|Bonta, James - Jim|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2011
Publication Date: 3/25/2011
Citation: Endale, D.M., Fisher, D.S., Owens, L.B., Jenkins, M., Schomberg, H.H., Tebes-Stevens, C.L., Bonta, J.V. 2011. Runoff water quality during drought in a zero-order Georgia Piedmont pasture: nitrogen and total organic carbon. Journal of Environmental Quality. 40:969-979. Interpretive Summary: Environmental impacts from grazing animals in low-input rotational systems are not well studied or documented, particularly in the southeastern USA. Recent relatively dry weather patterns have forced some modifications of agricultural and urban water usage in southern USA. To effectively manage nutrients in grazing systems there is a need for data during contrasting weather patterns. The objective of this study was to relate 11 years hydrologic and 10 years of nitrogen and total organic carbon runoff water quality data from a 19 acre Georgia Piedmont pasture, to low-input rotational grazing by black Angus cattle during a period with severe drought punctuated by some very wet months. The study was conducted from 1999 to 2009 by researchers at the USDA-ARS J. Phil Campbell Senior Natural Resource Conservation Center near Watkinsville, GA, with collaboration with others from USEPA in Athens, GA and USDA Agricultural Research Service in Coshocton, OH. Between 20 and 225 head of cattle grazed the pasture a total of 69 times for a third of the overall period, spending from 1 to 71 days each grazing period. Annual rainfall was 7 to 18 inches below the long-term average of 49 inches in 7 of the 11 years. Only 20 runoff events were recorded during 86 months of below average rainfall compared to 54 runoff events during the 46 months of at or above average monthly rainfall. The average nitrate-nitrogen and ammonium-nitrogen content in runoff that takes the variability in runoff into account (flow weighted concentration FWC) was less than 1 parts per million (ppm). For total nitrogen FWC was 3.7 ppm and for total organic carbon 9.1 ppm. The estimated nutrient mass lost in runoff in pounds per acre averaged 0.034, 0.028, 0.168, and 0.484 for nitrate, ammonium, total nitrogen and total organic carbon, respectively. These results suggest that nitrogen and organic carbon releases from Southern Piedmont pastures under low-input management may be of limited environmental concern. While, the data were obtained from a pasture that confined cattle away from creeks and during a period when the region was experiencing below average rainfall, the results should be of interest to a wide variety of individuals and groups with stake in improved stewardship of our environment. Approximately 11% of the Southern Piedmont (4.5 million acres) is used for pasture and hay production. These systems are mostly under low-input management.
Technical Abstract: Approximately 11% of the Southern Piedmont (1.8 million ha) is used for pasture and hay production. These systems are mostly under low-input management. Cattle manure can lead to enrichment of surface soils with nutrients raising concerns about water quality. We present 11 years (1999-2009) of hydrologic and 10 years of water quality data from a 7.8-ha, zero-order catchment with bermudagrass over-seeded with winter annuals rotationally grazed as pasture near Watkinsville in the Georgia Piedmont. Between 20 and 225 head of cattle grazed the pasture a total of 69 times for a third of the overall period, spending from 1 to 71 days each grazing period. Cattle days (number of cattle x number of days) varied from 8 to 611 (mean 182.4) ha-1 grazing-period-1. Annual rainfall was 176 to 463 mm below the long-term average of 1240 mm in 7 of the 11 years. Only 20 runoff events were recorded during 86 months of below average rainfall (deficit period) compared to 54 runoff events during the 46 months of the non-deficit period. Nutrient analyses were carried out for 43 to 47 out of 67 runoff events from 2000-2009. A forward step-wise regression was used to screen 23 independent variables for best predictors of nutrient load, runoff volume and peak runoff rate. Mean flow weighted concentration (FWC) in mg L-1 was 0.96 for nitrate-nitrogen (NO3-N), 0.97 for ammonium-nitrogen (NH4-N), 3.70 for total nitrogen (TN), and 9.12 for total organic carbon (TOC). Nutrient loads in kg ha-1 averaged 0.038 for NO3-N, 0.031 for NH4-N, 0.188 for TN, and 0.542 for TOC. Peak nutrient concentrations and loads occurred during calving season and/or when monthly rainfall was above average. Assuming these averages would hold for all 74 runoff events, total nutrients leaving the whole catchment in kg over 11 years would have been approximately 22 for NO3-N, 18 for NH4-N, 108 for TN, and 312 for TOC. These data suggest that nitrogen and organic carbon releases from Southern Piedmont pastures under low-input management may be of limited environmental concern. However, the data were obtained from a catchment that confined cattle away from creeks and during a period when the region was experiencing below average rainfall.