C-degrading Enzymes Research Articles

Unraveling the relationship between soil carbon-degrading enzyme activity and carbon fraction under biogas slurry topdressing

Biogas slurry, a by-product of the anaerobic digestion of biomass waste, predominantly consisting of livestock and poultry manure, is widely acclaimed as a sustainable organic fertilizer owing to its abundant reserves of essential nutrients. Its distinctive liquid composition, when tactfully integrated with a drip irrigation system, unveils immense potential, offering unparalleled convenience in application. In this study, we investigated the impact of biogas slurry topdressing as a replacement for chemical fertilizer (BSTR) on soil total organic carbon (TOC) fractions and carbon (C)-degrading enzyme activities across different soil depths (surface, sub-surface, and deep) during the tasseling (VT) and full maturity stage (R6) of maize. BSTR increased the TOC content within each soil layer during both VT and R6 periods, inducing alterations in the content and proportion of individual C component, particularly in the topsoil. Notably, the pure biogas slurry topdressing treatment (100%BS) compared with the pure chemical fertilizer topdressing treatment (CF), exhibited a 38.9% increase in the labile organic carbon of the topsoil during VT, and a 30.3% increase in the recalcitrant organic carbon during R6, facilitating microbial nutrient utilization and post-harvest C storage during the vigorous growth period of maize. Furthermore, BSTR treatment stimulated the activity of oxidative and hydrolytic C-degrading enzymes, with the 100%BS treatment showcasing the most significant enhancements, with its average geometric enzyme activity surpassing that of CF treatment by 27.9% and 27.4%, respectively. This enhancement facilitated ongoing and efficient degradation and transformation of C. Additionally, we screened for C components and C-degrading enzymes that are relatively sensitive to BSTR. The study highlight the advantages of employing pure biogas slurry topdressing, which enhances C component and C-degrading enzyme activity, thereby reducing the risk of soil degradation. This research lays a solid theoretical foundation for the rational recycling of biogas slurry.

Microbial nutrient limitation along a 2-million-year dune chronosequence

Long-term ecosystem development is characterized by a switch from nitrogen (N) to phosphorus (P) limitation of plant communities as soils age, which leads to changes in plant biomass, diversity, and foliar nutrient concentrations. Similar effects occur belowground, although the extent to which nutrient availability is the primary driver of long-term changes in soil microbial communities remains uncertain. To investigate this, we assessed proxies for soil microbial nutrient limitation along the Jurien Bay dune chronosequence in Western Australia, where a long-term decline in P availability is reflected in strong variation in plant and microbial community composition over 2 million years of soil development. We quantified carbon (C), N, and P in the soil microbial biomass, assayed soil enzymes involved in the acquisition of those nutrients from soil organic matter, and used microbial stoichiometric ratios and vector analysis of enzyme activities to assess whether long-term changes in soil nutrient availability are reflected in nutrient demand by soil microbes. Concentrations of microbial nutrients peaked in Holocene soils (1–6.5 ky), although differences in microbial P were not significant. There were no significant differences in microbial nutrient ratios (C:N, C:P, and N:P) along the chronosequence. Acid phosphatase activity increased continually throughout the chronosequence, while the activities of leucine aminopeptidase and β-glucosidase were greatest in Holocene soils and declined with increasing soil age. Corresponding vector lengths indicated greater investment in C-degrading enzymes in Holocene soils (≤6.5 ky), while vector angles indicated greater investment in the acquisition of P compared with N in Pleistocene soils (≥120 ky). Overall, these findings show that changes in nutrient availability during long-term pedogenesis at Jurien Bay are reflected in microbial investment in nutrient acquisition, but that the soil microbial biomass exhibits a remarkable degree of homeostasis despite major shifts in nutrient availability and community composition over 2 million years of ecosystem development.

