Evaluating soil biogeochemistry parameterizations in Earth system models with observations

Soils contain large reservoirs of terrestrial carbon (C), yet soil C dynamics simulated in Earth systems models show little agreement with each other or with observational data sets. This uncertainty underscores the need to develop a framework to more thoroughly evaluate model parameterizations, structures, and projections. Toward this end we used an analytical solution to calculate approximate equilibrium soil C pools for the Community Land Model version 4 (CLM4cn) and Daily Century (DAYCENT) soil biogeochemistry models. Neither model generated sufficient soil C pools when forced with litterfall inputs from CLM4cn; however, global totals and spatial correlations of soil C pools for both models improved when calculated with litterfall inputs derived from observational data. DAYCENT required additional modifications to simulate soil C pools in deeper soils (0-100cm). Our best simulations produced global soil C pools totaling 746 and 978 Pg C for CLM4cn and DAYCENT parameterizations, respectively, compared to observational estimates of 1259 Pg C. In spite of their differences in complexity and equilibrium soil C pools, predictions of soil C losses with warming temperatures through 2100 were strikingly similar for both models. Ultimately, CLM4cn and DAYCENT come from the same class of models that represent the turnover of soil C as a first-order decay process. While this approach may have utility in calculating steady state soil C pools, the applicability of this model configuration in transient simulations remains poorly evaluated. Key Points SOC estimates from ESMs show wide variation and are exceptionally low in CLM4cn After modifications DAYCENT parameterizations provide more realistic SOC pools SOC responses to warming suggest further evaluation of models are warranted