Process level significance of changes to soil nitrification due to nitrogen enrichment
Our ability to predict and model the consequences of N deposition depends on our understanding of the mechanisms of N cycling. The research detailed in this thesis investigates the impact of enhanced nitrogen loading on nitrification and mineralisation in soils from two contrasting sites in Britain. The first site is Deepsyke Forest, a Sitka spruce plantation situated in the Scottish borders. The second site is Pwllpeiran an upland grass heath in Mid Wales. The stable isotope, 15N, was used to measure gross mineralisation and nitrification rates at each site. This was used in conjunction with acetylene, a physiological block, which inhibits the oxidation of ammonium to nitrite; ascertain whether the dominant form of nitrification at each site was autotrophic or heterotrophic. Inorganic N concentrations at Deepsyke were highly variable. NO3- levels were low at both sites but particularly at Deepsyke. This had a major impact on the methodology used in the determination of gross transformation rates, and the subsequent analysis and interpretation of the results. Nitrification at Deepsyke was predominantly heterotrophic and rates whereas nitrification at Pwllpeiran was predominantly autotrophic. The results of a 20-day potential nitrification assay indicate that S deposition is inhibiting nitrification, while enhanced N loading is stimulating it. No other effects of N, S or acid deposition were detectable at Deepsyke. Similarly, no effects of N loading were found, however it was observed that heavy grazing reduced NH4+ concentrations.