Nine years of warming and nitrogen addition in the Tibetan grassland promoted loss of soil organic carbon but did not alter the bulk change in chemical structure

Abstract. Nitrogen (N) and warming effects on ecosystem carbon (C) budgets and stabilization are critical to understand as C sequestration is considered as a mechanism to offset anthropogenic CO2 emissions, which is important for accurately predicting ecosystem C sequestration and/or potential C loss, remaining controversial though. However, the relevant information, especially for the intervention of environmental controls on grassland soil, is limited in Tibetan Plateau (TP) regions. Here we used a 9-year two-way factorial experiment involving warming with open top chambers (+1.80 ∘C in the daytime and +0.77 ∘C in the nighttime at the soil surface) and multilevel nitrogen (N) enrichment treatments (0, 5, 10, and 15 gm-2yr-1) in the TP to investigate the changes in soil organic carbon (SOC) pool size and chemical structure. Nine-year warming treatment significantly decreased SOC stock in the Tibetan grassland. We observed decreasing SOC concentrations which may be related to changes in the C-degrading enzymes. Surprisingly, the SOC molecular structure remained unchanged in all N-enrichment and warmed plots, suggesting that both treatments had affected all forms of SOC, from simple and complex polymeric in a similar way. Our results suggest that long-term warming stimulates soil C loss but no preference in SOC loss with different chemical structures.

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

Climate and detritus influence soil organic carbon (SOC) and labile SOC fractions by affecting soil microbial communities. However, it is not clear how, or to what extent, different detritus treatments affect soil microbial communities and SOC content in karst landscapes during different seasons. Plots in a karst landscape were treated with different detritus input regimes (control, no litter, no roots, no litter or roots, and double litter), and samples were collected during the dry and rainy seasons. We used Illumina sequencing of 16S rRNA to examine shifts in the diversity and composition of the associated soil microbial communities. Additionally, labile SOC fractions, including dissolved organic carbon (DOC) and microbial biomass carbon (MBC), along with soil physicochemical properties and C-degrading enzyme activities, were analyzed. The results revealed that the responses of soil properties and labile SOC fractions to detritus treatments were more pronounced during the rainy season than during the dry season, which mainly reflected that the levels of available potassium (AK), DOC, and MBC were significantly increased during the rainy season. Moreover, SOC and total nitrogen (TN) demonstrated significant changes with the double litter (DL) treatment during the rainy season. The responses of soil microbial communities to detritus treatments varied with the season, as reflected primarily in changes in the relative abundance of Ascomycota, unclassified_K_fungi, Proteobacteria, and Actinobacteriota. Climate, detritus treatments, and their interactions had significant effects on the species richness of soil bacterial communities, but did not influence fungal community diversity. Furthermore, structural equation modeling (SEM) revealed that the soil bacterial composition had the largest total effects on SOC, DOC, and MBC. In addition to directly influencing SOC, DOC, and MBC, soil properties (TN, AK, and pH) indirectly affected SOC, DOC, and MBC by altering C-degrading enzyme activity and the microbial community. We conclude that detritus treatments affect the soil microbial community and labile carbon fractions during both the rainy and dry seasons. Relationships among SOC, labile SOC fractions, enzyme activities, microbial communities, and function differed between seasons and among treatment types. This research advances our knowledge of how variation in detritus treatments affects biogeochemical cycling in karst soils during the rainy and dry seasons.

Divergent Responses of Temperature Sensitivity to Rising Incubation Temperature in Warmed and Un-Warmed Soil: A Mesocosm Experiment from a Subtropical Plantation

We conducted a short-term laboratory soil warming incubation experiment, sampling both warmed and un-warmed soils from a subtropical plantation in southeastern China, incubating them at 20 °C, 30 °C, and 40 °C. Our aim was to study the SOC mineralization response to increasing temperatures. Our findings revealed that the temperature sensitivity (Q10) of SOC mineralization to short-term experimental warming varied between the warmed soil and the un-warmed soil. The Q10 of the un-warmed soil escalated with the temperature treatment (20–30 °C: 1.31, 30–40 °C: 1.63). Conversely, the Q10 of the warmed soil decreased (20–30 °C: 1.57, 30–40 °C: 1.41). Increasing temperature treatments decreased soil substrate availability (dissolved organic C) in both un-warmed and warmed soil. The C-degrading enzyme in un-warmed soil and warmed soil had different trends at different temperatures. In addition, warming decreased soil microbial biomass, resulting in a decrease in the total amount of phospholipid fatty acids (PLFAs) and a decrease in the abundance of fungi and Gram-negative bacteria (GN) in both un-warmed and warmed soil. The ratio of fungal to bacterial biomass (F:B) in un-warming soil was significantly higher than that in warmed soil. A drop in the microbial quotient (qMBC) coupled with a rise in the metabolic quotient (qCO2) indicated that warming amplified microbial respiration over microbial growth. The differential Q10 of SOC mineralization in un-warmed and warmed soil, in response to temperature across varying soil, can primarily be attributed to shifts in soil dissolved organic C (DOC), alterations in C-degrading enzyme activities, and modifications in microbial communities (F:B).

