Nitrogen Mineralization and Nitrification in a Cretaceous Shale and Coal Mine Spoils

Abstract

Abstract Knowledge of rates and amounts of N mineralization in geologic strata is needed to predict N availability to plants when these materials are disturbed by mining or construction and left as surface or subsurface plant‐growth media. Laboratory incubations were conducted to determine ammonification and nitrification rates in a Cretaceous shale, two strip‐mine coal spoils, and a soil from a strip mine in northwestern Colorado. The total N concentration in the soil (1193 ppm), a vegetated spoil (1,083 ppm), and the shale (1112 ppm) were similar, while that of the fresh spoil was lower (730 ppm). No net N mineralization was measured in the shale, and less than 6 ppm NO 3 ‐ ‐N accumulated in the fresh spoil during a 168‐day incubation. However, net mineralization of 49‐ppm NO 3 ‐ ‐N was found in the vegetated spoil and 75‐ppm NO 3 ‐ ‐N in the soil during the same incubation period. Nitrification of added NH 4 + (60 ppm NH 4 + ‐N) was measured in the shale, spoils, and soil. Accumulated levels of NO 3 ‐ ‐N were 100 ppm in the vegetated spoil and 117 ppm in the soil after a 168‐day incubation. However, only 36‐ppm NO 3 ‐ ‐N accumulated in the fresh spoil and no NO 3 ‐ ‐N accumulated in the shale during the same incubation period. Rates and total amounts of CO 2 evolution from the geologic materials resembled that from the soil, indicating considerable heterotropic microbial activity in these geologic materials. The addition of NH 4 + did not significantly affect the rate or amount of CO 2 evolution from the geologic or soil materials, indicating that heterotropic microorganisms were capable of using the forms of N present in the geologic materials.