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.

[Changes in Soil Microbial Carbon-Degrading Enzymes and Their Relationships with Carbon Pool Components During the Restoration Process of Robinia pseudoacacia].

To reveal the change in the characteristics of soil microbial C-degrading enzyme activities and the response to the components of C during the restoration process of Robinia pseudoacacia forests in the Loess Plateau, the components of the soil C pool, C-degrading enzyme activities, and microbial metabolic entropy of R. pseudoacacia in different restoration stages were studied, and the response relationship between C-degrading enzymes and soil C components was explored. The results showed that the microbial respiration (MR) first increased and then decreased with the restored years. We found that the microbial metabolic entropy (qCO2) decreased significantly with the restored years, but the microbial entropy (qMB) increased. Soil C-degrading enzymes increased significantly in the early-stage restoration of R. pseudoacacia; however, oxidizing enzymes (PO and PER) and cellobiohydrolase (CBH) decreased in the late stage of restoration. The soil organic C and recalcitrant organic C increased significantly with the restored years; however, there was no significant difference for the labile organic C. Correlation analysis and the partial least squares-path model (PLS-PM) showed that soil C-degrading enzymes and C components were significantly correlated with microbial respiration and entropy (qCO2 and qMB), respectively. The hydrolytic enzyme (BG+CBH) was significantly positively correlated with SOC, microbial biomass C, qMB, and recalcitrant and labile organic C. The oxidizing enzyme (PO+PER) was significantly positively correlated with the soil clay and qCO2. In addition, the recalcitrant organic C was the key driver of soil microbial metabolism affected by vegetation restoration. Overall, the ecosystem of R. pseudoacacia plantations would gradually stabilize with the increase in restored years and significantly increase the sequestration effect of soil C. These results will be helpful to understand the transformation rule and regulation mechanism of the soil C pool in vulnerable habitats and provide scientific basis for the restoration and management of vegetation in the Loess Plateau.

Plant Biomass and Soil Nutrients Mainly Explain the Variation of Soil Microbial Communities During Secondary Succession on the Loess Plateau.

Soil microorganisms play an important role in the circulation of materials and nutrients between plants and soil ecosystems, but the drivers of microbial community composition and diversity remain uncertain in different vegetation restoration patterns. We studied soil physicochemical properties (i.e., soil moisture, bulk density, pH, soil nutrients, available nutrients), plant characteristics (i.e., Shannon index [HPlant] and Richness index [SPlant], litter biomass [LB], and fine root biomass [FRB]), and microbial variables (biomass, enzyme activity, diversity, and composition of bacterial and fungal communities) in different plant succession patterns (Robinia pseudoacacia [MF], Caragana korshinskii [SF], and grassland [GL]) on the Loess Plateau. The herb communities, soil microbial biomass, and enzyme activities were strongly affected by vegetation restoration, and soil bacterial and fungal communities were significantly different from each other at the sites. Correlation analysis showed that LB and FRB were significantly positively correlated with the Chao index of soil bacteria, soil microbial biomass, enzyme activities, Proteobacteria, Zygomycota, and Cercozoa, while negatively correlated with Actinobacteria and Basidiomycota. In addition, soil water content (SW), pH, and nutrients have important effects on the bacterial and fungal diversities, as well as Acidobacteria, Proteobacteria, Actinobacteria, Nitrospirae, Zygomycota, and microbial biomass. Furthermore, plant characteristics and soil properties modulated the composition and diversity of soil microorganisms, respectively. Overall, the relative contribution of vegetation and soil to the diversity and composition of soil bacterial and fungal communities illustrated that plant characteristics and soil properties may synergistically modulate soil microbial communities, and the composition and diversity of soil bacterial and fungal communities mainly depend on plant biomass and soil nutrients.

[Leaf nutrient resorption characteristics of Robinia pseudoacacia at different ages and their response to soil nutrient availability]. / ä¸åŒæž—é¾„åˆºæ§å¶ç‰‡å »åˆ†é‡å¸æ”¶ç‰¹å¾åŠå ¶å¯¹åœŸå£¤å »åˆ†æœ‰æ•ˆæ€§çš„å“åº”.

To reveal nutrient resorption characteristics of Robinia pseudoacacia and their driving factors in hilly and gully regions, we measured the concentration of total nitrogen and total phosphorus in leaves and the concentrations and stoichiometry of organic carbon, total nitrogen, total phospho-rus, ammonium, nitrate and available phosphorus in soils of R. pseudoacacia plantations with different stand ages. We analyzed the relationship between leaf nitrogen and phosphorus resorption efficiencies and soil nutrient characteristics. The nutrients in plants and soil changed significantly with stand ages. The total and available phosphorus concentrations were low in the soil. Nitrogen resorption efficiency first increased and then decreased with the increases of stand age, with a range of 48.2%-54.0% and a mean value of 48.5%. Phosphorus resorption efficiency increased significantly with stand age, with a range of 45.2%-49.4% and a mean value of 46.9%. Nitrogen resorption efficiency showed negative response to soil nitrogen and N:P. Phosphorus resorption efficiency was significantly positively correlated to soil N:P and negatively correlated to soil available phosphorus. Our results indicated that soil nutrient availability negatively drove nutrient resorption efficiency. The strategies of leaf nutrient resorption responded strongly to soil N:P due to the N2-fixing effect and P-limitation of R. pseudoacacia.

[Responses mechanism of C:N:P stoichiometry of soil microbial biomass and soil enzymes to climate change.] / åœŸå£¤å¾®ç”Ÿç‰©ç”Ÿç‰©é‡ç¢³æ°®ç£·ä¸ŽåœŸå£¤é ¶åŒ–å­¦è®¡é‡å¯¹æ°”å€™å˜åŒ–çš„å“åº”æœºåˆ¶.

Microorganisms and soil enzymes are important drivers for biogeochemical cycles in terrestrial ecosystems. Understanding the role of microorganisms in the regulation of ecosystems and the response mechanisms of microbial biomass and soil enzymes to climate change are important topic in ecology. From the perspective of climatic factors, this review introduced the roles of microorganisms and soil enzymes in the carbon, nitrogen and phosphorus cycles of terrestrial ecosystems based on the theory of ecological stoichiometry. Moreover, we synthesized the responses mechanisms of soil microbial and soil enzyme stoichiometry, i.e., changes of microbial metabolic rate, enzymatic acti-vity, microbial community structure, ecological stoichiometry of soil microbial biomass and soil enzymes, and nutrient use efficiency. Finally, we analyzed the current research inadequacies and proposed the scientific problems in this field, i.e., to comprehensively elucidate the response mecha-nism of soil microbes and soil enzymes to climate change; to examine the nutrient coupling mechanism of soil microbes and extracellular enzymes; and to explore the adaptive strategies of C:N:P stoichiometry of soil microbial biomass and soil enzymes to climate change.