Mechanisms Controlling Nitrogen Losses in Dryland Ecosystems

Abstract:

Drylands are exposed to some of the highest rates of atmospheric nitrogen (N) deposition. Elevated N inputs can adversely affect ecosystems through acidification, nutrient imbalances, and eutrophication. Theory suggests that ecosystems have a finite capacity to assimilate N before they become “N saturated,” but it remains unclear how this concept (developed in mesic forests) applies to drylands, which cover 1/3 of the Earth’s surface and where N typically limits ecosystem productivity. To determine when drylands become N-saturated, we are studying N-saturation symptoms in three sites along a gradient of atmospheric N deposition (≤ 5 to ≥ 30kg N ha^-1 y^-1). We are measuring soil N emissions using automated chambers, a NOx analyzer, and an isotopomer N₂O analyzer coupled to measurements of nitrification and denitrification potentials to disentangle biotic and abiotic mechanisms controlling these emissions. We are also monitoring hydrological N losses to better capture the timing and magnitude of N losses as these ecosystems transition from dry-to-wet conditions—a period well-known to drive large N losses from drylands. Together, we are using these measurements to develop new frameworks to understand the mechanisms driving gaseous and hydrological losses in drylands, to determine when drylands are intermittently vs. chronically N saturated, and help predict ecosystem responses to elevated N inputs.