Development and evaluation of the carbon–nitrogen cycle module for the GPFARM-Range model

Rangelands cover approximately 50% of the terrestrial surface of the earth. Consequently, the soil carbon and nitrogen storage and turnover in rangeland systems are becoming increasingly important for sustainable grazing management and climate change. In this study, a carbon–nitrogen (C–N) cycle module from the N Leaching and Environmental Analysis Package (NLEAP) was added into the GPFARM-Range model. This linkage was tested against a 14-year forage dataset (1993–2006) with measurements of C and N in 1993, 2003, and 2006 near Cheyenne, WY. The results showed that the peak standing crop (PSC), and changes in soil organic carbon (SOC) and total soil organic nitrogen (TON) in the rangeland were reasonably simulated by the new GPFARM-Range model. The indices of agreement (d) were >0.85 and the mean bias errors (MBE) were <130 kg ha−1 for the PSC simulation. The SOC and TON in the soil profile were simulated with relative root mean squared error (RRMSE) <5% for the calibration treatment without grazing and <20% for the validation treatments with grazing, both showing no significant bias or error from the observed values. A sensitivity analysis showed that the model responded reasonably to changes in temperature and precipitation. A long-term dataset with more soil C and N measurement events on the rangeland is needed to further test the model, but it is not available at this time. Additional investigation is needed into the adequacy of root-to-shoot ratio approaches to simulate root growth for forage groups in these systems.

► We developed a C–N cycle module for the GPFARM-Range model based on NLEAP.
► The model was tested against a 14-year dataset with different grazing treatments.
► The peak standing crop, SOC, and TON were simulated in a reasonable manner.
► The simulated SOC storage responded reasonably to various climate patterns.