Palmitic acid triggers rhizosphere acidification to facilitate microbial nitrogen fixation: A perspective for phytoremediation of desertification alpine grassland via artificial root exudation mimicking.

Organic acids can enhance inorganic nitrogen transformation in rhizosphere soil, however the specific influence of long-chain organic acids on the community and functional structure of soil microorganisms that control nitrogen cycling is poorly understood. Organic acids have shown potential to increase the resilience of alpine ecosystems against desertification. To address this knowledge gap, we conducted a controlled experiment in a microcosm rhizosphere system. Using artificial roots (7.85 cm surface area), we amended Qinghai-Tibet Plateau soil with palmitic acid for 45 days at three secretion rates (½ × , 1 × , and 2 × the in situ rate of 3.20 μg C d cm), corresponding to 12.56, 25.12, and 50.24 μg C d root respectively, with a control group receiving only osmoregulation solution. The results revealed a remarkable 81 % increase in soil NH-N concentration at the highest addition rate of palmitic acid. Moreover, both microbial biomass and the abundance of nitrogen-fixing genes nifK (12.25 times), nifH (7.63 times), and nifD (27.33 times) increased significantly, alongside a substantial rise in the relative abundance of Azospirillum (12.9-fold). Notably, 82 % of biomarkers in the high-rate palmitic acid treatment were linked to nitrogen fixation. The structural equation model indicated a direct impact of nitrogen fixation on NH-N concentration. Our findings suggest that palmitic acid primarily enhances microbial biomass and drives nitrogen fixation processes through its decline to pH, rather than serving as a direct carbon source. Consequently, within the range of our experimental setup, the preferential selection of plant species with a high root exudation rate of palmitic acid can optimize NH-N retention in rhizosphere soil, thereby promoting plant growth and facilitating vegetation restoration across the desertification alpine grasslands in the Qinghai-Tibet Plateau.