The Impacts of Natural N Enrichment On Soil Organic Matter Dynamics On Nutrient Poor Sands.

Soil organic matter dynamics are very tightly controlled by the amount of available N.  Enrichment of N stocks through the presence of symbiotic N fixers has a significant impact on the characteristics and kinetics of soil organic matter.  Our study focuses on Sand Ridge State Forest (SRSF), a 3000 ha. mixed species series of plantations near Manito, Illinois. The area is a glacial floodplain from the late Woodfordian era consisting of alluvial deposits. SRSF was converted from a sand prairie to a plantation in the 1930s. Our studies to date have focused on the Pinus resinosa (red pine) and Pinus strobes (eastern white pine) stands on the Bloomfield-Plainfield association loamy sands.  Some areas were intermixed with black locust at the time of planting to establish a wind break and increase site fertility; these areas now have a mix of planted and volunteer black locust of varying ages. Sites interspersed with black locust were chosen because it offered a natural fertility gradient while controlling for land-use history and soil type. The effects of black locust on soil biota are a confounding factor, however, as it can alter not only soil fertility, but also other soil biological and chemical parameters. Soil organic matter was sampled across the sites to a depth of 25 cm.  As expected, soil C and N content was significantly greater under black locust than pine. The C turnover rates as determined by long term incubations were greater under black locust than pine. Microbial community composition was also affected by the presence of black locust. Further characterization of organic matter by examining the molecular structure of the C constituents using MiDIR on this site will provide detailed more complete understanding of soil organic matter chemistry and allow that understanding to be linked to kinetics.  This level of detail is necessary to model the impacts of natural N enrichment on nutrient poor soils and help understand the implications of N addition studies to increase C stocks in the face of climate change.