Nitrogen in soils, plants, surface water and shallow groundwater in a bahiagrass pasture of Southern Florida, USA

Despite substantial measurements using both laboratory and field techniques, little is known about the spatial and temporal variability of nitrogen (N) dynamics across the landscapes, especially in agricultural landscapes with cow-calf operations. This study was conducted to assess the comparative levels of total inorganic nitrogen, TIN (NO₃-N + NH₄-N) among soils, forage, surface water and shallow groundwater (SGW) in bahiagrass (Paspalum notatum, Flueggé) pastures. 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 spring and fall of 2004, 2005 and 2006, respectively. Bi-weekly (2004-2006) groundwater and surface water samples were taken from wells located at TS, MS, and BS and from the run-off/seepage area (SA). Concentrations of NH₄-N, NO₃-N, and TIN in SGW did not vary with landscape position (LP). However, concentrations of NH₄-N, NO₃-N, and TIN in water samples collected from the seep area were significantly (P ≤ 0.05) higher when compared to their average concentrations in water samples collected from the different LP. Average concentrations of NO₃-N (0.4-0.9 mg l⁻¹) among the different LP were well below the maximum, of 10 mg l⁻¹, set for drinking water. The maximum NO₃-N concentrations (averaged across LP) in SGW for 2004, 2005 and 2006 were also below the drinking water standards for NO₃-N. Concentration of TIN in soils varied significantly (P ≤ 0.05) with LP and soil depth. Top slope and surface soil (0-20 cm) had the greatest concentrations of TIN. The greatest forage availability of 2,963 ± 798 kg ha⁻¹ and the highest N uptake of 56 ± 12 kg N ha⁻¹ were observed from the TS in 2005. Both forage availability and N uptake of bahiagrass at the BS were consistently the lowest when averaged across LP and years. These results can be attributed to the grazing activities as animals tend to graze more at the BS. The average low soil test value of TN (across LP and soil depth) in our soils of 10.9 mg kg⁻¹ (5.5 kg N ha⁻¹) would indicate that current pasture management including cattle rotation in terms of grazing days and current fertilizer application (inorganic + feces + urine) for bahiagrass pastures may not have negative impact on the environment.