Seasonal patterns of nitrogen availability in contrasting soils in the Chiltern Hills

The availability of inorganic nitrogen in three strongly contrasting soils, which supported almost pure stands of Deschampsia caespitosa in the Chiltern Hills, was investigated during a study of the edaphic tolerance of the species. Seasonal changes in the rates of accumulation of nitrate- and ammonium-N, measured using an incubation method, were followed during 1971-72. The potential for nitrogen mineralization was assessed using incubations at 25? C while more realistic estimates of mineralization rates prevailing in the field were made with incubations at temperatures appropriate to the dates of collection. In the chalk soil (pH 7.6-8.3) and calcimorphic brown earth (pH 7.0-8.0) only nitrate-N accrued and at modest rates. The acid mull soil (pH 3.7-4.2) produced much more inorganic nitrogen; for much of the year this was substantially in the ammonium form but a pronounced spring peak in nitrification was observed during two successive years with some, although much less, nitrification evident for most of the study period. Nitrification could be detected in the calcareous soils even at temperatures as low as 2? C. The seasonal pattern of nitrogen release is discussed in relation to controlling environmental factors and microbial activity in the soil as well as the edaphic tolerance of D. caespitosa.