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

Abstract

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 15N fates after adding a 15N‐labeled cover crop under field management conditions. Fresh residue (478 g dry weight m−2) of 15N‐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 15N was ever found in microbial biomass, but NO−3–N was enriched with substantial 15N. Percentage recovery of the added 15N 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 15N 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 15N 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.