Maize-potato residue mixing in agricultural soils enhances residue decomposition and stable carbon content by modifying the potential keystone microbial taxa

Microbe-regulated organic residue decomposition is a central hub of carbon (C) cycling in ecosystems. However, little is known about the changes in organic C composition and its microbial mechanisms during mixed crop residue decomposition in agroecosystems. We conducted a field litterbag experiment to explore the changes in organic C chemical structure and their associations with microbial communities during mixed (1:1 maize:potato residues based on dry matter mass) and single-species residue decomposition in agricultural soils. Compared with single-species residues, mixed residues exhibited a positive non-additive effect, specifically higher mass loss, and more recalcitrant organic C composition. Structural equation modelling combined with network analysis suggested that residues mixing modified bacterial keystone taxa, augmented the activity of C-degrading enzymes such as β-glucosidase, and increased stable C content during mixed residues decomposition. Thus, this study highlights the potential advantages of applying residue mixtures to agri-cropping soils to increase the relative amount of persistent aromatic compounds derived from their decomposition and the importance of bacterial keystone taxa during residue decomposition.

Combining conservation tillage with nitrogen fertilization promotes maize straw decomposition by regulating soil microbial community and enzyme activities

Straw return can effectively improve farmland soil microenvironment and fertility. However, excessive straw in the topsoil adversely affects seed germination and crop growth. At present, the characteristics and key driving factors of straw decomposition in dry farmlands are unclear. Based on the interactions between tillage practices including zero tillage (ZT), chisel tillage (CT), and plow tillage (PT) and nitrogen (N) fertilization, i.e., low N (N1, 180 kg ha-1), normal N (N2, 240 kg ha-1), and high N (N3, 300 kg ha-1), quantitative polymerase chain reaction technology and an enzymatic detection kit were used to investigate the effects of key straw C-degrading enzyme activities and microbial abundance in soil on maize straw decomposition during the growth period of winter wheat in the winter wheat/summer maize double cropping system in a dry farmland of the Loess Plateau, China. Between 2018 and 2020, ZT and CT significantly increased winter wheat yield (by 10.94% and 12.79%, respectively) and straw decomposition velocity (by 20% and 26.67%, respectively), compared with PT. Compared to N1 and N3, N2 significantly increased wheat yield (by 4.65% and 5.31%, respectively) and straw decomposition velocity (by 26.33% and 13.21%, respectively). The partial least squares pathway modelling showed significant positive direct effects of soil moisture, NO3-, NH4+, total N, bacteria, and cellulase, laccase, and xylanase activities on straw decomposition, while soil pH, fungi, and Actinomycetes had significant negative direct effects. Overall, conservation tillage (ZT and CT) combined with N2 was beneficial for straw decomposition in the drylands of the Loess Plateau and improved straw resource utilization and basic soil fertility. The results of the study clarified the key drivers of straw decomposition in dry farmlands and provided new ideas for developing updated soil management practices and adaptive N application strategies to promote the resource utilization of straw and achieve the goals of carbon peaking and carbon neutrality.

Trade-offs in carbon-degrading enzyme activities limit long-term soil carbon sequestration with biochar addition.

