4.2 Combining microbial C and N metabolism in the SEM
Root exudates are ubiquitous and quantitatively important drivers of SOM turnover in flooded paddy soil ecosystems. The stoichiometric ratios of resources that are important as energy and nutrients are dominant drivers for the biogeochemical cycles of soil C (McGroddy et al., 2004; Anderson et al., 2005).
There are two stoichiometric processes that combine microbial C and N metabolism of root exudates in paddy soils (Figs. 5, 6, and 7). First, the continuous supply of root exudates caused a C-rich condition, and thus increased microbial activity via overflow respiration of root exudates. Different C/N stoichiometric ratios of root exudates affected the elemental stoichiometry demands for microorganisms to maintain the microbial biomass C/N stoichiometric balance (Haichar et al., 2014; Liu et al., 2020; Du et al., 2020; Zhu et al., 2021). Higher energy demand for microorganisms to acquire organic N from amino acids can further increase respiration. In the present study, the stoichiometric ratios of root exudates (i.e., C-only) for microbe-regulated C mineralization did not dependent on nutrient (N-containing compounds) limitation, as the lack of N content did not affect the allocation of energy through C mineralization. Second, a combination of exogenous C and N from root exudates is required for the adaptation of microbe catabolic activity by the enhancement of C- and N-acquiring enzyme activity and by causing catabolism of soil C (Liu et al., 2020; Mori et al., 2021). This was confirmed by the results of this study, which identified a significant positive relationship between CO2 emissions and enzyme stoichiometry. The stoichiometric ratios of root exudates provide a perspective on microbial stoichiometric requirement of nutrients and leads us to further understand the root exudate–soil–microbe interactions in the rhizosphere soil.
CONCLUSIONS
In this study, we used artificial root exudates with different C/N ratios to clarify the association of elemental stoichiometric ratios in microbial biomass and extracellular enzyme activities with soil C metabolism. We found that the addition of C-only decreased CO2 emissions attribute to the N limitation. Simultaneously, all three different C/N ratios of root exudates (CN6, CN10, and CN80) increased microbial activities and catabolism to meet the microbial biomass ratios, thus promoting SOM mineralization. Microorganisms preferred to use easily available low-molecular compounds that lead to increased CO2 emissions, corresponding with increased stoichiometric ratios of C- and N- hydrolases to meet microbial nutrient demands. It is, therefore, the input of root exudates with low C/N stoichiometric ratio stimulate soil C mineralization, while the inputs with high C/N stoichiometric ratio benefit soil C accumulation. These results highlight the fact that the stoichiometric of root exudates represent important derivers for understanding C cycling in plant-soil system.