Nitrogen mineralization in a sludge-amended sandy clay loam in South Africa

Abstract: Sewage sludge is a solid, semi-solid or liquid byproduct produced by waste water treatment plants. It contains both compounds of agricultural value (e.g. organic material, nitrogen, phosphorous, potassium and to a lesser extent calcium, sulphur and magnesium), and pollutants (e.g. heavy metals, pathogens and toxic organics). The quality of sludge is dependent on its origins, i.e. whether it is domestic or industrial. Sludge can be disposed in many ways, e.g. through land application, incineration and land filling. Generally the least cost option, land application, has become the most common disposal method in many countries. Sludge production has increased dramatically due to population growth and increased indutrialization and it is very important to reduce and utilize the digested sewage sludge and to minimize its environmental impact. One way to achieve this is to apply sludge on farm land. Land application has many advantages and is expected to become the dominant disposal method in the future. This is especially true for small communities which have high quality sludge in terms of lower concentrations of chemical pollutants. In sludge, N is in organic form but is mineralized to NH4+ and NO3- and therefore becomes plant available. The main aim of the present study was to compare N mineralization between two treatments (sludge amended soil (Sludge) and fertilization with ammonium nitrate (AN)) and a control (C) in a incubation experiment. A second aim was to quantify the effect of the treatments on soil pH. In both Sludge and AN, equal amounts of total N in organic and inorganic form were added. The upper limit for the amount of sludge farmers are allowed to apply on arable land in South Africa is 8 tonnes dry matter ha-1 year -1 and this was also the amount used in this incubation study. In all three treatments, mineral N changed significantly during the incubation. For Sludge and C, mineral N increased with time while it decreased for AN. This decrease in inorganic N was unexpected and might be due to gaseous N losses. The pH did not change significantly with time for Sludge and C but decreased significantly for AN as the soil became more and more acidified due to proton release during nitrification.