Formation and emissions of ammonia and greenhouse gases from stored and composted solid animal manure
Beside other branches of industry, agriculture is an important source of methane (CH4) and nitrous oxide (N2O) emissions and the most important source of ammonia (NH3). In spite of several investigations concerning ammonia and greenhouse gas emissions during storage and composting of liquid and solid animal manure, the real extent of these emissions is still unknown. This is mainly because of the lack of knowledge about causes and reasons of microbial trace gas formation in such biological substrates. Hence we investigated some important parameters influencing the growth and metabolism of the involved microorganisms and consequently the amount of trace gas emissions from solid manure.
Solid manure consists of solids (litter, excreta, feed residue), liquids (urea, water) and gas filled pores. The composition depends on the kind and age of the animals, the feeding practise and above all the amount of litter (predominant straw). In practice dry matter contents range between 12 and 30 % depending on the employed amount of straw.
The microbial processes occurring during the storage/composting period, which can last up to six month, depend on the conditions within the solid manure. The oxidation of organic matter under aerobic conditions causes heat leading to temperatures up to 75 degrees Celsius in the centre of the manure heap. Percolating water as a result of the decomposition process hinders the gas exchange and is therefore responsible for the generation of oxygen gradients among temperature gradients. Because of the different oxygen availability and different temperatures between outer and inner regions of a manure heap also the microbial processes differ leading to an inhomogeneous decomposition of the solid manure. Regular rotating of the manure in case of composting balance out the differences (Schuchardt and Hahne, 1996).
Structure and consistency of solid manures are mainly influenced by the amount of straw used: Manures containing small quantities of litter are wet and compact with little air filled pores and predominant anaerobic conditions. In this case methane and carbon dioxide are thought to be important end products of the biodegradation of organic material. With increasing structure material (straw) nitrification processes occur leading to the formation of nitrite and nitrate under conditions of sufficient oxygen availability and to nitrous oxide emissions at restricted oxygen supply. The metabolic products (nitrite and nitrate) can be transported by diffusion or mass flow in percolating water or by heap rotation from aerobic to anaerobic zones where denitrification processes set in resulting in further nitrous oxide formation.