Phosphorus, iron, and aluminum losses in runoff from a rotationally-grazed pasture in Georgia, USA

Authors: Endale, Dinku; Schomberg, Harry; FISHER, DWIGHT - Retired ARS Employee; OWENS, LLOYD - Retired ARS Employee; Jenkins, Michael; BONTA, JAMES - Retired ARS Employee

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2017
Publication Date: 6/12/2017
Citation: Endale, D.M., Schomberg, H.H., Fisher, D., Owens, L., Jenkins, M., Bonta, J. 2017. Phosphorus, iron, and aluminum losses in runoff from a rotationally-grazed pasture in Georgia, USA. Transactions of the ASABE. 60(3):861-875. Https://doi:10.13031/trans.12053.
DOI: https://doi.org/10.13031/trans.12053

Interpretive Summary: Pastures can be a source of phosphorus contributing to reduced water quality in water resources. We used 11-yr (1999-2009) of data from a 19-ac rotationally-grazed pasture near Watkinsville Georgia, to test for relationships between pasture management and environmental factors on runoff and losses of phosphorus (P), iron (Fe), and aluminum (Al) in runoff from the pasture. On average, 91 cattle grazed the pasture on 69 occasions for 19 days in each occasion. Approximately 58% of months had below average (deficit) and 42% had average or above average (non-deficit) monthly rainfall. Twenty runoff events were recorded during deficit and 54 during non-deficit months. Analyses for concentration and mass for dissolved P (DP), total P (TP), and Fe and Al were performed using 43 runoff event samples. Average concentration in ppm (parts per million) was 1.9 for DP, 2.4 for TP, 0.23 for Fe, and 0.06 for Al. Total nutrient mass exiting the pasture in pounds per acre was 3.7 for DP, 4.6 for TP, 0.6 for Fe, and 0.2 for Al. The presence of cattle increased P concentration. Phosphorus mass was larger during the non-deficit period. In the largest 6 of the 43 events, average concentration of P was 3 times and mass of P was 7 times that of the remaining events. A pond receiving runoff from the pasture reduced P concentration below levels considered of concern for water quality. Models were developed to help estimate nutrient losses from similar pastures. The study pointed to the need for additional measures to reduce nutrient losses, and provided valuable information to those involved in protecting water quality across the nation.

Technical Abstract: Pastures can be a source of phosphorus (P) contributing to eutrophication and impairment of water resources. Phosphorus is tightly held in soils that are highly weathered, acidic, and with high iron (Fe) and aluminum (Al) content like the Ultisols of southeastern USA. We used 11-yr (1999-2009) of data from a 7.8 ha rotationally-grazed pasture (W1) near Watkinsville, in the Georgia Piedmont, to test for relationships between pasture management and runoff and losses of P, Fe, and Al in runoff. Cattle numbering 21 to 224 (mean 91) grazed W1 on 69 occasions for 1 to 71 days (mean 19.2). Approximately 58% of months had below average (deficit) and 42% had average or above average (non-deficit) monthly rainfall. Twenty runoff events were recorded during deficit and 54 during non-deficit months. Analyses for flow-weighted concentration (FWC) and load for dissolved reactive P (DRP), total P (TP), and Fe and Al were performed using 43 runoff event samples. Event FWC (mg L-1) ranged from 0.4 to 7.1 for DRP (mean 1.9); 0.4 to 7.6 for TP (mean 2.4); 0.03 to 0.55 for Fe (mean 0.23); and 0.00 to 0.55 for Al (mean 0.06). Event load (kg ha-1) ranged from 0.00 to 0.45 for DRP (mean 0.10); 0.00 to 0.55 for TP (mean 0.12); 0.00 to 0.11 for Fe (mean 0.02); and 0.00 to 0.10 for Al (mean 0.01). The mean DRP to TP ratio was 0.77 for FWC and 0.80 for load. There was high correlation between loads of Fe and DRP (r = 0.87). The total load (kg ha-1) was 4.1 for DRP, 5.1 for TP, 0.7 for Fe, and 0.2 for Al. The FWC for DRP and TP was greater with cattle on than off W1 (p = 0.02; means 3.1 mg L-1 vs 1.9 mg L-1 for TP). Cattle presence did not increase loads for DRP and TP (p = 0.63). The FWC for DRP and TP were not significantly different between deficit and non-deficit periods (p = 0.30), but mean loads were 3- to 4-fold greater during the non-deficit than the deficit period (p = 0.01). Means from the six largest P losses (probability of exceedance < 0.15) were 3-fold greater for FWC and 7-fold greater for load than the remaining 37 events. A pond receiving runoff from W1 reduced P concentration to levels at the outlet to below eutrophication triggering levels. Factor analysis allowed identification of likely hydrologic and nutrient source drivers and the development of linear regression models for estimating nutrient losses. These results provide insight into environmental impacts of pasture management approach that represents a typically economically constrained producer farm making the data representative of much of the grazed areas of southeastern USA.