Urea as a promotive coupler of plant-herbivore interactions.

Growth responses of Kyllinga nervosa Steud., a sedge from the Serengeti short-grass plains, were examined in a factorial experiment which included clipped and unclipped plants, and nitrogen supplied as either urea or ammonium nitrate. Results were expressed in relation to three transfer processes: flow to grazers, flow to producers and flow to reproduction. Clipping increased biomass and nitrogen flow to grazers by significantly increasing nitrogen uptake, aboveground nitrogen flow, and the weights of and proportional allocation to green leaf production. This was at the expense of flow to vegetative and sexual reproduction, since the weights and proportional investments in roots, crowns and reproductive structures were reduced. Urea nutrition increased flow to grazers and plant reproduction through increases in green leaf weight, flower weight, allocation to green leaves, flowers and stems, and aboveground: belowground biomass ratios. Stimulation of aboveground productivity by urea was a consequence of increased tillering rates.Interactive responses of clipping and nitrogen source regulated plant growth, thus controlling flow to each transfer process. Combined effects of clipping and urea resulted in compensatory production of both green leaves and flowers, and maximized biomass and nitrogen flow to grazers. Both urea and clipping tightened herbivore-producer recycling by significantly reducing litter nitrogen and carbon masses. In contrast, when plants were unclipped and grown on NH4NO3, biomass allocation and weights of roots and crowns were increased at the expense of aboveground tissues, thus increasing flow to primary producers. Plant growth responses to experimental treatment combinations simulating nutritional status of grazed and ungrazed field plants indicate that urea represents a potential importance beyond it nitrogen contribution by introducing a positive feedback to herbivores.