Nitratation Promotion Process for Reducing Nitrogen Losses by N 2 O/NO Emissions in the Composting of Livestock Manure

Soils play multiple roles in the quality of life throughout the world, not only as the resource for food production, but also as the support for our structures, the environment, the medium for waste disposal, water, and the storage of nutrients. A healthy soil can sustain biological productivity, maintain environmental quality, and promote plant and animal health. Understanding the impact of land management practices on soil properties and processes can provide useful indicators of economic and environmental sustainability. The sixteen chapters of this book orchestrate a multidisciplinary composition of current trends in soil health. Soil Health and Land Use Management provides a broad vision of the fundamental importance of soil health. In addition, the development of feasible management and remediation strategies to preserve and ameliorate the fitness of soils are discussed in this book. Strategies to improve land management and relevant case studies are covered, as well as the importance of characterizing soil properties to develop management and remediation strategies. Moreover, the current management of several environmental scenarios of high concern is presented, while the final chapters propose new methodologies for soil pollution assessment. following:

distribute the manure from a limited to a wider area. Therefore, local pollution by livestock manure can be avoided. Moreover, the construction of a sustainable agricultural system is expected to be achieved by enhancing the circulation between the livestock industry and field husbandry via compost of livestock manure. It is important to improve composting techniques because substantial amounts of harmful gaseous compounds are emitted during the composting process (Fukumoto et al. 2003;Kuroda et al. 1996;Smet et al. 1999). Because of its high nitrogen contents, nitrogenous emission is substantial in livestock manure composting. Those nitrogenous emissions cause not only serious environmental risks, but also a decline in the compost's value as a fertilizer. Ammonia (NH 3 ) is one of the common nitrogenous emissions, which becomes a main cause of odor coming from the livestock industry (McGinn and Janzen, 1998). Moreover, NH 3 can be the cause of more extensive environmental pollution such as acid rain (Pearson and Stewart, 1993). Due to its high impact on the regional environment, there have been numerous attempts to reduce NH 3 emission from the composting process (Burrows, 2006;Kuroda et al. 2004;la Pagans et al. 2005;Lin, 2008;Yasuda et al. 2009). Nitrous oxide (N 2 O) is also one of the nitrogenous emissions arising from livestock manure composting (Beck-Friis et al. 2001;Czepiel et al. 1996;Osada et al. 2000;Sommer, 2001;Zeman et al. 2002). N 2 O is an important greenhouse gas, having a 296-fold stronger effect than carbon dioxide (IPCC, 2007). Additionally, a recent study disclosed that N 2 O has now become the largest ozone-depleting substance, surpassing chlorofluorocarbons (Ravishankara et al. 2009). Agriculture is the largest source of anthropogenic N 2 O emission (Oenema et al. 2005). Livestock activities in particular contribute to almost two-thirds of all anthropogenic N 2 O emissions, and 75-80 percent of agricultural emissions, which are mainly caused by manure (FAO, 2006). However, as of now, the numbers of countermeasures to reduce N 2 O emissions from composting are quite few compared with those established for NH 3 emission. This chapter presents recent knowledge of N 2 O reduction from the composting process. The technique developed for reducing N 2 O emission is termed the nitratation promotion process. The author will explain effects of the nitratation promotion process on N 2 O/NO emissions and nitrogen conservation during swine manure composting. Moreover, the collaboration effect of nitrogen conservation with another countermeasure for reducing NH 3 emission, and the possibility of an adaptation of the nitratation promotion process to other kinds of livestock manure (cattle and poultry) will also be discussed.  Nitrification is performed by different groups of microbes. Ammonia-oxidizing bacteria (AOB) converts NH 3 into nitrite (NO 2 -) (this reaction is called nitritation), and nitriteoxidizing bacteria (NOB) converts NO 2 -into nitrate (NO 3 -) (this reaction is called nitratation). Additionally, ammonia-oxidizing archaea (AOA) is thought to have important role in the oxidization of ammonia under various conditions, such as soil, sediments, and seawater (Francis et al. 2005;He et al. 2007;Leininger et al. 2006;Tourna et al. 2011;Wuchter et al. 2006). However, the evidence that AOA activity takes place in the composting process has not yet been confirmed (Maeda et al. 2010b). Nitrification is a prerequisite for N 2 O generation from stored manure because little nitrate nitrogen is contained in the manure immediately after excretion. Nitrate produced by nitrification has an opportunity to transfer to an anaerobic portion by disturbance of compost pile (turning). The nitrate is then reduced into dinitrogen gas (N 2 ) via N 2 O by denitrifiers. Inside the compost pile, both aerobic/anaerobic portions coexist, which makes it difficult to estimate the respective contributions of nitrification and denitrification in actual N 2 O emissions from composting (Maeda et al. 2011). It is known that there are several factors affecting N 2 O emission from composting, such as moisture content, amount of mixed bedding material, frequency of pile turning, compost pile scale, and so on (Beck-Friis et al. 2001;Fukumoto et al. 2003;Parkinson et al. 2004;Szanto et al. 2007). It is thought that these factors influence the N 2 O generation pathway directly or indirectly. As a result, the amount of N 2 O emitted from the composting would be decided by a synthetic influence of those factors. Therefore, reduction of gas emission by control of these factors is thought possible if there is a factor largely responsible for gas generation. The relative contributions of nitrification and denitrification to N 2 O emission do not become an important issue in the development of countermeasures, because N 2 O can be reduced by controlling such factors, whether the factor induces N 2 O generation via nitrification or denitrification. The more important issue is to find such a factor during the treatment process. However, when considering how to magnify the adaptable range of a countermeasure, clarifying the mechanism of the N 2 O generation would be an important issue that also has large scientific interest.