Biochar amendment is one of the most promising agricultural approaches to tackle climate change by enhancing soil carbon (C) sequestration. Microbial-mediated decomposition processes are fundamental for the fate and persistence of sequestered C in soil, but the underlying mechanisms are uncertain. Here, we synthesise 923 observations regarding the effects of biochar addition (over periods ranging from several weeks to several years) on soil C-degrading enzyme activities from 130 articles across five continents worldwide. Our results showed that biochar addition increased soil ligninase activity targeting complex phenolic macromolecules by 7.1%, but suppressed cellulase activity degrading simpler polysaccharides by 8.3%. These shifts in enzyme activities explained the most variation of changes in soil C sequestration across a wide range of climatic, edaphic and experimental conditions, with biochar-induced shift in ligninase:cellulase ratio correlating negatively with soil C sequestration. Specifically, short-term (<1year) biochar addition significantly reduced cellulase activity by 4.6% and enhanced soil organic C sequestration by 87.5%, whereas no significant responses were observed for ligninase activity and ligninase:cellulase ratio. However, long-term (≥1year) biochar addition significantly enhanced ligninase activity by 5.2% and ligninase:cellulase ratio by 36.1%, leading to a smaller increase in soil organic C sequestration (25.1%). These results suggest that shifts in enzyme activities increased ligninase:cellulase ratio with time after biochar addition, limiting long-term soil C sequestration with biochar addition. Our work provides novel evidence to explain the diminished soil C sequestration with long-term biochar addition and suggests that earlier studies may have overestimated soil C sequestration with biochar addition by failing to consider the physiological acclimation of soil microorganisms over time.

Different effects of litter and root inputs on soil enzyme activities in terrestrial ecosystems

Soil extracellular enzymes are involved in the decomposition of plant litter and soil organic matter, and their activities are of critical significance to soil carbon and nutrient turnover in terrestrial ecosystems. However, the coupling of litter and root inputs and soil enzyme activities across terrestrial ecosystems still remains not fully understood. In this study, we compiled 788 paired observations to evaluate changes in soil β-1,4-glucosidase (BG), β-d-cellobiosidase (CBH), phenol oxidase (PHO), β-1,4-N-acetyl-glucosaminidase (NAG), and acidic phosphatase (AP) activities after the removal of litter, living roots, and both litter and living roots in different ecosystems (forests, grasslands, croplands and tundra). Soil BG, NAG, and AP activities were decreased by 22.5 %, 16.6 % and 28.7 %, respectively, after removing both litter and living roots, but these effects varied in different ecosystems. Removing both litter and living roots decreased soil BG and CBH activities in forests and BG activity in croplands but increased soil CBH activity in croplands. Soil NAG activity was decreased by 15.1 % in forests but increased by 6.1 % in croplands after both litter and living roots were removed, while soil AP activity was decreased significantly across all of the studied ecosystems after both litter and living root removal. However, litter addition greatly increased soil BG, CBH, NAG and AP activities, suggesting that soil enzyme activities involved in carbon, nitrogen and phosphorus cycling were strongly controlled by plant carbon input, with an additive effect of litter and root inputs observed for C-degrading enzymes. Our synthesis also found that these enzymatic responses to plant carbon input were strongly controlled by edaphic and not climatic factors across terrestrial ecosystems. These results suggested that soil enzyme activities increased after litter addition to accelerate the decomposition of litter input but decreased after litter removal. Removing both litter and root inputs had a similar but additive effect on soil enzyme activities across sites. Although these responses varied greatly among ecosystems and litter types, edaphic factors predominantly controlled enzymatic responses over terrestrial ecosystems. Our results highlighted the importance of plant inputs in enzymatically involved carbon and nutrient cycles in terrestrial ecosystems.

Relative contributions of fungi and bacteria to litter decomposition under low and high soil moisture in an Australian grassland

Fungi and bacteria differ in their sensitivity to soil moisture, but their activity and contributions to soil carbon (C) cycling under different soil moisture conditions remain unclear. We manipulated soil fungi and bacteria with the fungicide captan and bactericide bronopol, respectively, and examined their effect on C dynamics at low and high moisture levels (40 and 80 % water holding capacity) in a grassland soil over a 21-day incubation period in the laboratory. 13C-labeled ryegrass was added with and without biocide treatments to separate ryegrass from soil-derived C in respiration, dissolved organic C (DOC) and soil C. We also measured activities of four C-degrading enzymes. Soil total- and ryegrass-respiration rates and microbial biomass C (MBC) decreased, while ryegrass-derived C in DOC and soil increased with decreased soil moisture, suggesting reduced microbial activity with decreased soil moisture. Initially, ryegrass respiration rates were suppressed by both biocides and both soil moisture levels. However, at low soil moisture, ryegrass respiration no longer was suppressed by bronopol after eight days of incubation, but remained suppressed with captan, suggesting that with low moisture fungi played a more important role in decomposing ryegrass litter than bacteria. On the other hand, at high soil moisture, biocide effects on ryegrass respiration mostly disappeared after eight days, suggesting that more labile components of ryegrass litter were no longer present, and that the surviving microorganisms adapted similarly to decomposing the more recalcitrant litter. Bacteria preferred to decompose labile substrates early on during the incubation, and fungi played a greater role in decomposing more recalcitrant ryegrass left over in soil at the end of the incubation. At low moisture with captan, a reduction in oxidative enzyme production contributed to the increase in the hydrolytic:oxidative enzyme ratio, further indicating the importance of fungi for decomposing more recalcitrant substances under low moisture conditions.

