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 ¹⁵N 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 ¹⁵N 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 ¹⁵N and agricultural soils retaining < 1%. In all four ecosystems, abiotic retention (retention in gamma irradiated soils) occurred after 1 d (15-20% of the ¹⁵N) and 21 d (up to 60% of ¹⁵N) of incubation. After 1 and 21 d, ¹⁵N 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 ¹⁵N 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.