Effect of nitrite accumulation on N 2 O emission
One important factor for N 2 O emission from the composting is that NO 2 -can be easily found. He et al. (2001) showed that the amount of N 2 O emission from food waste composting had increased when NO 2 -was accumulated. Moreover, a good NO 2 -− N 2 O correlation has been confirmed under various environmental conditions. Nitrite is an intermediate product of the nitrification process. Generally, NO 2 -is scarcely observed in the natural environment because nitrite oxidizers, such as Nitrobacter and Nitrospira, oxidize NO 2 -to NO 3 -immediately. However, it is also a fact that NO 2 -accumulation is observed under several environmental conditions (Burns et al. 1996;Corriveau et al. 2010;Silva et al. 2011). In swine manure composting, notable N 2 O emission begins after the thermophilic phase, because nitrifying bacteria cannot be active under thermophilic conditions, such as high temperature, high free ammonia and high organic matter content. After N 2 O emission starts, it tends to continue for a long time during the maturation phase of swine manure composting. It has been confirmed that NO 2 -accumulates in the composting material during this period of N 2 O emission. Because of the long duration of N 2 O emission in the maturation phase, the amount of N 2 O emission induced by NO 2 -accumulation accounts for a large portion of the total N 2 O emission in swine manure composting. Nitrite accumulation in swine manure composting is due to an inadequate nitrification process, i.e., the growth of indigenous NOB is inhibited while indigenous AOB is active immediately after the thermophlic phase, leading to a lower oxidization rate of NO 2 -. In this case, NO 2 -can be r e g a r d e d a s a c r i t i c a l f a c t o r o f N 2 O generation. Therefore, it is possible to develop a countermeasure for reducing N 2 O emission by the regulation of NO 2 -during the composting process. Two ways of avoiding NO 2 -accumulation can be considered. One method is to use a reagent of nitrification inhibitor, such as nitrapyrin. Its effect as a nitrification inhibitor that reduces N 2 O emission from soil has been confirmed in numerous studies (Bhatia et al. 2010;Dittert et al. 2001;Zaman and Blennerhassett, 2010). However, to our knowledge, there have been no studies investigating the effect of nitrification inhibitors on N 2 O emission during the composting process. Probably, disadvantages, such as ammonia accumulation and increasing treatment cost, would make it difficult to use in actual practice. Another way to reduce NO 2 -is to enhance NO 2 -oxidization, i.e., nitratation promotion. It is thought that the nitratation function is hurt or reduced when NO 2 -accumulates during nitrification. Therefore, for nitratation promotion, recovery of the nitratation function is necessary. In swine manure composting, the growth of NOB is inhibited, causing NO 2accumulation. There are two ways to recover the NOB growth. One way is to control the environment so that the composting material is suitable for NOB growth, e.g., decreasing the level of free ammonia. On the other hand, a bioremediational technique of adding NOB is also an effective candidate for the recovery of NOB growth. In fact, controlling the compost to be suitable for NOB growth is difficult. Therefore, the authors tried to develop a www.intechopen.com

Nitratation promotion process
In the first study conducted for reducing N 2 O emission from swine manure composting, effects from the addition of a NOB source on NO 2 -accumulation, and the state of nitrifying bacteria and N 2 O emission have been investigated using a laboratory-scale apparatus (Fukumoto et al. 2006).

