Plant growth strategies and microbial contributions to ecosystem nitrogen retention along a soil acidification gradient.

Nitrogen (N) retention is a critical ecosystem function associated with sustainable N supply. Lack of experimental evidence limits our understanding of how grassland N retention can vary with soil acidification. A N-labeling experiment was conducted for 2 years to quantify N retention by soil pathways and plant functional groups across a soil-acidification gradient in a meadow. The N added to the ecosystem was mainly intercepted by the soil (up to 87.3%). Within the soil, N recovery in ammonium, dissolved organic N, microbial biomass, and amino sugars (a proxy for microbial necromass) represented approximately 46% of soil-retained N. N recovery in these N fractions increased with acidification, highlighting the complexity of microbial N transformations that affect ecosystem N retention. Plant N-retention increased in sedges, decreased in forbs, and was unaffected in grasses with acidification, reflecting their divergent associations with mycorrhizas and sensitivities to soil acidification. Soil microbial biomass was the key variable delineating soil N retention, while sedges were critical for plant N retention, resulting in a clear trade-off and competition in N retention between the two compartments. Overall, acidification might curb N losses by strengthening microbial retention and shifting plant N retention among different plant growth strategies.