Characteristics of bacterial and fungal communities and their impact during cow manure and agroforestry biowaste co-composting

Microbial communities and environmental conditions are both of great importance for efficient utilization of agroforestry resources. Nevertheless, knowledge about the role of soluble nutrients and enzymatic properties, and their inner links with microbial communities remain limited. This is especially the case for the co-composting of agricultural and forestry biowaste. Here, we investigate the succession of key microbes during co-composting (sawdust + cow manure, SA; straw + cow manure, ST), employing amplicon sequencing, enzyme assays, and physicochemical analyses. N-fixing bacteria (Pseudomonas) and C-degrading fungi (Acaulium) have been identified as dominant taxa during such co-composting. Although eight antibiotic resistance genes were found to persist during composting, pathogenic microbes declined with composting time. NO3−-N content was screened as a determinant structuring the bacterial and fungal communities, with importance also shown for C-degrading enzymes such as cellulose, laccase, and peroxidase activity.These results identify the key microbial taxa and their main interactive environmental factors, which are potentially valuable for the development of a mixed microbial inoculant to accelerate the maturation of agroforestry biowastes composting.

Changes in Soil Organic C Fractions and C Pool Stability Are Mediated by C-Degrading Enzymes in Litter Decomposition of Robinia pseudoacacia Plantations.

Litter decomposition is the main source of soil organic carbon (SOC) pool, regarding as an important part of terrestrial ecosystem C dynamics. The turnover of SOC is mainly regulated by extracellular enzymes secreted by microorganisms. However, the response mechanism of soil C-degrading enzymes and SOC in litter decomposition remains unclear. To clarify how SOC fraction dynamics respond to C-degrading enzymes in litter decomposition, we used field experiments to collect leaf litter and SOC fractions from the underlying layer in Robinia pseudoacacia plantations on the Loess Plateau. Our results showed that SOC, easily oxidizable organic C, dissolved organic C, and microbial biomass C increased significantly during the decomposition process. Litter decomposition significantly decreased soil hydrolase activity, but slightly increased oxidase activity. Correlation analysis results showed that SOC fractions were significantly positively correlated with the litter mass, lignin, soil moisture, and oxidase activity, but significantly negatively correlated with cellulose content and soil pH. Partial least squares path models revealed that soil C-degrading enzymes can directly or indirectly affect the changes of soil C fractions. The most direct factors affecting the SOC fractions of topsoil during litter decomposition were litter lignin and cellulose degradation, soil pH, and C-degrading enzymes. Furthermore, regression analysis showed that the decrease of SOC stability in litter decomposition was closely related to the decrease of soil hydrolase to oxidase ratio. These results highlighted that litter degradation-induced changes in C-degrading enzyme activity significantly affected SOC fractions. Furthermore, the distribution of soil hydrolases and oxidases affected the stability of SOC during litter decomposition. These findings provided a theoretical framework for a more comprehensive understanding of C turnover and stabilization mechanisms between plant and soil.