Materials and methods
Two kinds of NOB sources were used. One was incubated mature swine compost, in which NOB had been concentrated at a high density by an incubation process (NOB density: 10 11 cell/g). The other NOB source was normal mature swine compost (NOB density: 10 6 cell/g). These materials were added to the composting swine manure after the thermophilic phase because NOB activity is strongly restricted under high temperature conditions. Gas emission was monitored continuously using an infrared photoacoustic detector.

Results and discussion
After the addition of an NOB source to the composting swine manure, the establishment of an NOB population at the cell density of 10 5 -10 7 cell/g in the composting material was confirmed, while the absence of NOB continued for 5-6 weeks after the start of AOB growth in the composting bin without the addition of NOB (control) (Fig. 2). Because NOB was absent, NO 2 -accumulated for a long duration in the control (precisely, the absence of NOB resulted in the cell number of NOB being under the detection limit of 10 2 cell/g). On the other hand, a prolonged NO 2 -accumulation was not observed in the composting with an NOB source addition (Fig. 3). Moreover, the effect on NO 2 -avoidance was also obtained by the addition of normal mature compost, which meant that no specific treatment, such as incubation of NOB, was necessary for preparation of the NOB source. The required cell number of NOB for the oxidation of NO 2 -to NO 3 -seems to be more than 10 5 cell/g compost. Blouin et al. (1990) showed a similar result regarding the required cell number for complete NO 2 -oxidization in swine manure. However, because the population size of NOB grows after a source addition, it is not always necessary for NOB to exceed 10 5 cells per gram compost at the time of the NOB source addition. The pattern of N 2 O emission from swine manure composting agreed well with changes in the NO 2 -concentration (Fig. 4). Therefore, the duration of NO 2 -accumulation affects the amount of N 2 O emission. In the normal composting (control), the N 2 O emission continued until NO 2 -disappeared. As a consequence, the amount of N 2 O emission throughout the composting became large (N 2 O emission rate: 88.5 g N 2 O-N/kg TN initial ). On the other hand, because of its shortened duration of NO 2 -accumulation, the amount of N 2 O emission was reduced in the compost with an NOB source addition (N 2 O emission rate: 17.5-20.2 g N 2 O-N/kg TN initial ). The rate of N 2 O emission decrease by the addition of an NOB source was calculated as 77-80% in this study (Table 1)

Nitrogen conservation
The nitrogen that had avoided being lost as an N 2 O emission by applying the NIPRO process seemed to be preserved in the form of nitrate nitrogen in the compost product. Fig. 6 shows the changes in the NH 4 + , NO 2 -and NO 3 -contents during swine manure composting. During the thermophilic phase (0-3 week), NH 4 + accounted for most of the inorganic nitrogen. After the start of nitrification, NO 2 -/NO 3 -nitrogen began to increase, while NH 4 + declined. However, a remarkable difference in NO 3 -increase between the two treatments was observed. In the composting without an NOB addition (control), NO 3 -content increased slowly. In particular, the rate of NO 3 -increase became slower during NO 2 -accumulation. On the other hand, NO 3 -in the composting with the NIPRO process increased quickly after the addition of an NOB source, and then it reached a concentration in the final product higher than that of the control. Therefore, it was hypothesized that the NIPRO process contributes to nitrogen conservation. To quantify the effect of the NIPRO process on nitrogen conservation, a nitrogen balance in swine manure composting was investigated (Fukumoto and Inubushi, 2009).

Materials and methods
Fresh swine manure was mixed with sawdust to make it suitable for aerobic decomposition. Sixteen kilograms of mixture as wet matter (WM) was piled inside a laboratory-scale apparatus. In the NIPRO run, 500 g WM of mature swine compost was added at turning on day 18 to avoid decreasing the number of NOB added at high temperature.

