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

Optimal utilization of cover crop (CC) residues in conservation tillage 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 environmental variables to which surface residues are exposed. As a result, these models are not practical as a decision support tool used by producers. An improved surface residue water potential ( ψ residue ) module that provides fast estimates of hourly ψ residue using easily available weather information was developed and integrated into the existing 'Cover Crop Nitrogen Calculator (CC-NCALC)'. Specific dynamics of surface residue decomposition were accounted for by adjusting decomposition rates based on ψ residue and temperature dynamics, N limitations, and fractional residue mass in contact with the soil. The modified CC-NCALC tool was calibrated and validated using on-farm litter bag decomposition data collected across 99 site-years during 2017–2019 from conservation tillage-based 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, relative RMSE (rRMSE) = 27%, Willmott's index of agreement (d) = 0.98; validation: RMSE = 483 kg ha⁻¹, rRMSE = 33%, d = 0.97] and N (calibration: RMSE = 9.1 kg ha⁻¹, rRMSE = 34%, d = 0.93; validation: RMSE = 15 kg N ha⁻¹, rRMSE = 48%, d = 0.93) remaining on the soil surface over time were simulated reasonably well by the modified CC-NCALC tool. 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 estimates. We propose that the modified CC-NCALC tool 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 conservation tillage systems. [ABSTRACT FROM AUTHOR]