Ammonia volatilization from Chihuahuan Desert habitats

Little is known about the loss of NH3 from non- agricultural soils. Many native soils in arid regions have alkaline pH and may be a significant source of NH3 to the atmosphere. In the absence of exogenous inputs, NH4+ in soils is derived from the mineralization of soil organic N. Loss of NH3 is affected by the rate of nitrification, cation exchange and biotic immobilizations that compete for NH4+. Plant growth in many desert ecosystems is limited by available N during the wet season, and gaseous losses of N represent an important potential loss of soil fertility. We have examined the processes controlling the loss of gaseous NH3 from Chihuahuan Desert soils. We hypothesized that if the rate of nitrification limits the loss of NH3 from desert soils, then the rate of NH3 volatilization should increase when nitrification is inhibited experimentally. Similarly, if microbial immobilization of NH4+ inhibits the loss of NH3, then the loss of NH3 should decline when labile organic C is added. If the availability of NH4+ determines the loss of NH3, then losses should increase when NH4+ is added or when added water stimulates the rate of N mineralization from native organic matter. Ammonia volatilization was measured at three sites in the Chihuahuan Desert of southern New Mexico, U.S.A. In dry soils, ammonia volatilization ranged from 9 to 11 micrograms of nitrogen per square meter per day, but rates increased to 95 micrograms of nitrogen per square meter per day in a shrubland site after an experimental addition of water. Ammonia volatilization also increased with experimental additions of NH4Cl and decreased with additions of sucrose. Competition by nitrifiers for available NH4+ had little effect on NH3 volatilization: N-Serve, added to inhibit nitrification, decreased NH3 volatilization in a grassland site and had little effect at other sites. We suggest that NH3 volatilization is controlled by the rate of mineralization of NH4+ from soil organic matter, and mineralization is stimulated by rainfall. Overall rates of NH3 volatilization from undisturbed desert ecosystems appear to be much lower than those reported for rangeland and agricultural soils