Accounting for Greenhouse Gas Emissions in the Agricultural System of China Based on the Life Cycle Assessment Method
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A carbon footprint is a quantification of greenhouse gas emissions [41], including the direct and indirect carbon emissions associated with the entire life cycle of a product or service activity [42]. Referring to the global warming potential (GWP) values stated in the Fifth IPCC Bulletin, CO2, N2O, and CH4 emissions are converted to CO2-eq units, where their respective GWP values are 1, 28, and 265.
CFT,i,j = CFin,i,j + CFpro,i,j
where CFT,i,j is the total carbon footprint (KgCO2-eq) of crop i in area j from sowing to harvesting, CFin,i,j is the carbon footprint of crop i in the production stage of agricultural materials in area j, and CFpro,i,j is the carbon footprint of crop i in the production stage of planting in area j.
CFin,i,j = ∑jADi,k × EFk
where k is the agricultural materials or emission sources including direct energy and indirect energy utilization and ADi,k is the consumption of the k agricultural materials or emission sources for crop i, where diesel consumption is measured based on the ratio of the fuel power fee to the average price of diesel fuel in the current year. The electricity consumption for irrigation is calculated based on the ratio of the electricity cost for irrigation to the current year’s electricity price and EFk is the emission factor of the various agricultural materials and consumption sources (Table 1).
Table 1.
Carbon emission factors for various agricultural inputs.
Table 1.
Carbon emission factors for various agricultural inputs.
| Carbon Sources | Carbon Emission Coefficients |
Unit | Coefficient Source |
|---|---|---|---|
| Seeds | Rice: 1.84 Wheat: 0.58 Corn: 1.93 |
kgCO2-eq/kg | ecoinvent 2.2 [43] |
| Nitrogen fertilizers | 1.53 | CLCD 0.7 [44] | |
| Phosphorus fertilizers | 1.63 | ||
| Potash fertilizers | 0.65 | ||
| Compound fertilizers | 1.77 | ||
| Pesticides | 18.09 | [45] | |
| Agricultural films | 22.72 | CLCD 0.7 [44] | |
| Diesel fuel burning | 0.89 | ||
| Electricity for irrigation | Northwestern: 0.97 | kgCO2-eq/3.6 MJ | |
| North China: 1.23 | |||
| South China: 0.82 |
CFpro,i,j = CFM,i,j + CFN2O,i,j + CFStraw,i,j + CFCH4,j
CFM,i,j = ADDies,i,j × EFDies
where CFM,i,j denotes the carbon footprint of CO2 in the production stage of crop i in area j, including CO2 emitted directly from the combustion of diesel fuel consumed by the use of agricultural machinery in the processes of sowing, plowing, and harvesting, etc. The GHGs generated from the calculation of diesel fuel inputs at the agricultural stage have already been included the emissions from the production stage and therefore will not be added twice. ADDies,i,j denotes the diesel fuel consumption of crop i in area j; EFDies denotes the CO2 emission factor for diesel combustion.
CFN2O,i,j = CFDirect,i,j + CFIndirect,i,j
where CFN2O,i,j represents the carbon footprint of N2O from agricultural land during the production stage of crop i in area j, including both direct and indirect emissions. CFDirect,i,j is the carbon footprint of N2O direct emission from agricultural land of crop i in area j, which is the emission caused by the nitrogen input of agricultural land in the current season, and the input nitrogen includes nitrogen fertilizer, manure, and straw return. Since fertilizer application mainly dominates in agricultural production activities and manure use data are difficult to calculate accurately [46], only the direct N2O emissions from agricultural land from nitrogen fertilizer and straw return to the field were calculated. The formula is as follows:
CFDirect,i,j = (ADN,i,j + ADstraw,i,j) × EFDirect,j × 44/28 × 265
where ADN,i,j denotes the amount of nitrogen fertilizer used by crop i in area j, including the amount of nitrogen contained in nitrogen fertilizers and compound fertilizers. ADstraw,i,j denotes the amount of nitrogen inputs from straw returned to the field by crop i in area j. EFDirect,j denotes the coefficient of direct emission of N2O from agricultural land in area j (Table 2).
In this case, the formula for calculating the amount of nitrogen input for straw return is as follows:
ADstraw,i,j = (Mi,j/∂i − Mi,j) × βi × Ki + Mi,j/∂i × Li × Ki
where Mi,j denotes the crop seed yield of crop i in area j; ∂i denotes the economic coefficient of crop i; βi denotes the straw return rate of crop i; Ki denotes the nitrogen content of straw of crop i; and Li denotes the root–crown ratio of crop i. The values of specific parameters are shown in Table 3:
CFIndirect,i,j = IN,i,j × 0.1 × EH + (ADN,i,j + ADstraw,i,j) × 0.2 × EL] × 44/28 × 256
In the formula, CFIndirect,i,j denotes the carbon footprint of N2O indirect emissions from the agricultural land of crop i in area j and EH and EL are the coefficients of N2O indirect emissions from agricultural land for nitrogen volatilization and nitrogen leaching and runoff, respectively. According to the “Guidelines for the Preparation of Provincial-level Greenhouse Gas Inventories (Trial),” the rate of volatilization of nitrogen inputs is 0.1 and the ratio of nitrogen leaching and runoff is 0.2.
CFStraw,i,j = AIi,j × δi,j × ηi × EFStraw
In the formula, CFStraw,i,j denotes the carbon footprint produced by the crop burning of crop i in area j; AIi,j shows the straw yield of crop i in area j, which is calculated by the economic yield and the ratio of grass to grain; δi,j denotes the ratio of straw burning of crop i in area j; ηi denotes the efficiency of straw burning of crop i; and EFStraw denotes the emission coefficient of crop straw burning. In this paper, we mainly calculate the emission of CO2 and CH4 produced by straw burning.
CFCH4,j = ARj × EFj × 28
where CFCH4,j denotes the carbon footprint produced by rice in area j; ARj denotes the sown area of rice in area j; and EFj denotes the CH4 emission factor of rice in area j (Table 4).
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