Nitrogen availability as a master control of plant-derived vs. microbial-derived soil carbon accumulation – insights from a novel model

Current models of soil biogeochemistry are facing difficulties to match the observed amount and composition of soil organic carbon (SOC). An important omission is that nitrogen (N) can directly and interactively affect microbial decomposition processes, rather than merely being a nutrient for plant production and a passive subordinate of C flow based on stoichiometric coupling. In the past decades, many litter manipulation studies have shown that SOC stocks do not increase significantly in response to experimental increases of litter C input. Moreover, many N fertilization studies showed that SOC accrual is predominantly driven by changes in heterotrophic respiration instead of litter production; i.e., soluble N directly affects multiple processes such as the activities of various C-cycling enzymes, microbial growth (e.g., via carbon use efficiency, CUE), and necromass production.

In order to reconcile empirical insights with simulated patterns of SOC and N dynamics, we developed variants of the Century soil submodel coupled with the stand-scale forest gap model ForClim. We implemented equations to capture neglected processes, including 1) the effect of available N on lignin decomposition; 2) the effect of available N on low C:N substrates (e.g., protein) degradation; 3) the effect of available N on microbial CUE dynamics. We tested the full combination of model variants comprising new or alternative equations against multi-level observational patterns, over large gradients of climate and fertility in Swiss forests.

The models reconcile that factors affecting C output (decomposition) predominantly control the site-to-site variation of equilibrium SOC stocks, instead of factors affecting litter C input. We further identified the individual processes essential for shaping the geographical pattern (across different climates, forests, and soil types) of the amount and composition of SOC. We conclude that taking into account the direct effects of available N on microbial processes is essential to improve the realism and accuracy of soil models under global change (i.e., reconciling the mainstream theory of plant-derived vs. microbial-derived SOC accumulation pathways), thus avoiding the need of frequent parameter tuning inherent in “C-centric” models.