Soil Organic Matter Decomposition Research Articles

Sources, distribution and decomposition of soil organic matter based on an effective biomarker in the pastoral areas of Zoige Plateau, China

The degradation of organic matter in soils plays an important role in the carbon cycle. Lignin is the main source of soil organics and it can be used to trace the source, distribution and turnover of organic matter. In this study the distribution and degradation of lignin were investigated to identify the source and degradation of soil organic matter during the succession of China's Zoige Plateau. Lignin monomers were determined by gas chromatography-mass spectrometry with alkaline CuO oxidation and the soils' δ13C and δ15N contents were interpreted to explore the turnover rate of lignin and organic matter. The main source of organics was identified as C3 non-woody angiosperm tissues. Lignin in the topsoil (0–30 cm) was derived from litter and roots, and it then migrated vertically to the deep soil (30–80 cm). Correlations of δ13C/δ15N with the soil's elemental composition showed that the organics degraded more quickly in meadow soil than in bog soil. The soil communities in the meadow and bog soils were generally similar, but there were certain differences in the dominant microbial phyla at different depths. The meadow soil's effectiveness as a carbon sink was gradually weakened, while that of the bog soil strengthened with depth. These results provide a scientific basis for accurately assessing the carbon sink capacity of the soils in Zoige Plateau.

Asynchronous responses of microbial CAZymes genes and the net CO2 exchange in alpine peatland following 5 years of continuous extreme drought events

Peatlands act as an important sink of carbon dioxide (CO2). Yet, they are highly sensitive to climate change, especially to extreme drought. The changes in the net ecosystem CO2 exchange (NEE) under extreme drought events, and the driving function of microbial enzymatic genes involved in soil organic matter (SOM) decomposition, are still unclear. Herein we investigated the effects of extreme drought events in different periods of plant growth season at Zoige peatland on NEE and microbial enzymatic genes of SOM decomposition after 5 years. The results showed that the NEE of peatland decreased significantly by 48% and 26% on average (n = 12, P < 0.05) under the early and midterm extreme drought, respectively. The microbial enzymatic genes abundance of SOM decomposition showed the same decreasing trend under early and midterm extreme drought, but an increasing trend under late extreme drought. The microbial community that contributes to these degradation genes mainly derives from Proteobacteria and Actinobacteria. NEE was mainly affected by soil hydrothermal factors and gross primary productivity but weakly correlated with SOM enzymatic decomposition genes. Soil microbial respiration showed a positive correlation with microbial enzymatic genes involved in the decomposition of labile carbon (n = 18, P < 0.05). This study provided new insights into the responses of the microbial decomposition potential of SOM and ecosystem CO2 sink function to extreme drought events in the alpine peatland.

Linking soil organic carbon mineralization with soil variables and bacterial communities in a permafrost-affected tussock wetland during laboratory incubation

Microtopography can cause great soil heterogeneity in both abiotic and biotic factors, which leads to different soil organic matter decomposition rates. However, the quantitative effects of soil variables and microbial properties on soil organic carbon (SOC) mineralization in microfeatures in permafrost wetlands largely remain unknown. Here, soil samples of 0–10 cm from hummocks and hollows in a permafrost-affected tussock wetland were collected for laboratory incubation. The results showed that the temperature sensitivities (Q10 value) of SOC mineralization were 3.14 and 2.44 for hummocks and hollows, respectively. After an 80-day laboratory incubation, the microfeatures had no significant effects on the relative abundance of bacteria, but significantly affected bacterial α-diversity (Shannon indices) of soil. The incubation temperature also separated the bacterial community structure of the two microfeatures. Laboratory warming and the microfeatures influenced the SOC mineralization rate by regulating bacterial communities and labile substrates, with a joint contribution of 72 % of the total variances in SOC mineralization. Our study quantified the differences in soil variables, bacterial community indices, and their contribution to SOC mineralization between the two microfeatures in a permafrost-affected wetland, and thus improved our understanding of the microbial processes driving soil carbon cycling in permafrost regions.

