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SOIL BIOLOGY & BIOCHEMISTRY

Abiotic Nitrate Retention in Agroecosystems and a Forest Soil

Penn State Univ., 2042 Peach Orchard Dr. #317, Falls Church, VA 22043

^* Corresponding author (fricksbe{at}yahoo.com).

Agricultural ecosystems are the primary source of nitrate (NO₃^–) 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 NO₃^– 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.

Abbreviations: 1-d, one day incubations • 21-d, 21 day incubations • 15-min, 15 minute incubations • conventional, conventionally managed agroecosystem according to Penn State recommendations • DON, dissolved organic nitrogen • FST, Farming Systems Trial • legume, organically managed agroecosystem with leguminous cover crops for N source • manure, organically managed system with composted dairy manure for N source • TSP, Tryptic Soy Broth