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 abioric 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 similar to 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.