Nitrogen Mineralization of Cover Crop Residue Depends on Carbon to Nitrogen Ratio and Soil Temperature

Groundwater nitrate contamination is largely attributed to fertilizer and intensive livestock manure inputs in agricultural systems. California’s Salinas Valley is an area where state policy is aiming to reduce nitrate leaching. Non-legume winter cover crops can help decrease nitrate leaching by scavenging residual soil nitrogen (N) during winter fallow periods following the cropping season. However, the ability of fall-incorporated cover crops to decrease nitrate leaching and recycle N to subsequent cash crops is unknown. We conducted a 112-d laboratory soil incubation experiment using ‘Merced’ rye (Secale cereale) cover crop shoot biomass, with four carbon (C/N) ratios (10, 14, 19, and 30), at three temperatures (10°C, 15°C, and 20°C). Destructive soil sampling was done at six intervals during the incubation to measure plant-available nitrogen (NO3-N + NH4-N). Rye biomass with the lowest C/N ratio (10:1) had the highest average nitrogen mineralization (Nmin) rate (56%) at the warmest temperature (20 °C). Conversely, biomass with the highest C/N (30) showed net nitrogen immobilization at 10 °C and 15 °C during the incubation, transitioning to net mineralization only at 20°C. We found a linear correlation between soil temperature and percent Nmin (at 112-d) for higher C/N ratios. Furthermore, doubling the soil mineral nitrogen content had a negligible impact on the percent mineralization of the C/N 30 residue. These results provide useful information to help farmers and policymakers understand mineralization dynamics from fall-, winter-, or spring-terminated cereal cover crops.