Abiotic reactions play a role in nitrate retention in three agroecosystems

Recent evidence suggests abiotic reactions may rapidly convert inorganic nitrogen to organic forms, at rates comparable to microbial immobilization.  To date, this research has focused on forest soils; however, agroecosystems are more important sources of nitrogen pollution, especially nitrate loss.  Our study examined nitrate processing in four ecosystems at the Rodale Institute:  1) an organically managed agroecosystem fertilized with composted manure, 2) an organically managed agroecosystem using legumes for N fertilization, 3) a conventionally managed agroecosystem, and 4) a forest on a similar soil type near these agroecosystems.

We added K¹⁵NO₃ to sterile (gamma radiation) and live soils from all four systems and measured the quantity of ¹⁵N that became insoluble after minutes, days, and weeks.  In all cases, we found ¹⁵N in the insoluble soil pools, with as much as 60% of our original addition of ¹⁵N in the insoluble pool during the 3 week long sterile incubations.  However, preliminary analyses show no significant differences among the three agricultural systems.   In all the agricultural systems, there was no ¹⁵N found in the insoluble pool in either sterile or live incubations after 15 minutes, while in the forested systems there was about 3% of the labeled N in the insoluble pool in both  live and sterile samples.   Our data suggest abiotic reactions do play a role in transformation of nitrate into insoluble N; yet future studies examining transformation processes, composition and stability of insoluble N species are needed.