Implications of carbon saturation model structures 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 concentration and mineralogy. While C saturation the- ory has advanced our ability to predict soil C stabilization, few biogeochemical ecosystem models have incorporated C saturation mechanisms. In biogeochemical models, C and ni- trogen (N) cycling are tightly coupled, with C decomposi- tion and respiration driving N mineralization. Thus, changing model structures from non-saturation to C saturation dynam- ics can change simulated N dynamics. In this study, we used C saturation models from the literature and of our own design to compare how different methods of modeling C saturation affected simulated N mineralization dynamics. Specifically, we tested (i) how modeling C saturation by regulating ei- ther the transfer efficiency (", g C retained g 1 C respired) or transfer rate (k/ of C to stabilized pools affected N min- eralization dynamics, (ii) how inclusion of an explicit mi- crobial pool through which C and N must pass affected N mineralization dynamics, and (iii) whether using " to imple- ment C saturation in a model results in soil texture controls on N mineralization that are similar to those currently in- cluded in widely used non-saturating C and N models. Mod- els were parameterized so that they rendered the same C bal- ance. We found that when C saturation is modeled using ", the critical C : N ratio for N mineralization from decompos- ing plant residues (rcr/ increases as C saturation of a soil increases. When C saturation is modeled using k, however, rcr is not affected by the C saturation of a soil. Inclusion of an explicit microbial pool in the model structure was neces- sary to capture short-term N immobilization-mineralization turnover dynamics during decomposition of low N residues. Finally, modeling C saturation by regulating " led to simi- lar soil texture controls on N mineralization as a widely used non-saturating model, suggesting that C saturation may be a fundamental mechanism that can explain N mineralization patterns across soil texture gradients. These findings indicate that a coupled C and N model that includes saturation can (1) represent short-term N mineralization by including a micro- bial pool and (2) express the effects of texture on N turnover as an emergent property.