Nitrogen additions (NaNO 3 and NH 4NO 3) at Aber forest, Wales: I. Response of throughfall and soil water chemistry

Abstract

Nitrogen was sprayed weekly onto the forest floor of a Sitka spruce stand for 2.5 years in a replicated block design experiment to investigate the impacts of chronic atmospheric nitrogen deposition, in both oxidised and reduced form, on these ecosystems. The nitrogen was applied as sodium nitrate at 35 or 75 kg N ha −1 year −1 and as ammonium nitrate at 35 kg N ha −1 year −1, and the response of the throughfall and soil water chemistry determined. The response to increased water availability was also determined in a zero-N treatment. Increased water availability resulted in decreased concentrations of ammonium and phosphate and increased hydrogen ion concentration in the throughfall above the height of the spray, indicating tree growth is limited by water at this site. No effect of increased nitrogen availability on throughfall or stemflow chemistry above that of the water response was observed. Ambient nitrogen inputs in throughfall plus stemflow were 17 kg N ha −1 year −1 of which 11 kg N ha −1 year −1 was leached. Retention capacity of additional nitrogen applied to the forest floor was found to differ for nitrate-N and ammonium-N. There was no retention capacity for incoming nitrate-N but 100% retention capacity for applied ammonium-N. Nitrate leaching losses increased in parallel with nitrate inputs and were independent of ammonium addition. There was no apparent transformation of ammonium to nitrate in the ammonium nitrate treatment. Total dissolved nitrogen losses were therefore lower in the ammonium nitrate 35 kg N ha −1 year −1 treatment relative to the sodium nitrate 35 kg N ha −1 year −1. As the soil exchange complex is dominated by aluminium, the increase in salt loading and lack of nitrate retention in both treatments was associated with increased soil water aluminium concentrations due to ion exchange between the incoming sodium or ammonium ions and aluminium on the soil exchange complex. No increase in soil water concentrations of any other cation, with exception of sodium in the sodium nitrate treatments, was observed. These findings suggest that any increase in nitrate deposition will result in increased nitrate and aluminium leaching at this site which can perhaps best be described as ‘nitrate saturated’. Elevated ammonium deposition will also result in increased aluminium leaching if accompanied by a mobile anion. More long term monitoring is required to evaluate these changes in soil water chemistry on tree vitality and the internal nitrogen cycle.