THE CONSEQUENCES OF CHRONIC NITROGEN ADDITIONS ON N CYCLING AND SOILWATER CHEMISTRY IN A SITKA SPRUCE STAND, NORTH WALES

Major changes in dissolved nitrogen (N) leaching losses occurred durin g 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 (NaNO3,) at rates of 35 and 75 kg N ha(-1) yr(-1) and ammonium nitrate (NH4NO3,) at 35 kg N ha(-1) y r(-1) representing either a 125% or 270% increase in total N depositio n. There was a close relationship between NO3, inputs and NO3, leachin g losses from below the rooting zone over the 5-yr period indicating t his stand was NO;(3)-saturated pretreatment. In contrast, NH4, was ret ained within the soil profile. Some evidence for mobilisation of reduc ed N from the forest floor N, in excess of inputs, was observed in all treatments in the initial years of N applications. However, this disa ppeared in later years and all mobilised N from the forest floor was r etained 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 NaNO3, treatment beca use 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 th e NH4,NO3, 35-kg-N-ha(-1)-yr(-1) treatment due to acidification associ ated with NH4, retention. No significant changes in dissolved organic carbon or base cation concentrations were observed in organic or miner al horizon soil solutions in any N treatment. The results demonstrated that any future increases in oxidised N deposition to similarly matur e Sitka spruce stands will result in immediate increases in NO3, leach ing. In acid-sensitive areas, this will result in both eutrophication and acidification of stream water due to the link between NO3, leachin g and Al concentrations. The speed of response to increased deposition of reduced N will depend on the N status of the ecosystem. These resu lts are discussed within the context of responses observed at other NI TREX sites, and provide information on the rate and sequence of respon ses to chronic N deposition by forest ecosystems. (C) 1998 Elsevier Sc ience B.V.