Plant and microbial feedbacks maintain soil nitrogen legacies in burned and unburned grasslands

Nitrogen (N) availability is a well-known driver of ecosystem structure and function, but as air quality regulations continue to reduce atmospheric N deposition, there is a need to understand how managed and unmanaged ecosystems respond to widespread decreases in terrestrial N availability. Historical N eutrophication, from pollution or fertilisation, may continue to constrain contemporary responses to decreases in available N because of altered plant and microbial feedbacks. Thus, while certain management practices like prescribed fire remove N from grassland ecosystems, the role of fire supporting ecosystems recovering from chronic N input is unknown. To address this knowledge gap, we ceased a 30-year N-fertilisation treatment at a field experiment in a tallgrass prairie ecosystem crossed with burned and fire-suppressed (unburned) treatments. We established subplots within each previously fertilised, recovering plot, fertilised at the same historical rate (10 g N m-2 year-1 as NH4NO3), to compare plant and soil properties in recovering plots with control (never-fertilised) and still-fertilised treatments within different fire regimes. We document different N-fertilisation legacies among ecosystem properties in burned and unburned prairies recovering from N-fertilisation. Soil N availability, nitrification and denitrification potentials in recovering plots remained higher than controls for 3-5 years-indicative of positive legacies-in both burned and unburned prairies, but burning did not reduce this legacy. In burned prairies, however, a positive legacy in above-ground plant production persisted because a more productive grass species (switchgrass) replaced the previously dominant species (big bluestem) even though root C:N, but not soil C:N, increased to return back to control levels. Consequently, the main N loss pathways in burned and unburned prairies (pyrovolatilisation and microbially mediated processes, respectively) led to similar losses of soil total N (20-28 g N m-2) over 5 years. Synthesis: Our results indicate that N eutrophication induces positive legacies of ecosystem functions that can persist for at least half a decade. N-induced legacies arise because of shifts in soil microbial N-cycling and plant functional traits. As a result, different management practices may elicit similar trajectories of ecosystem recovery in terms of total and available soil N because of different plant and microbial feedbacks. At a tallgrass prairie experiment crossed with burned and unburned treatments, nitrogen (N)-cycling processes and above-ground productivity remained high after ceasing long-term N-fertilisation, evidence of positive legacies. These responses-contributed by material and information legacies-resulted in similar soil N losses in both systems. Collectively, different land management practices could yield similar ecosystem recovery from eutrophication.image