Application of Farmyard Manure Rather Than Manure Slurry Mitigates the Net Greenhouse Gas Emissions from Herbage Production System in Nasu , Japan

In Japan, it is important to recycle the nutrients in manure for forage production because most dairy cattle are fed inside, mainly with imported grain and home-grown roughage. To understand the overall effect of manure use on grassland on the net greenhouse gas (GHG) emission and GHG intensity of herbage production systems, the integrated evaluation of emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) is essential. The objective of this study was to compare the net GHG emissions (expressed in CO2-eq ha−1 y−1) and GHG intensity (expressed in CO2-eq Mg–1 dry matter yield) of herbage production based on manure slurry + synthetic fertilizer (slurry system) with that based on farmyard manure + synthetic fertilizer (FYM system). Calculations of net GHG emissions and GHG intensity took into account the net ecosystem carbon balance (NECB) in grassland, the CH4 and N2O emissions from grassland, and GHG emissions related to cattle waste management, synthetic fertilizer manufacture, and fuel consumption for grassland management based on literature data from previous studies. The net GHG emissions and GHG intensity were 36% (6.9 Mg CO2-eq ha−1 y−1) and 41% (0.89 Mg CO2-eq Mg−1), respectively, lower in the FYM system.


Introduction
Recent greenhouse gas (GHG) profiles of the agriculture sector in Japan show that major GHG sources in terms of carbon dioxide equivalents (CO 2 -eq) are rice cultivation (methane (CH 4 ), 41%), enteric fermentation (CH 4 , 22%), manure management (CH 4 and nitrous oxide (N 2 O), 19%), and soils (N 2 O, 16%) [1].In Japan, most dairy cattle are fed inside, mainly with imported grain and home-grown roughage.Around 70% of the dairy cattle waste is composted [2] for use in crop and forage production.Therefore, it is important to apply manure to meadows and pastures, which account for 13.4% of the total agricultural land area of Japan [3].In the year 2015, 570,475 Mg-N y −1 was excreted by livestock in Japan, of which 304,285 Mg-N y −1 was applied to agricultural soil (4,496,000 ha) [1].
The application of farmyard manure (FYM) to grassland increases the net ecosystem carbon balance (NECB) relative to manure slurry application [4].This is mainly because the amount of C input to grassland from FYM is greater than that from slurry, but the decomposition of FYM is slower than that of slurry [4].Consequently, FYM application has a greater potential to improve the C stock in grassland soil than slurry application.However, emissions of CH 4 and N 2 O from manured grassland [5] need to be considered in evaluating the overall effect of manure application on the net GHG emissions from grassland [3].The soil of grassland usually acts as a sink of atmospheric CH 4 , and manure application only temporarily increases CH 4 emission from grassland [3].In contrast, N 2 O emission increases with increasing the N surplus in grassland soil [6].Therefore, judicial application of organic and inorganic N is necessary to mitigate the N 2 O emission from grassland [3].Cattle waste management, including slurry storage [7] and composting [8], is another source of GHG, and in addition to CH 4 and N 2 O emissions from cattle waste management, farm machinery used for composting FYM also emits CO 2 [9].To maintain productive sward, supplemental fertilizers are used to make up for nutrient insufficiencies in manure (e.g., N and P in the case of cattle manure) [5], but their manufacture also emits GHG [10].Furthermore, GHG emissions from fuel consumption for grassland management also need to be taken into consideration [11].
To assess the net GHG emissions (i.e., integrated evaluation of CO 2 , CH 4 and N 2 O) and GHG intensity (GHGI) of herbage production systems, an integrated evaluation of the above processes is necessary (Table S1).On that basis, the identification of important processes with significant contributions is necessary in order to determine the priority of countermeasures to mitigate GHG.To date, it is recognized that the quality and quantity of organic materials applied have great influence on soil organic carbon [4,12]; however, insufficient information is available on the effect of manure type (i.e., slurry or FYM) on the net GHG emissions and GHGI of herbage production systems.
The objectives of this study were: (1) to investigate the net GHG emissions (expressed in CO 2 -eq ha -1 y −1 ) and GHGI (expressed in CO 2 -eq Mg -1 dry matter yield) of herbage production systems based on manure slurry + synthetic fertilizer (slurry system) and on FYM + synthetic fertilizer (FYM system); (2) to show the relative contributions of each process in GHG emission; and (3) to show how farming practices can be adjusted to minimize emissions.My hypotheses were that the FYM system reduces the net GHG emissions in comparison with the slurry system, and that the contributions of grassland soil and cattle waste management to the net GHG emissions of herbage production systems are greater than the other processes.