Native soil labile organic matter influences soil priming effects

The input of exogenous labile carbon into the soil is an important factor affecting soil organic carbon dynamics by accelerating or inhibiting the decomposition of native soil organic matter, known as the priming effect. Many factors affect the priming effect, including biological factors, such as soil microorganisms, plants, and soil fauna, as well as, abiotic factors, such as soil structure, nutrients, water, and temperature. Although the mechanisms responsible for the occurrence and maintenance of priming effects are well-understood, most previous studies only considered the impact of input of exogenous available organic carbon on priming effects. How the native soil labile organic matter (LOM) affects the direction and magnitude of the priming effect remains unclear. In this study, mineral layer soil was collected from a Chinese fir plantation in Qianyanzhou, Jiangxi. Native LOM concentrations in the soil were adjusted using vacuum soil solution extraction. The priming effect was quantified by adding 13C-labeled glucose at two different concentrations. Changes in the concentration of native LOM affected the native microbial biomass in the soil, in turn altering the response of soil microorganisms to exogenous available organic carbon. In the absence of glucose, soil with a high concentration of native LOM contained high levels of soil microbial biomass carbon. After adding glucose, soil microbial biomass increased, which promoted the decomposition of soil organic carbon and produced a positive priming effect. At two glucose addition levels, soil with the native LOM concentration of 50 % showed a significantly lower priming effect with values of 1.00 ± 3.10 and 0.69 ± 2.99 mg C g−1 SOC at 5 % and 20 % MBC, respectively. The priming effects did not significantly differ between the other native available organic matter concentration treatments. Thus, the influence of the native LOM concentration on the soil priming effect may be much more complex than previously considered.

Effects of earthworm (Eisenia fetida) and arbuscular mycorrhizal fungi improving plant hormones and antioxidant enzymes under simulated acid rain stress

Acid rain is considered one of the three most serious environmental disasters worldwide, disrupting the normal physiological metabolism of plants and inhibiting their growth. Earthworms alleviate plant stress by regulating soil physical, chemical and biological characteristics, and mycorrhizal fungi can promote plant nutrient absorption and improve plant resistance via symbiosis with plants. We used Illumina high-throughput sequencing to conduct a relevant redundancy analysis (RDA) of soil microbial community structure and plant growth factors, and we constructed a structural equation model (SEM) for maize biomass and a biological index to study the mechanisms by which earthworms and mycorrhizal fungi affect maize stress resistance under simulated acid rain stress. The results showed that earthworms promoted the decomposition of soil organic matter; promoted the growth of the root system; and increased the hormone levels of gibberellin A3 (GA3), abscisic acid (ABA) and indole-3-acetic acid (IAA), ultimately improving the stress resistance of maize. Mycorrhizal fungi increased the relative abundance of plant growth-promoting rhizosphere bacteria, increased the levels of plant hormones and antioxidant enzymes, and improved the stress resistance of maize. Earthworms promoted infection by mycorrhizal fungi, and the interaction between earthworms and mycorrhizal fungi increased the root IAA content and the Shannon index of rhizosphere bacteria and alleviated the simulated acid rain stress of the aboveground part of the maize. This funding is crucial to understanding above- and belowground feedback in plant–soil systems under abiotic stress.

Species shifts induce soil organic matter priming and changes in microbial communities

Invasion of plant species with functional traits that influences the rhizosphere can have significant effects on soil organic matter (SOM) dynamics if the invasive species stimulates soil microbial communities with, for example, an enhanced supply of labile carbon and oxygen. We evaluated these effects along a Phragmites invasion chronosequence spanning over 40 years. Using a δ13C and δ15N enriched substrate, we separated SOM-derived and substrate-derived carbon (C) and nitrogen (N) mineralization in surface (top 15 cm), shallow (30–45 cm), and deep (65–80 cm) soils collected from established, newly invaded, and native plant communities. We found all soils were susceptible to SOM priming, but priming profiles differed between vegetation communities, being highest at the surface in native assemblage soils, whereas highest at depth under invasive plants. Changes in functional microbial community composition at depth in Phragmites soils, evidenced by an increase in relative fungal laccase abundance, explained the SOM priming in these deep invaded soils. Our results show that invasive Phragmites maintains a microbial community at depth able to degrade SOM faster than that under native vegetation, evidencing that plant species shifts can fundamentally change soil biogeochemistry, altering element cycling and decreasing SOM residence time. Furthermore, our experimental design allowed to quantify real-time SOM-C and SOM-N gross mineralization, resulting in a new model relating C and N mineralization in these wetland soils and providing new insights on how SOM decomposition impacts N availability and cycling across wetland N pools.