Land-use-driven change in soil labile carbon affects microbial community composition and function

The conversion of natural forests to plantations can lead to a substantial loss of soil organic carbon (SOC) and threaten the function of soil microbial communities. However, the effect of forest conversion on the relationship between SOC dynamics and shifts in the composition and function of the microbial community remain poorly understood, although such information is critical for predicting the long-term effect of land-use change on global SOC dynamics and storage. To fill this knowledge gap, our study investigated the influence of two decades of converting natural forest to a tea plantation on SOC storage, microbial community composition, C-degrading enzyme activity and the abundance of C-degrading microbial functional genes in a subtropical region in China. Two decades after converting natural forest to tea plantation, SOC storage decreased by 30%, and labile SOC [including easily mineralized C (Cmin), readily oxidizable C (ROC), dissolved organic C (DOC), and microbial biomass C (MBC)] decreased by 21–71%. Land-use change decreased the relative abundance of copiotrophic bacteria but increased the relative abundance of oligotrophic bacteria and lignin-degrading fungi. This resulted in a decrease in cellulase activity and cbhI gene abundance involved in cellulose degradation and conversely an increase in ligninase activity involved in lignin degradation after the conversion. Such shifts in microbial community composition and function associated with land-use change were mediated by the decrease in the labile C pool rather than the total SOC. Our study provided new insights into not only shifts in microbial communities but also specific C functional genes and related enzymes after forest conversion. These findings highlight that labile SOC, as an important functional soil component for microbial communities, significantly affects SOC dynamics and storage.

Effect of forest thinning on soil organic carbon stocks from the perspective of carbon-degrading enzymes

Forest thinning greatly affects the soil organic carbon (SOC) pool by changing the soil microenvironment, organic matter input, and microbial metabolism. However, the heterogeneity of thinning intensity, recovery stage, and microclimatic conditions increases uncertainty about the effect of thinning on SOC. The activities of C-degrading enzymes (ligninases and cellulases) secreted by soil microorganisms can be used to explore the effects of thinning on soil C storage. Our meta-analysis showed that forest thinning significantly increased soil temperature (ST), soil moisture (SM), and pH, but significantly decreased litter and fine root biomass. Moderate thinning and recovery time extension increased soil C storage, SOC, and recalcitrant organic C (ROC). An increase in the thinning intensity significantly improved ligninase activity, while decreasing cellulase activity and extending recovery time after thinning. Increased SOC, soil labile organic C (LOC), and ROC with thinning were associated with increased ligninase activity that was significantly positively correlated with soil pH, SM, and ST after thinning. The variation in soil ligninase: cellulase ratio induced by thinning was significantly positively correlated with the increases of soil ROC: LOC ratio and SOC after thinning. Therefore, forest thinning enhanced SOC pool stability through the effect of ligninases and cellulases on SOC sequestration. The global pattern of soil C-degrading enzymes in response to thinning has deepened our understanding of the mechanism by which thinning affects the SOC cycle.

HupZ, a Unique Heme-Binding Protein, Enhances Group A Streptococcus Fitness During Mucosal Colonization.

Group A Streptococcus (GAS) is a major pathogen that causes simple and invasive infections. GAS requires iron for metabolic processes and pathogenesis, and heme is its preferred iron source. We previously described the iron-regulated hupZ in GAS, showing that a recombinant HupZ-His6 protein binds and degrades heme. The His6 tag was later implicated in heme iron coordination by HupZ-His6. Hence, we tested several recombinant HupZ proteins, including a tag-free protein, for heme binding and degradation in vitro. We established that HupZ binds heme but without coordinating the heme iron. Heme-HupZ readily accepted exogenous imidazole as its axial heme ligand, prompting degradation. Furthermore, HupZ bound a fragment of heme c (whose iron is coordinated by the cytochrome histidine residue) and exhibited limited degradation. GAS, however, did not grow on a heme c fragment as an iron source. Heterologous HupZ expression in Lactococcus lactis increased heme b iron use. A GAS hupZ mutant showed reduced growth when using hemoglobin as an iron source, increased sensitivity to heme toxicity, and decreased fitness in a murine model for vaginal colonization. Together, the data demonstrate that HupZ contributes to heme metabolism and host survival, likely as a heme chaperone. HupZ is structurally similar to the recently described heme c-degrading enzyme, Pden_1323, suggesting that the GAS HupZ might be divergent to play a new role in heme metabolism.