System Boundary and Functional Units
The system boundary comprised the following processes: the NECB in grassland, emissions of CH 4 and N 2 O from grassland, and GHG emissions related to cattle waste management (i.e., slurry storage and composting FYM), synthetic fertilizer manufacture, and fuel consumption for grassland management operations.The functional unit was defined as ha −1 y −1 of grassland or Mg −1 of dry matter yield.The study did not take into account the GHG emissions related to the manufacture of farm machinery and buildings, transport of synthetic fertilizers, or indirect N 2 O emissions related to leaching of nitrate (NO 3 − ) and redeposition of volatilized ammonia (NH 3 ).

NECB and Emissions of CH 4 and N 2 O from Grassland
The NECB and emissions of CH 4 , and N 2 O from grassland (1 ha) treated with slurry (65.8 to 66.4 Mg ha −1 y −1 ) or FYM (36.5 to 39.2 Mg ha −1 y −1 ) were based on previous studies [4,5] in which slurry or FYM was applied to the upper limit based on K requirement for herbage production.Annualized values of NECB, emissions of CH 4 , and N 2 O were calculated by averaging the information of two years.

GHG Emissions Related to Cattle Waste Management
Emissions of CH 4 and N 2 O from stored slurry were calculated from emission factors (EFs, 3.90% and 0.02%, respectively) in Japan [1].Emissions of CH 4 and N 2 O from composting of applied FYM were estimated from a farm study [13] and EFs (3.8% and 2.38% to 2.39%, respectively) in Japan [1].Biogenic CO 2 losses from manure were excluded (i.e., C neutral), but emissions of CO 2 due to the consumption of electricity or fuel for composting FYM were estimated from a farm study [13] and EFs [14].Emissions of CH 4 , N 2 O and CO 2 per unit area of grassland (1 ha) were calculated by multiplying these emissions per unit weight of slurry or FYM and the weight of slurry or FYM annually applied to grassland (Mg ha −1 y −1 ).

GHG Emissions Related to Synthetic Fertilizer Manufacture
GHG emissions from the manufacture of N and P fertilizers were estimated from the SimaPro 7.1 database (PRé Consultants, Amersfoort, Netherlands).Emissions of CH 4 , N 2 O and CO 2 per unit area of grassland (1 ha) were calculated by multiplying these emissions per unit weight of synthetic fertilizer and the weight of synthetic fertilizer annually applied to grassland (kg ha −1 y −1 ).No K fertilizer was used, because the applied slurry or FYM covered the K requirement for herbage production [4,5].

GHG Emissions Related to Grassland Management
GHG emissions due to fuel consumption by farm machinery for loading and spreading of manure and fertilizers and for cutting and harvesting of herbage were estimated from a previous Japanese study in the 1990s [15] and EF [14].The emission of CO 2 per unit area of grassland (1 ha) was calculated by multiplying the fuel consumption per unit of operation, the operation unit necessary for management of grassland (1 ha), and EF.

Overall Net GHG Emissions and GHGI of Herbage Production
Emissions of CH 4 and N 2 O were converted to CO 2 -eq by using values of the 100-year global warming potential, assumed to be 1 for CO 2 , 25 for CH 4 , and 298 for N 2 O [16].The net GHG emissions (CO 2 -eq ha -1 y −1 ) were calculated by considering the NECB and emissions of CH 4 and N 2 O from grassland (Section 2.2.) and the GHG emissions related to cattle waste management (Section 2.3.), synthetic fertilizer manufacture (Section 2.4.), and grassland management (Section 2.5.) on an area basis.The GHGI (CO 2 -eq Mg -1 ) was calculated by dividing the net GHG emissions by the dry matter yield of grassland receiving slurry or FYM [4].

NECB and Emissions of N 2 O and CH 4 from Grassland
The NECB of the slurry system (−12.8Mg CO 2 -eq ha −1 y −1 ) was far lower than that of the FYM system (−1.8Mg CO 2 -eq ha −1 y −1 ); that is, the FYM system contributed more to improving the C stock in grassland than the slurry system (Figure 1).(Please note that negative NECB values represent net CO 2 emission from grassland to the atmosphere.)The NECB of the slurry and FYM systems were similar to previously measured values in Japanese grasslands that respectively received only synthetic fertilizers or FYM + synthetic fertilizers [17], suggesting that slurry C had limited capacity for maintaining soil organic C in comparison with FYM C [12].The emissions of N 2 O from grassland were not significantly different between slurry and FYM systems (2.2 Mg CO 2 -eq ha −1 y −1 in slurry system, 2.3 Mg CO 2 -eq ha −1 y −1 in FYM system), because synthetic N fertilizer was also applied [18,19].The emissions of CH 4 were much smaller than those of CO 2 and N 2 O (+0.034 Mg-CO 2 -eq ha −1 y −1 in slurry system, −0.032 Mg-CO 2 -eq ha −1 y −1 in FYM system).

