Validation of spring wheat responses to elevated CO2 , irrigation, and nitrogen fertilization in the Community Land Model 4.5

The Community Land Model (CLM) started to incorporate crop growth models since version 4.0 in 2012. Since then, the crop model in CLM has been evolved remarkably, but some of the key crop growth responses to environmental conditions (such as the elevated CO2) have not been well validated. Here, we set up single point simulations with CLM (version 4.5) and validated spring wheat growth response against the Maricopa spring wheat Free Air CO2 Enrichment (FACE) experiment that consisted of multiyear paired treatments to understand the growth response to elevated CO2, irrigation, nitrogen fertilization, and their interactions. Overall, CLM showed too positive growth response to elevated CO2 but insufficient growth response to irrigation. The overestimated growth response to elevated CO2 may be due to ignoring factors (e.g., leaf traits) that will limit crop growth under elevated CO2. The insufficient response to irrigation is due to CLM simulating lower latent heat flux during April and May, which resulted in higher soil moisture. In response to nitrogen fertilization, CLM underestimated leaf area index increase but overestimated grain yield increase. In terms of energy fluxes, CLM showed decreased latent heat flux in response to elevated CO2 but increased latent heat flux in response to nitrogen fertilization, but the response magnitude was much smaller than the observations. Based on these validations, we summarized further model developments for CLM to better simulate crop growth process.