Results and discussion
In this composting experiment, the amount of N 2 O emission was reduced by 70% by applying the NIPRO process, while there was little difference in NH 3 emission (Fig. 7). At the start of this composting experiment, approximately 130 g of nitrogen was contained in the initial compost pile of 16 kg (WM). The total mass of nitrogen emitted as N 2 O in the control and the NIPRO run were 12.3 g and 4.0 g, respectively, and the amount of NH 3 -N emission, which occurred mostly in the thermophilic phase, was 13.0 g in both runs. Therefore, the amounts of nitrogen loss by NH 3 and N 2 O emission in the control and the NIPRO run were calculated as 25.2 g and 16.9 g, respectively. However, total nitrogen loss in the control and the NIPRO run were 36.8 g and 17.6 g, respectively (Fig. 8). Nitrogen loss mainly occurred after the thermophilic phase, with the exception of NH 3 and N 2 O, and its magnitude became very small in the NIPRO run process that prevented prolonged NO 2accumulation. Therefore, it is considered that the unexplained nitrogen loss is expanded by NO 2 -accumulation during the composting, and that the effect of the NIPRO process on nitrogen conservation has a possibility to become larger than expected. When the composition of nitrogen components in the final compost product between the control and the NIPRO run was compared, the organic nitrogen content made little difference between the runs, but NO 3 -nitrogen, which is a fast release fertilizer, increased greatly in the NIPRO run (   Kester et al. (1997) reported that higher NO 2 -concentrations enhanced both N 2 O and nitric oxide (NO) emissions in the continuous cultures of nitrifiers and denitrifiers. Therefore, the emission of NO was measured during swine manure composting to clarify the components of the unknown nitrogen emissions. As a result of the measurement, it was revealed that significant NO emission begins after the thermophilic phase and is enhanced by NO 2accumulation during the composting, as with N 2 O. Therefore, NO emission will also be reduced by applying the NIPRO (Fig. 9). However, the portion of NO emission from the total nitrogen loss tended to be small compared with NH 3 and N 2 O emissions, even in the control, especially when the moisture content of the composting material was high. In one example, the level of nitrogen loss as an NO emission was only one-tenth the magnitude of the N 2 O emission in our composting experiment. However, there was also a case in which the amount of NO emission had become half of the N 2 O emission under comparatively dry conditions (Fukumoto et al. 2011a). It is known that moisture content is an important factor related to NO emission (del Prado et al. 2006), and these results seemed to have reflected it. Information concerning NO emissions from composting is limited (Hao and Chang, 2001), though it exerts a strong impact on chemical and physical processes in the atmosphere.

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Therefore, further research is warranted to assess the environmental risk caused by NO emission from livestock activity.

Collaboration with struvite crystallization
Ammonia emission during the thermophilic phase is thought to be a principal cause of nitrogen loss during composting, though N 2 O, NO, and other emissions during the maturation phase have an important role in nitrogen loss. Therefore, reducing NH 3 emission www.intechopen.com In the composting process, struvite crystallization used for reducing NH 3 emission is prompted by the addition of magnesium (Mg) and phosphate (PO 4 ) salts. The struvite crystallization reaction is accelerated when t h e p H i s b e t w e e n 8 a n d 9 . D u r i n g t h e thermophilic phase, the pH of composting material generally rises to over 8 by the increase of NH 4 + nitrogen, which is generated by the decomposition of organic nitrogen. Therefore, the adjustment of pH is not necessary. The effect of struvite crystallization on reducing NH 3 emission has been confirmed in the composting of food waste, and of poultry and swine manure. Struvite is a valuable slow-release fertilizer. The ingredients of struvite are released under acidic conditions. Nitrate nitrogen that is generally contained in mature compost is a fast-release type fertilizer; therefore, the struvite crystallization is thought to add a new and different value to the compost product, as well as reducing environmental risks. As reported in several papers, the reagent addition of struvite crystallization has a negative effect on the composting microorganisms decomposing in organic matter. Therefore, there is a possibility that nitratation promotion would be affected by the reagent addition of struvite crystallization in the case that these two countermeasures are applied simultaneously.

Materials and methods
To quantify the combined effect of struvite crystallization (MAP) and the NIPRO process on the reduction of nitrogenous emissions, laboratory-and mid-scale composting experiments of swine manure were conducted (Fukumoto et al. 2011a). The dose of Mg and PO 4 sources is an important issue related to struvite crystallization in composting (Jeong and Hwang, 2005;Lee et al. 2009). A higher dose of Mg and PO 4 could reduce the amount of nitrogen loss; however, there would be adverse effects on the decomposing organic matter and treatment costs would be increased. In our study, the respective doses of Mg and P placed into 0.045 and 0.030 mol/kg of raw feces was decided according to our past study, considering the balance of three factors (N conservation, degradation of organic matter and cost). The reagents for struvite crystallization were added at the start of composting, and then the mature swine compost (NOB source) was added after the thermophilic phase for nitratation promotion.

