Increased nutrient inputs to ecosystems frequently decrease plant biodiversity and shift community composition in grasslands. These changes can persist for decades after cessation of N addition, suggesting the presence of alternative high and low-diversity stable states. Soil nitrate is also elevated with N fertilization, while soil N concentration variably increases. Restoration efforts often target reducing N levels, however, the recovery of these soil N conditions remains understudied. Our work aims to determine (1) whether soil N concentration and nitrate recover after fertilization is stopped; (2) if partial or no recovery is observed, whether management interventions such as burning, carbon amendments, litter removal, or seed addition can return soil N conditions to pre-fertilization levels; and (3) if restoration of soil conditions is achieved, is there immediate recovery of plant diversity or is there hysteresis in the system? This work utilizes two grassland experiments that experienced N fertilization and cessation. One field received restoration treatments of seed addition, C addition, or litter removal, while the second was burned. Soil nitrate, soil N concentration, and species richness were sampled from subset of plots including never fertilized controls, fertilized controls, C addition, litter removal, seed addition, and fire treatments.
Results/Conclusions
Our findings indicate a potential lack of recovery of soil N conditions following cessation of fertilization. Soil nitrate was elevated with fertilization and showed no significant recovery after fertilization stopped. N concentration was not significantly affected by fertilization, cessation, or restoration treatments. Of the considered management interventions, only burning significantly decreased soil nitrate and increased species richness. Other management interventions such as C amendments, litter removal, and seed addition produced no significant effects on soil nitrate or species richness, although seed addition tended to reduce soil nitrate. Overall, our findings suggest that burning may be an effective management tool to aid the recovery of species diversity following cessation of N inputs.