Fate of Fall and Spring Applied 15N-Labeled Aqueous Ammonia in Porous, Buried Bags

Application of nitrogen fertilizers, particularly anhydrous NH₃ is frequently practiced in the northern sections of the Corn Belt to spread out the work load and take advantage of lower fertilizer prices. However, nitrogen losses through nitrification of the fertilizer followed by leaching or denitrification have been a continuing concern. The work described here was designed to evaluate the fate of Fall and Spring applied NH₃ under field conditions using ¹⁵N labeled fertilizer. Because of limitations in the amount of labeled fertilizer available, and to minimize sampling problems, small porous bags (125 cm³) were filled with soil, injected with ¹⁵N aqueous NH₃ and were placed either under fallow or under corn (Zea mays L.) on a Piano silt loam soil. The bags were retrieved at intervals and the soil and plants analyzed for ¹⁵N. Recovery of Spring applied NH₃ in the corn plants was between 58 and 69%, and about 7% remained in the soil, primarily as organic N and fixed NH₄. Recovery decreased with an increase in the amount of N applied. About half of the Fall applied fertilizer N incorporated into the organic-N fraction was released the following spring, but fertilizer-derived organic-N from Spring-applied NH₃ was largely unavailable to soil microorganisms. From 60 to 80% of the Spring applied fertilizer N fixed by day was released during cropping. The major pathway for loss of fertilizer N was presumed to be either leaching or denitrification. Some fertilizer-derived (NO₂ + NO₃)-N was leached as much as 60 to 75 cm below the point of placement by the following Spring. At this depth it would be unavailable to the following crop until the roots had penetrated deeply. The results suggest that fall application of NH₃ is not desirable, from an agronomic or an environmental point of view, because of potential N fertilizer losses, particularly by leaching. At least in the soil studied, Fall application has an advantage in that less fertilizer N is retained in an unavailable form.