GHG Emissions Related to Cattle Waste Management
The GHG emissions related to cattle waste management were due mainly to CH4 (2.9 Mg CO2eq ha −1 y −1 , Table 1) in the slurry system and to both CH4 (4.2 Mg CO2-eq ha −1 y −1 , Table 2) and N2O (2.0 Mg CO2-eq ha −1 y −1 , Table 3) in the FYM system (Figure 2).The emission of CO2 related to energy consumption (0.15 Mg CO2-eq ha −1 y −1 , Table 4) was smaller than the emission of CH4 and N2O by composting.This is because the amount of CO2 emission for composting 1 Mg of FYM was only 4.1 kg.The emission of N2O by slurry storage (0.028 Mg-CO2-eq ha −1 y −1 , Table 5) was much smaller than the emission of N2O by composting FYM (2.0 Mg CO2-eq ha −1 y −1 , Table 3), mainly due to the small EF for slurry storage (0.02%) in comparison with composting FYM (2.4%). 1 Organic matter content in slurry was determined as 1.764 × C content. 2 Emission factor of CH4 from slurry storage was 2.38-2.39%(g-CH4 g-OM −1 ) [1]. 3 The 100-year global warming potential of CH4 was assumed to be 25 [16].

GHG Emissions Related to Cattle Waste Management
The GHG emissions related to cattle waste management were due mainly to CH 4 (2.9 Mg CO 2 -eq ha −1 y −1 , Table 1) in the slurry system and to both CH 4 (4.2Mg CO 2 -eq ha −1 y −1 , Table 2) and N 2 O (2.0 Mg CO 2 -eq ha −1 y −1 , Table 3) in the FYM system (Figure 2).The emission of CO 2 related to energy consumption (0.15 Mg CO 2 -eq ha −1 y −1 , Table 4) was smaller than the emission of CH 4 and N 2 O by composting.This is because the amount of CO 2 emission for composting 1 Mg of FYM was only 4.1 kg.The emission of N 2 O by slurry storage (0.028 Mg-CO 2 -eq ha −1 y −1 , Table 5) was much smaller than the emission of N 2 O by composting FYM (2.0 Mg CO 2 -eq ha −1 y −1 , Table 3), mainly due to the small EF for slurry storage (0.02%) in comparison with composting FYM (2.4%).

GHG Emissions Related to Cattle Waste Management
The GHG emissions related to cattle waste management were due mainly to CH4 (2.9 Mg CO2eq ha −1 y −1 , Table 1) in the slurry system and to both CH4 (4.2 Mg CO2-eq ha −1 y −1 , Table 2) and N2O (2.0 Mg CO2-eq ha −1 y −1 , Table 3) in the FYM system (Figure 2).The emission of CO2 related to energy consumption (0.15 Mg CO2-eq ha −1 y −1 , Table 4) was smaller than the emission of CH4 and N2O by composting.This is because the amount of CO2 emission for composting 1 Mg of FYM was only 4.1 kg.The emission of N2O by slurry storage (0.028 Mg-CO2-eq ha −1 y −1 , Table 5) was much smaller than the emission of N2O by composting FYM (2.0 Mg CO2-eq ha −1 y −1 , Table 3), mainly due to the small EF for slurry storage (0.02%) in comparison with composting FYM (2.4%). 1 Organic matter content in slurry was determined as 1.764 × C content. 2 Emission factor of CH4 from slurry storage was 2.38-2.39%(g-CH4 g-OM −1 ) [1]. 3 The 100-year global warming potential of CH4 was assumed to be 25 [16].1.9 1 Based on previous research on the dairy farm from which FYM was collected in this study [13]. 2 Based on the National Greenhouse Gas Inventory Report of Japan [1]. 3 N content was assumed to be 0.4% in feces and 0.8% in urine [1]. 4 Emission factor of N 2 O from composting of dairy cattle excreta was assumed to be 2.4% (g-N 2 O-N g-N −1 ) [1]. 5 FYM production was 168.8 Mg per farm per 30 d [13]. 6The 100-year global warming potential of N 2 O was assumed to be 298 [16].  1 Based on previous research on the dairy farm from which FYM was collected in this study [13]. 2 378 g-CO 2 was assumed to be emitted by 1 kWh of electricity consumption [14]. 32619 g-CO 2 was assumed to be emitted by consumption of 1 L light diesel oil [14]. 4CO 2 emission per unit area of grassland was calculated by multiplying the CO 2 emission per unit weight of FYM and the weight of FYM applied to 1 ha of grassland.
Table 5. Emission of N 2 O related to storage of cattle slurry.

