Different fates and retention of deposited NH4+ and NO3− in the alpine grasslands of the Qinghai-Tibet plateau

The fate and redistribution of nitrogen in terrestrial ecosystems are of paramount significance to nitrogen cycling. Nevertheless, the destiny and content of nitrate (NO3–) and ammonium (NH4+) in grassland ecosystems in the context of atmospheric nitrogen deposition remain unclear. This study utilized field-based 15N tracing experiments to investigate the fate of NO3– and NH4+ in a high alpine meadow on the Tibetan Plateau over three growing seasons, elucidating the patterns of nitrogen allocation in plant stems, roots, and soil. The results indicate that: 1) NO3– and NH4+ exhibit distinct behaviors and distribution patterns in the high alpine grassland ecosystem, with significantly higher plant uptake capacity for NH4+ than for NO3–; 2) Nitrogen was primarily concentrated in the 0–15 cm soil layer during the first two growing seasons, shifting to the 15–30 cm soil layer in the third growing season; 3) Total nitrogen recovery decreased progressively from the first to the third growing season. In summary, with the progression of the growing season, the majority of nitrogen is retained in the soil, and the retained nitrogen is redistributed to aboveground and root components of plants through the soil, with plants exhibiting a preference for NH4+ uptake.