The consequences of chronic nitrogen additions on N cycling and soilwater chemistry in a Sitka spruce stand, North Wales

Abstract

Major changes in dissolved nitrogen (N) leaching losses occurred during 5 yr of weekly applications of N to the forest floor of a 30-yr-old Sitka spruce ( Picea sitchensis (Bong.) Carr.) stand in Aber forest, N. Wales. N was applied as sodium nitrate (NaNO 3) at rates of 35 and 75 kg N ha −1 yr −1 and ammonium nitrate (NH 4NO 3) at 35 kg N ha −1 yr −1, representing either a 125% or 270% increase in total N deposition. There was a close relationship between NO 3 inputs and NO 3 leaching losses from below the rooting zone over the 5-yr period indicating this stand was NO 3-saturated pretreatment. In contrast, NH 4 was retained within the soil profile. Some evidence for mobilisation of reduced N from the forest floor N, in excess of inputs, was observed in all treatments in the initial years of N applications. However, this disappeared in later years and all mobilised N from the forest floor was retained in lower mineral horizons. In the mineral soil, both forms of N application resulted in elevated H + and/or total Al concentrations. These effects were transitory in the high-dosage NaNO 3 treatment because a new equilibrium was established between concentrations of Na in soil solution and amounts of Na on the soil exchange complex. However, continuing reductions in base cation to Al ratios were observed in the NH 4NO 3 35-kg-N-ha −1-yr −1 treatment due to acidification associated with NH 4 retention. No significant changes in dissolved organic carbon or base cation concentrations were observed in organic or mineral horizon soil solutions in any N treatment. The results demonstrated that any future increases in oxidised N deposition to similarly mature Sitka spruce stands will result in immediate increases in NO 3 leaching. In acid-sensitive areas, this will result in both eutrophication and acidification of stream water due to the link between NO 3 leaching and Al concentrations. The speed of response to increased deposition of reduced N will depend on the N status of the ecosystem. These results are discussed within the context of responses observed at other NITREX sites, and provide information on the rate and sequence of responses to chronic N deposition by forest ecosystems.