Restoration of nitrogen and carbon cycling in an Appalachian mine spoil

Abstract

Nitrogen deficiencies have long been acknowledged as a factor limiting the restoration of ecosystems destroyed by surface mining in the Appalachian Region of the U.S. The fundamental ecological structure and function common to intact terrestrial ecosystems are largely lacking in mine soils. Reliable guidelines for effective long-term restoration require a detailed understanding of the ecological processes occurring within the mine-soil system. The objective of this study was to determine the extent to which inorganic N fertilization, native topsoil replacement, or whole-tree wood-chip amendment affected the restoration and reforestation of an Appalachian mine-soil system through changes in C and ~ dynamics. Eighteen concrete tank lysimeters filled with mine spoils served as experimental microcosms to test hypotheses set forth in this study. Treatment effects on soil N and C pools, herbaceous biomass production, N uptake, N fluxes between pools, net leachate N losses, and early growth of pitch x loblolly hybrid pines were evaluated at regular intervals between July 1987 and October 1989. Inorganic N fertilization increased aboveground herbaceous biomass yield and N uptake by 87 and 71%, respectively, during the first growing season, but did not significantly affect yield or N uptake thereafter. During the first growing season, biomass production was 38% higher in the topsoil-amended mine soil than the unamended control. This resulted in an additional 17.4 kg N ha-1 sequestered in comparison to the control. Biomass yield was 270 and 19% lower in the wood-chip-amended mine soil than the unamended control after the first and third growing seasons, respectively. This resulted in 63 and 25% less N uptake, respectively, than the control. Survival of pitch x loblolly pine after two growing seasons was 90% in the N-fertilized mine soil and 71% with the fertilizer control treatment. This difference in survival was the result of lower water potential in the unamended mine soil during the growing season in which the trees were planted. Nitrogen fertilization did not significantly affect tree growth or nutrition. Pine survival after two growing seasons was 83, 98, and 60% for the unamended control, wood-chip, and topsoil treatments, respectively. By the end of the second growing season, the wood-chip treatment also resulted in greater tree height, ground-line diameter, and stem-volume index by 30, 49, and 203% respectively, when compared to the control. Increased survival and growth in the wood-chip-amended mine soil were directly related to higher soil water potential than the control or topsoil treatments. Total inorganic N leaching loss from N-fertilized mine soil was 47.64 kg ha-1 yr-1 higher than the control during the first growing season. However, N fertilization losses were not significantly higher during the remainder of the study period. Drainage was significantly higher during all three growing seasons in the wood-chip-amended mine soil. This resulted in lower N sequestering during the third growing season when precipitation was most abundant. Topsoil amendment did not significantly affect N leaching losses. Inorganic N fertilization did not significantly affect total organic C, total N, or N availability indices in the mine soil. Following topsoil addition, mine-soil total N was 294% higher than the unamended control. Wood-chip effects on the soil organic-matter pool were more gradual; however, by the end of the study, total N and total organic C were 18 and 95% higher, respectively in the wood-chip-amended mine soil than in the unamended control. Aerobic incubation of soil samples collected near the end of the second growing season showed that the topsoil and wood-chip amendments increased the N mineralization potential by 101 and 55%, respectively, in comparison to the unamended control. Furthermore, the mineralization rate constant of the wood-chip-amended mine soil was 44% lower than the control. This shows a slower rate of N turnover and more stable mine-soil N pool with the wood-chip treatment. This study shows that inorganic N fertilizer effects on N and C dynamics were rapid but transient. In contrast, the surface-applied amendments of native topsoil and whole-tree wood chips improved the potential for successful restoration of forests by increasing the N cycling capacity of the developing mine-soil system.