Phosphorus, Iron, and Aluminum Losses in Runoff from a Rotationally Grazed Pasture in Georgia

Well-managed grazing systems can provide valuable ecosystem services, such as reducing sediment and phosphorus (P) loading to nearby waterways. However, the available long-term data to fully support this hypothesis are limited. In this article, we describe flow-weighted concentrations (FWCs) and loads for dissolved reactive P (DRP), total P (TP), iron (Fe), and aluminum (Al) over 11 years (1999-2009) from a 7.8 ha rotationally grazed pasture (W1) near Watkinsville, Georgia. The region is characterized by Fe and Al rich and acidic Ultisols. Cattle numbering 21 to 224 (mean 91) grazed W1 on 69 occasions for 1 to 71 d (mean 19.2). Of 74 runoff events, 20 occurred when the monthly rainfall was below the long-term average (deficit period) and 54 occurred during non-deficit periods. Samples were collected from 43 of 74 runoff events for nutrient analyses. Event FWC (mg L-1) ranged from 0.38 to 7.07 for DRP (mean 1.91), from 0.36 to 7.60 for TP (mean 2.43), from 0.03 to 0.55 for Fe (mean 0.23), and from 0.43 to 553. g L-1 for Al (mean 65. g L-1). Event load (kg ha(-1)) ranged from 0.00 to 0.45 for DRP (mean 0.10), from 0.00 to 0.55 for TP (mean 0.12), from 0.00 to 0.11 for Fe (mean 0.02), and from 0.00 to 0.10 for Al (mean 0.01). The total load (kg ha(-1)) was 4.12 for DRP, 5.12 for TP, 0.71 for Fe, and 0.25 for Al. DRP accounted for 80% of the TP FWC and load. Cattle presence increased sediment load, but the difference was not statistically significant. There was high correlation between Fe and DRP loads (r = 0.87), a likely indicator of erosion-induced losses due to cattle treading. Cattle presence increased FWCs but not loads for DRP and TP. The FWCs for DRP and TP were not different between deficit and non-deficit periods, but mean loads were 3-fold to 4-fold greater during non-deficit periods. Means from the six largest P loss events were 3-fold greater for FWC and 7-fold greater for load than the remaining 37 events. These six large events accounted for 53% of the total P load. Less than 1% of the inorganic P applied and redeposited through manure was lost in runoff. The study demonstrated that hydrologic transport processes were the dominant drivers of pollutant fluxes and highlighted the possible mitigation of pollutant fluxes through grazing management that includes maintenance of good grass cover, effective rotational grazing, and limited fertilization.