Biochar-composting substantially reduces methane and air pollutant emissions from dairy manure

Dairy manure is one of the largest sources of methane (CH4) emissions and air pollution from agriculture. In a previous study, we showed that composting dairy manure with biochar substantially reduces CH4 emissions and could help the dairy industry meet climate goals. However, it remained unclear whether biochar could also mitigate the emission of air pollutants and odor during composting. Here, we conducted a full-scale composting study at a dairy farm and monitored the emission of greenhouse gases (CO2, CH4, N2O) and air pollutants (H2S, VOCs, NO x , NH3) from compost piles amended with or without biochar. We found that amending compost with biochar significantly reduced total CH4 emissions by 58% (±22%) and cut H2S, VOCs, and NO x emissions by 67% (±24%), 61% (±19%) and 70% (±22%), respectively. We attribute this reduction in emissions to improved oxygen diffusion from the porous biochar and the adsorption of gas precursors to the biochar surface. Interestingly, NO x fluxes from the composting dairy manure were much higher than the few values reported in the literature, suggesting that dairy manure could also be a significant source of NO x emissions. We estimate that biochar-composting of dairy manure would reduce the social cost of manure emissions from this farm by over $66 000 annually. Results from this study suggest that composting dairy manure with biochar, in addition to reducing CH4 emissions, may help to improve air quality and the health and wellbeing of rural communities, but further studies are needed to test the quantitative impacts.


Introduction
Livestock manure is a leading source of anthropogenic methane (CH 4 ) emissions and air pollution from agriculture [1][2][3][4][5][6].In the U.S., manure accounts for 9% of total CH 4 emissions and is responsible for 43% of all premature deaths from food-related air pollution-nearly 7000 people per year [7,8].As the global demand for animal products rises, there is an increasing need for novel strategies that mitigate emissions from manure as governments work to fight climate change and improve rural air quality [8][9][10].The dairy industry represents a large opportunity to mitigate both CH 4 and air pollutant emissions due to the large amount of manure produced and stored on-site in anaerobic lagoons and stockpiles at intensive dairies [11][12][13][14].Due to the low oxygen present in these environments, manure managed through these strategies generates substantial CH 4 emissions and is also a large source of odorous air pollutants such as ammonia (NH 3 ), volatile organic compounds (VOCs), and hydrogen sulfide (H 2 S) [15][16][17].In California, approximately 45% of total anthropogenic CH 4 emissions come from dairies, and the state's short-lived climate pollution reduction law (SB 1383) will require a 40% reduction in dairy methane by 2030 [18,19].Alternatives to traditional manure management practices are desperately needed that can cut CH 4 emissions, reduce air pollution and transform dairy manure from a polluting waste into a valuable organic resource [18].
Anaerobic digestion is one strategy that can mitigate CH 4 by capturing it from anaerobic lagoons for use in renewable energy production [20].While this strategy may substantially reduce CH 4 from liquid manure, manure solids that are separated from liquids entering digesters are often stored in stockpiles, and remain a large source of CH 4 [11,13,16,17].In a previous study, we show that biocharcomposting, which consists of composting manure together with biochar, is an effective alternative to stockpiling or composting separated manure solids and can reduce CH 4 emissions by 84% relative to composting without biochar [11].This method may also be used for mitigating CH 4 from solid dairy manure at dairies without digesters, which often include smaller farms that are not equipped to house digesters [11].We estimate that if biocharcomposting replaces manure stockpiling at intensive dairies globally, this strategy can increase the maximum CH 4 mitigation potential of dairy manure from 4.5 Tg CH 4 yr −1 -6.1 Tg CH 4 yr −1 [11].Previously, we also found that the dairy manure biochar-compost product, when used as an organic soil amendment, had agronomic and environmental advantages over applying biochar or dairy manure compost separately [21,22].
Pairing anaerobic digestion together with biochar-composting creates a system in which both liquid and solid dairy manure is managed for CH 4 mitigation [11].However, composting organic waste, in general, can be a large source of air pollution [23].A recent study by Nordahl et al showed that, while composting had the lowest global warming potential (GWP) of all the organic waste management strategies considered in their analysis, it also had the highest social cost due to the negative health impacts resulting from the high rate of air pollutants emitted during composting [24].At the local scale, odor from composting, produced from air pollutants such as NH 3 , VOCs, and H 2 S, can act as an environmental stressor in addition to harming respiratory health [24][25][26][27].At the regional scale, these pollutants, along with nitrogen oxides (NO x ), can react in the atmosphere to form both fine particulate matters (PM 2.5 ) and ozone (figure 1) [24,28,29].In California, over 80% of dairies are located in the state's rural Central Valley which regularly exceeds federal PM 2.5 and ozone standards [14,30].Due to the high number of disadvantaged communities in the Central Valley, vulnerable communities may have limited capacity to cope with the potential health impacts from additional air pollution from composting [31][32][33].
While evidence is limited, some studies suggest that, in addition to reducing CH 4 emissions during composting, amending compost with biochar may also reduce the emission of some air pollutants [34][35][36][37].However, we are aware of no full-scale field studies that simultaneously measure both greenhouse gases (GHGs) and air pollutants during biocharcomposting of dairy manure.If biochar-composting can reduce air pollution from dairies, this would result in a significant co-benefit of this powerful CH 4 mitigation strategy.
In this study, we test whether biochar-composting can function as both a CH 4 mitigation and air pollution reduction strategy for dairy manure management.In a full-scale field experiment at a dairy, we measured the emission of GHGs (CO 2 , CH 4 , N 2 O) and air pollutants (H 2 S, VOCs, NO x , NH 3 ) during the composting of separated solid dairy manure with and without biochar.We hypothesized that incorporating biochar into the composting process would mitigate both CH 4 and air pollutant emissions primarily through an increase in pile aeration and the adsorption of gases and/or their precursors on the surface of biochar.We also use the emission data from our composting experiment, along with an integrated assessment model, to estimate the social cost of each composting strategy to assess their potential impact on public health.

