Implications of Carbon Saturation Model Structure for Simulated Nitrogen Mineralization Dynamics.

Carbon (C) saturation theory suggests that soils have a limited capacity to stabilize organic C and that this capacity may be regulated by intrinsic soil properties such as clay content and mineralogy. While C saturation theory has advanced our ability to predict soil C stabilization, we only have a weak understanding of how C saturation affects N cycling.  In biogeochemical models, C and N cycling are tightly coupled, with C decomposition and respiration driving N mineralization.  Thus, changing model structures from non-saturation to C saturation dynamics can change simulated N dynamics.  Carbon saturation models proposed in the literature calculate a theoretical maximum C storage capacity of saturating pools based on intrinsic soil properties, such as clay content.  The extent to which current C stocks fill the storage capacity of the pool is termed the C saturation ratio, and this ratio is used to regulate either the efficiency or the rate of C transfer from donor to receiving pools.  In this study, we evaluated how the method of implementing C saturation and the number of pools in a model affected net N mineralization from decomposing plant residues.  In models that use the C saturation ratio to regulate transfer efficiency, C saturation affected N mineralization, while in those in which the C saturation ratio regulates transfer rates, N mineralization was independent of C saturation.  When C saturation ratio regulates transfer efficiency, as the saturation ratio increases, the threshold C:N ratio at which positive net N mineralization occurs also increases because more of the C in the residue is respired.  In a single-pool model where C saturation ratio regulated the transfer efficiency, predictions of N mineralization from residue inputs were unrealistically high, missing the cycle of N immobilization and mineralization typically seen after the addition of high C:N inputs to soils.  A more realistic simulation of N mineralization was achieved simply by adding a second pool to the model to represent short-term storage and turnover of C and N in microbial biomass.  These findings increase our understanding of how to couple C saturation and N mineralization models, while offering new hypotheses about the relationship between C saturation and N mineralization that can be tested empirically.