Warming causes variability in SOM decomposition in N‐ and P‐fertiliser‐treated soil in a subtropical coniferous forest

AbstractBoth temperature and nutrient availability have essential roles in regulating the decomposition of soil carbon (C) and nitrogen (N), the main controls on organic matter accumulation in forest ecosystems. However, there is a lack of information about how N deposition and phosphorus (P) additions might impact soil C and N decomposition rates in subtropical forests under climate warming. We measured soil organic C and N mineralisation rates and corresponding exoenzyme activities in a subtropical forest soil that had received N and/or P additions for six years in experimental conditions at a range of temperatures between 10 and 40°C. Our results showed that soil organic C and N decomposition rates were positively correlated with the activities of their corresponding enzymes, which suggests that the extracellular enzyme activities could be the main influence on soil organic matter (SOM) decomposition rates. N additions had a significant positive effect on soil organic C mineralisation and enhanced oxidase and hydrolase activities. P additions had little effect on soil organic C and N decomposition rates. These results challenge the assumptions that soil microorganisms are N‐rich and P availability restricts organic matter decomposition, and provides additional evidence that N, not P, regulates organic matter decomposition in subtropical forests. While N additions significantly influenced the soil C and N decomposition rates, they had little effect on their sensitivity to temperature. In contrast, P additions had a significant effect on the temperature sensitivities of SOM decomposition and the βG and NAG Vmax. Overall, our results show that SOM decomposition is vulnerable to both N and P additions, and both should be considered when predicting how SOM decomposition and C cycling might change under warming.Highlights The effect of N and/or P additions on the temperature sensitivity of SOM decomposition was explored. N stimulated soil organic C decomposition rates and corresponding enzyme activities. P promoted the sensitivity of SOM decomposition to temperature changes. N and P additions could affect the stability of SOM in subtropical forests in the future.

Terrestrial carbon sequestration under future climate, nutrient and land use change and management scenarios: a national-scale UK case study

Carbon sequestration (Cseq) in soils and plant biomass is viewed as an important means of mitigating climate change. Recent global assessments have estimated considerable potential for terrestrial Cseq, but generally lack sensitivity to climate warming, nutrient limitations and perspective on local land use. These are important factors since higher temperatures can accelerate the decomposition of soil organic matter, nutrient availability affects plant productivity, while land use pressures put broader constraints on terrestrial organic matter inputs and storage. Here, we explore the potential for Cseq under changing land use, climate and nutrient conditions in a UK-based national scale case study. We apply an integrated terrestrial C–N–P cycle model with representative ranges of high-resolution climate and land use scenarios to estimate Cseq potential across the UK. If realistic UK targets for grassland restoration and afforestation over the next 30 years are met, we estimate that an additional 120 TgC could be sequestered by 2100 (similar to current annual UK greenhouse gas emissions or roughly 7% of net emission cuts needed in meeting net zero), conditional on climate change of <2 °C. Conversely, we estimate that UK arable expansion would reduce terrestrial carbon storage by a similar magnitude. The most pessimistic climate trajectories are predicted to cause net losses in UK soil carbon storage under all land use scenarios. Warmer climates substantially reduce the potential total terrestrial carbon storage gains offered by afforestation and grassland restoration. We conclude that although concerted land use change could make an important moderate contribution to national level Cseq for countries like the UK, soil Cseq only provides a contribution if we are on a low emission pathway, and is therefore conditional on deep global cuts to emissions from fossil fuels, deforestation and soil degradation.

