Submitted to: Agronomy for Sustainable Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2011
Publication Date: 7/2/2012
Citation: Sigua, G.C., Coleman, S.W., Chase, C.C., Albano, J. 2012. Absence of negative environmental effects of increased soil P levels in cattle congregation zones. Agronomy for Sustainable Development. 32:693-701. Interpretive Summary: Studies on landscape attributes, on slope aspect in particular, have produced contrasting results. Foresters have traditionally viewed south aspects as less productive than north aspects, yet there is substantial evidence that the assumption of lower site productivity on south aspects may be incorrect. Despite substantial measurements using both laboratory and field techniques, little is known about the spatial and temporal variability of phosphorus and nitrogen dynamics across landscape location, especially in agricultural ecosystem with cow-calf operations. There is a continued interest in estimating the grazing-induced changes in soil levels of phosphorus and nitrogen. We hypothesized that cattle congregation site may have higher concentrations of phosphorus and/or nitrogen than in the grazing site and forest site. In order to assess our hypothesis, soil samples were collected during the spring and fall of 2005 to 2007 from three adjoining landscape sites within a beef cattle operation. These sites consisted of three locations: congregation, grazing and forest sites. The objective of this study was to assess the levels of Mehlich-1 extractable phosphorus, total inorganic nitrogen, and soil phosphorus saturation in relation to landscape location in subtropical beef cattle pasture. Averaged across years and soil depth, congregation sites had the highest concentration (mg kg-1) of soil phosphorus (36.1) followed by grazing sites (17.7) and forest sites (8.2). Spatial distribution of total inorganic nitrogen (mg kg-1) across the landscape was in this order: CS (2.3) > FS (0.9) > GS (0.7). The degree of soil phosphorus saturation in the three locations were below the environmental threshold of phosphorus saturation (DPS = 60%), suggesting that phosphorus buildup and/or release is not a predicament anywhere in the landscape, including the congregation and forest sites. These results may have significant implications for the transport of phosphorus and nitrogen to surface water and our ability to predict and model losses of phosphorus and nitrogen from congregation zone or grazing zone of pastures with cow-calf operations.
Technical Abstract: Determining soil nutrient distribution in pasture with beef cattle operation is critical to identifying which area is at risk of nitrogen or phosphorus build-up and loading. Characterizing spatial variability of soil nutrients in relations to landscape location is important for understanding the effects of future land use change on soil nutrients and water pollution. We hypothesized that cattle congregation site may have higher concentrations of phosphorus and nitrogen than in the pasture and grazing site or the adjoining forest site. This study assessed levels of Mehlich-1 extractable P, total inorganic N, and soil P saturation in relation to landscape locations in subtropical beef cattle pasture. Soil samples were collected during the spring and fall of 2005 to 2007 from three 19 adjoining landscape sites that are associated with beef cattle operation. These sites consisted of three locations: congregation, grazing and forest sites. The levels of extractable P, total inorganic N, and P saturation in soils varied with landscape location. Congregation site had the highest concentration of extractable P, of 36.1 mg kg-1, followed by grazing site, of 17.7 mg kg-1 and forest site, of 8.2 mg kg-1. Spatial distribution of total inorganic nitrogen across the landscape was higher for congregation site (2.3 mg kg-1) than forest site (0.9 mg kg-1) and grazing site (0.7 mg kg-1). The overall spatial distribution of extractable P from congregation site to forest site can be described by P = -4.2x + 45.8; (R2 = 0.97**); the best-fit models for total inorganic N was 0.04x2 - 0.6x + 3.5; (R2 = 0.89**) and for soil P saturation was -3.6x + 36.2; (R2 = 0.92**). Results show that the levels of extractable P, total inorganic nitrogen, and soil phosphorus saturation were decreasing from the congregation site to forest site. Although our results may have had supported our hypothesis that congregation site typical on Florida ranchers have greater concentrations of extractable P than in grazing site and forest site, the average extractable P at all three landscape locations did not exceed the crop requirement threshold of 36 mg kg-1 and the water quality protection threshold of 150 mg kg-1. Our current pasture management including cattle rotation in terms of grazing days and current fertilizer application had thus no negative environmental impact on landscape with cow-calf operation.