Soil nutrients and soil carbon storage : modulators and mechanisms

It is well recognized that the capacity of soils to sequester carbon (C) is strongly influenced by nitrogen (N) and phosphorus (P) availability because of the strong stoichiometric links between these biogeochemical cycles. Human disturbance (e.g., deposition, fertilization, and mining), has, and continue to have, caused large imbalances between the biogeochemical cycles and changing nutrient availabilities identified as a key uncertainty in predicting future ecosystem C sequestration. Despite this knowledge, key gaps exist in our understanding of the mechanisms that drive the interactions between C and nutrient cycles, which limit our ability to predict the impacts of change on future soil C sequestration. In this chapter, we discuss N, P, and other nutrients as key modulators of soil C storage. We consider two contrasting mechanistic theories—microbial nutrient mining (MNM) theory and basic stoichiometric decomposition theory—driving these modulators, as well as the impact that environment and land-use change is likely to have, and the expected consequences for soil C storage. Overwhelmingly, experimental evidence from the micro- to global-scale support MNM theory as the main mechanism of the impact of nutrients on soil C storage, and suggest that increasing N availability inhibits enzymes responsible for recalcitrant C degradation and thus promotes long-term C storage in soils. A better mechanistic understanding of the interactions between resource and biomass stoichiometry, microbial nutrient use efficiency, and litter chemistry, as well as improved knowledge of P mineralization, sorption, limitation, and stoichiometry is needed to improve parameterization of N- and P cycling into C-cycling models.