Nitrate fluctuations in tropical soils

Abstract

The soil nitrate content has been determined at fortnightly intervals on bare fallow plots and on plots under crops and natural vegetation in the forest and savannah regions of Ghana. Nitrifiable nitrogen, ammonium and soil moisture contents have also been determined. It has been found that the changes in nitrate content are related to changes in the content of nitrifiable nitrogen. During the dry season, a partial sterilization of the soil occurs and consequently the content of nitrifiable nitrogen rises. This nitrifiable nitrogen is slowly converted to nitrate in the dry season and rapidly converted once the rains commence. Nitrate is then lost from the soil by leaching and denitrification. Under bare fallow conditions leaching is the most important cause of nitrate loss, but it is probable that denitrification makes some contribution on cropped soils and those under permanent vegetation. There is no indication of microbial nitrate absorption under a crop. This is presumably due to the rapidity with which nitrate is lost by leaching and denitrification, and to the greater quantity of organic matter required in these free-draining tropical soils than in most temperate ones to produce an equivalent absorption of nitrate. The high levels of nitrate and nitrifiable nitrogen which are found under natural forest probably represent an equilibrium level typical of a cycle in which nitrogen fixation and nitrification as well as nitrate losses proceed very rapidly. Natural savannah grassland suppresses nitrogen mineralization in the field almost entirely, but the nitrifiable nitrogen content is at times high, in spite of the very high carbon/nitrogen ratio of the soil of over 20. This confirms the view that little nitrate absorption takes place in spite of the presence of what might be considered excess carbonaceous material. The suppression of mineralization under grass is considered due to an excretion of the plant roots which is toxic to the nitrification process. The failure of organic matter to build up in these savannah soils, although mineralization is suppressed, is most probably associated with their low general fertility.