Effects of different forms of nitrogen addition on microbial extracellular enzyme activity in temperate grassland soil

BackgroundNitrogen (N) deposition alters litter decomposition and soil carbon (C) sequestration by influencing the microbial community and its enzyme activity. Natural atmospheric N deposition comprises of inorganic N (IN) and organic N (ON) compounds. However, most studies have focused on IN and its effect on soil C cycling, whereas the effect of ON on microbial enzyme activity is poorly understood. Here we studied the effects of different forms of externally supplied N on soil enzyme activities related to decomposition in a temperate steppe. Ammonium nitrate was chosen as IN source, whereas urea and glycine were chosen as ON sources. Different ratios of IN to ON (Control, 10:0, 7:3, 5:5, 3:7, and 0:10) were mixed with equal total amounts of N and then used to fertilize the grassland soils for 6 years.ResultsOur results show that IN deposition inhibited lignin-degrading enzyme activity, such as phenol oxidase (POX) and peroxidase (PER), which may restrain decomposition and thus induce accumulation of recalcitrant organic C in grassland soils. By contrast, deposition of ON and mixed ON and IN enhanced most of the C-degrading enzyme activities, which may promote the organic matter decomposition in grassland soils. In addition, the β-N-acetyl-glucosaminidase (NAG) activity was remarkably stimulated by fertilization with both IN and ON, maybe because of the elevated N availability and the lack of N limitation after long-term N fertilization at the grassland site. Meanwhile, differences in soil pH, soil dissolved organic carbon (DOC), and microbial biomass partially explained the differential effects on soil enzyme activity under different forms of N treatments.ConclusionsOur results emphasize the importance of organic N deposition in controlling soil processes, which are regulated by microbial enzyme activities, and may consequently change the ecological effect of N deposition. Thus, more ON deposition may promote the decomposition of soil organic matter thus converting C sequestration in grassland soils into a C source.

Nitrogen and phosphorus addition exerted different influences on litter and soil carbon release in a tropical forest

Terrestrial soils release large amount of carbon dioxide (CO2) each year, which are mainly derived from litter and soil carbon (C) decomposition. Nutrient availability, especially nitrogen (N) and phosphorus (P), plays an important role in both litter and soil C decomposition. Therefore, understanding the underlying mechanism is crucial for mitigating CO2 emission and climate changes. Here, we assessed patterns of litter and soil C decomposition after 11 yrs. in-situ N and P addition in a tropical forest where corn leaves or corn roots were added as litter C. The total CO2 efflux was quantified and partitioned using 13C isotope signatures to determine the sources (litter or soil C) every three months. In addition, Changes in C-degrading enzyme activities: β-1,4-glucosidase (BG), phenol oxidase (PHO) and peroxidase (PER), and microbial biomarkers were assessed to interpret the underlying mechanism. Total C-release was enhanced up to17% by the long-term N addition but inhibited up to 15% by P addition. Precisely, N addition only accelerated the litter decomposition and increased about 42% and 6% of the litter C release at 0–5 cm and 5–10 cm soil depths, respectively; while P addition only impeded the soil C decomposition and decreased about 9% and 11% of the soil C release at 0–5 cm and 5–10 cm, respectively. The enhanced C release under N addition might be attributed to the enhanced microbial biomass, the ratio of fungi to bacteria and C-degrading enzyme activities. However, P addition resulted in the reverse result in microbial properties and C-degrading enzyme activities, associated with a decreased C release. Our study suggests that the long-term N and P addition selectively affected the litter and soil C decomposition because of their different physiochemical properties and this tendency might be more pronounced in tropical forests exposed to increasing atmospheric N deposition in the future. The study indicates that the different patterns of litter and soil C decomposition under climate change should be taken account in the future C management strategies.

Land-Use Change Enhanced SOC Mineralization but Did Not Significantly Affect Its Storage in the Surface Layer.

