The Effects of Clear-cutting on Soil and Groundwater Nitrate Pools on Martha’s Vineyard

Abstract: This study examined the mechanisms responsible for high nitrate losses to groundwater from clear-cut forest sites on Martha’s Vineyard, Massachusetts. The clear-cut sites are experimentally managed as part of an effort to restore native grassland. My goal was to determine whether or not a zone of nitrification existed below the surface of the mineral soil of the disturbed sites where excess nitrate was being produced. I also explored whether or not the nitrate in the groundwater could be accounted for by nitrate in precipitation leaching unprocessed through the soil profile. I collected samples throughout the soil profile to a depth of 2 m, and determined the pool sizes of nitrate and ammonium, rates of mineralization, nitrification, and potential nitrification, and carbon to nitrogen ratios. I also compared the δ15N isotopic values of precipitation and groundwater samples, and calculated a nitrate budget for one of the clear-cut sites. I found that the nitrate in the groundwater is produced within the soil profile and cannot be accounted for by nitrate deposited in precipitation. In clear-cut sites, the extent of plant re-growth and plant uptake of nitrate are the primary factors controlling nitrate losses. However, low levels of nitrification do occur below the surface of the mineral soil in the clear-cut sites, and these low rates can account for the total nitrate appearing in the aquifer