through increased storage of C and N. A study to analyze the short-term impact of the addition of biosolids on the release of nutrients and the microbial activity in agricultural soil was carried out. The microbial biomass C (MB-C), urease activity (UA), and mineralization of N and C at different application rates of biosolids were evaluated (0 mg, 100mg and 200 mg of N-NH4+ kg-1). In addition, a biosolid-only treatment was tested. It was observed an increase in C and N mineralization, NH3 volatilization, and MB-C content, according to the application rate of biosolids. In biosolid treatments, UA increased 100% on average in the first seven days of incubation. These results suggest that the nutrient content in the soil is improved and microbial activity is positively stimulated.

Effects of short‐term tillage managements on rhizosphere soil‐labile organic carbon fractions and their hydrolytic enzyme activity under a double‐cropping rice field regime in Southern China

AbstractThe soil‐labile organic carbon (C) fractions and their hydrolytic enzyme activity play an important role in maintaining or increasing soil quality and soil fertility, which are affected by adopting various tillage managements. But there is still limited information about how rhizosphere soil‐labile organic C fractions and their hydrolytic enzyme activity respond to different tillage managements under a double‐cropping rice (Oryza sativa L.) (early rice and late rice) in a field in Southern China. We hypothesized that rhizosphere soil‐labile organic C fractions and their hydrolytic enzyme activity in the double‐cropping rice field were modified by combined application of tillage with crop residue incorporation. Therefore, the main objectives of this study were to explore change in rhizosphere SOC content, soil‐labile organic C fractions [mineralizable C (Cmin), microbial biomass C (MBC), dissolved organic C (DOC), particulate organic C (POC), light fraction organic C (LFOC), permanganate oxidizable C (KMnO4‐C)] and their hydrolytic enzyme activity during 6‐years tillage management in a double‐cropping rice system in Southern China by using the fluorometric method. The relationship between proportion of each labile organic C fraction to SOC content were analysis using the redundancy analysis (RDA) method. The experiments included four tillage treatments: rotary tillage with all crop residues removed as a control (RTO), conventional tillage with crop residue incorporation (CT), rotary tillage with crop residue incorporation (RT), and no‐tillage with crop residue retention (NT). The results showed that combined application of tillage with crop residue incorporation managements significantly increases rhizosphere SOC content and soil‐labile organic C fractions compared without crop residue input treatment. Compared with RTO treatment, rhizosphere soil Cmin, KMnO4‐C, POC, DOC, LFOC, and MBC contents with CT treatment increased by 170.97%, 37.22%, 52.53%, 14.46%, 20.29%, and 27.35%, respectively. The proportion of labile C with CT, RT, and NT treatments was higher than that of RTO treatment. This results demonstrated that rhizosphere soil hydrolytic enzyme activity with CT, RT, and NT treatments were higher than that of RTO treatment. Compared with RTO treatment, rhizosphere soil polyphenol oxidase and peroxidase activities with NT treatment increased by 75.80% and 40.90%, respectively. They had significantly positive correlation between rhizosphere soil hydrolytic enzyme activity and SOC content and its labile organic C fractions. Rhizosphere soil polyphenol oxidase and peroxidase activities had significantly positive correlation with labile organic C fractions. The RDA results indicated that CT and RT treatments were significantly different from NT and RTO treatments, and all of them were clearly separated from each other. As a result, combined application of tillage with crop residue incorporation is a beneficial nutrient management for improving rhizosphere soil‐labile organic C fractions and their hydrolytic enzyme activity in the double‐cropping rice fields of Southern China.

Microbial biomass in forest soils under altered moisture conditions: A review

AbstractMicrobial biomass is known to decrease with soil drying and to increase after rewetting due to physiological assimilation and substrate limitation under fluctuating moisture conditions, but how the effects of changing moisture conditions vary between dry and wet environments is unclear. Here, we conducted a meta‐analysis to assess the effects of elevated and reduced soil moisture on microbial biomass C (MBC) and microbial biomass N (MBN) across a broad range of forest sites between dry and wet regions. We found that the influence of both elevated and reduced soil moisture on MBC and MBN concentrations in forest soils was greater in dry than in wet regions. The influence of altered soil moisture on MBC and MBN concentrations increased significantly with the manipulation intensity but decreased with the length of experimental period, with a dramatic increase observed under a very short‐term precipitation pulse. Moisture effect did not differ between coarse‐textured and fine‐textured soils. Precipitation intensity, experimental duration, and site standardized precipitation index (dry or wet climate) were more important than edaphic factors (i.e., initial water content, bulk density, and clay content) in determining microbial biomass in response to altered moisture in forest soils. Different responses of microbial biomass in forest soils between dry and wet regions should be incorporated into models to evaluate how changes in the amount, timing, and intensity of precipitation affect soil biogeochemical processes.

