Modeling surface residue decomposition and N release using the Cover Crop Nitrogen Calculator (CC-NCALC)

Authors: THAPA, RESHAM - North Carolina State University; CABRERA, MIGUEL - University Of Georgia; REBERG-HORTON, CHRIS - North Carolina State University; DANN, CARSON - University Of Georgia; Balkcom, Kipling; Fleisher, David; GASKIN, JULIA - University Of Georgia; HITCHCOCK, RICHARD - University Of Georgia; PONCET, AURELIE - University Of Arkansas; Mirsky, Steven; Schomberg, Harry; Timlin, Dennis

Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2023
Publication Date: 8/11/2023
Citation: Thapa, R., Cabrera, M., Reberg-Horton, C., Dann, C., Balkcom, K.S., Fleisher, D.H., Gaskin, J., Hitchcock, R., Poncet, A., Mirsky, S.B., Schomberg, H.H., Timlin, D.J. 2023. Modeling surface residue decomposition and N release using the Cover Crop Nitrogen Calculator (CC-NCALC) . Nutrient Cycling in Agroecosystems. https://doi.org/10.1007/s10705-022-10223-3.
DOI: https://doi.org/10.1007/s10705-022-10223-3

Interpretive Summary: Cover crops adoption has gained traction among no-till farmers to improve soil and water quality, conserve soil moisture, suppress weeds, and for offsetting nitrogen (N) fertilizer inputs. In no-till fields, the efficacy of cover crops to provide these benefits depends on how long the residues persist on the soil surface (the rate of residue decomposition) and N release. To assist farmers and agricultural professionals regarding their residue and fertility management decisions, various models have been calibrated and later converted into decision support tools. One such publicly available tool is the Cover Crop N Calculator (CC-NCALC). While the CC-NCALC effectively simulates decomposition and N release from incorporated residues (i.e., conventional tillage systems), it tends to overpredict decomposition and N release from residues on the soil surface (i.e., no-till systems). We adapted the CC-NCALC for no-till cropping systems by adjusting the model's decomposition rates based on residue environmental conditions (residue moisture and temperature), N availability (C:N ratio), and residue mass in contact with the underlying soil. The modified CC-NCALC was calibrated and validated using 99 site-years of on-farm litter bag decomposition studies from across the mid-Atlantic and Southeastern US states. Calibration resulted in a good agreement between measured and modeled residue mass and N remaining on the soil surface over time. The calibrated, modified CC-NCALC is publicly accessible for use by farmers, agricultural professionals, researchers, and policy makers. Farmers using no-till systems can now use the model to shape their residue and N management decisions. The tool will allow growers to optimize cover crop benefits and hence, make their cropping systems more sustainable, eco-friendly, and profitable.

Technical Abstract: Optimal utilization of cover crop (CC) residues in no-till cropping systems require fast-running crop-soil simulation models that can accurately predict surface residue decomposition through time, which in turn determines both nitrogen (N) availability for subsequent cash crop and the longevity of residue cover for effective soil protection, soil moisture conservation, and weed suppression. However, existing models either have long execution times or do not consider the environmental variables to which surface residues are exposed. As a result, these models are not practical for a decision support tool to be used by producers. An improved surface residue water potential (residue) module that provides fast estimates of hourly residue using easily available weather information was developed and integrated into the existing ‘Cover Crop N Calculator (CC-NCALC)’. Specific dynamics of surface residue decomposition were accounted for by adjusting decomposition rates based on residue and temperature dynamics, N limitations, and fractional residue mass in contact with the soil. The modified CC-NCALC model was calibrated and validated using on-farm litter bag decomposition data collected across 99 site-years during 2017-2019 from no-till corn (Zea Mays L.) systems in the mid-Atlantic and Southeastern USA. Both residue mass [calibration: root mean square error (RMSE)=403 kg ha-1, Willmott’s index of agreement (d)=0.98; validation: RMSE=483 kg ha-1, d=0.97] and N (calibration: RMSE=9.1 kg ha-1, d=0.93; validation: RMSE=15 kg N ha-1, d=0.93) remaining on the soil surface over time were simulated reasonably well by the modified CC-NCALC model. Accurate accounting of leaching and gaseous losses from high-quality CC residues (i.e., > 5% N) and initial N immobilization from poor-quality CC residues could further improve model simulations. We propose that the modified CC-NCALC model can be used as a decision support tool to help inform farmers and land managers regarding their residue and N management decisions in CC-based no-till cropping systems.