The Community Land Model underestimates land-use CO₂ emissions by neglecting soil disturbance from cultivation

Abstract. The Community Land Model (CLM) can simulate planting and harvesting of crops but does not include effects of cultivation on soil carbon decomposition. The biogeochemistry model DayCent does account for cultivation and provides a baseline for evaluating the CLM. With the goal of representing cultivation effects on soil carbon decomposition, we implemented the DayCent cultivation parameterization in the CLM and compared CLM and DayCent simulations at eight Midwestern United States sites with and without the cultivation parameterization. Cultivation decreases soil carbon by about 1350 gC m⁻² in the CLM and 1660 gC m⁻² in DayCent across the eight sites from the first cultivation (early 1900s) to 2010. CLM crop simulations without cultivation have soil carbon gain, not loss, over this period, in contrast to the expected declining trends in agricultural soil carbon. A global cultivation simulation for 1973–2004 reduces ecosystem carbon by 0.4 Pg yr⁻¹ over temperate corn, soybean, and cereal crop areas, which occupy approximately 1/3 of global crop area. Earth System Models may improve their atmospheric CO₂ and soil carbon simulations by accounting for enhanced decomposition from cultivation.