Microplastics are transferred by soil fauna and regulate soil function as material carriers

The ecotoxicological effects of microplastics, a new and widespread ecosystem pollutant, have been extensively reported. However, it remains unclear whether soil fauna transfer microplastics and whether migration behaviours influence subsequent ecological functions in terrestrial ecosystems. We investigated the transfer patterns of microplastics and their adsorbed substances by soil animals (the springtail, Folsomia candida) and the effect of the transfer on the decomposition of soil organic matter through a standardized cotton strip assay. The results showed that springtails had a strong ability to transfer microplastics into the soil. The adsorbed nutrient (nitrogen; N), pollutant (cadmium; Cd), and green fluorescent Escherichia coli (GFP-E. coli) were also transferred with the microplastics. In addition, cotton strip decomposition was accelerated when the microplastics adsorbed N, but the adsorption of Cd decreased decomposition. These ecological effects were particularly strong for small microplastics. Microplastic transfer regulated soil bacterial communities, promoting the growth of Ascomycota fungi and inhibiting that of Basidiomycota, leading to cotton strip decomposition. Thus, microplastic pollution may occur at one site, but microplastics can be transferred anywhere in terrestrial ecosystems by soil animals and adsorb other substances, including nutrients and pollutants, that affect ecosystem function. Therefore, more studies on the migration behaviour of microplastics are necessary.

Temperature Sensitivity of Topsoil Organic Matter Decomposition Does Not Depend on Vegetation Types in Mountains.

Rising air temperatures caused by global warming affects microbial decomposition rate of soil organic matter (SOM). The temperature sensitivity of SOM decomposition (Q10) may depend on SOM quality determined by vegetation type. In this study, we selected a long transect (3.6 km) across the five ecosystems and short transects (0.1 km) from grazed and ungrazed meadows to forests in the Northwest Caucasus to consider different patterns in Q10 changes at shift of the vegetation belts. It is hypothesized that Q10 will increase along altitudinal gradient in line with recalcitrance of SOM according to kinetics-based theory. The indicators of SOM quality (BR:C, respiration per unit of soil C; MBC:C, ratio of microbial biomass carbon to soil carbon; soil C:N ratio) were used for checking the hypothesis. It was shown that Q10 did not differ across vegetation types within long and short transects, regardless differences in projective cover (14–99%) and vegetation species richness (6–12 units per plot). However, Q10 value differed between the long and short transects by almost two times (on average 2.4 vs. 1.4). Such a difference was explained by environmental characteristics linked with terrain position (slope steepness, microclimate, and land forms). The Q10 changes across studied slopes were driven by BR:C for meadows (R2 = 0.64; negative relationship) and pH value for forests (R2 = 0.80; positive relationship). Thus, proxy of SOM quality explained Q10 variability only across mountain meadows, whereas for forests, soil acidity was the main driver of microbial activity.

Soil organic matter molecular composition with long-term detrital alterations is controlled by site-specific forest properties.

Forest ecosystems are important global soil carbon (C) reservoirs, but their capacity to sequester C is susceptible to climate change factors that alter the quantity and quality of C inputs. To better understand forest soil C responses to altered C inputs, we integrated three molecular composition published data sets of soil organic matter (SOM) and soil microbial communities for mineral soils after 20 years of detrital input and removal treatments in two deciduous forests: Bousson Forest (BF), Harvard Forest (HF), and a coniferous forest: H.J. Andrews Forest (HJA). Soil C turnover times were estimated from radiocarbon measurements and compared with the molecular-level data (based on nuclear magnetic resonance and specific analysis of plant- and microbial-derived compounds) to better understand how ecosystem properties control soil C biogeochemistry and dynamics. Doubled aboveground litter additions did not increase soil C for any of the forests studied likely due to long-term soil priming. The degree of SOM decomposition was higher for bacteria-dominated sites with higher nitrogen (N) availability while lower for the N-poor coniferous forest. Litter exclusions significantly decreased soil C, increased SOM decomposition state, and led to the adaptation of the microbial communities to changes in available substrates. Finally, although aboveground litter determined soil C dynamics and its molecular composition in the coniferous forest (HJA), belowground litter appeared to be more influential in broadleaf deciduous forests (BH and HF). This synthesis demonstrates that inherent ecosystem properties regulate how soil C dynamics change with litter manipulations at the molecular-level. Across the forests studied, 20 years of litter additions did not enhance soil C content, whereas litter reductions negatively impacted soil C concentrations. These results indicate that soil C biogeochemistry at these temperate forests is highly sensitive to changes in litter deposition, which are a product of environmental change drivers.

