Landscape Position and Buffer Effects on Groundwater Nitrogen.

Landscape Position and Buffer Effects on Groundwater Nitrogen  

Nitrate, from both natural and anthropogenic sources, is possibly the most widespread contaminant in groundwater. The main objective of this study was to measure the effects of slope position, on NO₃-N and total N (TN) transport to groundwater beneath two land cover treatments. The experimental design consisted of two, rotationally grazed small watersheds with Menfro silt loam (fine silty, mixed superactive, mesic Typic Hapludalfs). Land cover in one watershed was tall fescue (Festuca arundinacea Schreb.), red clover (Trifolium pretense L.), and lespedeza (Kummerowia stipulacea Maxim.) and the other one had the same pasture composition plus a row of poplar trees (Populus tremula L.) at the foot slope. In each watershed, a transect of wells was installed across a catena sequence from summit to the foot slope-position. Soil core samples collected during well installation were analyzed for texture, total and inorganic N and P forms. Groundwater samples were collected weekly from piezometers and analyzed for NO₃-N and TN concentration. Average NO₃-N concentrations ranged from 0.06 to 3.05 ppm, and average TN concentrations ranged from 0.44 to 3.35 ppm, with the lowest concentrations in the foot slope well. Comparisons between land cover for the foot slope wells showed that average NO₃-N concentrations were 0.007 ppm in the pasture with poplar trees and 0.08 ppm in the pasture only treatment. Average TN concentrations were 0.31 ppm in the pasture with poplar trees and 0.48 ppm in the pasture only watershed. Results also showed decreasing NO₃-N and TN concentrations with decreasing depth to the water table. These results indicated that denitrification was the likely reason for the extremely low NO₃-N concentrations in the foot slope wells and that uptake of NO₃^- by poplar trees could further reduce NO₃-N contamination in groundwater compared to the pasture only treatment.

 Key words: Cattle grazing, Deep wells, Menfro soils, Nitrate