NH4NO3 fertilizer induces DON leaching from Sphagnum magellanicum

Boreal peatlands globally function as long-term sinks for atmosheric CO₂. Mosses in the genus Sphagnum dominate the understory of boreal peatlands, especially in ombrotrophic bogs for which the sole sources of new nitrogen (N) is atmospheric deposition and biological N₂-fixation. Sphagnum mosses in undisturbed regions generally are thought to be N-limited because of slow nutrient turnover and low rates of atmospheric N deposition. In Alberta, Canada, mean background N deposition rates are less than 1 kg N ha^-1yr^-1, but increasing atmospheric N deposition is occurring as a consequence of oil sands development in northern Alberta. In some European countries, anthropogenic practices have increased annual N deposition rates to levels in excess of 20 kg N ha^-1yr^-1. Increased rates of atmospheric N deposition have been shown to alter N cycling processes of Sphagnum mosses. Current hypotheses propose that Sphagnum mosses leach dissolved organic nitrogen (DON) from their tissues to mitigate N saturation stress in high N deposition areas. The goal of this study was to determine whether Sphagnum magellanicum leaches DON when exposed to elevated concentrations of NH₄NO₃ applied experimentally as synthetic rainfall. Sphagnum magellanicum was collected from a fen in Driftwood, Pennsylvania (41°13'58"N 78°11'44"W). Intact cores (10.5cm x 5.5cm) were grown in 500 mL culture flasks in a controlled environment chamber at Villanova University. Ammonium nitrate was applied to moss treatments eight times over 73 days at concentrations equivalent to 6 kg N ha^-1yr^-1, 12 kg N ha^-1yr^-1, and 18 kg N ha^-1yr^-1 (n=5 replicates per treatment, 400 mL H₂O per N addition). Leachates (freely drained water samples) were analyzed for dissolved total nitrogen (TN), ammonium (NH₄⁺-N), nitrate (NO₃^--N), and DON (TN-DIN) concentrations. Results show that after 73 days, increasing DIN resulted in: 1) a shift from net N mineralization to net NH₄⁺ immobilization (Control: 376.3 ± 15.2 ug/g; High N: -377.1 ± 51.1 ug/g), 2) a shift from net nitrification to net NO₃^- immobilization (Control: 228.5 ± 12.9 ug/g; High N: -89.3 ± 31.3 ug/g) and 3) an increase in DON production (Control: 178.8 ± 15.3 ug/g; High N: 299.8 ± 54.6 ug/g). Increasing N deposition, as is occurring in northern Alberta, has the potential to cause significant changes in how Sphagnum mosses process N inputs.