An investigation of composting poultry manure in relation to nitrogen conservation and phosphate rock dissolution : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University

Gaseous losses of nitrogen (N) through ammonia (NH3) volatilization and denitrification diminish the fertilizer value of animal and poultry manures and form atmospheric pollutants. Appropriate methods of composting can improve the fertilizer value by conserving manure N and minimise the risk of environmental pollution. Additionally, acids produced during composting and nitrification of conserved N in manure, provide a source of protons (H+) which have the potential to dissolve phosphate rock (PR) when composted with manure. The objective of this research was to examine the methods of composting poultry manure with different amendments in relation to N conservation and PR dissolution. Firstly, the methods for preserving and measuring N in manure samples were evaluated. The results suggested that freeze drying and inclusion of a strong oxidizing agent (KMnO4) prior to routine Kjeldahl digestion were required to achieve complete recovery and accurate measurement of N in manure samples. Methods for reducing the loss of N through NH3 volatilization and denitrification during aerobic and anaerobic composting of poultry manure with different amendments were investigated in controlled 'flow through' incubation experiments. The amendments included carbonaceous bedding materials (woodchip, paper waste, wheat straw and peat), acidifier (elemental sulphur-S°) and adsorbents (zeolite and soil). The loss of N through NH3 volatilization from aerobic composting was about 17% of the manure N which was reduced by 90-95% under anaerobic composting. Under aerobic composting the addition of various amendments reduced the volatilization loss by 33 to 60%. Although the rate of denitrification was negligible (<1µg kg-1 day-1) in fresh manure, it increased enormously (3.7mg kg-1 day-1) during composting. The presence of nitrate (NO3-) was found to be a rate determinant for denitrification in manure. Amongst the treatments, the addition of S° was very effective in reducing NH3 volatilization and denitrification. The dissolution of PR during composting with poultry manure was examined using radioactively (32P) labelled synthetic francolite and North Carolina phosphate rock (NCPR). The use of 32P labelled francolite indicated that PR dissolution in poultry manure/PR composts could be measured more accurately from the increases in NaOH extractable phosphorus (∆NaOH-P) than from the decreases in HCl extractable P (∆HCl-P). Low levels of francolite and NCPR dissolution (