A molecular analysis of betaproteobacterial ammonia oxidizers and a N 2 O isotopomer analysis were conducted to study the sources of N 2 O emissions during the cow manure composting process. Much NO 2 ؊ -N and NO 3 ؊ -N and the Nitrosomonas europaea-like amoA gene were detected at the surface, especially at the top of the composting pile, suggesting that these ammonia-oxidizing bacteria (AOB) significantly contribute to the nitrification which occurs at the surface layer of compost piles. However The very sensitive greenhouse gas nitrous oxide (N 2 O) has a 296 times higher impact than CO 2 (39) and is also responsible for ozone depletion (10). Agricultural activities such as the use of nitrate fertilizers, livestock production, and manure management, including composting, are known to be important sources of N 2 O emissions (18). To devise a strategy to mitigate N 2 O emissions, it is essential to understand its sources in detail. However, the sources of N 2 O emissions during the composting process are still largely unclear.In the composting process, a part of NH 4 ϩ -N is known to be processed through nitrification-denitrification and emitted as N 2 and N 2 O. Nitrous oxide is known to be generated through both the nitrification and denitrification processes as intermediate products or by-products. Nitrous oxide emission is a very complex process because denitrifying bacteria are phylogenetically diverse (60), and nitrifiers are also known to utilize the denitrification process even under aerobic conditions (42). It is thus very difficult to estimate the relative contributions of nitrification and denitrification in actual N 2 O emissions from the environment. Until now, there has been insufficient knowledge about the relative contributions of these processes to N 2 O emissions during the animal manure composting process. Measurement of the actual contributions of N 2 O emissions from compost piles in the field is therefore critical to establishing a strategy of mitigating N 2 O emissions.Recently, a high-precision analytical technique for determining intramolecular 15 N site preference in asymmetric molecules of N 2 O was developed (47). Since N 2 O has two N atoms within the molecule (central and outer N
SummaryComposting is the major technology in the treatment of animal manure and is a source of nitrous oxide, a greenhouse gas. Although the microbiological processes of both nitrification and denitrification are involved in composting, the key players in these pathways have not been well identified. Recent molecular microbiological methodologies have revealed the presence of dominant Bacillus species in the degradation of organic material or betaproteobacterial ammonia‐oxidizing bacteria on nitrification on the surface, and have also revealed the mechanism of nitrous oxide emission in this complicated process to some extent. Some bacteria, archaea or fungi still would be considered potential key players, and the contribution of some pathways, such as nitrifier denitrification or heterotrophic nitrification, might be involved in composting. This review article discusses these potential microbial players in nitrification–denitrification within the composting pile and highlights the relevant unknowns through recent activities that focus on the nitrogen cycle within the animal manure composting process.
The greenhouse gas (GHG) [methane (CH 4 ) and nitrous oxide (N 2 O)] mitigation effects of mixing dried grass into passively-aerated manure during the composting process (which accounts for 68.7% of Japanese dairy manure management) were assessed. Gaseous emissions [CH 4 , N 2 O, carbon dioxide (CO 2 ) and ammonia (NH 3 )] from about 4 t of fresh dairy manure with or without 400 kg of dried grass mixed in were measured by the dynamic chamber method. The addition of dried grass contributed to a decrease in GHG emissions from 20.8 AE 1.3 g kg À1 volatile solids (VS) to 5.4 AE 1.4 g kg À1 VS (74.3% mitigation) for CH 4 and from 7.4 AE 2.6 g N 2 O-N kg À1 N initial to 2.7 AE 0.4 g N 2 O-N kg À1 N initial (62.8% mitigation) for N 2 O. By applying this strategy, the expected reduction of GHG emission would be 70,466 t CH 4 yr À1 and 1379 t N 2 O-N yr
À1(1907 Gg CO 2 eq. yr À1 in total) in the Japanese dairy sector. On the other hand, it was showed that CO 2 and NH 3 emissions increase [from 424.4 AE 214.9 g CO 2 kg À1 VS to 603.8 AE 99.6 g CO 2 kg À1 VS for CO 2 and from 16.9 AE 7.1 g ammonium-nitrogen (NH 3 -N) kg À1 N initial to 38.3 AE 3.5 g NH 3 -N kg À1 N initial for NH 3 ] by this method. Moreover, the mechanism of this significant N 2 O mitigation effect cannot be explained, and a better understanding of this effect could further improve the GHG mitigation strategy.
The diversity and dynamics of the denitrifying genes (nirS, nirK, and nosZ) encoding nitrite reductase and nitrous oxide (N(2)O) reductase in the dairy cattle manure composting process were investigated. A mixture of dried grass with a cattle manure compost pile and a mature compost-added pile were used, and denaturing gradient gel electrophoresis was used for denitrifier community analysis. The diversity of nirK and nosZ genes significantly changed in the initial stage of composting. These variations might have been induced by the high temperature. The diversity of nirK was constant after the initial variation. On the other hand, the diversity of nosZ changed in the latter half of the process, a change which might have been induced by the accumulation of nitrate and nitrite. The nirS gene fragments could not be detected. The use of mature compost that contains nitrate and nitrite promoted the N(2)O emission and significantly affected the variation of nosZ diversity in the initial stage of composting, but did not affect the variation of nirK diversity. Many Pseudomonas-like nirK and nosZ gene fragments were detected in the stage in which N(2)O was actively emitted.
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