Small-scale heterogeneity in a semi-arid North American grassland. I : Tillering, N uptake and retranslocation in simulated urine patches

1. We examined the response of two grass species differing in growth form and photosynthetic pathway to simulated cattle urine deposition in a native semi-arid grassland. The species were Agropyron smithii, a spreading, rhizomatous C3 grass, and Bouteloua gracilis, a cespitose C4 grass. They are codominants in this grassland and A. smithii is the most important forage species in the study site. 2. Simulated urine increased tiller density of both species, with B. gracilis increasing in the first growing season while A. smithii was not increased until the second year. Both species had increased tiller densities and above-ground N concentrations in a ring outside the area wetted by urine. The percentage of retranslocated leaf N decreased, and standing dead litter N concentration increased in both species after urine deposition. 3. Above-ground biomass, N yield, and tiller height of A. smithii increased in urine patches. Leaf production was not affected by treatment. Both the increase in the live:dead biomass ratio and the higher number of live leaves at the end of the first growing season suggested urine deposition delayed senescence of A. smithii. 4. The implications of plant growth form in the spread of urine, the potential duration of urine effects in semi-arid grasslands, and the role of herbivore urine deposition in promoting grassland structural and functional heterogeneity are discussed.

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