Short Report : Effects of Biochar Addition on Manure Composting and Associated N 2 O Emissions

Recent interests in biochar stem from its agronomic benefits and carbon sequestration potentials in soil applications. As a not fully understood newer concept, adding biochar as a bulking agent to animal manure composting has the potential to enhance the performance of composting process and reduce associated N2O emissions. This short report presents emerging trends and knowledge gaps in this research area, and provides an introduction to understand the mechanism by which biochar impacts manure composting performance and N2O fluxes.


Introduction
Nitrous oxide (N 2 O) is a potent greenhouse gas that contributes to global warming, climate change, and stratospheric ozone depletion [1].Globally averaged N 2 O concentration in the air in 2012 reached 325.1 ppb, which was 120% of the pre-industrial level (270 ppb) [2].Agriculture has been one of the major sources of global N 2 O emissions.Emissions from soil and associated nitrogen (N) inputs, such as synthetic fertilizer, animal manure and crop residue, are the main agricultural N 2 O sources, contributing 90% of the total [3].Mosier et al. [4] estimated that animal manure applied to soils directly contributed 0.3 Gt CO 2 -eq/yr (i.e., 10%) to global N 2 O emissions.Nitrogen losses in the form of N 2 O from land application of animal manure are of global and regional importance to air quality and climate change [5].
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As an alternative to direct land application, composting is one of the widely accepted technologies for recycling organic wastes in agriculture: it can minimize some of the disadvantages associated with direct application of raw wastes, e.g., phytotoxicity, leaching and denitrification of mineralized organic N [6].Composting consists of the transformation of organic matters (OM) into a relatively well-stabilized product through rapid succession of microbial populations under aerobic conditions.During that process, part of the OM is mineralized to CO 2 , whereas the rest is transformed to humic substances, which represent a valuable index of OM stabilization [7].However, the emission of greenhouse gases from composting of organic wastes is a serious problem.For example, annual global N 2 O emissions from composed organic wastes have been estimated at 1.2 × 10 6 metric tons or approximately 0.4 Gt CO 2 -eq [8].At this scale, composting poses serious environmental risks by contributing to global warming and ozone depletion.When applied to soils, composted manure is also known to increase N 2 O emissions by stimulating nitrification and denitrification [9].Compared with N 2 O emissions from chemical fertilizers, N 2 O emissions from manure are of greater duration and emission intensity [10].
The authors believe that one solution to the above challenges is to compost animal manure with biochar and apply this composted biochar-manure (CBM) to soils to enhance crop production and minimize N 2 O emission.Biochar is charcoal produced from biomass via pyrolysisor gasification [11] [12].Biochar can effectively retain NH 3 , 4 NH + , and 3 NO − in animal manure [11].Recent studies demonstrated that bulking manure with biochar reduced N loss while simultaneously enhancing humification, thereby producing mature composts with a high fertilizer value [13]- [16].

The Formation of N 2 O during Manure Composting
Composting of high organic content wastes has been shown to produce N 2 O by the microbial processes of nitrification and denitrification, and N 2 O generation is found to depend on the transformation of different nitrogen states in the composting mixture [1].As shown in Figure 1, under aerobic conditions, 4 NH + -N from manure is rapidly converted into N 2 O by incomplete nitrification.But at the conditions of low O 2 contents, 3 NO − -N in the manure emits N 2 O through incomplete denitrification process [17] [18].Considering the shortage of oxygen for most composting piles, denitrifiation is responsible for the most part of N 2 O generation in composting, while nitrification makes a substantial contribution to the N 2 O emission at the surface of composting pile where O 2 is adequate and temperature is suitable [19].
Manure properties such as moisture content, NO x -N content and carbon-to-nitrogen ratio (C:N) along with process management such as aeration, temperature regime, turning, covering and compacting can significantly affect N 2 O emissions during composting [20].For example, it was found that there were large amounts of N 2 O emissions during the early stage of composting with high N materials because of NO x -N denitrifying in the early stage.Conversely, at this period, nitrification that limits N 2 O emission is restricted owing to the unsuitable microbial activities at the environment of high temperature and nitrogen/oxygen [21] [22].The research in dairy cattle manure and swine waste composting confirmed that the cumulative emissions of N 2 O increased significantly by the use of mature compost that contains nitrate and nitrite [17] [23].In general, materials with a low C:N is desirable for low N 2 O emission composing [24] [25].Moisture content in composting mixtures is another important parameter affecting the quality of the compost because it affects the metabolic and physiological activities of microorganisms.High moisture content enhanced nutrient transport [24], but too high moisture may cause anaerobic conditions, which provides the beneficial conditions to generate N 2 O through the incomplete denitrification pathway of NO x -N.It also prevents and halts the ongoing composting activities [24] [26].On the other hand, very low moisture content would cause early dehydration during composting and becomes a limiting factor for the aerobic degradation, thus giving physically stable but biologically unstable composts [24].In general, 50% -60% moisture content is identified as suitable for effective composting and inhibiting N 2 O emission [24] [27].
In addition to manure properties, various environmental variables also affect composting and its N 2 O emission.Temperature of the composting process is widely considered as a significant factor for composting efficiency and N 2 O emission because microbial metabolism and activities are all temperature sensitive and dependent.It was found that temperature of composing below 20˚C or in excess of 60˚C would slow and even stop composting owing to impeded microbial activity [24], which explains why there was lower or even no generation of N 2 O during the thermophilic phase (temperature of higher than 50˚C) in the early stage of composting [28].Substantial N 2 O emissions usually start in the middle stage of composting when the temperature of the composting pile begins to decline [29].Aeration is another important factor because composting is basically an aerobic transformation of organic matters where O 2 is necessary, and the supply/distribution of O 2 in the composting pile also affects the production and emission of N 2 O. Usually, with increasing O 2 the emission of N 2 O increases first and then decreases, therefore proper aeration is beneficial for reduction of N 2 O emissions [27] [30].Similarly, controlling the air void of the composting pile through compacting or adding porous materials affects N 2 O emission [27].Besides, other factors such as pile size, pH and available nutrients have also been shown to impact on composting performance and N 2 O emission [24] [29].

