Biological Cycling of 15N-Labeled Fertilizer Nitrogen in Lignite Minesoil Materials

Soil profile and overburden core materials from a lignite minesite in east central Texas were amended with ¹⁵N-labeled Ca(NO₃)₂ fertilizer and cropped to forage grasses in the greenhouse to determine their relative potential for supporting an active biological N cycle. Soils and overburden materials were sampled before and after cropping and analyzed for distribution of total N and fertilizer ¹⁵N in various chemical compartments. Several soil properties which affect N cycling were also measured. Plant tops from several harvests plus roots and crowns from the final harvest were analyzed for total N and ¹⁵N content. Results were compared among soil and grass treatment combinations, for uncropped and cropped samples, to determine any differences in N behavior, particularly between surface (A horizon) soils and subsurface strata materials. Nitrogen fertilizer recovery by plants averaged 65% of that applied. Approximately 21% of ¹⁵N was unaccounted for and assumed lost through denitrification by the end of the cropping period. There were no significant differences in either measurement due to soils or grasses. There were significant differences (p = 0.05) in amounts of ¹⁵N remaining in soils after cropping. Total indigenous soil N was composed of chemically more stable organic forms in the subsurface materials, with generally higher C:N ratios than in the surface soils. Distribution of the more recently-incorporated fertilizer ¹⁵N among soil N forms was similar in surface and most subsurface materials, with a greater proportion found in chemically less stable forms. The acid lignitic clay was the one exception in that ¹⁵N distribution within the acid-soluble fractions was different from that of other soils. The amount of indigenous soil N mineralized during aerobic incubation was significantly greater in surface than subsurface samples, measured either before or after cropping. Percent mineralization of immobilized fertilizer ¹⁵N was greater on the average than that of total soil N residual. The relative amounts of ¹⁵N mineralization among soils was no different, however, than that for indigenous soil N, even though the ¹⁵N was similarly incorporated into less stable forms in each. Apparently differences in the C:N ratio were strongly determinant of differences in ¹⁵N disposition observed among soils. The measured tendancy for the C:N ratio to decrease with cropping demonstrated the potential to increase N incorporation and cycling through proper fertilizer management on minesoils constructed from materials evaluated in this study.