This study evaluated the interactive effects of combined phosphorus (P) and potassium (K) additions on canopy nitrogen (N) retention (CNR) and subsequent canopy leaching at a long-term N manipulation site on Whim bog in south Scotland. Ambient deposition is 8 kg N ha −1 year −1 and an additional 8, 24, and 56 kg N ha −1 year −1 of either ammonium (NH 4 + ) or nitrate (NO 3 − ) with or without P and K has been applied over 11 years. Throughfall N deposition below Calluna vulgaris and foliar N and P concentrations were assessed. Results showed that 60% for low dose and 53% for high dose of NO 3 − contrasting with 80% for low dose and 38% for high dose of NH 4 + onto Calluna was retained by Calluna canopy. The CNR was enhanced by P and K addition in which 84% of NO 3 − and 83% of NH 4 + for high dose were retained. CNR for NO 3 − increased the canopy leaching of dissolved organic N (DON) and associated organic anions. NH 4 + retention increased canopy leaching of magnesium and calcium through ion exchange. Even over 11-years N exposure without P and K, foliage N:P ratio of Calluna did not increase, suggesting that N exposure did not lead to N saturation of Calluna at Whim bog. Our study concluded that increases in P and K availability enhance CNR of Calluna but accelerate the associated canopy leaching of DON and base cations, depending on foliar N status.
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