Site description and experimental design
The composting experiment was conducted over 35 d from 24 October-27 November 2022, at a dairy in Madera, California.While composting can take place over much longer periods of time, intensive dairies are limited in space for composting and have high rates of manure production, so shorter on-farm composting times may be necessary [11,38].In our previous study, we determined that 35 d of composting allowed dairy manure compost to reach maturity and to achieve thermophilic temperatures long enough to kill most pathogens [11,39,40].
On the first day of the experiment, fresh dairy manure solids were collected from a sloped screen separator (US Farms Dual Sloped Screen Separators) that separated manure solids from liquids that flow into an anaerobic digester.From this manure, two full-scale windrows (10 m long, 2 m wide, 1 m tall) were prepared on-site at the dairy.Both piles were turned on day 0 using a mechanical compost turner (HCL Machine Works CT-12 Compost Turner), and biochar was mixed into one pile during the first turning process at a rate of 20% by volume or 13% by mass.The manure-only pile was composed of approximately 10.23 t fresh manure (2.66 t dry weight) while the biochar-compost pile had the same amount of manure amended with 1.36 t biochar (1.32 t dry weight).Biochar was produced from almond shells through slow pyrolysis at 475 • C and characteristics of the biochar are listed in table S1.Both piles were aerated and homogenized using a mechanical compost turner 4 times over the course of the experiment (days 0, 8, 23, and 31).

