Institute for Arctic and Alpine Research, University of Colorado, Boulder, Boulder, Colorado, USA.
Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York, USA.
Ecol Appl. 2022 Jun;32(4):e2528. doi: 10.1002/eap.2528. Epub 2022 Mar 21.
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 NH and NO ), from the literature to evaluate the N cycles in the land model components of two ESMs. The two land models evaluated here, E3SM Land Model version 1 (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 approximately two-fold to three-fold differences in their underlying global mineralization, immobilization, plant N uptake, nitrification, and denitrification fluxes. Both models dramatically underestimate the immobilization of NO by soil bacteria compared with literature values and predict dominance of plant uptake by a single form of mineral nitrogen (NO for ELM, with regional exceptions, and NH for CLM5.0). CLM5.0 strongly underestimates the global ratio of gross nitrification:gross mineralization and both models are likely to substantially underestimate the ratio of nitrification:denitrification. Few experimental data exist to evaluate this last ratio, in part because nitrification and denitrification are quantified using different techniques and because denitrification fluxes are difficult to measure at all. More observational constraints on soil nitrogen fluxes such as nitrification and denitrification, as well as greater scrutiny of the functional impact of introducing separate NH and NO pools into ESMs, could help to improve confidence in present and future simulations of N limitation on the carbon cycle.
地球系统模型（ESMs）已经实施了氮（N）循环，以说明陆地碳吸收对 N 的限制。然而，在 ESM 中表示 N 的输入、损失和再循环是具有挑战性的。在这里，我们使用文献中全球关键土壤 N 通量（包括硝化、反硝化、矿化、淋溶、固定和植物吸收（NH 和 NO ）的速率和比例）来评估两个 ESM 中陆地模型组件的 N 循环。这里评估的两个陆地模型，E3SM Land Model version 1（ELMv1）-ECA 和 CLM5.0，来自于一个共同的模型，但在其对植物-微生物竞争土壤 N 的表示上已经出现分歧。这两个模型预测了相似的全球总初级生产力（GPP）速率，但在其基础的全球矿化、固定、植物 N 吸收、硝化和反硝化通量方面存在大约两倍到三倍的差异。这两个模型都大大低估了土壤细菌对 NO 的固定作用，与文献值相比，并且预测了单一形式的矿物氮（ELM 中的 NO ，但有区域例外，以及 CLM5.0 中的 NH ）对植物吸收的主导作用。CLM5.0 强烈低估了总硝化作用：总矿化作用的全球比值，这两个模型都可能大大低估了硝化作用：反硝化作用的比值。几乎没有实验数据可以评估最后一个比值，部分原因是硝化作用和反硝化作用是使用不同的技术来量化的，而且反硝化作用通量根本难以测量。对土壤氮通量（如硝化作用和反硝化作用）的更多观测约束，以及对将单独的 NH 和 NO 库引入 ESMs 的功能影响的更严格审查，有助于提高对当前和未来对碳循环中 N 限制的模拟的信心。
