Nutrient concentration ratios and co-limitation in South African grasslands


  • Joseph M. Craine,

    1. Department of Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN 55108, USA;
    2. Environmental Studies Program, Dartmouth College, Hanover, NH 03755, USA;
    3. (present address) Division of Biology, Kansas State University, Manhattan, KS 66506, USA;
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  • Carl Morrow,

    1. Department of Botany, University of Cape Town, Cape Town, South Africa;
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  • William D. Stock

    1. Centre for Ecosystem Management, Edith Cowan University, Perth, Western Australia, Australia
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Author for correspondence:
Joseph M. Craine
Tel:+1 785 532 3062
Fax:+1 785 532 6653


  • Assessing plant nutrient limitation is a fundamental part of understanding grassland dynamics. The ratio of concentrations of nitrogen (N) and phosphorus (P) in vegetation has been proposed as an index of the relative limitation of biomass production by N and P, but its utility has not been tested well in grasslands.
  • At five sites in Kruger National Park, South Africa, across soil and precipitation contrasts, N and P were added in a factorial design to grass-dominated plots.
  • Although the N:P ratio of unfertilized vegetation across all sites (5.8) would have indicated that production was N-limited, aboveground production was consistently co-limited by N and P. Aboveground production was still greater in plots fertilized with N and P than in those fertilized with just N, but the N:P ratio did not exceed standard thresholds for P limitation in N-fertilized vegetation. Comparisons among sites showed little pattern between site N:P ratio and relative responses to N and P.
  • When combined with results from other grassland fertilization studies, these data suggest that the N:P ratio of grasses has little ability to predict limitation in upland grasslands. Co-limitation between N and P appears to be much more widespread than would be predicted from simple assumptions of vegetative N:P ratios.