Climate impacts on belowground N cycling, N2O fluxes, and N leaching: Development of a mechanistic N reactive transport model in CLM

Nitrogen (N) availability is a dominant factor controlling the terrestrial carbon response to climate and climate change. Further, N₂O from managed and unmanaged ecosystems is an important greenhouse gas that is not, or poorly, represented in global climate models. Here we describe the development, testing, and application of a vertically-resolved, multi-phase reactive transport N cycle model integrated in CLM4, the land-surface model in the Community Earth System Model (CESM). This model development builds on our integration of vertically-resolved soil C dynamics in CLM by including process representations of nitrification, denitrification, microbial activity, leaching, equilibrium partitioning between aqueous and gaseous phases, sorption, and exchanges with the atmosphere. We first present tests of the belowground reactive transport model against several agricultural datasets with varying levels (0 - 250 kg N ha^-1) and types (NH₄NO₃; (NH₄)₂SO₄; CO(NH₂)₂; KNO₃) of fertilizer application, water additions, and plant types. Comparisons are also made to N cycle observations in unmanaged forests and grasslands and to two atmospheric N₂O inversions. We analyze the sensitivity of our regional predictions to uncertainty in N deposition and fertilizer application, belowground model parameters, and model representation of plant N processes. Finally, we present an analysis of the impact of incorporation of these N cycling processes on soil C dynamics.