Forest succession, soil carbon accumulation, and rapid nitrogen storage in poorly remineralized soil organic matter

Substantial nitrogen (N) retention by temperate terrestrial ecosystems results from the rapid storage of newly deposited N in stable soil organic matter. Yet, we poorly understand the ecosystem properties that regulate the kinetics of this process. We applied mineral ¹⁵N to temperate hardwood forest soils to test the hypothesis that N stabilization is faster owing to greater stocks of soil carbon (C) in late-successional than in young forests. Within 26 minutes of addition, about 30% of tracer N was stored in stable form in organichorizon soil with a median residence time of >29 years. About 5-10% of tracer N was stored in a soluble organic form. An additional 30% of tracer N was recovered within hours from organic-horizon soils in a remineralizable (labile) form, apparently derived from microbial biomass. Over the following year, tracer N storage in stable and soluble organic pools remained constant while recovery from labile and microbial pools declined. Tracer storage was greater in older forests with larger soil C pools, supporting our hypothesis that the accumulation of soil C with forest succession promotes ecosystem N retention. Rapid storage of stable soil N in the O horizon may create a source for chronic dissolved organic N losses from watersheds.