Glacier forelands reveal fundamental plant and microbial controls on short-term ecosystem nitrogen retention

Human activities have massively increased the amount of reactive nitrogen in the biosphere, which is leading to increased nitrogen (N) inputs in terrestrial ecosystems. The retention of N is a crucial ecosystem function of both managed and natural ecosystems, and there is a long history of experimental, observational, and conceptual studies identifying its major controls. Yet, the plant and soil microbial controls on the retention of added N remain elusive. Here, we used three ecosystem chronosequences in front of retreating glaciers in the European Alps to test our hypothesis that the retention of added reactive N-15 increases as succession proceeds, and to identify the plant and microbial controls on ecosystem N retention. We found that the uptake and retention of N did not change during succession, despite consistent changes in plant, soil, and microbial properties with increasing time since deglaciation. Instead, we found that plant and microbial properties that remained constant during succession controlled N-15 uptake and retention: low root and microbial C/N ratios, as well as high root biomass, increased plant and microbial uptake of added N. In addition, high soil concentrations of nitrate and ammonium reduced the uptake of N in microbes and roots, respectively. Synthesis. Our results demonstrate that plant and microbial N demand, as well as soil N availability, drive the short-term retention of added N during succession in glacier forelands. This finding represents an advance in our understanding of the fundamental controls on ecosystem N retention and the role of plant-microbial interactions in this process. Such understanding is crucial for predicting and mitigating the response of terrestrial ecosystems to the ever-increasing amounts of reactive N in the biosphere.