Results and discussion
By the addition of Mg and PO 4 salts, the amount of NH 3 emission could be reduced by 25-43% compared with the control. To confirm the struvite formation, the amount of nitrogen fixed in struvite crystals was measured according to the procedure of Tanahashi et al. (2010).
As the result of these analyses, the amount of struvite nitrogen contents in the final product of sole struvite crystallization treatment became larger than those in the control. Therefore, it was confirmed that struvite crystallization had been enhanced by the addition of reagents during swine manure composting. However, in the treatment of two combined countermeasures (MAP and NIPRO), the struvite nitrogen content had become lower than those in the control despite the addition of Mg and PO 4 salts. To evaluate the effect reagent addition has on struvite crystallization, changes in the struvite nitrogen content were investigated. During the thermophilic phase (0-28 days), the struvite nitrogen content in the treatment of two combined countermeasures had changed more than those in the control, which indicated that the struvite crystallization was enhanced by the addition of reagents. However, after the addition of mature swine compost for nitratation promotion, the struvite nitrogen content was suddenly decreased, and became lower than that of the control. During the thermophilic phase, the material pH rose to over 8 due to the high NH 4 + contents. However, the material pH generally declined due to the start of nitrification. Particularly, because more NO 3 -nitrogen is accumulated in the compost product by the NIPRO process, the material pH tended to become lower than what was found in normal composting. Due to the lower pH, the struvite crystals formed during the thermophilic phase are considered to have been dissolved again during the maturation phase (Fig. 10). On the other hand, the effect of struvite crystallization on reducing N 2 O emission was small (decreasing rate of N 2 O was 10%). When the NIPRO process was applied simultaneously, the amount of N 2 O emission could be reduced by 52-80%. However, the struvite crystallization showed a reducing effect on NO and other nitrogenous emissions during the maturation phase, as well as the NIPRO process. Conserving nitrogen in the form of struvite crystals has a benefit for stable nitrogen conservation because microbes cannot use the nitrogen in the struvite before it is dissolved. Therefore, it is considered that the struvite crystallization had a reduction effect on nitrogen losses during the maturation phase (Fig.  11).
www.intechopen.com The amount of total nitrogen loss was reduced by 60% by applying the two combined countermeasures, opposed to 50% by the struvite crystallization alone. No adverse effects from adding reagents to the growth of NOB were observed. However, the NIPRO process dissolved struvite crystals due to a decline in pH. Therefore, the effectiveness of struvite as a slow-release fertilizer cannot be expected when the struvite crystallization is applied with the NIPRO process.

Cattle and poultry manure composting
In the sections above, all composting experiments were conducted using swine manure. This is because swine manure has a natural property that is suitable for applying the NIPRO process. A precondition for showing the maximum effect of the NIPRO process is that N 2 O induced by NO 2 -accumulation contributes to a large portion of its total emission. In swine manure composting, because the decreasing rate of N 2 O emission by applying the NIPRO process is comparatively high (average decreasing rate is 60%), the portion of N 2 O induced by NO 2 -accumulation is considered to be large. However, such information is inapplicable in the composting of cattle and poultry manure. Thereupon, to inspect the adaptability of the NIPRO process, composting experiments of cattle and poultry manure, and incubation tests for the quantification of ammonium generation potential (AGP) in respective animal manure, were conducted (Fukumoto et al. 2011b).

Materials and methods
In composting experiment of cattle manure, the house-shaped dynamic chamber system, which was constructed in a former study (Fukumoto et al. 2011a), was used. On the other hand, the laboratory-scale composting apparatus was used for poultry manure composting.
To estimate the maximum ammonium generation potential (AGP), incubation tests of respective livestock manure were conducted. The mixed solution of fresh manure (3-5% w/v) and nitrification inhibitor was agitated continuously at 25 o C in the dark, and the concentration of NH 4 + -N in the solution was periodically measured until the rising curve became a plateau. The AGP was calculated as the amount of generated NH 4 + -N per gram of organic matter