N in Slurry
1st year 2nd year 1st year 2nd year 1st year 2nd year 1 Emission factor of N 2 O from slurry storage was 0.02% (g-N 2 O-N g-N −1 ) [1]. 2 The 100-year global warming potential of N 2 O was assumed to be 298 [16].

GHG Emissions Related to Fertilizer Manufacture
The GHG emission related to fertilizer manufacture in the FYM system (1.5 Mg CO 2 -eq ha −1 y −1 , Table 6) was almost double that for the slurry system (0.82 Mg CO 2 -eq ha −1 y −1 , Table 6), mainly owing to the difference in N application rate.Slurry contains substantial amount of readily available N, however, most of N in FYM is in organic form.Therefore, the amount of N fertilizer supplemented to grassland in FYM system (159 to 177 kg-N y −1 ) was greater than that in slurry system (90 kg-N y −1 ).

GHG Emissions Related to Grassland Management
The GHG emissions related to grassland management were similar between the FYM and slurry systems (0.49 vs. 0.47 Mg CO 2 -eq ha −1 y −1 , Table 7), because the herbage yields were not significantly different between the slurry (8.8 Mg y −1 ) and FYM (9.5 Mg y −1 ) systems.The GHG emissions related to grass cutting, turning and harvesting, bailing and wrapping were greater than those related to loading and spreading of slurry or FYM.This is mainly because slurry and FYM were spread twice and once a year, respectively; however, grass cutting, turning and harvesting, bailing and wrapping was performed four times a year in both the slurry and FYM systems. 1 Loading, transport, and spreading of 30 Mg-FYM was assumed to be 1 operating unit [15]. 2 Grassland was assumed to be 500 m from cowshed [15]. 3Loading, transport, and spreading of 80 Mg-slurry was assumed to be 1 operating unit [15]. 4Distribution of 500 kg fertilizer was assumed to be 1 operating unit [15]. 5Bailing of 7 Mg-DM haylage was assumed to be 1 operating unit [15]. 6Wrapping of 3 Mg-DM haylage was assumed to be 1 operating unit [15]. 7Transport of 7 Mg-DM haylage was assumed to be 1 operating unit [15]. 82619 g-CO 2 was assumed to be emitted by consumption of 1 L light diesel oil [14].

Overall Net GHG Emissions and GHGI
The net GHG emissions was 19 Mg CO 2 -eq ha −1 y −1 in the slurry system and 12 Mg CO 2 -eq ha −1 y −1 in the FYM system (Figure 3).The GHGI was 2.2 Mg CO 2 -eq Mg −1 in the slurry system and 1.3 Mg CO 2 -eq Mg −1 in the FYM system.Thus, the net GHG emissions of the FYM system was 36% (6.9 Mg CO 2 -eq ha −1 y −1 ) less and the GHGI of the FYM system was 41% (0.89 CO 2 -eq Mg −1 ) less than that of the slurry system.

