Effects of Phosphorus Nutrition and Defoliation on C4 Graminoids from the Serengeti Plains

Kyllinga nervosa (Cyperaceae) and Digitaria macroblephara (Poaceae), C₄ graminoids, were obtained from the Serengeti National Park of Tanzania. Kyllinga is most abundant on high pH, high calcium, low phosphorous, carbonatitic ash—derived soils, and Digitaria is more abundant on neutral, lower calcium, higher P soils. Both species come from intensely grazed grasslands. Plant responses to different nutritional levels and defoliation were measured to evaluate potential limiting interactions between energy and nutrient flows in grazing ecosystems. Plants were grown hydroponically at solution phosphorus concentrations of 10 and 100 μmol/L and were either unclipped or clipped weekly to a height of 3 cm for 5 wk. At that time, the kinetics of P uptake were measured with ³ ²P, plants were harvested, and dry masses and the tissue concentrations of P and N were determined. Clipping and P concentration of the growth medium both had major effects upon yields of all plant components. Total yield of Kyllinga was highest in unclipped plants at high levels of P supply; yields under all other conditions were lower and indistinguishable from one another. In contrast, yield of Digitaria, which had been collected in areas with soils of higher phosphorus availability, was unaffected by defoliation but was almost 21/2 times greater when plants were grown at the higher P level. Root growth of Digitaria was stimulated by defoliation when P was abundant. Maximum yield to grazers from Kyllinga under conditions of this experiment would be achieved if grazers foraged only at the end of the period; maximum yield of Digitaria to grazers would be achieved if grazers defoliated plants at frequent intervals. A variety of morphological changes associated with maintaining similar leaf areas whether clipped or unclipped were associated with Digitaria's ability to compensate for simulated grazing. Yield to producers (the residual live biomass present at harvest) and yield to grazers were positively correlated, indicating that conditions promoting plant yield caused a proportional increase in yield to higher trophic levels. Maximum concentrations of P and N in the tissues of both species occurred in plants that were clipped and were grown at the high P level. N concentration was positively correlated with P concentration when plants were grown at high P, but was unrelated to tissue P concentrations when plants were grown at low P. That pattern reflects luxury N uptake of plants grown at low P. Nutrient uptake rate per unit root mass was higher in Digitaria than in Kyllinga. Defoliation stimulated nutrient uptake rate in both species, primarily due to decrease Kₘ rather than changes in Vₘ ₐ ₓ. Total P accumulation per plant (predicted from uptake rates measured with ³ ²P, root mass, and nutrient mass, and nutrient solution P concentrations) was closely and positively correlated with measured P accumulation. However, P accumulation over the experiment fell far below predicted amounts. That disparity was likely due to low root biomasses early in the experiment and nutrient depletion between solution renewals.