NITRATE RETENTION BENEATH NITROGEN-LIMITED VEGETATION IN ARID AND SEMIARID ENVIRONMENTS AND IMPLICATIONS FOR RAPID MOBILIZATION

A limited, yet geographically widespread, compilation of deep soil-water profiles from the arid-semiarid western United States reveals a large, previously unaccounted for pool of bioavailable N (as much as 10⁴ kg-N ha^-1, as nitrate) retained in deep vadose zones. This subsoil pool could raise estimates of vadose zone N inventories by 14-62% for arid and semiarid regions worldwide and 3-14% globally. These findings have important implications for (1) plant-nutrient dynamics in nitrogen water-limited ecosystems, and (2) vadose-zone nitrate mobilization following environmental change. Previous studies demonstrate the high N-use efficiency of desert plants and identify N availability as a limiting factor in arid and semiarid ecosystem productivity. Yet, large quantities of accumulated subsoil N (as nitrate) imply that vegetation does not utilize all N available in the soil zone. In some cases, average long-term N soil leaching rates are of the same magnitude as atmospheric inorganic N deposition rates. We propose a conceptual model to explain the accumulation of subsoil nitrate after the transition from the wetter, cooler climate of the Pleistocene to the drier, hotter climate of the Holocene in the southwestern U.S. The conceptual model consists of episodic wetting events that flush soil nitrate from the root zone, separated by sustained upward water vapor transport toward the plant-atmosphere continuum, thus removing subsoil water while leaving non-volatile nitrate behind. Disturbance to the natural desert ecosystem may induce the quick release of thousands of years of accumulated subsoil nitrate to groundwater. Evidence of downward translocations of subsoil nitrate inventories demonstrates the potential for rapid mobilization of nitrate at desert sites that have experienced reservoir construction and conversion to irrigated agriculture.