Spatial Redistribution of Nitrogen by Cattle in Semiarid Rangeland
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, cattlemediated
N losses are often poorly understood. We used measurements 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 and spatial redistribution of N into pasture corners and
areas near water tanks, and used previous studies to calculate ammonia volatilization from urine patches. Using measurements
of plant biomass and N content inside and outside grazing cages over 13 yr, we estimate that cattle stocked at 0.65 animal unit
months (AUM) ha1 consumed 3.34 kg N ha1 yr1. Using an independent animal-based method, we estimate that cattle
consumed 3.58 kg N ha1 yr1 for the same stocking rate and years. A global positioning system 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 ha1 during the summer
can remove 0.60 kg N ha1 in cattle biomass gain and spatially redistribute 0.73 kg N ha1 to areas near corners and water
tanks. An additional 0.17 kg N ha1 can be lost as NH3 volatilization from urine patches. Cumulatively, these cattle-mediated
pathways (1.50 kg N ha1) may explain the imbalance between current estimates of atmospheric inputs and trace gas losses.
While NOx emission remains 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.
Key Words: grazing management, Great Plains, large herbivores, nitrogen cycling, semiarid grassland, shortgrass steppe