To achieve carbon (C) neutrality and mitigate climate change, it is crucial to understand how converting natural forests to agricultural plantations influences soil organic C (SOC) mineralization. In this study, we investigated the impact of converting evergreen broadleaf forests (EBF) to extensively managed Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau) plantations (MBP) in subtropical China on SOC mineralization rate; the concentrations of labile SOC fractions such as dissolved organic C (DOC), microbial biomass C (MBC), and readily oxidizable C (ROC); the activities of C-degrading enzymes (cellobiohydrolase and phenol oxidase); and the abundance of C-degrading enzyme-encoding genes (cbhI and lcc). Three paired soil samples were taken from the surface layer (0–20 cm) of adjacent EBF-MBP sites in Anji County, Zhejiang province. Results showed that converting EBF to MBP significantly increased the SOC mineralization rate as well as soil pH, MBC, cellobiohydrolase, and phenol oxidase activities, and cbhI gene abundance, but did not change other soil properties described above. In addition, structural equation modelling (SEM) showed that the conversion increased SOC mineralization rate through increasing soil pH, cbhI gene abundance, MBC, and cellobiohydrolase and phenol oxidase activities. Our novel finding that converting EBF to extensively managed MBP enhanced SOC mineralization via increasing the activities of C-degrading enzymes suggests that C-degrading enzymes were a key factor regulating SOC mineralization in the extensively managed subtropical bamboo plantations.

Microbial community structure and functional genes drive soil priming effect following afforestation

Afforestation substantially modifies native soil organic carbon (SOC) decomposition via plant carbon inputs (the priming effect), and in turn, triggers vital biogeochemical processes that influence the regulation of soil carbon dynamics. Soil microbes are crucial in regulating the direction and magnitude of the priming effect. In the present study, we performed metagenomic sequencing and 13C-glucose labeling analyses of microbial communities and priming effects across a Robinia pseudoacacia afforestation chronosequence (14-, 20-, 30-, and 45-year-old stands) in the Loess Plateau in China, with adjacent farmland being selected as a control. Our results revealed that the cumulative priming effect across five sites along the afforestation chronosequence initially increased and approached a peak value in the 20-year-old stand, after which it declined. The priming effect was predominantly driven by the microbial community structure (i.e., the fungal-to-bacterial ratios and relative abundances of Proteobacteria and Actinobacteria), and stable C decomposition genes and C-degrading enzymes. Specifically, among the key functional genes correlated with priming effect, which were identified in orders Rhizobiales and Pseudonocardiales, considerably promoted SOC priming. Overall, our findings indicate that afforestation alters soil microbial community structure and function, particularly with respect to enhancing stable soil C decomposition genes, which may promote SOC priming. The findings of the present study could enhance our understanding of fresh C input-induced changes associated with C mineralization in the context of the revegetation of ecologically fragile areas.

Linking carbon-degrading enzyme activity to microbial carbon-use trophic strategy under salinization in a subtropical tidal wetland

Although increasing studies have reported that microorganisms play an important role in carbon (C)-degrading enzyme activities under salinization, there is little experimental evidence for the associations between C-degrading enzyme activities and microbial community structure. Here, we examined the C-degrading enzyme activities, bacterial and fungal community abundance and composition (16S and ITS rRNA gene sequencing), soil organic carbon (SOC) storage, carbon/nitrogen (C/N) ratios, labile organic carbon (LOC) levels, and LOC/SOC ratios along an estuarine salinity gradient [range: 0.1–2.2 part per thousand (ppt)] of a subtropical tidal wetland. To help isolate salinity effects, sampling sites were selected to be similar in plant community composition, soil texture, and tidal influence. Our results indicated that the SOC storage, LOC levels, and LOC/SOC ratios decreased, whereas C/N ratios increased with salinization. The activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase increased by 485%, 147%, 699%, and 868%, respectively, with increasing salinity. The α-diversities of bacterial and fungal communities did not change, whereas the abundances of bacterial and fungal communities decreased with increasing salinity. Both bacterial and fungal oligotrophs/copiotrophs ratios were positively associated with salinity, suggesting that bacterial and fungal communities shifted from being copiotroph-dominated to oligotroph-dominated as salinity increased. All C-degrading enzyme activities were correlated with the relative abundance of typical bacterial and fungal oligotrophs, e.g., α-Proteobacteria, δ-Proteobacteria, Chloroflexi, Acidobacteria, and Basidiomycota. Overall, the C-degrading enzyme activities were co-determined by the C/N and LOC/SOC ratios and the fungal and bacterial oligotrophs/copiotrophs ratios. Our observations indicated that salinization influenced C-degrading enzyme activities via modulating substrate quality and lability and causing shifts in microbial C-use trophic strategies. Our proposed oligotrophic–copiotrophic scheme could be a novel method for incorporating vast taxonomic data of microbial communities into enzyme activity prediction in an ecologically meaningful manner.