Frequent carbon input primes decomposition of decadal soil organic matter

Soil organic matter (SOM) decomposition in response to global change represents a critical uncertainty in coupled carbon (C) cycle-climate models. Much of this uncertainty arises from our limited mechanistic knowledge of the effects of organic C input frequency on SOM decomposition. Based on a three-source-partitioning isotopic approach (14C glucose addition to soil continuously labeled by C4 plants over 21 years) and literature review of 86 observations, we assessed the priming of fast- and decadal-SOM decomposition after occasional and frequent (every 12 vs 60 days) labile C input in a grassland soil. Frequent glucose input accelerated SOM decomposition over 200 days, but the occasional input reduced this positive priming by 2.6 times. The positive priming by occasional C input resulted in 139 μg C g−1 soil net C sequestration corresponding to 38% of C input. During the 200 days incubation, the primed fast-cycling C (younger than 21 years) under occasional C addition was 52–94% greater compared with frequent addition. Conversely, the priming of decadal SOM (older than 21 years) by frequent C input was 463 μg C g−1 after the 200 days incubation, which represents 5.3% of the initial decadal-cycling soil C pool. This was 1.5 times higher than by occasional input because of the stronger N mining caused by the continuously high activity of microbial K strategists under frequent C input. Our global scale assessment showed that occasional C input reduces the priming of decadal SOM decomposition by 89–127 Tg per year compared to frequent input and thus, sequestrated an additional 102 Tg C per year. Overall, the vulnerability of decadal SOM to increasing C input frequency thus weakening the C sink of grassland, whilst reduced C input frequency (e.g. by drought) suppresses the SOM priming and reduces the positive feedback of SOM decomposition to global change. This points out the necessity to assess the age of soil C when predicting the consequences of altered soil C input frequency under global change.

Functional Guilds, Community Assembly, and Co-occurrence Patterns of Fungi in Metalliferous Mine Tailings Ponds in Mainland China.

Metalliferous mine tailings ponds are generally characterized by low levels of nutrient elements, sustained acidic conditions, and high contents of toxic metals. They represent one kind of extreme environments that are believed to resemble the Earth's early environmental conditions. There is increasing evidence that the diversity of fungi inhabiting mine tailings ponds is much higher than previously thought. However, little is known about functional guilds, community assembly, and co-occurrence patterns of fungi in such habitats. As a first attempt to address this critical knowledge gap, we employed high-throughput sequencing to characterize fungal communities in 33 mine tailings ponds distributed across 18 provinces of mainland China. A total of 5842 fungal phylotypes were identified, with saprotrophic fungi being the major functional guild. The predictors of fungal diversity in whole community and sub-communities differed considerably. Community assembly of the whole fungal community and individual functional guilds were primarily governed by stochastic processes. Total soil nitrogen and total phosphorus mediated the balance between stochastic and deterministic processes of the fungal community assembly. Co-occurrence network analysis uncovered a high modularity of the whole fungal community. The observed main modules largely consisted of saprotrophic fungi as well as various phylotypes that could not be assigned to known functional guilds. The richness of core fungal phylotypes, occupying vital positions in co-occurrence network, was positively correlated with edaphic properties such as soil enzyme activity. This indicates the important roles of core fungal phylotypes in soil organic matter decomposition and nutrient cycling. These findings improve our understanding of fungal ecology of extreme environments.

The Effects of Different Tillage Techniques and N Fertilizer Rates on Nitrogen and Phosphorus in Dry Land Agriculture

