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.