Managing symbiotically-fixed nitrogen on mined land for tree crops

Young mine soils constructed following surface-mining for coal contain low levels of organic matter and nitrogen. It was hypothesized that nitrogen-fixing plants could be incorporated into a reforestation system in a manner that would meet the short term and long-term nitrogen needs of crop trees while rebuilding the soil and litter nitrogen pools and restoring a stable nitrogen cycle. The nitrogen status of two interplanting studies containing legume ground covers was examined.

The first study site was mined prior to 1940 and subsequently abandoned. In 1988, the land was reclaimed, and an interplanting study was established in 1990. Pitch x loblolly pines (Pinus x rigitaeda) and eastern white pines (Pinus strobus L.) were interplanted with four nitrogen-fixing species: black alder (Alnus glutinosa L. IGaertn.l), black locust (Robinia pseudoacacia L.}, bicolor lespedeza (Lespedeza bicolor) and autumn olive (Elaeagnus umbel/ata Thwnb.). Pitch x loblolly pine survival was good; however, eastern white pine survival was only fair, averaging 54%. All of the nurse tree species with the exception of black alder had fair survival rates. Black alder survival was extremely low at 19%. After the third growing season, a ground cover dominated by legumes averaged 79% cover. The black mine spoil on the site was extremely rich in nitrogen, averaging 5,115 mg·kg-1. When the black mine spoil was mixed with the native topsoil, a mine soil that was created had an extremely variable total nitrogen content. However, low mineralizeable nitrogen levels suggested that the nitrogen in the mine spoil was not generally available. Pine responses to the four nitrogen-fixing species were compared. Nurse trees had no effect on pine growth after three years. Pine foliar nitrogen levels were adequate across the study site even in the control treatments and in the microsites where few legumes were present. Microsites surrounding 50 pines of each species were studied. The pitch x loblolly pines grew larger at lower pH levels and higher coarse fragment content reflecting the pines' preference for moderately acid soils, coarse-textured soils. Some competition was present between the crop trees and herbaceous cover, but it was not a controlling factor in pine tree growth.