Representation of carbon, nitrogen, and phosphorus interactions in the ACME Land Model using the Equilibrium Chemistry Approximation

Abstract

The carbon cycle, and therefore C-Climate feedbacks, of most terrestrial ecosystems are limited by nitrogen (N), phosphorus (P), or a combination of N and P. We present recent results here comparing some common ESM approaches to representing the multi-nutrient, multi-competitor environment and test those approaches with 15N and 32P tracer studies in natural ecosystems. The concepts of Relative Demand (as applied in CLM) and microbial dominance are unable to capture the fundamental patterns of nutrient acquisition observed in these tracer studies. However, the new approach we have integrated in the ACME Land Model (ALM), called the Equilibrium Chemistry Approximation, both qualitatively and quantitatively captures the features of these experimental observations. We describe integration of the ECA approach in ALM, which requires fundamental changes in (1) integration of leaf traits that affect photosynthesis, and their controls by leaf N and P content, (2) mineral surface interactions of P, and (3) explicit representation of root traits for nutrient acquisition. We also present global ALM CNP simulations and evaluation in preparation for the upcoming ACME BGC experiments, which will apply this new ECA approach as one axis of those simulations.