Predicting Nitrogen Mineralization in Chaparral Soils

Nitrogen mineralization is a key process in supplying N for plant growth in terrestrial ecosystems. The objectives of this study were to evaluate N mineralization in chaparral soils and to evaluate an N mineralization simulation model. The chaparral soils contained 0.4 to 54 ppm potentially mineralizable N, which is considerably less than typical agricultural soils (generally > 100 ppm potentially mineralizable N). The quality of the organic matter with respect to N mineralization (N mineralization/total N) was low for chaparral soils relative to agricultural soils; this may be due to soil toxins (allelopathic compounds) or a highly resistant organic matter in chaparral soils. The potentially mineralizable N was highly correlated to total soil N ( r 2 = 0.82) but poorly correlated to C/N ratio ( r 2 = 0.07). The simulation model is based on laboratory soil N mineralization curves developed from long‐term soil incubations with adjustments for field soil temperature and moisture. Because of temperature and moisture limitations, simulated field N mineralization rates were only 3 to 27% of optimal rates; environment is as important as substrate quantity and quality in estimating field N mineralization. The simulation model estimates of annual N mineralization in two chaparral stands agreed to within ± 10% with independent estimates of N mineralization developed by solving a system N balance equation. The proposed N mineralization model should be useful for estimating annual soil N availability, which is an important step in developing N fertilizer prescriptions for intensively managed lands.