Nitrate distribution in tropical soils

Abstract

Many investigators in the tropics report high nitrate content in the topsoil during the drier part of the year (Schofield 9, Hardy 3, Meiklejohn6, Jones4). This phenomenon is not as yet understood, since soil temperatures are generally believed to be too high and soil water content too low during such periods for biological processes to take place. This has. been confirmed in laboratory studies by Meiklejohn 6 in which maximum temperatures for nitrification were lower than found in the field at the time and place of accumulation. According to Meiklejohn6, there is some circumstantial evidence from field experiments in Kawanda, Uganda, for support of the photo-chemical oxidation hypothesis, as suggested by Dhar et al.2. Viz (i) the greatest accumulation took place in bare fallow soil exposed to sunlight, (ii) nearly all nitrate was in the top inch of the soil, and (iii) maximum temperatures were regarded as being too high and soil water content too low for bacterial activity. This phenomenon of increase in nitrate content in the top soil during the dry season has been observed at the Katherine Research Station, Northern Territory, Australia, on bare fallow plots of Tippera clay loam. These plots are a part of an experiment in which certain aspects of the nitrogen cycle are being studied. The soil, derived from limestone, is a sandy clay loam overlying clay subsoils, with a pH of 6.5 in the topsoil (Stewart n). At 8 inches depth