Greenhouse gas and air pollutant flux measurements
Gas fluxes from the compost piles were measured using a static chamber system connected to several gas analyzers.For each pile, fluxes were measured from three locations (south side, top, and north side) daily for the first week, when pile conditions were most dynamic, and every other day for the remainder of the experiment.The collar for each chamber was inserted 3 cm into the compost and left to sit for at least 30 min before measurement to allow the initial pulse of gas released from the compost after chamber installation to dissipate.Measurements were made by placing a chamber (12 271.9 cm 3 ) that was connected to the gas analyzer and circulation pump.
GHGs (CO 2 CH 4 , N 2 O) and NH 3 were sampled for three minutes from each chamber and measured using a cavity ring-down laser spectrometer (Picarro G2508, Picarro Inc.Santa Clara, CA).GHG and NH 3 fluxes were calculated using Picarro's soil flux Processor program using a linear model or the Hutchinson and Mosier exponential model when changes in concentration were nonlinear [41].Using an identical sampling strategy, NO x (NO + NO 2 ) was measured using a chemiluminescent NO x analyzer (Serinus 40 oxides of nitrogen analyzer, Acoem, Richmond, VA), and H 2 S was measured with a portable gas analyzer equipped with an electrochemical H 2 S sensor (Analytical Technology Inc., D-16 PortaSens III, Collegeville, PA).Changes in NO x and H 2 S concentration were determined through linear regression, and fluxes were calculated using the change in concentration and the chamber temperature and dimensions [42,43].Non-methane VOCs were sampled by drawing air from each chamber at a rate of 0.1 l min −1 for 7 min into VOC sorbent tubes (Markes International, Bridgend, UK), which were capped and later analyzed using a thermal desorption system (UNITY-xr, Markes International, Bridgend, UK) coupled to a gas chromatography mass spectrometer (G7077BA, Agilent), see details given in the supplement.The VOC mass collected over each 7 min sampling period was converted to a flux by dividing by the chamber area and the sampling time [44].
Gas fluxes were converted from a per area basis to a per dry feedstock mass basis by multiplying each flux by the area of the compost pile and dividing by the dry mass of the compost pile.We assume a rectangular prism shape for the windrow and assume that emissions come from the top of the pile, as a result of the chimney effect (figure S1) [45].We measured the pile area at the base of the pile (top of rectangular prism) and measured initial dry feedstock mass gravimetrically from compost samples taken immediately following pile creation and homogenization through mechanical turning.Flux measurements on the sides of the compost pile were multiplied by a correction factor to account for the difference in the mass of compost below side measurements and top measurements (figure S1).The cumulative emission of each gas from each pile over the course of the experiment was estimated by calculating the area under each average flux curve.From the cumulative emissions, we also calculate the GWP for each pile using both 100 year and 20 year GWPs [46].
Due to the large scale of our experiment, we do not test replicate piles, which is a common limitation of field-scale composting studies [47][48][49][50].However, we took measures to eliminate systematic differences between piles and to improve the robustness of results.The compost windrows were parallel, separated by 100 m, and oriented east to west, to ensure that there was no shading between piles and that each pile experienced equal warming throughout the day.The first pile measured each day was also alternated, to reduce bias that could occur as temperatures change throughout the day.Wind at the site ran from north to south.Piles were constructed south of a large (approximately 10 m tall, 200 m long) mound of soil, concrete, and dry manure, that served as a wind barrier.We also allow for instruments to reach ambient gas concentrations, recorded each day ∼200 m away from piles, before taking measurements for each pile, which along with the large spatial separation between piles, minimize potential systematic differences imposed by a pile being upwind of the other.Finally, we take measurements at a high frequency (daily for the first week and every other day following) to capture changes in emissions during the dynamic composting process at a high temporal resolution.
We also acknowledge that, though widely used in the composting literature, there are limitations to chamber-based flux measurements.Artifacts of chamber measurements include an induced pressure gradient between the compost and headspace, a buildup of gases in the headspace resulting in a lower gas diffusion rate, and an increase in headspace temperature [47,51].We took measures to limit pressure changes by using a dynamic chamber method, which returns air back to the chamber, when using the Picarro G2508 (GHGs and NH 3 ) and by installing a vent tube on the top of the chambers (1.5 mm × 16 cm) to prevent pressure increases when measuring gases whose instruments did not allow for the use of a dynamic chamber (H 2 S, VOCs, NO x ).We also use short measurement times (3-7 min) which reduce the likelihood of gas concentrations, in particular CO 2 , from reaching high enough concentrations in the headspace to cause non-linear changes in concentration.In order to minimize any potential increase in temperature due to chamber installation, we used white, polyethylene chambers, along with short measurement times.Finally, while we did not measure every pathway through which C and N could be lost from the compost piles, we performed a mass balance on the system to check that our emission measurements did not overestimate emissions and aligned with trends observed during the composting process (see table S5 and 'Mass balance check on emissions' in the supplement).

