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Nitrification, denitrification, and competition for soil N: Evaluation of two earth system models against observations.

Earth System Models (ESMs) have implemented nitrogen (N) cycles to account for N limitation on terrestrial carbon uptake. However, representing inputs, losses and recycling of N in ESMs is challenging. Here, we use global rates and ratios of key soil N fluxes, including nitrification, denitrification, mineralization, leaching, immobilization and plant uptake (both NH4 + and NO3 - ), from the literature to evaluate the N cycles in the land model components of two ESMs. The two land models evaluated here, ELMv1-ECA and CLM5.0, originated from a common model but have diverged in their representation of plant/microbe competition for soil N. The models predict similar global rates of gross primary productivity (GPP) but have ~2 to 3-fold differences in their underlying global mineralization, immobilization, plant N uptake, nitrification and denitrification fluxes. Both models dramatically underestimate the immobilization of NO3 - by soil bacteria compared to literature values and predict dominance of plant uptake by a single form of mineral nitrogen (NO3 - for ELM, with regional exceptions, and NH4 + for CLM5.0). CLM5.0 strongly underestimates the global ratio of gross nitrification:gross mineralization and both models likely substantially underestimate the ratio of nitrification:denitrification. Few experimental data exist to evaluate this last ratio, in part because nitrification and denitrification are quantified with different techniques and because denitrification fluxes are difficult to measure at all. More observational constraints on soil nitrogen fluxes like nitrification and denitrification, as well as greater scrutiny of the functional impact of introducing separate NH4 + and NO3 - pools into ESMs, could help improve confidence in present and future simulations of N limitation on the carbon cycle.