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Yield and species composition of a mesic grassland savanna in South Africa are influenced by long-term nutrient addition

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Abstract

Species composition and productivity of natural grasslands are influenced by soil nutrient status. With high resource availability, productivity is expected to increase, and competition is assumed to gain prominence with predicted exclusion of species of lower competitive ability. During 2010 and 2011 we used the dry weight rank method to measure above-ground phytomass production of herbage in 96 plots (9 m × 2.7 m) fertilized for 60 years with two forms of nitrogen (N as limestone ammonium nitrate or ammonium sulphate at four levels: 0, 7.1, 14.1, 21.2 g m−2), phosphorus (P as superphosphate at two levels: 0, 33.6 g m−2), and lime (two levels: 0, 225 g m−2). Light attenuation was measured as the proportion of photosynthetically active radiation reaching the lower leaf layers of the grasses and the ground surface. Light conditions beneath the grass layer were reduced by nutrient addition to 30% of full sunlight but remained above 60% in non-fertilized plots. Grass total above-ground phytomass production increased with nutrient addition. The strongest yield responses were attained with N plus P addition. Species responses showed that Themeda triandra and Hyparrhenia hirta decreased in above-ground phytomass production with nutrient addition while Panicum maximum, Eragrostis curvula and E. plana increased. These findings are discussed in terms of competitive interactions among species, their position in the grass canopy and their physiological tolerances to high nitrogen environments.

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