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

Author
item Sigua, Gilbert
item Hubbard, Robert
item Coleman, Samuel
item WILLIAMS, MARY - USDA,NRCS,GAINESVILLE,FL

Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2009
Publication Date: 3/1/2010
Citation: Sigua, G.C., Hubbard, R.K., Coleman, S.W., Williams, M. 2010. Nitrogen in soils, plants, surface water and shallow groundwater in a bahiagrass pasture of southern Florida, USA. Nutrient Cycling in Agroecosystems. 86(2):175-187.

Interpretive Summary: Despite substantial measurements using both laboratory and field techniques, little is known about the spatial and temporal variability of N dynamics across the 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 (BG, Paspalum notatum Flueggé)-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, respectively. Forage yield and N uptake of BG were also measured from the TS, MS, and BS. Bi-weekly (2004-2006) groundwater and runoff/surface samples were taken from wells located at TS, MS, and BS and from the run-off/seepage area (SA). Overall, there was no buildup of soil TIN. Under these management conditions, livestock operations contribute negligible loads of inorganic N to SGW (1.89 %) and surface water (1.4%). The greatest forage yield of 2,963 ± 798 kg/ha and the highest N uptake of 56 ± 12 kg N/ha were observed from the TS in 2005. Both forage yield and N uptake of BG at the BS were consistently the lowest when averaged across landscape positions and years, which can be attributed to the grazing activities. Animals tend to graze more at the bottom slope, leaving more number of animal excretions and this behavior may lead to an increase in the concentration of soil N.

Technical Abstract: Better understanding of soil N dynamics and other crop nutrient changes in pasture-based cow-calf management systems should allow us to better predict potential impact on adjacent surface and ground waters. Despite substantial measurements using both laboratory and field techniques, little is known about the spatial and temporal variability of N dynamics across the entire landscape, 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 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, middle slope, and bottom slope) in the fall and spring of 2004 to 2006, respectively. Overall, there was no buildup of soil TIN in bahiagrass-based pasture. The average TIN gained by soil was reasonably low (11.84 kg/ha) by agronomic/crop requirements. Therefore, results of this study may help to renew the focus on improving fertilizer efficiency in subtropical beef cattle systems, and maintaining a balance of N removed to N added to ensure healthy forage growth and minimize N runoff. Our results indicate that current pasture management including cattle rotation in terms of grazing days and current fertilizer application (inorganic + manures + urine) for bahiagrass pastures offer little potential for negatively affecting the environment. Properly managed livestock (this study) operations contribute negligible loads of inorganic N to shallow groundwater (1.89 %) and surface water (1.4%). The amount of estimated total inorganic nitrogen that was gained by the soil perhaps would suggest that the current recommendations for N might be somewhat low to adequately maintain and sustain growth of bahiagrass. Periodic applications of additional N may be necessary to sustain agronomic needs and to offset the export of N due to animal production. Further studies are needed to determine whether the environmental and ecological implications of grazing and haying in forage-based pastures are satisfied over the longer term.