Social cost analysis
We used the air pollution emission experiments and policy (version 3) integrated assessment model to quantify the social cost from the emission of air pollutants during dairy manure composting and biochar-composting [52].This model provides social cost multipliers (in U.S. dollars) for air pollutant sources for specific U.S. counties, taking into account factors such as regional air chemistry, local population, mobile versus non-mobile sources, and the height of the pollution source [52].We also use the Biden administration's recommended social costs for CH 4 and N 2 O emissions [53].While H 2 S is an odorous and toxic air pollutant, the social cost of H 2 S is not reported in environmental or public health literature, so it is not included in this analysis.The cumulative mass of each gas emitted from each pile is multiplied by its respective social cost multiplier (listed in table S2) to get the estimated public health damages resulting from each pile's emissions.Social costs were calculated in U.S. dollars per metric ton of compost and were scaled up to the farm scale using average per farm manure production for California dairies [13,14].

Compost and biochar characterization
Compost samples were taken weekly after compost piles were homogenized through turning of the windrows.Samples were taken from a minimum of six locations on the sides and top of the windrow at depths ranging from 0 to 20 cm.Compost moisture was determined gravimetrically through oven drying at 55 • C for 72 h.Oven dried samples were ground and analyzed on an elemental analyzer (Costech 4010, Costech Analytical Technologies Inc., Valencia, CA) to determine total carbon (C) and nitrogen (N).The NH 4 + -N and NO 3 − N concentration of compost samples were measured by analyzing 2 M KCl compost extracts through colorimetry on a microplate reader (Synergy HTX Multimode Reader, Agilent Technologies Inc., Santa Clara, CA).Compost pH and EC was determined in a 1:5 (w/v) compost to DI water suspension.Proximate analysis of feedstocks and final composts, which determines moisture content, volatile matter and ash content, and consists of measuring changes in the mass of materials after heating to specific temperatures, was performed according to ASTM D3172-13 [54].For final composts, seed germination indexes, a measurement of the phytotoxicity of composts based on radicle growth and germination rates, were determined following recommendations by Luo et al [55].
Biochar samples were characterized using the same methods detailed above for moisture, total C and N, NH 4 + -N and NO 3 − N, pH and EC, and proximate analysis.The specific surface area of biochar was determined through the Brunauer, Emmett, and Teller method on a TriStar II Plus (Micromeritics, Norcross, GA) [56].Total O and H of biochar were also measured on a thermal conversion elemental analyzer (TC/EA High Temperature Conversion Elemental Analyzer Thermo Scientific, Waltham, MA).

Statistical tests
All statistical analyses were performed in R [57].Differences in physical and chemical properties between compost and biochar-compost over the course of the experiment were tested for significance using general linear models with treatment and time used as fixed factors.Differences in cumulative gas emissions were tested for significance using Welch's t-test.Pearson correlation coefficients were used to analyze the relationship between compost characteristics and gas emissions.Significance for all analyses was set at p < 0.05.Assumptions of normality and homoscedasticity were checked before each analysis and non-normal data was transformed if necessary.Data presented are the average of 3 replicate measurements per compost pile and error bars represent one standard error.

Compost physical and chemical characteristics
After achieving thermophilic temperatures, both composts remained above 55 • C for the remainder of the experiment, except for turning days when compost temperature dropped substantially for both piles for approximately 24 h (figure 2(a)).The biocharcompost pile had significantly lower moisture content (p = 1.4 × 10 −8 ) and higher porosity (p = 0.0016) compared to the manure-only pile (figures 2(b) and (c)).NO 3 − in both composts were relatively low and remained similar throughout the composting process, but the biochar-compost had significantly less NH 4 + than the manure-only compost (p = 0.00491) (figures 2(g) and (h)).Both composts were determined to be mature at the end of the experiment which was demonstrated by a C/N ratio less than 25, a germination index above 50 and NH 4 + -N less than 400 mg kg −1 (table S3) [39].

Social cost analysis
Results from our social cost analysis show that the public health damages from composting and biocharcomposting are $47.21 t TS −1 and $37.6 t TS −1 , respectively.The annual social cost per farm for composting and biochar-composting was found to be $325 670 yr −1 and $259 455 yr −1 , respectively.Based on this analysis, dairies that use biochar-composting instead of composting to manage their manure could reduce social costs by $66 215 per farm annually.

