Simulating Soil Organic Carbon Dynamics with Residue Removal Using the CQESTR Model

Concerns about CO2 emissions and fossil fuel supplies have enhanced interest in using crop residues for biofuel production; however, maintaining soil organic C (SOC) through residue return is vital for maintaining soil productivity. Our objectives were to simulate long-term SOC dynamics using CQESTR and to examine the effect of residue harvest on SOC stocks under disking (DT) and conservation tillage (CS). A long-term study in the mid-Coastal Plain region of South Carolina was used to simulate four residue harvest rates (0, 50, 66, and 90%) during two harvest periods. The yearly variation of SOC was predicted well (r2 = 0.84, P < 0.0001). Without residue removal, average increases of 0.10 and 0.39 g SOC kg–1 yr–1 were predicted under DT and CS, respectively, consistent with observed increases of 0.12 and 0.44 g SOC kg–1 yr–1. After 23 yr, simulated SOC stock gain was more than threefold greater under CS than DT (9.0 vs. 2.4 g SOC kg–1). The model predicted 1.86 and 4.47 g SOC kg–1 losses in the top 5 cm of soil under DT and CS, respectively, during 23 yr of 66% residue harvest compared with no residue harvest. The predicted SOC stocks under CS were 5 g SOC kg–1 greater than under DT, however, even with 90% residue harvest. The quantities of crop residue that can be sustainably harvested were directly influenced by the initial SOC concentration and tillage practices. While CS can somewhat mitigate the loss of soil C, residue harvest from loamy sand soils may have an adverse impact on SOC stocks.