Influence of biosolid stability, temperature and water potential on nitrogen mineralisation in biosolid amended soils

Abstract

Soils with inherently low soil fertility, and nutrient depletion of fertile soils, are the root causes of declining per capita food production in Africa. On the other hand, demand for better water quality and strict environmental laws have led to an increase in biosolid production. Accumulation of this waste poses an increasing environmental pollution risk. Disposal methods like incineration, ocean dumping and land filling are causing enormous environmental and economic problems. Therefore, municipal authorities have been challenged with the environmental management of biosolids, whilst many farmers are facing a problem of soil fertility decline. Biosolids of “Exceptional quality class A” contain high organic matter, plant nutrients and have few restrictions on use for land application. Therefore, it is a valuable resource. Beneficiation of sewage sludge through land application is an optional solution to address both soil fertility and environmental problems. Scientific management of sewage sludge utilization must be observed to minimize environmental problems. The study of N release and the rate of nitrification from biosolids is essential to improve nutrient use efficiency, as well as to prevent environmental pollution. Mineralization and nitrification processes are influenced by several factors, for instance, the origin and quality of organic material, and soil environmental conditions, of which moisture and temperature are the most important factors. The study aims to: (i) evaluate biosolid stability, temperature and soil water effects on net N release from municipal and industrial sludge amended soil, and (ii) generate important parameters for modeling N dynamics (rate constants and half life). This dissertation consists of two major experiments: The first experiment was a fifty six day laboratory incubation study to assess N release and nitrification rate constants in a biosolid amended soil, as well as the biosolid’s half life time. The experiment was conducted using three types of biosolids originated from three different wastewater treatment processes, subjected to three levels of temperature and three of soil water potentials. The second experiment was an investigation on sample handling strategy for accurate nitrate (NO3-) and ammonium (NH4+) determinations. Different handling procedures: Direct field extraction, Field drying extraction and Laboratory drying extraction were tested on biosolid amended soils. In conclusion, biosolid stability, temperature and soil water interaction significantly influence mineralization and nitrification processes. Unstable sludges had higher N mineralization rate constant and shorter half life times compared to stable sludge, and the Direct field extraction procedure proved to be the most representative sample handling strategy for determination of N speciation in soils and biosolid amended soils to get representative time specific data.