Greenhouse gas emissions
Biogenic CO 2 released during composting is considered to have no net climate impact, but it is an important composting metric that provides insight into decomposition rates, aeration, and stability [39].Over the course of the experiment, the biocharcompost pile emitted 41% (±14%) less CO 2 than the manure-only pile (figure 3(e)).This could be due to the adsorption of labile C on the biochar surface, the precipitation of CO 2 onto the high pH biochar, or the absorption of CO 2 into the biochar's extensive pore network (table S1) [58].Despite a reduction in CO 2 emissions, the biochar-compost pile reached thermophilic temperatures faster than the manure-only pile, suggesting that biochar likely enhanced microbial activity.
We found that the biochar-compost pile emitted 58% (±22%) less CH 4 than the control over the course of the experiment, which is consistent with previous biochar-composting studies (figure 3(d)) [11,36,59,60].The primary mechanism driving this reduction is likely an increase in oxygen diffusion in the biochar-compost.We observed significantly higher porosity and lower moisture in the biocharcompost, which could reduce anaerobic hotspots and potentially increase CH 4 oxidation by methanotrophs [61][62][63] (figures 2(b) and (c)).Though not statistically significant, CH 4 was positively correlated with moisture (p = 0.246) and negatively correlated with porosity (p = 0.202), which agrees with our understanding of the drivers behind CH 4 emissions during composting (figure S2) [61,62].While other compost bulking agents may also improve aeration, biomass that is not pyrolyzed can provide a readily available source of C for methanogenesis, whereas biochar likely provides very little, further restricting CH 4 production [11].
Cumulative N 2 O emissions from both piles were very low, and differences between piles were not statistically significant (figure 3(f)).We previously reported very low N 2 O fluxes during the composting of dairy manure with or without biochar, and we attribute this to the inhibition of nitrification due to the high temperatures maintained over the composting experiment [11,47].Interestingly, N 2 O emissions were mostly negative for both composts during the first half of the experiment when the majority of CH 4 was emitted, which suggests complete denitrification under very anaerobic conditions.While the biocharcompost pile emitted slightly more N 2 O, total N 2 O emissions were still very low, and the climate benefit of the reduction in CH 4 was much larger.
In addition to reducing GHG emissions during biochar-composting, the production of biochar and the application of biochar-compost to soils offer additional climate benefits (figure 1) [11].For example, facilities that produce biochar often also generate renewable energy from the pyrolysis process, which can displace fossil fuel use [64].Biochar production also serves as a sustainable management strategy for biomass that may have otherwise generated additional GHG emissions though burning or landfilling [11,65].While the pyrolysis process requires energy and generates some GHGs, emissions from this stage are typically negligible compared to other lifecycle stages because pyrolysis is an exothermic reaction that requires little initial energy and releases mostly biogenic CO 2 , which has no net climate impact [66].Incorporating biochar into the composting process also enhances its soil carbon sequestration potential as the majority of biochar carbon is likely to persist in soils long-term [67].

