ASA Southern Branch 2008 Annual Meeting
February 3-5, 2008
Dallas, TX
Monday, February 4, 2008 - 3:35 PM
Levels of Nitrogen in Soils, Plants and Shallow Groundwater Beneath Bahiagrass-Based Pastures with Cow-Calf Operations.
Gilbert C. Sigua1, Robert K. Hubbard2, and Samuel W. Coleman1. (1) USDA-ARS, Subtropical Agricultural Research Station, 22271 Chinsegut Hill Rd., Brooksville, FL 34601, (2) Southeast Watershed Research Laboratory, USDA-ARS, 2390 Rainwater Rd. P.O. Box 748, Tifton, GA 31793
One of the first steps in assessing soil and water nitrogen levels on any farm is to consider total nitrogen inputs and outputs. Despite substantial measurements using both laboratory and field techniques, little is known about the spatial and temporal variability of nitrogen dynamics across the entire landscapes, especially in agricultural landscapes with cow-calf operations. This study was conducted to assess the levels of total inorganic nitrogen, TIN (NO3-N+NH4-N) in soils, forage and shallow groundwater (SGW) beneath a bahiagrass-based pasture with cow-calf operation. Soil samples were collected at 0-20, 20-40, 40-60 and 60-100 cm across the pasture’s landscape (top slope, TS; middle slope, MS; and bottom slope, BS) in the fall and spring of 2004 to 2006. Forage yield and nitrogen uptake (NU) were also measured from the TS, MS and BS. Groundwater and runoff/surface samples were taken bi-weekly (2004-2006) from wells located at TS, MS and BS and from the seepage area, respectively. Soil’s TIN varied significantly (p≤0.001) with landscape positions (TS > MS = BS) and with soil depths. The higher levels of soil’s TIN at the top of the landscape may have been associated with the presence of animal congregation sites (mineral feeders, water troughs, and shades). Levels of TIN in SGW were not affected by landscape position. The average TIN in 2004 of 1.01±0.2 mg/L was statistically comparable with TIN in SGW in 2005 (0.85±0.2 mg/L) and 2006 (0.88±0.2 mg/L). TIN in surface water (averaged across year) was significantly higher than those of TIN in SGW samples. Forage yield and NU were affected by the interaction of year and landscape position (p≤0.001). The greatest forage yield of 6,842±542 kg/ha and the highest NU of 118±55 kg N/ha were observed from the TS in 2005, respectively. Both forage yield and NU at the BS were consistently the lowest when averaged across landscape positions and years which can be attributed to the grazing activities and preferences of beef cattle across the bahiagrass-based pasture positions.