Spatial redistribution of nitrogen by cattle in semiarid rangeland

Authors: Augustine, David; MILCHUNAS, DANIEL - Colorad0 State University; Derner, Justin

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2012
Publication Date: 1/2/2013
Citation: Augustine, D.J., Milchunas, D.G., Derner, J.D. 2013. Spatial redistribution of nitrogen by cattle in semiarid rangeland. Rangeland Ecology and Management. 66:56-62.

Interpretive Summary: Nitrogen (N) availability can strongly influence forage quality and the capacity for semiarid rangelands to respond to increasing atmospheric CO2. Although many pathways of nitrogen input and loss from rangelands have been carefully quantified, cattle-mediated N losses are often poorly understood. We synthesized studies of cattle N consumption rate, weight gains, and spatial distribution in shortgrass rangeland of northeastern Colorado to evaluate the influence of cattle on rangeland N balance. Specifically, we estimated annual rates of N loss via cattle weight gains, ammonia volatilization from urine patches, and spatial redistribution of N into pasture corners and areas near water tanks. Using measurements of plant biomass and N content inside and outside grazing cages over 13 years, we estimate that cattle stocked at 0.65 AUM ha-1 consumed 3.34 kg N ha-1yr-1. An independent animal-based method estimated 3.58 kg N consumed ha-1yr-1 for the same stocking rate and years. A GPS tracking study revealed that cattle spent an average of 27% of their time in pasture corners or adjacent to water tanks, even though these areas represented only 2.5% of pasture area. Based on these measurements, we estimate that cattle stocked at 0.65 AUM ha-1 during the summer can remove 0.60 kg N ha-1 in cattle biomass gain and spatially redistribute 0.73 kg N ha-1 to areas near corners and water tanks. An additional 0.17 kg N ha-1 can be lost as NH3 volatilization from urine patches. Cumulatively, these cattle-mediated pathways of N loss (1.50 kg N ha-1) can explain the imbalance between atmospheric inputs and trace gas losses. While NOx emissions remain the largest pathway of N loss, spatial N redistribution by cattle and N removed in cattle biomass are the second and third largest losses respectively. Management of cattle-mediated N fluxes should be recognized as one means to influence long-term sustainability of semiarid rangelands.

Technical Abstract: Nitrogen (N) availability can strongly influence forage quality and the capacity for semiarid rangelands to respond to increasing atmospheric CO2. Although many pathways of nitrogen input and loss from rangelands have been carefully quantified, cattle-mediated N losses are often poorly understood. We synthesized studies of cattle N consumption rate, weight gains, and spatial distribution in shortgrass rangeland of northeastern Colorado to evaluate the influence of cattle on rangeland N balance. Specifically, we estimated annual rates of N loss via cattle weight gains, ammonia volatilization from urine patches, and spatial redistribution of N into pasture corners and areas near water tanks. Using measurements of plant biomass and N content inside and outside grazing cages over 13 years, we estimate that cattle stocked at 0.65 AUM ha-1 consumed 3.34 kg N ha-1yr-1. An independent animal-based method estimated 3.58 kg N consumed ha-1yr-1 for the same stocking rate and years. A GPS tracking study revealed that cattle spent an average of 27% of their time in pasture corners or adjacent to water tanks, even though these areas represented only 2.5% of pasture area. Based on these measurements, we estimate that cattle stocked at 0.65 AUM ha-1 during the summer can remove 0.60 kg N ha-1 in cattle biomass gain and spatially redistribute 0.73 kg N ha-1 to areas near corners and water tanks. An additional 0.17 kg N ha-1 can be lost as NH3 volatilization from urine patches. Cumulatively, these cattle-mediated pathways of N loss (1.50 kg N ha-1) can explain the imbalance between atmospheric inputs and trace gas losses. While NOx emissions remain the largest pathway of N loss, spatial N redistribution by cattle and N removed in cattle biomass are the second and third largest losses respectively. Management of cattle-mediated N fluxes should be recognized as one means to influence long-term sustainability of semiarid rangelands.