Soil carbon and nitrogen mineralization with flexible soil and microbial C:N ratios
Microbial assimilation of carbon and nitrogen (C-N) and the physicochemical protection of soil organic matter (SOM) play fundamental roles in regulating land-atmosphere interactions. However, these microbial and physicochemical processes are not explicitly represented in most regional/global terrestrial ecosystem models, e.g., the Community Land Model (CLM). Fixed C:N ratios in SOM pools are usually adopted in CLM due to the lack of explicit representation of microbial functions. Thus current soil C-N model in CLM configuration may be inadequate to model the effects of litter inputs or fertilization on soil carbon and nitrogen mineralization and linkages between plant litter C:N ratios and soil C:N ratios. We propose a coupled C-N model that allows for flexible C:N ratios in SOM and microbial pools. A mineral-associated organic matter pool is included to account for the physicochemical protection of SOM. Multiple microbial functional groups are incorporated into the Convergent Trophic Cascade (CTC) reaction network to mediate the decomposition and N mineralization/immobilization processes. Microbial dormancy is included to represent microbial physiological states coping with environmental stress, such as substrate and soil moisture availability. Two hypotheses (i.e., N inhibition and C overflow) accounting for mineral N limitation are reconciled into the C-N coupled model.
Our preliminary analysis has shown that the C:N ratios in SOM, dissolved organic matter (DOM), and microbes can be well constrained by the new C-N model and the microbial functional groups can be distinguished from each other. We are using this new model framework to evaluate the soil carbon and nitrogen mineralization processes under conditions of excess organic C, excess organic N, or limited mineral N.