Effects of groundwater and seasonal streamflow on the symbiotic nitrogen fixation of deep-rooted legumes in a dryland floodplain

Abstract

Biological nitrogen fixation of deep-rooted legumes is a major pathway of nitrogen (N) inputs to drylands. 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. We established two sampling transects at the southern fringe of the Taklimakan Desert (Xinjiang province, north-west China) to study the effects of both the groundwater depth and distance to a seasonally active river on the N fixation rate of Alhagi sparsifolia, a dominant deep-rooted 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. The N fixation rate of A. sparsifolia decreased with groundwater depth and increased with distance from the river channel. Instead of variation in the water source or nutrient concentrations, we found that the gradient in salinity formed by groundwater flow and streamflow was most responsible for the spatial pattern in N fixation of A. sparsifolia. 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 N fixation in dryland ecosystems is deserving of attention.