Changes in Soil Organic Carbon and Its Labile Fractions after Land Conversion from Paddy Fields to Woodlands or Corn Fields

Land use change could significantly affect soil organic carbon (SOC) and other soil chemical properties. However, the responses of soil labile C fractions at different soil depths to land-use change are not still clear. The aim of this study was to investigate the effect of paddy field conversion on woodlands or corn fields on total soil organic C (TOC) and its labile C fractions including particulate organic C (POC), microbial biomass C (MBC), and potassium permanganate-oxidizable C (KMnO4–C) along a 0–100 cm soil profile. Our results indicate that soil TOC concentrations increased by 3.88 g kg−1 and 3.47 g kg−1 in the 0–5 cm soil layer and 5.33 g kg−1 and 4.68 g kg−1 in the 5–20 cm soil layer during 13 years after the conversion from paddy fields to woodlands and corn fields, respectively. In the 20–40 cm soil layer, the woodlands had the highest TOC concentration (12.3 g kg−1), which was 5.13 g kg−1 and 3.5 g kg−1 higher than that of the paddy and corn fields, respectively. The increase in TOC was probably due to the absence of soil disturbance and greater root residue input into the woodland soil. In corn fields, pig manure addition contributed to the increase in soil organic C concentrations. In addition, the proportion of soil KMnO4–C increased after conversion from paddy fields to woodlands or corn fields in the 0–40 cm soil layer, ranging from 39.9–56.6% for the woodlands and 24.6–32.9% for the corn fields. The soil POC content was significantly higher in woodland and corn field soils than in paddy field soils at lower soil depths (5–40 cm). However, there were no differences in MBC contents in the whole soil profile between the woodlands and paddy fields. The KMnO4–C and MBC was the most important factor affecting the CMI values through the whole 0–100 cm soil profile. Overall, converting paddy fields to woodlands or corn fields increased the TOC and labile C fractions in the 0–40 cm soil layer. Future studies should focus on the response of the deeper soil C pool to land-use change.

Identification of barley genetic regions influencing plant\u2013microbe interactions and carbon cycling in soil

PurposeRhizodeposition shapes soil microbial communities that perform important processes such as soil C mineralization, but we have limited understanding of the plant genetic regions influencing soil microbes. Here, barley chromosome regions affecting soil microbial biomass-C (MBC), dissolved organic-C (DOC) and root biomass were characterised.MethodsA quantitative trait loci analysis approach was applied to identify barley chromosome regions affecting soil MBC, soil DOC and root biomass. This was done using barley Recombinant Chromosome Substitution Lines (RCSLs) developed with a wild accession (Caesarea 26-24) as a donor parent and an elite cultivar (Harrington) as recipient parent.ResultsSignificant differences in root-derived MBC and DOC and root biomass among these RCSLs were observed. Analysis of variance using single nucleotide polymorphisms genotype classes revealed 16 chromosome regions influencing root-derived MBC and DOC. Of these chromosome regions, five on chromosomes 2H, 3H and 7H were highly significant and two on chromosome 3H influenced both root-derived MBC and DOC. Potential candidate genes influencing root-derived MBC and DOC concentrations in soil were identified.ConclusionThe present findings provide new insights into the barley genetic influence on soil microbial communities. Further work to verify these barley chromosome regions and candidate genes could promote marker assisted selection and breeding of barley varieties that are able to more effectively shape soil microbes and soil processes via rhizodeposition, supporting sustainable crop production systems.

Predicting soil C changes after pasture intensification and diversification in Brazil

Globally, poorly managed pasture can contribute to increasing greenhouse gas (GHG) emissions. In Brazil, sustainable management systems are being proposed to reduce carbon dioxide (CO2) emissions and increase the soil C stock under degraded pasture. However, despite the potential benefits in the adoption of sustainable management systems, few studies have been carried out seeking to analyze their long-term effects on the soil C cycle. In this study, we used the DayCent model to simulate the effects of converting poorly managed pastures (PMP) to more-intensive and diversified systems of pasture management [fertilized pasture (FP), integrated crop-livestock (ICL) and integrated livestock-forest (ILF)] on long‐term soil C stocks and microbial biomass C (MBC). We also evaluated the effects of different pasture management scenarios for FP (fertilization frequency), ICL (time of implementation of the crop phase) and IFL (spacing between the tree rows). The DayCent model estimated that the conversion of PMP to FP, ICL and ILF increases the soil C stocks by 0.95, 0.04–0.70 and 0.16 Mg ha−1 yr−1, respectively. Similarly, the MBC contents also increased with conversion, mainly for ICL and ILF. In addition, the fertilization of the pasture every year (FP), the implementation of the crop phase within two years (ICL) and the spacing between the tree rows of 15 m (ILF) showed the highest soil C stocks and MBC contents. FP, ICL and IFL were also GHG sinks of 43, 57 and 116 Mg CO2eq ha−1, respectively. These results can help national initiatives associated with the recovery of degraded pasture in Brazil.