Processes governing soil organic matter (SOM) decomposition and mineralization are important for soil fertility and ecosystem sustainability. However, in the sub-Saharan region, limited work has been conducted on SOM dynamics; therefore, there was an imperative need for this study. The objective of this study was to determine the phosphorus (P) and nitrogen (N) dynamics in soil under different tillage and fertilizer management practices. The field trial was arranged as a randomized split plot design, with tillage forming the whole plot and the fertilizer application rate being the sub-plot. The tillage techniques were no-till (NT), annual tillage (CT-ANNUAL) and conventional tillage every fifth season (CT-Y5), whereby NT was practiced for four consecutive seasons, and in the fifth season, conventional tillage was employed. For all these tillage techniques, urea fertilizer was applied in amounts of 60, 120 and 240 kg N ha−1 with a control for each tillage treatment. Ammonium and nitrate levels were determined calorimetrically. Sulfuric acid, at 1 M, along with 0.057 M ascorbic acid and molybdate reagent were used to extract the organic P from the soil and extractable P. The total N, ammonium, total P, organic P and extractable P in the surface soil under NT were higher (p &lt; 0.05) compared to the values of both CT-Y5 and CT-ANNUAL. The high levels of total N, N mineralization, total P, organic P and extractable P under NT, compared to CT-Y5 and CT-ANNUAL, at the depth of 0–10 cm may be attributed to the accumulation of crop residues on the surface and less soil disturbance. Furthermore, nitrate was found in higher (p &lt; 0.05) concentrations under CT-ANNUAL compared to CT-Y5 at all the application rates and depths. The ploughing of soil under CT-ANNUAL improves the aeration, which accelerates the decomposition of organic material and mineralization of organic N and P into soluble forms. No-till, at 60 kg N ha−1 and a 0–10 cm soil depth, had optimum total N, nitrate, ammonium, total P and organic P values, thus showing its salient impact on the maintenance of soil fertility. However, the conclusion drawn from this study is that CT-Y5, due to its average N and P pools, can be recommended to under-resourced farmers in sub-Saharan preferentially over both NT and CT-ANNUAL, because it does not require advanced technology and equipment and it sustains an average soil fertility.

Simulated Hydrological Dynamics and Coupled Iron Redox Cycling Impact Methane Production in an Arctic Soil

AbstractThe fate of organic carbon (C) in permafrost soils is important to the climate system due to the large global stocks of permafrost C. Thawing permafrost can be subject to dynamic hydrology, making redox processes an important factor controlling soil organic matter (SOM) decomposition rates and greenhouse gas production. In iron (Fe)‐rich permafrost soils, Fe(III) can serve as a terminal electron acceptor, promoting anaerobic respiration of SOM and increasing pH. Current large‐scale models of Arctic C cycling do not include Fe cycling or pH interactions. Here, a geochemical reaction model was developed by coupling Fe redox reactions and C cycling to simulate SOM decomposition, Fe(III) reduction, pH dynamics, and greenhouse gas production in permafrost soils subject to dynamic hydrology. We parameterized the model using measured CO2 and CH4 fluxes as well as changes in pH, Fe(II), and dissolved organic C concentrations from oxic and anoxic incubations of permafrost soils from polygonal permafrost sites in northern Alaska, United States. In simulations of repeated oxic‐anoxic cycles, Fe(III) reduction during anoxic periods enhanced CO2 production, while the net effect of Fe(III) reduction on cumulative CH4 fluxes depended on substrate C availability. With lower substrate availability, Fe(III) reduction decreased total CH4 production by further limiting available substrate. With higher substrate availability, Fe(III) reduction enhanced CH4 production by increasing pH. Our results suggest that interactions among Fe‐redox reactions, pH and methanogenesis are important factors in predicting CH4 and CO2 production as well as SOM decomposition rates in Fe‐rich, frequently waterlogged Arctic soils.

Spring soil CO2 and CH4 emissions in the Yellow River Delta wetland, China

Climate change has become a global concern. Coastal wetlands play an important role in the global carbon cycle and significantly contribute to the emission and absorption of the greenhouse gases CO2 and CH4. In this study, the temporal and spatial variations of soil CO2 and CH4 emission fluxes, soil total carbon, total organic carbon, and carbon and nitrogen stable isotopes of four vegetation areas (Suaeda salsa, Phragmites australis, Tamarix chinensis, and farmland) and a bare beach in the Yellow River Delta wetland were observed in the spring of 2011–2013. Soil organic carbon δ13C ranged from −25.43–-23.69‰ and soil organic carbon content from 0.311 to 1.42%, and the two were significantly negatively correlated, indicating a low decomposition degree of soil organic matter. The soil CO2 emission fluxes in the study period were carbon emissions. In the area with mainly Tamarix chinensis, the 3-year average CO2 emission flux was the highest, in the area with predominantly Phragmites australisthe lowest. The soil CH4 emission flux in the spring of 2011 was a carbon absorption, whereas it was a carbon emission in the spring of 2012 and 2013. Its 3-year average was the highest in the bare beach area and the lowest in the Tamarix ramosissima area. Based on Pearson correlation analysis combined with RDA and multiple linear regression analysis, we found that the main factors affecting soil CO2 and CH4 emission fluxes may be the fast decomposition rate of soil organic matter, the content of soil carbon and organic carbon, and the low degree of soil organic matter decomposition.