Results and discussion
In cattle manure composting, the trends of nitrogen transition and N 2 O emission were different from those of swine manure composting (Fig. 12). Most N 2 O in cattle manure composting tends to be generated in the thermophilic phase, whereas it occurs in the maturation phase in swine manure composting. Similar results of N 2 O emission in cattle manure composting were also observed in other studies (Maeda et al. 2010a;Maeda et al. 2010b). Moreover, the delayed growth of indigenous NOB, which is a cause of prolonged NO 2 -accumulation in swine manure composting, was not observed in cattle manure composting. Therefore, it is considered that the activity of nitrification, which becomes the starting point of N 2 O g e n e r a t i o n f r o m l i v e s t o c k m a n u r e , i s h i g h e v e n d u r i n g t h e thermophilic phase in cattle manure composting. The level of NH 4 + -N in cattle manure is usually lower than that of swine manure, which also indicates that the inhibitory effect of free ammonia on nitrification is low. A high NH 4 + -N level is thought to be one of the causes of delayed growth of indigenous NOB in swine manure composting. The quick recovery of indigenous NOB, i.e., complete nitrification, in cattle manure composting seems to be due to the lower influence of inhibitors, such as free ammonia. Therefore, the prolonged NO 2 -accumulation after the start of nitrification did not occur in cattle manure composting, which indicates that the NIPRO process may be not suitable for cattle manure composting. With the poultry manure composting, no obvious nitrification activity could be observed even in the long maturation phase (over 300 days). The excretion mechanism of poultry is www.intechopen.com different from other livestock animals. The bird excretes urine together with feces in the form of uric acid, making nitrogen content in the manure extremely high. Therefore, it is considered that nitrification was completely inhibited by a high concentration of free ammonia in poultry manure composting. In fact, any growth of nitrifiers was not observed in this study, except for some temporary detections of AOB. Because nitrification could not be initiated, no significant N 2 O emissions were confirmed. Therefore, it is considered that the NIPRO process, which reduces the amount of N 2 O generated via inadequate nitrification, has no positive effect on poultry manure composting. These three kinds of livestock manure each showed characteristic nitrogen transitions and N 2 O emission patterns. It is considered that the respective unique nitrogen turnover of each livestock depends on the amount of active nitrogen generated via decomposition of manure. Therefore, as one of the factors affecting nitrogen turnover, the AGP of fresh livestock manure was investigated (Fig. 13). From the results of the incubation tests, poultry manure showed the highest AGP value among the three kinds of manure, as expected. On the other hand, the lowest AGP value was observed in cattle manure, which was approximately one-fiftieth the value of poultry manure. Swine manure showed an intermediate value between cattle and poultry manure. The AGP value is thought to affect nitrification. In poultry manure composting, large amounts of ammonium are generated, which inhibits the activity of nitrification completely (Anthonisen et al. 1976). On the other hand, because of the low AGP, the effect of inhibitory factors, such as free ammonia, would be small in cattle manure composting, which would lead to the quick recovery of complete nitrification. Therefore, in cattle or poultry manure composting, it is thought that prolonged NO 2 -accumulation scarcely occurs, although when it does, it is for completely different reasons. This indicates that the quantity of N 2 O emission induced by NO 2 -accumulation is small. Therefore, the effect of the NIPRO process on reducing N 2 O emission would be slight in cattle and poultry manure composting. On the other hand, the level of AGP in swine manure might be suitable for NO 2 -accumulation. That is, it is considered that the AGP of this level does not inhibit the activity of the first half of nitrification (i.e., nitritation), but would have a serious influence on the latter half of nitrification (i.e., nitratation) because the tolerance of NOB on inhibition factors, such as free ammonia, is lower than that of AOB. It is thought that the AGP would become one of the decisive materials when choosing to apply the NIPRO process to the composting of organic wastes.

Conclusions
The present report showed the effect of the nitratation promotion (NIPRO) process on nitrogenous emissions during the composting of livestock manure. The NIPRO process can reduce the nitrogenous emissions induced by NO 2 -accumulation by the addition of an NOB source such as mature compost. In swine manure composting, a remarkable reducing effect on N 2 O, NO and other nitrogenous emissions (probably N 2 ) by applying the NIPRO process, was confirmed. As a result, more nitrogen could be preserved in the final compost product in the form of NO 3 -nitrogen, which is expected to improve the compost's value as a fertilizer. Moreover, the NIPRO process can collaborate with the struvite crystallization which reduces NH 3 emission in the thermophilic phase. However, it was revealed that struvite crystals formed during the thermophilic phase have been dissolved during the maturation phase due to a pH decline induced by NO 3 -accumulation in the case applying the NIPRO process. In cattle or poultry manure composting, it seemed to be difficult to apply the NIPRO process for reducing nitrogen losses, because prolonged NO 2accumulation would be difficult to maintain in those manure composts. Before putting these findings to practical use, however, some issues still remain. For instance, how to decide the timing of NOB addition, and how to evaluate effects and outcomes in an actual case. Therefore, further study is indeed necessary.

Acknowledgement
The works constituting this chapter have been supported by the Ministry of Agriculture, Forestry and Fisheries of Japan, the Ministry of the Environment of Japan, and the Ministry of Education, Culture, Sports, Science and Technology of Japan.