Abiotic Nitrate Retention in Agroecosystems and a Forest Soil

Agricultural ecosystems are the primary source of nitrate (NO 3 − ) pollution in many watersheds. We used a laboratory 15 N tracer study with live and gamma irradiated soils to examine the role of land management in abiotic retention of NO 3 − in three agroecosytems and a forest soil. Nitrogen retention was defined as the quantity of 15 N that remained in soil following salt extractions. Land management only affected N retention after 15 min of incubation, with forested soils retaining <4% of the added 15 N and agricultural soils retaining <1%. In all four ecosystems, abiotic retention (retention in gamma irradiated soils) occurred after 1 d (15–20% of the 15 N) and 21 d (up to 60% of 15 N) of incubation. After 1 and 21 d, 15 N retention in irradiated soils was larger than in live soils, revealing a methodological artifact associated with gamma irradiation. While our results show that abiotic reactions can occur in the laboratory, it is unlikely that such high rates of abiotic 15 N retention occur in the field. Gamma irradiation appears to impart methodological artifacts that limit its use in partitioning microbial and abiotic N retention in soils.