How does nitrogen control soil organic matter turnover and composition? – Theory and model

Abstract. Nitrogen (N) enrichment triggers diverse responses of different soil organic carbon (SOC) pools, but a coherent mechanism to explain them is still lacking. To address this, we formulated several hypothesized N-induced decomposer responses in a dynamic soil model (irrespective of plant responses), i.e., decomposition retardation under increasing N excess and stimulation under decreasing N-limitation, N-responsive microbial turnover and carbon use efficiency (CUE), and a priming effect driven by changing microbial biomass. To evaluate their relevance on SOC turnover, they were incrementally combined into multiple model variants, and systematically tested against data from meta-analyses of N addition experiments and SOC fraction data from contemporary temperate forests spanning wide environmental gradients. Our results support the hypothesis that N directly influences multiple C pools by changing decomposition and microbial physiology, which are in turn driven by stoichiometric imbalances. Under N addition, only the model variants that incorporated both (1) decomposition retardation with increasing N-excess and (2) decomposition stimulation with decreasing N limitation were able to qualitatively reproduce the common observation of a greater increase of surface organic layer (LFH) relative to topsoil SOC, and of particulate organic carbon (POC) relative to mineral-associated carbon (MAOC). We attributed this to the accelerated decomposition of N-limited detritus by N addition, thereby supplying processed C to intermediate pools (i.e., POC and FH organic horizon). In addition, excess N retarded the decomposition of these processed C and MAOC that have lower C:N ratios. This concurrently explains the organic horizon and POC accumulation under contemporary N deposition in temperate forests, albeit with smaller effect sizes than in N addition experiments. Furthermore, incorporating N-responsive microbial turnover and CUE helped reproduce microbial biomass reduction, and improved the modelling of microbial biomass C:N homeostasis and hence, the estimation of microbial N-limitation and excess in turn. Collectively, our model experiment provided robust mechanistic insights into the stoichiometric control of soil N-C interaction. We recommend our simple model for further testing and incorporation into other soil CN models.