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Nitrogen nutrition of grazing-tolerant grasses.
Swemmer, Tony*,1, Stock, William2, Bond, William2, 1 Kansas State University, Manhattan, Kansas2 University of Cape Town, Cape Town, Western Cape, South Africa
ABSTRACT- Heavy grazing of grass communities is generally considered to increase rates of nutrient cycling. This has been shown as increased rates of net nitrogen (N) mineralization for the grazing lawns of the Serengeti ecosystem (east Africa) and a number of North American grasslands. This study tested the hypothesis that grazing-tolerant grasses which dominate heavily-grazed communities rely on a high supply of N to cope with frequent herbivory. The species used are dominant on grazing lawns in the mesic savanna of the Hluhluwe-Umfolozi Park, South Africa. Grazing lawns here are similar in structure to those described for the Serengeti, and support a similar density of large grazers during the growing season. An initial sand culture experiment revealed no significant effects of N source (nitrate, ammonium or a combination) for three grazing lawn species. To test for the effects of N amount, these species and one additional grazing lawn species, were grown in a series of solution culture experiments (with biweekly clipping). N was supplied as ammonium nitrate at a range of concentration levels. The four species showed considerable variation in relative growth rate (RGR) and biomass allocation patterns in response to N concentration. For five of six pairwise, species comparisons significant differences were found for maximum RGR and RGR response curves. Many biomass allocation variables, including root mass fraction, leaf area ratio and specific leaf area, also showed significant differences between species. While growth responses under controlled conditions do not necessarily reflect responses under natural conditions, these results strongly suggest that grazing-tolerant grasses do not share a similar, high demand for inorganic N. This calls into question the generality of the accepted pattern of increased N availability with grazing, and the significance of any such increases for grazing-tolerant grasses.
Key words: nitrogen, biomass allocation, grazing-tolerance, RGR