Soil nitrate-nitrogen in forested versus non-forested ecosystems in a mixed-use watershed

Abstract

In tracking nutrients that enter the Gulf of Mexico via the Suwannee Basin, a disproportionate amount of the nitrate-nitrogen (NO 3-N) has been shown to originate in the Santa Fe River Watershed (SFRW). This study investigated soil NO 3-N distributions across the range of land-use and soil order combinations that exist in the SFRW with a focus on comparing NO 3-N levels in forested versus non-forested land-uses. The SFRW consists of 52% forested land-uses (i.e. pine plantation, forest regeneration, upland forest, and forested wetland), 47% non-forested land-uses (i.e. agriculture, rangeland, and urban), and 1% water. Soil samples were collected from four depth intervals (0–30, 30–60, 60–120, 120–180 cm) at 101 to 141 sites with a stratified-random design in six sampling events (Sept. 2003, Jan. 2004, May 2004, Jan. 2005, May 2005, and Sept. 2005). No samples were collected in Sept. 2004 due to flooding associated with two hurricanes. Nitrate-nitrogen was significantly lower in forested than non-forested land-uses across all sampling events, depth intervals, and for profile average data. Within the non-forested land-use category, NO 3-N levels were highest in row crop agriculture and improved pasture sites. In terms of soil order, NO 3-N values were generally highest in Ultisols and Spodosols, but soil order explained less of the variation in the NO 3-N data than did land-use or sampling date. Nitrate-N concentrations were considerably altered by Hurricanes Frances and Jeanne which passed over the SFRW in late summer of 2004. In the post-hurricane sampling events, NO 3-N was significantly lower in both forested and non-forested sites. A year later, however, NO 3-N concentrations in forested sites remained quite low, while concentrations in non-forested sites had begun to increase.