Fates and Losses of Nitrogen from a Nitrogen‐15‐Labeled Cover Crop in an Intensively Managed Vegetable System

Cover crops are known to decrease leaching of NO − 3 –N during the winter fallow period in vegetable crop systems, but soil N dynamics following cover crop incorporation are not well understood. The ability of microbes and plants to assimilate and retain N from cover crop residue was studied in a sandy soil in a field under intensive vegetable production in California. The purpose was to describe changes in soil responses and 15 N fates after adding a 15 N‐labeled cover crop under field management conditions. Fresh residue (478 g dry weight m −2 ) of 15 N‐labeled phacelia ( Phacelia tanacetifolia Benth.) (C/N of 19) was added to miniplots contained within large cylinders. Microbial biomass and NO − 3 –N increased rapidly, then began to decline 2 wk later. Microbial biomass C declined faster than microbial biomass N. Only a small amount of 15 N was ever found in microbial biomass, but NO − 3 –N was enriched with substantial 15 N. Percentage recovery of the added 15 N after 4 mo was 60.7% as soil organic N; 20.7% in plants; 1.4% as inorganic N; 1.4% in microbial biomass; 4.7% in ion exchange resin (IER) bags, leached below a depth of 60 cm; and 11.1% as unexplained loss. Losses of 15 N during the first lettuce ( Lactuca sativa L.) crop after the cover crop were relatively low, most likely due to low rainfall and appropriate scheduling of fertilizer and irrigation. Total soil 15 N at 0‐ to 30‐cm depth declined for the first 7 mo, and thereafter cover crop N was apparently no longer readily mineralizable. Microbes may have assimilated C from the plant residue, met their N demand mainly with soil‐derived N, and released cover crop‐derived N that was rapidly mineralized and nitrified. The resulting NO − 3 –N was either taken up by plants, leached, or denitrified. Proper management of water and fertilizer inputs after incorporation of low C/N plant material is important for avoiding N loss before plants are established, especially since NO − 3 –N is readily available, and microbes do not retain much of the cover crop N in this intensively managed soil.