Fate of Nitrogen-15-Labeled Fertilizer Nitrogen in Revegetated Cretaceous Coal Spoils

Western wheatgrass (Agropyron smithii L.) was grown in a greenhouse study on two Cretaceous coal spoils and a topsoil to describe the short-term behavior of several levels of added ¹⁵N-labeled (NH₄)₂SO₄ (0, 60, 120, and 240 mg N kg⁻¹ soil). Recovery and distribution of fertilizer N was measured at harvest in the soil materials and in the harvested tops, roots, crowns, and rhizomes of the western wheatgrass. Fertilizer N uptake by plants grown in the topsoil increased significantly with increasing fertilizer rate, but a significant increase in aboveground biomass did not occur with the 240 mg N kg⁻¹ treatment. In contrast, aboveground biomass on the two spoils increased significantly with the addition of 60 mg N kg⁻¹, but additional increases in plant biomass with higher fertilizer rates did not occur, and a significant decrease in plant biomass occurred with the 240 mg N kg⁻¹ treatment. Significantly lower water holding capacities and cation exchange capacities resulted in significantly higher NH₄⁺ concentrations in solution in the spoil materials as compared with the topsoil. Thus, suppressed seedling emergence and establishment in the spoil materials was attributed to NH₃ toxicity and/or phytotoxicity due to NO₂⁻ accumulation in the spoil. Substantial fractions of added fertilizer N (65–72%) were immobilized by the plants and into soil organic matter in the topsoil, and only at the highest fertilizer rate did fertilizer N accumulate in mineral form. Under field conditions, this immobilized N should be available for recycling and plant uptake in subsequent years and thus help reestablish a functional N-cycling ecosystem. In comparison, percent recovery of added N decreased in plant biomass and increased in NO₃⁻ form in the two spoils as the rate of fertilizer application increased. Thus, 40 to 50% of added fertilizer N could be susceptible to leaching losses under field conditions.