The interaction of defoliation and nutrient uptake in Sporobolus kentrophyllus, a short-grass species from the serengeti plains.

Sporobolus kentrophyllus, a grazing-tolerant C4 grass from the southeastern Serengeti Plains, was grown in solution culture to examine the effects of clipping on the uptake, preference and subsequent transport of varying nitrogen forms. Clipping reduced offtake mass, crown mass ane root mass, resulting in a 58% decline in plant mass. Proportional biomass allocation to roots decreased with clipping, while tillering rates increased. Clipping also increased the nitrogen concentrations of all tissues, and plant nitrogen uptake (nitrogen accumulated throughout the experiment per gram root). The 15N concentrations (% atom excess) of all tissues were higher in clipped compared with unclipped plants, and the average 15N uptake rate of clipped plants was twice that of unclipped plants. The relative 15N allocation to aboveground mass, a measure of canopy sink strength, was higher in clipped plants. Plants fed 15N-ammonium or 15N-nitrate during the 15N pulse experiment had greater 15N tissue concentrations compared with urea-fed plants, and 15N uptake rates were higher in ammonium-fed and nitrate-fed plants, compared with urea-fed plants. The relative magnitudes of these differences were higher when plants were clipped. Clipped plants had higher uptake rates for potassium, phosphorus and sodium, while differences between clipping treatments for calcium, iron, and magnesium were indistinguishable. Rapid uptake rates for species on the southeastern Serengeti plains, particularly during grazing periods, have important implications for nutrient cycling in this system.