Ammonia volatilization during drought in perennial C4 grasses of tallgrass prairie.

We measured foliar NH3 volatilization as part of our study of the decrease (up to 40%) in shoot N concentration during drought in three perennial C4 grasses of tallgrass prairie. Volatilization of recently expanded leaves was quantified using cuvettes and acid traps for Spartina pectinata, Andropogon gerardii, and Schizachyrium scoparium, a mesic, intermediate, and xeric species, respectively. In general, volatilization decreased during drought, approaching zero as stomates closed, and increased with plant N status and drought tolerance. Prior to drought, NH3 volatilization was greater in xeric than mesic species (179 and 131 vs. 115 ng m-2 s-1 for individual leaves of S. scoparium and A. gerardii vs. Sp. pectinata). During a 2-3 week drought, whole-shoot volatile N losses can exceed 5% of total plant N in these species, accounting for 2-10% of the decrease in shoot percent N (again, xeric > mesic). Drought-induced N retranslocation of shoot N to roots and rhizomes is responsible for c. 63% of the decrease in percent N in Sp. pectinata, 28% in A. gerardii, and 8% in S. scoparium. The remainder of the decrease in percent N is attributable to growth dilution of existing shoot N, accounting for 34, 65, and 87% of the decrease in shoot percent N during drought in Sp. pectinata, A. gerardii, and S. scoparium, respectively. Thus, the relative importance of volatilization, retranslocation, and dilution in decreasing foliar percent N during drought in prairie grasses is species dependent and related to drought tolerance.