Novel studies of the dynamics of mineral N species transformations and their mobilities relevant to assessing risks to drainage water.

ABSTRACT The biogeochemical cycling of nitrogen has been studied in detail at a predominantly acid grassland nature reserve, Hob Moor, just outside the city of York in the UK. Because of the risk of more frequent and/or extended summer droughts as a potential consequence of climate change, particular attention was paid to the influence of drying and rewetting upon the mobility of mineral N species. It was found that ammonification proceeds after net nitrification has ceased, and that subsequently nitrate is immobilized when the drying period is protracted. It is suggested that this is probably due to uptake by drought-tolerant microorganisms. The experimental work showed that much of the first flush of nitrate after a period of drying is therefore attributable to stored nitrate, as long as the drying out period is not excessively long. The mobility of ammonium-N in soils from Hob Moor was studied to test the hypothesis that in the heavily N-impacted soils at the site it would be more mobile than most soil scientists would predict, by measuring adsorption/desorption characteristics. The absorption isotherms confirmed that ammonium in these soils is potentially mobile, and when mobilized below the rooting depth may pass to the adjacent stream around the edge of the site. This helps explain the high ammonium-N and nitrate-N concentrations observed in this stream. A developing interest in the Gaia hypothesis prompted the author to make a brief preliminary investigation of the idea that deciduous trees have evolved naturally to provide a close match between the dynamics of N release by litter decomposition and the dynamics of plant N requirement. The experiment showed that initially the fresh litter with a high C/N ratio immobilized nitrate especially in the forest soil. Under the relatively warm conditions of the experiment decomposition was rapid, and the immobilization was not sustained as would be predicted. Further evaluation of this concept is advocated. The extent of immobilization by litter prompted a study of long-term seasonality of trends in nitrate concentration throughout the River Derwent in North Yorkshire using data obtained from the Environment Agency. It was thought that a ban on straw burning in 1993 might have reduced winter annual nitrate concentration peaks and possibly increased summer minima. The data partly supported this idea, but the timing match was not perfect, and it was thought that the foot and mouth disease impact and farmers’ responses to environmental concerns and policy and to increasing fertilizer and energy costs were probably also important. Finally a study was made of the importance of storage conditions upon extractable ammonium and nitrate concentrations in soils. Surprisingly nitrification was not sufficiently inhibited in some soils stored under refrigerated conditions overnight, and it is concluded that volumetric sampling and immediate extraction in the field may be a preferred option.