Mitigation Option of Greenhouse Gas Emissions from Livestock Manure Composting

Composting of livestock manure is a significant source of greenhouse gases such as nitrous oxide (N2O), but the complexity of the N2O generation pathway hinders efforts to develop effective countermeasures. In this paper, we present one mitigation option for N2O emissions from composting. Nitrite (NO2) has an important role in N2O generation. It has been observed that significant N2O emissions from composting are induced when NO2 is accumulated during nitrification. Accordingly, in our technique, nitrite-oxidizing bacteria (NOB), as nitrifying bacteria, are added in the middle of composting fermentation to prevent NO2 accumulation. Adding NOB prevents prolonged NO2 accumulation, which had resulted in low N2O emissions. This technique is also cost-effective, because mature compost can be used as an NOB source. Mitigating N2O emissions allows more nitrate nitrogen (NO3), of high value as fertilizer, to be preserved in the compost product. Moreover, it was confirmed that this technique could be combined with that for NH3 mitigation using chemical reagents. Before actual use, it is necessary to investigate the methods used to determine optimal timing to add NOB source and prevent N2O from being generated from an NO3 reduction. Discipline: Agricultural environment, animal industry Additional key words: nitrification, nitrite-oxidizing bacteria, nitrous oxide Present address: 2 Animal Feeding and Management Research Division, National Institute of Livestock and Grassland Science, National Agriculture and Food Research Organization (NARO) (Nasushiobara, Tochigi 329-2793, Japan) *Corresponding author: e-mail yasuyuki@affrc.go.jp Received 23 October 2014; accepted 22 January 2015. Introduction Composting is a principal means of treating organic waste such as livestock manure. However, during the composting process, substantial amounts of harmful environmental gases are emitted, including greenhouse gases such as nitrous oxide (N2O) (Czepiel et al. 1996, Fukumoto et al. 2003a, Osada et al. 2000, Sommer 2001). Accordingly, reducing the emission of harmful gases from the composting process has become increasingly important. N2O is a powerful greenhouse gas, with a global warming effect around 300-fold stronger per molecule than carbon dioxide (IPCC 2001). Moreover, N2O also impacts on ozone layer depletion (Crutzen 1981). Agriculture is the largest source of anthropogenic N2O emissions, of which livestock activity in particular makes a significant contribution (FAO 2006). Accordingly, it is important to develop a technique to reduce N2O emissions from livestock activity, including composting of livestock manure. However, there are relatively few countermeasures to reduce N2O emissions from composting compared to NH3 emissions. The authors have been trying to develop countermeasures to reduce N2O emissions from composting of livestock manure. In this paper, we present the mitigation option and discuss issues concerning its actual usage. N2O generation in the composting process N2O is generated via both nitrification and denitrification processes as intermediate products or by-products during the composting process. The lack of nitrite/nitrate contained in fresh manure means the nitrification start is a prerequisite for N2O generation from the composting process. Nitrification is performed by two kinds of bacteria, i.e.