Salinity mediates the effects of groundwater and seasonal streamflow on the symbiotic nitrogen fixation of a dryland phreatophyte

Abstract Purpose Biological nitrogen fixation of deep-rooted legumes is a major pathway of nitrogen (N) inputs to drylands ecosystems. Water sources including groundwater and streamflow are vital to the growth of deep-rooted legumes in drylands, but it is unclear how their different availability and biochemical properties may influence plant N fixation. Methods We established two intersecting sampling transects at the southern fringe of the Taklimakan Desert to study the effects of the groundwater depth and distance to a seasonally active river channel on the N fixation rate of Alhagi sparsifolia, a dominant phreatophytic legume species. We quantified the N fixation rate of A. sparsifolia using the 15N natural abundance method, and related variation in N fixation to water source use and soil biochemical properties along the two transects. Results The N fixation rate of A. sparsifolia decreased with groundwater depth, and increased with distance from the river channel. Instead of variation in water source or nutrient concentrations, we found that gradients in salinity formed by groundwater flow and streamflow were most associated with the spatial pattern in N fixation of A. sparsifolia. Conclusions In many dryland regions, water availability controls the spatial variability of symbiotic N fixation along hydrological gradients. Our study system provides a contrasting condition where salinity most strongly regulates N fixation. In addition to understanding the important role of variation in water source and nutrient availability, our study suggests that the impact of soil salinity in mediating the N fixation of dryland ecosystems is deserving of increased attention.