Contrasting effects of maize litter and litter-derived biochar on the temperature sensitivity of paddy soil organic matter decomposition.

Organic matter input regulates the rate and temperature sensitivity (expressed as Q10) of soil organic matter (SOM) decomposition by changing microbial composition and activities. It remains unclear how the incorporation of litter-made biochar instead of litter affects the Q10 of SOM decomposition. Using a unique combination of two-and three-source partitioning methods (isotopic discrimination between C3/C4 pathways and 14C labeling), we investigated: (1) how maize litter versus litter-made biochar (of C4 origin) addition influenced the Q10 of SOM (C3 origin) under 10°C warming, and (2) how the litter or biochar amendments affected the Q10 of 14C-labeled fresh organic matter (FOM) after long-term incubation. Compared with biochar addition, litter increased the rates and Q10 of mass-specific respiration, SOM and FOM decomposition, as well as the contents of SOM-derived dissolved organic C (DOC) and total phospholipid fatty acids (PLFA). Litter-amended soils have much higher activities (Vmax) of β-glucosidase, N-acetyl-β-glucosaminidase, and leucine aminopeptidase, suggesting larger enzyme pools than in soils with biochar. The Q10 of enzyme Vmax (1.6–2.0) and Km (1.2–1.4) were similar between litter-and biochar-amended soils, and remained stable with warming. However, warming reduced microbial biomass (PLFA) and enzyme activity (Vmax), suggesting decreased enzyme production associated with smaller microbial biomass or faster enzyme turnover at higher temperatures. Reductions in PLFA content and enzyme Vmax due to warming were larger in litter-amended soils (by 31%) than in the control and biochar-amended soils (by 4–11%), implying the active litter-feeding microorganisms have a smaller degree of heat tolerance than the inactive microorganisms under biochar amendments. The reduction in enzyme activity (Vmax) by warming was lower in soils with biochar than in the control soil. Our modeling suggested that the higher Q10 in litter-amended soils was mainly caused by faster C loss under warming, linked to reductions in microbial biomass and growth efficiency, rather than the slightly increased SOM-originated substrate availability (DOC). Overall, using straw-made biochar instead of straw per se as a soil amendment lowers the Q10 of SOM and FOM by making microbial communities and enzyme pools more temperature-tolerant, and consequently reduces SOM losses under warming.

Natural revegetation over ∼ 160 years alters carbon and nitrogen sequestration and stabilization in soil organic matter on the Loess Plateau of China

Natural revegetation has been reported to play a very active role in ecosystem carbon (C) and nitrogen (N) sinks in degenerated ecosystems. However, the responses of C and N sequestration and stabilization in soil organic matter (SOM) to natural revegetation remain inadequately understood. In this study, we analyzed C and N contents and δ13C and δ15N values of SOM, free light fraction (FLF), intra-aggregate particulate organic matter (IPOM) and mineral-associated organic matter (MAOM) along ∼ 160 years of natural revegetation on the Loess Plateau of China. The results showed that natural revegetation significantly (P < 0.05) enhanced soil C and N contents in FLF, IPOM, MAOM, SOM in the surface soil (0–20 cm) during the later stages of revegetation, which exhibited smaller impact on the deeper soil (20–60 cm). Natural revegetation significantly increased distribution proportions of C and N in FLF (i.e., 18.00–36.00%, 8.46–22.57%, respectively), and that in IPOM (i.e., from 11.60 to 25.38%, 10.89–26.92%, respectively), while it decreased that in MAOM (i.e., from 56.25 to 37.00%, 67.69–50.24%, respectively) in the surface soil, thereby altering C and N stabilization in SOM (0–20 cm). The climax Quercus liaotungensis forest exhibited the highest C distribution proportion in FLF and IPOM, and the lowest C distribution proportion in MAOM in the surface soil. The δ13C and δ15N were enriched with the decomposition of SOM and soil profile depths at each revegetation stage. The highest δ13C and δ15N values (0–60 cm) and the lowest C:N ratio (0–20 cm) of SOM, FLF, and IPOM were found in the farmland. In conclusion, ∼160 years of natural revegetation substantially promoted C and N sequestration in SOM, whereas altered C and N stabilization in SOM of the surface soil through shifting C and N towards more non-protected and pure physically protected SOM fractions rather than the most stable MAOM. Meanwhile, soil δ13C and δ15N in SOM and its fraction were changed along with natural revegetation. The most enriched soil δ15N in the farmland implied that soil N cycle in the farmland was more open to N losses relative to the other revegetation stages.

