Predicting soil nitrogen dynamics after incorporating cereal cover crop residues

Authors: GEISSELER, DANIEL - University Of California; HARMON, MEGAN - University Of California; SMITH, RICHARD - University Of California - Cooperative Extension Service; Brennan, Eric

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Cover cropping is a best management practice to protect the groundwater from nitrogen leaching in high-value vegetable production systems. Cover crops can also provide many other benefits such as improving soil quality or health, suppressing weeds, and diversifying crop rotations to reduce soil borne diseases. This study includes several incubation experiments where cover crops residues of rye and triticale of various growth stages were mixed into the soil to understand the decomposition of the cover crops and the release of nitrogen to the soil. The growth stages included cover crops that were younger with more green tissue versus those with older tougher tissues when the plants were closer to maturity. The data we collected from this study allowed us to develop a model to predict how the cover crops decompose and release nitrogen in the cover crops back to the soil where the nitrogen can be used by vegetables. This model will help farmers in the central coast region to make fertilizer decisions in fields following vegetable cover cropping.

Technical Abstract: Recently adopted regulations for water resource protection on California’s Central Coast incentivize cover-cropping, creating a need for simple, reliable tools that determine soil nitrogen (N) dynamics after the incorporation of cover crop residues. Three microcosm experiments were conducted using common winter cover crop shoot residues (Pacheco triticale and Merced rye) to determine the impacts of residue carbon to N (C:N) ratio, which ranged from 9.1 to 42.9, and temperature on net N turnover (mineralization and immobilization) rates over time, as well as to determine the impacts of temperature on gross N turnover rates. Residue C:N ratio was found to be inversely correlated with N mineralization, while temperature was found to be positively correlated with N mineralization. This data was further utilized to model N dynamics of cover crops terminated at different growth stages, revealing the best predictor of N turnover to be residue biomass and C:N ratio, rather than a residue’s cell wall components (lignin, cellulose, hemi-cellulose). Our results indicate that few inputs are required to reliably simulate N release from incorporated cereal cover crop residue, providing a framework for the creation of future support tools that will guide Central Coast vegetable growers in their fertilization decisions in fields following cover crops.