Changes in soil organic carbon fractions in response to wheat straw incorporation in a subtropical paddy field in China

AbstractBackground: Optimal wheat straw management in paddy fields is important for evaluating soil organic carbon (SOC) dynamics and sustaining soil quality.Aims: We investigated the effects of wheat straw incorporation in paddy soil on the total SOC content, soil labile organic C fractions [water soluble organic C (WSOC), hot‐water soluble organic C (HWSOC), easily oxidizable C (EOC), microbial biomass C (MBC), particulate organic C (POC), and light fraction organic C (LFOC)], and C pool management index (CPMI).Methods: Rice (var. Qingjiao 307) was grown from June to November 2019 in a subtropical paddy field, under four different treatments: without any amendments (control; CK); with N, P, and K fertilizers (NPK); with NPK and a moderate rate of wheat straw application (NPKS1); and with NPK and a high rate of wheat straw application (NPKS2). Soil samples were collected from depths of 0–5, 5–10, 10–20, and 20–30 cm after the rice was harvested.Results: In all soil layers, the bulk density was 10.79–51.85% lower and the total SOC, WSOC, HWSOC, EOC, MBC, POC, and LFOC contents were 13.87–145.97% higher in the NPKS2 treatment than in the CK treatment (p < 0.05). The CPMI was highest in the NPKS2 treatment in the top 20 cm soil. SOC correlated positively with labile C fractions and CPMI in the 0–5 and 5–10 cm soil layers (p < 0.05), with the exception of WSOC and LFOC in the 5–10 cm soil layer. The sensitivity of each soil labile organic C fraction to the different treatments varied in the 0–5 cm soil layer. Rice yield was highest in the NPKS2 treatment.Conclusions: Overall, the NPKS2 treatment had a positive effect on SOC stocks and higher soil quality in subtropical paddy soils in China.

Microbial-derived carbon components are critical for enhancing soil organic carbon in no-tillage croplands: A global perspective

Soil organic carbon (SOC) is an important component of the global carbon (C) cycle, while tillage has been shown to significantly impact SOC stocks, its impact on the different SOC fractions is not well understood at global level, making it difficult to predict the behavior of SOC over time. Using data from 95 studies on a global level, we investigated the changes of various SOC fractions in 0-400 mm [dissolved organic C (DOC), particulate organic C (POC), easily oxidizable organic C (EOC), microbial biomass C (MBC), and mineral-associated organic C (MOC)] with NT application. This study also quantified the roles of different environmental and agronomic factors (e.g., climatic conditions, soil texture and nutrient conditions, study duration, and cropping intensity) in the changes of these SOC fractions. Compared to conventional tillage (CT), NT and reduced tillage (RT; tillage systems that are less intensive, with fewer trips in the field than CT) increased SOC concentrations, which were 7.4% higher in NT than in RT (percentage change based on weighted response ratio). Compared with CT, NT significantly increased DOC (17.6%), POC (11.7%), EOC (14.8%), MBC (33.1%), and MOC concentration (16.0%). Furthermore, POC, MBC, and MOC concentrations were 8.2, 34.1, and 4.4% higher in NT than in RT, respectively. Importantly, the increased SOC concentrations under NT were significantly correlated with MBC (R2 = 0.62) and POC (R2 = 0.55). Soil depth and mean annual temperature were the dominant factors affecting the response ratios of SOC fractions (DOC, EOC, MBC, MOC, and POC) under NT, followed by the duration of NT application. Our results suggest that SOC fractions, especially those driving soil biological activities (rather than total SOC), should be considered when evaluating the effects of conservation tillage practices on SOC sequestration. We highlight the development of site-specific strategies to effectively manage C stocks and to enhance local parameterization while developing biogeochemical and Earth system models.

Changes in plant community and soil ecological indicators in response to Prosopis juliflora and Acacia mearnsii invasion and removal in two biodiversity hotspots in Southern India

Invasion of alien plant species can alter local plant diversity and ecosystem processes closely linked to soil organic carbon (SOC) and nutrient dynamics. Soil ecosystem processes such as microbial respiration and enzyme activity have been poorly explored under alien plant invasion and especially following invasive plant species removal. We studied the impact of Prosopis juliflora and Acacia mearnsii invasion and subsequent removal on local plant community composition and diversity and on soil microbial respiration and enzyme activity in two biodiversity hotspots in Southern India. Removal of Prosopis promoted recolonisation of local vegetation as indicated by a 38% and 28% increase in species richness and ground vegetation cover, respectively, compared to an unremoved site. Prosopis and Acacia removal led to a significant reduction in soil microbial biomass C (MBC), respiration, dehydrogenase and urease activity due to increased microbial respiration and N mineralisation rate. Higher metabolic quotients qCO2 in soil at Prosopis and Acacia removed sites indicate that MBC pools declined at a faster rate than SOC, resulting decreased MBC/SOC ratios compared to their respective removed sites. Natural and undisturbed ecosystems maintain more SOC through increased belowground and aboveground C input in the soil, resulting in a higher MBC content per unit SOC. Our results indicate that the interaction between above- and below-ground communities is a critical factor determining the structure and dynamics of local plant communities, especially in ecosystems affected by plant invasions.

