Microbial Immobilization Shapes the Non-Linear Response of Allochthonous Nitrogen Retention to Grassland Acidification Within Soil Aggregates

Soil nitrogen (N) retention plays a crucial role in determining the ecosystem capacity to buffer anthropogenic N inputs and provides a sustainable N supply. However, the effect of acidification, driven by atmospheric deposition of N and sulfur (S), on the retention and fate of allochthonous N across soil aggregate size classes remains poorly understood. We utilized a soil-acidification gradient induced by 0–50 g S m⁻² year⁻¹ addition to investigate ¹⁵N recovery in soil N pools within aggregates 21 days after labeling in a Eurasian meadow. Macroaggregates showed higher ¹⁵N recovery in microbial biomass, amino acids, amino sugars, and therefore total N (TN), as well as greater sensitivity of the former two fractions to acidification compared to microaggregates. This was accompanied by higher N hydrolases and net N mineralization in macroaggregates, supporting the aggregate hierarchical theory. Under moderate acidification (pH decrease from 7 to 6), ¹⁵N retention in hydrolyzable ammonium, amino sugars, non-hydrolyzable N, and TN decreased, likely due to lower microbial immobilization and entombing of allochthonous N. Conversely, severe acidification (pH decrease below 6) enhanced ¹⁵N retention in these N fractions through stabilization of microbial necromass, revealing a non-linear relationship between acidification and ¹⁵N retention. Concentrations of autochthonous organic N fractions remained unchanged after five-year acidification. These findings underscore the mediating role of soil microbes across aggregates in allochthonous ¹⁵N retention among N fractions with contrasting bioavailability under acidification.