How does nitrogen control soil organic matter composition? – A theory and model

Current soil biogeochemical models have difficulties matching the observed composition of soil organic matter (i.e., the relative proportions of deadwood, raw litter, organic horizon, particulate organic carbon, and mineral-associated organic carbon). In reality, nitrogen (N) controls microbial decomposition and physiological processes, whereas in most models it is merely considered a plant nutrient. In addition, many N fertilization studies have shown that N exerts different effects on different C pools via changing exoenzyme activities, microbial growth, and necromass production via microbial turnover. These divergent effects control SOM composition and have C-cycle consequences.

We expanded the CENTURY model by incorporating multiple hypothesized microbial responses to nitrogen availability, including 1) decomposition reduction of recalcitrant substrates when N is in excess; 2) decomposition stimulation of high C:N substrates when N limitation is alleviated; 3) microbial adaptation of turnover rate; 4) microbial adaptation of CUE; and 5) secondary feedback to decomposition via changes in microbial biomass in response to N. We systematically tested multiple model variants using two sets of simulations, one along a natural N gradient in Swiss forests, and another one with artificially increased N input (i.e., simulating an N-fertilization experiment). We evaluated the simulated outputs using data on soil organic matter fraction stocks, their relative proportions, and temporal responses under N addition.

From the simulation results, we identified the necessary processes to explain the temporal response pattern of different C pools to N addition, in accordance with findings from meta-analyses. In addition, we identified patterns of SOM composition over a natural gradient of N supply (no artificial N addition), which can again be explained by the N-driven processes we implemented. We conclude that considering the direct effects of nitrogen as a key additional constraint on microbial processes is essential to improve the realism and accuracy of soil biogeochemistry models.