The Effect of Microbial Diversity and Biomass on Microbial Respiration in Two Soils along the Soil Chronosequence.

Microbial diversity plays an important role in the decomposition of soil organic matter. However, the pattern and drivers of the relationship between microbial diversity and decomposition remain unclear. In this study, we followed the decomposition of organic matter in soils where microbial diversity was experimentally manipulated. To produce a gradient of microbial diversity, we used soil samples at two sites of the same chronosequence after brown coal mining in Sokolov, Czech Republic. Soils were X-ray sterilized and inoculated by two densities of inoculum from both soils and planted with seeds of six local plant species. This created two soils each with four levels of microbial diversity characterized by next-generation sequencing. These eight soils were supplied, or not, by litter of the bushgrass Calamagrostis epigejos, and microbial respiration was measured to assess the rate of decomposition. A strong positive correlation was found between microbial diversity and decomposition of organic matter per gram of carbon in soil, which suggests that microbial diversity supports decomposition if the microbial community is limited by available carbon. In contrast, microbial respiration per gram of soil negatively correlated with bacterial diversity and positively with fungal biomass, suggesting that in the absence of a carbon limitation, decomposition rate is controlled by the amount of fungal biomass. Soils with the addition of grass litter showed a priming effect in the initial stage of decomposition compared to the samples without the addition of litter. Thus, the relationship between microbial diversity and the rate of decomposition may be complex and context dependent.

Profile Distribution of Available Plant Nutrients in Western Hilly Tracts of Cuttack District, Odisha

Aims: An investigation was conducted to examine the distribution of available plant nutrients and the relationships between soil properties and available nutrient status in soil profiles in the Western Hilly Tracts of the Cuttack District, Odisha.&#x0D; Place and Duration of Study: The research area, i.e. the Narasinghpur block, is located in the western part of the Cuttack district in Odisha and is characterized by lateritic uplands and mountainous terrain. Three soil profiles were exposed i.e. upland, medium land and low land before the rainy season (February, 2020).&#x0D; Methodology: For experiments, five layers demarcated at 20cm intervals up to a depth of 100cm were sampled, processed, and stored. Several parameters, including particle size, pH, EC, OC, and available Ca, Mg, S, Fe, Mn, Cu, and Zn were analysed and interpreted using standard protocols.&#x0D; Results: Sand content decreased with pedon depth, whereas clay content showed the opposite trend. With increasing soil depth in all pedons, soil pH increased while EC and soil organic carbon content declined. The available Ca, Mg, and S in surface soils varied from 4.11 to 6.56 [cmol (p+)/kg], 2.15 to 3.54 [cmol (p+)/kg] and 9.85 to 12.06 mg kg-1, respectively. The corresponding subsurface ranges for these nutrients were 4.31 to 8.52 [cmol (p+)/kg], 2.41 to 4.52 [cmol (p+)/kg], and 6.95 to 11.41 mg kg-1, respectively. The available Fe, Mn, Cu, Zn, and B range in surface soils were 127.85-278.81, 68.48-144.98, 0.64-0.9, 0.54-0.63 and 0.37-0.47 mg kg-1, respectively. The subsurface ranges for these nutrients were 102.32–234.46, 53.21–118.28, 0.28–0.89, 0.17–0.50, and 0.29–0.91 mg kg-1. The status of micronutrients in the present study region was as follows: Fe &gt; Mn &gt; Cu &gt; Zn &gt; B. With increasing soil depth, available Ca and Mg increased, but available S, Fe, Mn, Cu, Zn and B declined. Positive correlations between available Fe, Mn, Cu, and Zn with the soil organic carbon content of the soil and negative correlations with the soil pH were observed.&#x0D; Conclusion: Plant nutrients in the research area varied with topography, although the differences between upland, medium land and lowland wasn't substantial. However, accelerated decomposition of soil organic matter and agricultural residues likely led to highermicronutrient concentrations at the soil's surface than subsurface.