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 dynamic soil CN models integrating several hypothesized N-induced decomposer responses (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 induced by changing microbial biomass. To evaluate the relevance of each response on SOC turnover, they were incrementally combined into multiple model variants, and systematically tested against diverse observations from meta-analyses of N addition experiments and SOC fraction data from forests spanning wide environmental gradients.

Our results support the idea that N directly controls the response of multiple C pools via changing decomposition and microbial physiology. Under N addition, only the model variants that incorporated both the responses of 1) decomposition retardation with increasing N-excess and 2) decomposition stimulation with decreasing N limitation were able to reproduce the common observation of a greater increase of surface organic horizon (LFH) relative to topsoil SOC, and of particulate organic carbon (POC) relative to mineral-associated carbon (MAOC). In addition, cold and warm forests respectively experienced more decomposition retardation and stimulation under N addition. Furthermore, incorporating N-responsive microbial turnover and CUE helped reproduce microbial biomass reduction, and the latter was also critical for microbial biomass C:N homeostasis, which in turn constrained the estimation of N-limitation and excess.

Synthesizing the model findings and literature, we propose that N addition accelerates the decomposition of N-limited detritus, which supplies C to intermediate processed pools (i.e., light fraction C), and retards the decomposition of processed organic matter with lower C:N ratios (both light fraction C and MAOC). This explains the large light fraction C accumulation under N addition or contemporary N deposition in temperate forests. Collectively, our model experiment provided robust mechanistic insights on soil N-C interaction, and challenged the common model assumption of plant being the primary respondent to N. We recommend our simple model for further testing and ecological applications.