Recovery of soil nitrogen pools in species‐rich grasslands after 12 years of simulated pollutant nitrogen deposition: a 6‐year experimental analysis

Nitrogen (N) deposition from anthropogenic sources is a global problem that can reduce biodiversity and impair ecosystem functioning through effects on soil eutrophication and acidification. While increasing controls on emissions of oxides of nitrogen (NOx) have reduced European N deposition rates from their peak in the late 20th Century, little is known about the legacy effects of N deposition in soils or the reversibility of N-induced shifts in ecosystem processes. We studied species-rich limestone and acidic grasslands, located in a highly polluted region that received over 3000 kg N deposition ha−1 throughout the 20th Century, followed by a decline of ∼50% in NOx deposition rate in the past two decades. We investigated the effects on seasonal and annual mean concentrations of soil mineral N in experimental plots established in 1990 receiving simulated enhanced N deposition (0–140 kg N ha−1 yr−1) until 2002, both in the final year of treatment, and the subsequent 5 years of ‘recovery’ following cessation of treatments. Winter–summer cycles of N mineralization–immobilization were strongly amplified by simulated N deposition rates through the final year of treatments and into the first year of recovery, with winter concentrations of ammonium-N in the acidic grassland and nitrate in the limestone grassland enhanced by up to 360% and 450%, respectively. Both the magnitude of the seasonal variations and the residual effects of the treatments on soil mineral N concentrations decreased progressively in the first 5 years after treatments ceased, although dose-dependent trends remained in the acidic grassland. This study establishes that reducing N deposition rates in species-rich grasslands can reverse eutrophication, even in soils that have experienced prolonged high rates of deposition. It provides new insight into the rates of recovery following, and effects of, declining N deposition rates with implications for restoration of species-rich grasslands.