Abiotic and biotic drivers of tree trait effects on soil microbial biomass and soil carbon concentration

Forests are critical ecosystems to understand the global carbon budget, due to their carbon sequestration potential in both above- and belowground compartments, especially in species-rich forests. Soil carbon sequestration is strongly linked to soil microbial communities, and this link is mediated by the tree community, likely due to modifications of micro-environmental conditions (i.e., biotic conditions, soil properties, and microclimate). We studied soil carbon concentration and the soil microbial biomass of 180 local neighborhoods along a gradient of tree species richness ranging from 1 to 16 tree species per plot in a Chinese subtropical forest experiment (BEF-China). Tree productivity and different tree functional traits were measured at the neighborhood level. We tested the effects of tree productivity, functional trait identity and dissimilarity on soil carbon concentrations, and their mediation by the soil microbial biomass and micro-environmental conditions. Our analyses showed a strong positive correlation between soil microbial biomass and soil carbon concentrations. Besides, soil carbon concentration increased with tree productivity and tree root diameter while it decreased with litterfall C:N content. Moreover, tree productivity and tree functional traits (e.g. root fungal association and litterfall C:N ratio) modulated micro-environmental conditions with substantial consequences for soil microbial biomass. We also showed that soil history and topography should be considered in future experiments and tree plantations, as soil carbon concentrations were higher where historical (i.e., at the beginning of the experiment) carbon concentrations were high, themselves being strongly affected by the topography. Altogether, these results imply that the quantification of the different soil carbon pools is critical for understanding microbial community–soil carbon stock relationships and their dependence on tree diversity and micro-environmental conditions.

森林是解析全球碳收支的关键生态系统，因其地上与地下组分均具备碳固存（carbon sequestration）潜力，而物种丰富的森林尤具优势。
土壤碳固存（carbon sequestration）与土壤微生物群落紧密相关，该关联由树木群落介导，其潜在机制为树木群落可调控微环境条件——包括生物条件、土壤属性与微气候。本研究依托中国亚热带森林生物多样性与生态系统功能实验（BEF-China），在180个局域邻域样地中开展调查，样地的树木物种丰富度梯度为1至16种/样地，测定了其中的土壤碳浓度与土壤微生物生物量（soil microbial biomass）。研究同时在邻域尺度测定了树木生产力及多种树木功能性状，以此检验树木生产力、功能性状同一性与功能性状差异性对土壤碳浓度的影响，以及土壤微生物生物量与微环境条件在此过程中的介导作用。
分析结果显示，土壤微生物生物量与土壤碳浓度之间存在显著正相关关系。此外，土壤碳浓度随树木生产力与树木根径的增加而升高，随凋落物C:N比的升高而降低。进一步研究发现，树木生产力与功能性状（如根系真菌共生属性、凋落物C:N比）可调控微环境条件，进而对土壤微生物生物量产生显著影响。
本研究还指出，未来实验与人工林经营中应纳入土壤历史状况与地形因素考量：实验初始阶段碳浓度较高的样地，其当前土壤碳浓度也更高，而初始碳浓度本身受地形的强烈调控。
综上，本研究结果表明，量化不同土壤碳库对于解析微生物群落-土壤碳储量关系及其对树木多样性与微环境条件的依赖性至关重要。