Air pollutant emissions
While some studies have shown that biocharcomposting can reduce CH 4 emissions, there are very few studies that have investigated how biochar influences H 2 S during composting, despite both gases forming under similar, anaerobic conditions [34,61,62].We found that biochar significantly reduced H 2 S during composting by 67% (±24%), likely through the same mechanisms through which it mitigated CH 4 emissions (figure 4(d)).This odorous gas is toxic and can act as an environmental stressor, so H 2 S mitigation from dairies could improve the health and wellbeing of farmworkers and rural communities living near dairies [68][69][70].A reduction in H 2 S emissions may have also reduced the number of stable flies (Stomoxys calcitrans), a harmful cattle pest attracted to decomposing manure, which we observed in fewer numbers on the biochar-compost (figure 5) [71].
VOCs released during composting are rarely reported in the literature, despite their potential toxicity and contribution to both malodor and tropospheric ozone formation [23].We found that total non-CH 4 VOCs were significantly reduced by 61% (±19%) in the biochar-compost treatment, suggesting that biochar application may be an effective strategy to reduce VOCs during composting, which could benefit both farmworker and community health [28].Biochar has been shown to be an effective sorbent for VOCs, which could explain their reduction in the biochar-compost treatment [37,72,73].Anaerobic hotspots in the compost pile can also be a source of VOCs, so biochar's aeration effect may also limit VOC formation by increasing oxygen diffusion in the compost [48,74].While different VOCs can have varying environmental impacts, and different composts may produce different types of VOCs, we report the total mass of 30 VOCs identified by GC-MS to provide an emission factor that can be used to compare with other emission factors in the literature  [23].We also provide emission data for individual VOCs in the supplementary material (table S4).
NO x emissions from composting are typically assumed to be negligible, and there exist very few measurements of NO x fluxes during composting [49,[75][76][77][78].However, in our study, NO x emissions from both compost piles were surprisingly high.When considered on an area basis, the peak NO x flux from the manure-only pile was 101 ng NO x -N m −2 s −1 while the peak flux for the biochar-compost pile was 43 ng NO x -N m −2 s −1 (figure 4(b)).This is comparable to large NO x fluxes observed from agricultural and arid soils, suggesting that manure management and composting could be a significant source of NO x , which should be investigated further [42,43,79,80].
Although NO x emissions were relatively high from both piles, the biochar-compost pile emitted 70% (±22%) less NO x than the manure-only pile (figure 4(g)).While this is the first time that biochar has been found to reduce NO x emissions during composting, some studies have reported that biochar mitigated NO x emissions when applied to soils [81][82][83][84].In our study, NO x fluxes from both piles peaked after piles were aerated and cooled through mechanical turning, suggesting that nitrification was the dominate NO x production pathway.This hypothesis is also supported by the very low NO x fluxes we recorded when compost piles reached thermophilic temperatures and when CH 4 emissions were high, as high temperatures and low oxygen can inhibit nitrification [11,47].Interestingly, the biochar-compost pile had much lower NH 4 + concentrations than the compost pile throughout the experiment, while NO 3 − concentration in both piles were similar (figures 2(g) and (h)).This suggests that electrostatic adsorption of NH 4 + onto the biochar surface did not play a significant role in NO x mitigation.Instead, we suggest that biochar chemisorption of NH 4 + was the dominate NO x mitigation mechanism.Chemisorption of NH 4 + through covalent bond formation has been found to be an important process for N retention on biochar surfaces and would explain the lower KCl-extractable NH 4 + in the biochar-compost [85][86][87].It is likely that much of the NH 4 + was chemisorbed early in the experiment, as indicated by the large difference in NH 4 + between the compost and biochar-compost on day zero, which was also was the day of the largest NH 3 flux from the biochar-compost (figure 2(g)).Under high NH 3 concentrations, chemisorption might happen relatively quickly.Previous studies have found that chemisorption of NH 3 by biochar can happen rapidly (over several hours), even at near-ambient conditions [85,87].While chemisorbed NH 3 -N on biochar may slowly become plant available, other properties of biocharcompost, such as the formation of a mixed-charge organo-mineral layer on the composted biochar surface, may prove to be more important for helping to retain and later deliver nutrients to plants [21,88].
While amending compost with biochar significantly reduced NO x emissions, it surprisingly did not mitigate NH 3 emissions (figure 4(h)).Multiple studies have reported lower NH 3 emissions during biochar-composting compared to composting without biochar [34,62,89,90].However, biochar has also been found to increase NH 3 emissions from soils and composts by increasing pH [83,[91][92][93].Even though biochar may have adsorbed NH 4 + during composting, it may have increased the rate of volatilization of non-adsorbed NH 4 + by increasing the pH of the compost.This tradeoff may explain why differences in NH 3 emissions between the biocharcompost pile and the manure-only pile were not statistically significant.NH 3 is an important air pollutant released during composting, so further research that tests which biochar feedstocks and production techniques optimize NH 3 mitigation during composting is needed to improve the potential of biocharcomposting to reduce air pollution.
Biochar production may also mitigate air pollution by providing an alternative to the burning of agricultural biomass, a common waste management strategy on farms (figure 1).Biomass burning is a large source of PM 2.5 pollution and is a significant contributor to poor air quality in rural regions [6].Compared to combustion, pyrolysis emits far less PM 2.5 and PM 2.5 precursors, especially when gases are trapped for renewable energy production [94,95].In order to improve rural air quality, California plans to ban nearly all biomass burning of agricultural residues in the San Joaquin Valley by 2025 [96].Biochar production offers a sustainable management pathway for this agricultural waste that adds value to the biomass and could help develop a circular economy based on biomass transformation and reuse in agriculture (figure 1) [97].Biochar use in dairy manure management would further the air pollution reduction potential of this system and could provide a reliable market for biochar companies.