Overall Net GHG Emissions and GHGI
The net GHG emissions was 19 Mg CO2-eq ha −1 y −1 in the slurry system and 12 Mg CO2-eq ha −1 y −1 in the FYM system (Figure 3).The GHGI was 2.2 Mg CO2-eq Mg −1 in the slurry system and 1.3 Mg CO2-eq Mg −1 in the FYM system.Thus, the net GHG emissions of the FYM system was 36% (6.9 Mg CO2-eq ha −1 y −1 ) less and the GHGI of the FYM system was 41% (0.89 CO2-eq Mg −1 ) less than that of the slurry system.
The contribution of grassland soil, cattle waste management, fertilizer manufacture and grassland management to the net GHG emissions were 78% (CO2: 66%, N2O: 11% and CH4: 0.2%), 15% (CO2: 0.0%, CH4: 15%, N2O: 0.1%), 4% and 2% in the slurry system, and 33% (CO2: 14%, N2O: 19% and CH4: −0.3%), 51% (CO2: 1%, N2O: 16%, CH4: 34%), 12% and 4% in the FYM system, respectively.These results collectively suggest that NECB and the N2O emissions from grassland and the CH4 and N2O emissions related cattle waste management are crucial to the control of net GHG emission and GHGI.The FYM system reduced the net GHG emissions and GHGI relative to the slurry system (Figure 3).The net reduction was due largely to the improvement of C stock in grassland (Figure 1).Although the emissions of GHG related to cattle waste management and fertilizer manufacture were greater in the FYM system than in the slurry system (Figure 3), the FYM system maintained an advantage in net GHG emissions and GHGI, due mainly to the difference in NECB in grassland (Figure 1)-that is, the persistent organic matter in FYM decomposed slowly in the soil and contributed to the improvement of C stock, but the labile organic matter in slurry decomposed quickly in the soil and was released to the atmosphere as CO2 [4].Our results support the validity of FYM application for the mitigation of GHG emissions [20,21], not only from grassland, but also during herbage production.In the slurry and FYM systems, N and P were supplemented based on the fertilizer recommendation.Therefore, the yields in the slurry (8.8 Mg y −1 ) and FYM (9.5 Mg y −1 ) systems were comparable to the standard yield (8-10 Mg y −1 ) in Nasu, Japan [4].

Adjustment of Farming Practices
These results show that the FYM system improved the net GHG emissions and GHGI relative to the slurry system (Figure 3).In Japan, a substantial amount of manure is derived from imported feed, and thus represents the net import of organic matter, which must be used with care for fertility management [3].Making maximum use of manure in consideration of N, P, and K requirements for herbage production to reduce synthetic fertilizer rates to the absolute minimum is crucial to mitigating overall GHG emissions [18,22].Applying manure and synthetic N fertilizer in excess of demand increases N2O emissions from grassland [23,24].Therefore, the tightening of N application rates can limit overall GHG emissions.Mixing low-quality dried grass as a bulking agent into FYM reduced CH4 and N2O emissions [2] and could further improve the C stock in grassland.For this goal, The FYM system reduced the net GHG emissions and GHGI relative to the slurry system (Figure 3).The net reduction was due largely to the improvement of C stock in grassland (Figure 1).Although the emissions of GHG related to cattle waste management and fertilizer manufacture were greater in the FYM system than in the slurry system (Figure 3), the FYM system maintained an advantage in net GHG emissions and GHGI, due mainly to the difference in NECB in grassland (Figure 1)-that is, the persistent organic matter in FYM decomposed slowly in the soil and contributed to the improvement of C stock, but the labile organic matter in slurry decomposed quickly in the soil and was released to the atmosphere as CO 2 [4].Our results support the validity of FYM application for the mitigation of GHG emissions [20,21], not only from grassland, but also during herbage production.In the slurry and FYM systems, N and P were supplemented based on the fertilizer recommendation.Therefore, the yields in the slurry (8.8 Mg y −1 ) and FYM (9.5 Mg y −1 ) systems were comparable to the standard yield (8-10 Mg y −1 ) in Nasu, Japan [4].

Adjustment of Farming Practices
These results show that the FYM system improved the net GHG emissions and GHGI relative to the slurry system (Figure 3).In Japan, a substantial amount of manure is derived from imported feed, and thus represents the net import of organic matter, which must be used with care for fertility management [3].Making maximum use of manure in consideration of N, P, and K requirements for herbage production to reduce synthetic fertilizer rates to the absolute minimum is crucial to mitigating overall GHG emissions [18,22].Applying manure and synthetic N fertilizer in excess of demand increases N 2 O emissions from grassland [23,24].Therefore, the tightening of N application rates can limit overall GHG emissions.Mixing low-quality dried grass as a bulking agent into FYM

Figure 2 .
Figure 2. Annualized emissions of N 2 O, CH 4 , and CO 2 related cattle waste management.

Table 1 .
Emission of CH4 related to storage of dairy cattle slurry.

Table 1 .
Emission of CH 4 related to storage of dairy cattle slurry.

Table 1 .
Emission of CH4 related to storage of dairy cattle slurry.

Table 2 .
Emission of CH 4 related to composting.

Table 3 .
Emission of N 2 O related to composting dairy FYM.

Emission per Unit Weight of FYM 5,6 N 2 O Emission per Unit Area of Grassland kg
head −1 d −1 Mg-CO 2 -eq ha −1 y −1

Table 4 .
Emission of CO 2 related to energy consumption for composting dairy FYM.

Table 6 .
Emissions of GHG related to synthetic fertilizer manufacture.

Table 7 .
Emission of CO 2 related to fuel consumption for grassland management and transport.