Spatial distribution of chloride and nitrate within an unsaturated dune sand of a cold-arid desert: Implications for paleoenvironmental records

Based on chloride mass balance approach, we used two tracers (Cl− and NO3−) to study the interstitial water in four profiles of an unsaturated dune sand. The aim was to reconstruct the recharge history and use this to infer the long-term related climate changes and vegetation transitions in a cold-arid desert region of northwestern China. The estimated recharge rate of 0.21 to 0.59 mm year− 1 represented approximately 0.2 to 0.7% of the annual precipitation in the western Badain Jaran desert. The orientation of a megaduneʹs slopes appeared to greatly affect recharge variability. Below the near-surface peaks, the total cumulative NO3–N inventories ranged from 61 to 121 kg ha− 1. The near-surface peaks in NO3–N, with larger NO3–N/Cl− molar ratios, reflected active modern N-fixation by cyanobacteria, and the nitrate enrichment deeper in the unsaturated moisture zone indicated drier periods with less vegetation, limited nitrate fixation, or a combination of the two.