Net nitrogen mineralization,nitrification and CO2 production in alternating moisture conditions in an unfertilized low-humus sandy soil from the Sahel

In this study the rates of net mineralization, net immobilization and net nitrification have been quantified under laboratory conditions in a sandy low-humus soil from a semi-arid region, in absence of plant growth. Incubation experiments were carried out under constant humidity and under alternating wet and dry conditions to simulate field conditions during the rainy season. The ammonium and nitrate content of the incubates were determined and their CO₂ production measured.The rate of net mineralization at field capacity was 0.6 kg N ha^–1d^–1 during the first 40 days and decreased to 0.06 kg N ha^–1d^–1 after 400 days. This rate was twice as high on wet days under alternating wet and dry conditions. The rate of net nitrification during alternating wet and dry conditions was also higher (1.9 kg N ha^–1d^–1) than at constant field capacity (1.3 kg N ha^–1d^–1) until the ammonium was almost completely depleted. These rates of net mineralization and net nitrification are in agreement with field observations.Net immobilization did not occur in the experiments, unless glucose was added to the soil.The data on CO₂ production and net mineralization showed that the C/N ratio of the degraded material was around 9 or below. It is much lower than the ratio of total carbon over total nitrogen in the soil. This indicates that microorganisms and compounds high in nitrogen were mineralized. Certainly after about 30 days the only growth taking place is based on turnover of material of the microbial biomass itself.A decrease in the amount of inorganic nitrogen was observed upon drying of the soil, while it returned to the original content after rewetting. It is postulated that this might be due to temporary uptake of nitrogen in an inorganic form in microorganisms as part of their osmoregulation.