Impacts of slash burning on soil carbon pools vary with slope position in a pine plantation in subtropical China

Slash burning is commonly used to clear harvest residues and improve forest health in tropical and subtropical forest plantations. However, whether the effects of slash burning on soil carbon pools (C) varied with slope position remains unknown. In this study, soils samples were collected at three depths (0–5, 5–10 and 10–20 cm) in three slope positions (upper, middle, and lower) one day before the slash burning and one week afterwards in a Pinus massoniana plantation in southeastern China. We measured soil physiochemical properties (bulk density, pH and moisture) and six soil C pools (total C, dissolved organic C (DOC), microbial biomass C (MBC), coarse particulate organic C (CPOC), fine particulate organic C (FPOC), and mineral-associated organic C (MAOC)). The results showed that slash burning increased soil pH (p < 0.001) and total C content (p < 0.001), but decreased soil DOC content (p < 0.001). Soil BD and moisture were not altered by the slash burning (p > 0.05). Slope position significantly affected soil moisture, DOC, MBC and MAOC. Soil moisture at 0–5 cm depth was significantly higher in the lower than upper slope position. The effects of slash burning on soil MBC varied with slope position and soil depth. Slash burning significantly decreased soil MBC at the 5–10 cm depth in the middle and lower slope position, but not in the upper slope position. Slash burning significantly decreased soil MBC at the 0–5 cm depth, while had no influence on soil MBC at the 10–20 cm depth irrespective of slope position. Averaged across slope positions for each soil depth, slash burning resulted in 48.0%, 41.3% and 25.7% decrease in soil MBC content at the 0–5, 5–10 and 10–20 cm depth, respectively. Slash burning had no effect on the distribution of the mass, but altered the C content in soil physical fractions. Slash burning increased FPOC (p < 0.01), but had no influence on CPOC and MAOC. The effects of slash burning on soil FPOC did not vary with slope position (p > 0.05). Our findings suggested that slope position should be considered as an important factor influencing the impact of slash burning on soil microbial mediated C pools, and future research is needed to investigate the post-fire recovery of vegetation and soil C dynamics.

Water Table Drawdown Alters Soil and Microbial Carbon Pool Size and Isotope Composition in Coastal Freshwater Forested Wetlands

Loss of coastal wetlands is occurring at an increasingly rapid rate due to drainage of these wetlands for alternative land-uses, which also threatens carbon (C) storage in these C-rich ecosystems. Wetland drainage results in water table drawdown and increased peat aeration, which enhances decomposition of previously stabilized peat and changes stable C isotope profiles with soil depth. The effect of water table drawdown on the pool size and δ13C signature of plant C, soil organic C (SOC) and microbial biomass C (MBC) across a range of organic and mineral soils has not previously been reported in coastal freshwater forested wetlands. To this end, litter, roots, and soils were collected from organic and mineral soil horizons in two coastal freshwater forested wetlands in North Carolina with different hydrological regimes: 1) a natural bottomland hardwood forest (natural); and 2) a ditched and drained, intensively-managed wetland for loblolly pine silviculture (managed). We found that hydrology and soil horizon, and to a lesser degree micro-topography, was important in shaping observed differences in size and 13C signature of soil and microbial pools between the natural and managed wetland. The natural wetland had higher SOC and MBC concentrations in the litter, surface organic, and mineral horizons compared to the managed wetland. In the managed wetland, 13C of SOC was enriched across most of the soil profile (Oa and mineral soil horizons) compared to the natural wetland, suggesting enhanced decomposition and incorporation of microbially-derived inputs to soils. Root C concentration decreased with soil depth, while root 13C signature became enriched with soil depth. In the litter and Oe horizon of the natural wetland, MBC was higher and 13C of MBC and SOC was enriched in hummocks compared to hollows. The 13C of MBC and SOC tended to be enriched in upper soil horizons and depleted in lower soil horizons, particularly in the managed wetland. We conclude that drainage of these coastal wetlands has enhanced the breakdown of previously stabilized C and has the potential to alter regional C storage, feedbacks to climate warming, and ecosystem responses to changing environmental conditions.