Biochar in Manure Composting and Its Effect on N 2 O Emissions
The mechanisms of N 2 O formation described in Section 2 can help us understand the effect of biochar addition in manure composting.The authors believe that biochar, produced from high carbon content solid biomass, is one of the best bulking agents for reducing N 2 O emission in manure composting for the following reasons.First of all, its high porosity results in increased aeration in the composting process, which enhances the supply and distribution of O 2 in the composting pile, and may lead to reduction of N 2 O as previously mentioned reasons [16].Secondly, the high porosity and high surface area of biochar also enables it to absorb and retain large amounts of water which results in decreased N 2 O emission by altering redox conditions and denitrifying communities.High moisture content also enhances the metabolic and physiological activities of microorganisms by transporting dissolved nutrients [12] [24].Thirdly, NH 3 or water-soluble 4 NH + , adsorbed by biochar signifi- cantly reduces NH 3 and 3 NO − losses during composting, further to reduce the emission of N 2 O, which also of- fers a mechanism for developing slow release fertilizers [31].Steiner et al. [11] incorporated biochar to poultry manure and composted them over a 42-d period, and found that NH 3 emissions decreased by 47% -55% as the rate of biochar incorporation increased, confirmed that biochar was effective to alter N transformation and fate.These beneficial effects may have been caused by the decrease in N availability for denitrification, as biochar can efficiently adsorb and retain ammonia gas and ammonium as well as nitrate ions [32] [33].Furthermore, other recent studies confirmed that bulking poultry manure with biochar lessened N loss and improved N retention, while simultaneously enhanced humification, thereby produced mature composts with a high fertilizer value [11] [34] [35].Fourthly, biochar with a higher pH alters the abundance of denitrifying bacteria significantly in manure composting, resulting in less N 2 O producing but more N 2 O-consuming bacteria communities [12].Although the benefits regarding the use of biochar as a bulking agent for composting have been demonstrated, research in understanding its role in reducing N 2 O emission is still scarce.The mechanism by which biochar impacts N 2 O fluxes over the entire composting period is also poorly defined.
In addition to the reported results of biochar reducing N 2 O emission in manure composting, biochar also provides benefits on accelerating composting.For example, it not only provides structural support to prevent the physical compaction of the pile and increases air voids allowing the aeration of the pile [36], but also acts as a biodegradable carbon and energy source for supporting microbial activity and balancing the initial C:N ratio of the mixture [37].Besides, the addition of biochar to the composting process can reduce the activity of methanogen (CH 4 production) and increase methylotroph (CH 4 oxidation) activity of microbes.Moreover, non-carbon neutral CO 2 can be mitigated by the strong carbon sequestration ability of biochar, which is beneficial for reducing the environmental load of GHG emissions [16].
The question on how manure interacts with biochar and alters biochar properties is scientifically interesting, though currently little is known.For example, composting may facilitate surface oxidation of biochar by the elevated temperature, especially at the beginning of the composting process.It also changes biochar properties biotically by the high microbial activity or the co-metabolic decay during the degradation of available carbon sources [38] [39].In addition, the nutrient contents of biochar can be enriched by co-composting with nutrientrich manure.Biochar absorbs leachate generated during the composting process, resulting in increased moisture content.With the leachate, biochar also absorbs organic matter and nutrients, resulting in increased contents of water-extractable organic carbon, total soluble nitrogen, plant-available phosphorus and plant-available potassium, therefore increasing nutrient retention capability of the composted material.However, it should be noted that the surface area of biochar might decline during the composting process due to the clogging of micropores by adsorbing compost-derived material [40].Besides, the sorption of organic matter like humic acid from manure could lead to an increase of oxidized functional groups, e.g., carboxylic groups, on the biochar surface, which further increases surface oxidation and absorptivity [32].Thus, co-composting manure with biochar is considered a promising method that can generate a nutrient-and humus-rich soil amendment agent or slow release fertilizer [35] [41].

Conclusion
With limited literature available, biochar has demonstrated its potential in enhancing manure composting and reducing associated N 2 O emissions.This can be attributed to the high porosity and high surface area of biochar that enables absorption/adsorption and retention of water, NH 3 or water-soluble 4 NH + , as well as nitrate ions, leading to desirable metabolic and physiological activities of microorganisms.The authors believe that co-composting manure with biochar is a promising method for both slow-release fertilizer production and greenhouse gas mitigation; however, further research is needed to understand the role of biochar in the composting process and the interaction between manure, biochar, and microbes.