Social cost analysis
We found that biochar-composting could reduce the social cost from dairy manure composting emissions by over $66 000 per year, at this particular farm.This is likely a conservative estimate since we do not consider the public health damages from H 2 S emissions, which was the largest source of non-GHG air pollution during the experiment and which was reduced by 67% (±24%) in the biochar-compost treatment.There are also other air pollution and climate benefits in the biochar-composting lifecycle that are not quantified in this analysis, such as reduced biomass burning, that would further reduce social cost.We also acknowledge the limitations of using social cost to quantify public health impact, as communities vary in their capacity to cope with pollution.For example, rural areas tend to have a higher proportion of socioeconomically disadvantaged communities, and members of these communities are much more likely to lack the economic and social means to deal with air pollution compared to members of middle class or affluent communities [31][32][33].Furthermore, due to the large scale of this experiment, compost piles were not replicated in this study, which was conducted on one farm and used one type of biochar applied at a single application rate.Additional replicate studies conducted at other dairy farms could help to better constrain the potential benefit of biochar-composting to public health, as variations in dairy manure and biochar characteristics, climate, regional air chemistry, and the population living near a dairy could significantly influence air pollutant emissions from biochar-composting and/or the impact of these emissions on nearby communities.

Conclusion
Previously, we found that biochar-composting of dairy manure solids is an untapped CH 4 mitigation strategy that serves as a complement to the anaerobic digestion of liquid manure and offers multiple climate benefits over its lifecycle [11].Here, we again found high potential for biochar to reduce CH 4 from dairy manure while also highlighting a significant co-benefit of this strategy-its potential to reduce air pollution from dairies.In addition to substantially mitigating CH 4 emissions, biocharcomposting could improve the health of rural, disadvantaged communities, especially in California's Central Valley which suffers from poor air quality and where many of the nation's dairies are concentrated.Sourcing biochar feedstock from agricultural biomass that would have otherwise been burned offers an additional opportunity to reduce air pollution from agriculture.Biochar-composting could offer governments and farmers a rare chance to tackle both climate change and air pollution simultaneously.However, further research is needed that optimizes biochar production and application for NH 3 mitigation during composting.Furthermore, due to the lack of pile replication in this study, additional field-scale biochar-composting studies at dairies are needed before biochar-composting can be recommended as an air pollutant emission mitigation tool at dairies.

Figure 1 .
Figure 1.Conceptual model showing the flow of orchard biomass to a biochar production facility, biochar to a dairy farm, and biochar-compost back to the orchard for use as an organic soil amendment.The impacts at the local, regional, and global scale from the reduction in the emission of CH4 and air pollutants during composting are also reported.

Figure 2 .
Figure 2. Average (n = 3) physical and chemical characteristics of the composts over the course of the experiment including (a) temperature in • C (b) moisture content (c) porosity (d) C in g kg −1 (e) N in g kg −1 (f) C/N ratio (g) NH4 + -N in mg kg −1 (h) NO3 -N in mg kg −1 (i) pH.The black lines are the biochar-compost treatment, and the yellow lines are the manure-only compost.Error bars are ±1 standard error.Compost piles were turned on days 0, 8, 23, and 31.

Figure 5 .
Figure5.Photographs of (a) a gas sampling collar on the manure-only compost and (b) a gas sampling collar on the biochar-compost.The photographs were taken on the same day, moments apart from each other.We observed substantially fewer stable flies (Stomoxys calcitrans) on the biochar-compost, which could be due to the reduction of H2S emissions following biochar application.