Calculating ANPS Pollution Emissions from Agricultural Sources

We calculate the ANPS pollution emissions from crops, animal breeding, aquaculture, and rural community sources (shown in Table 1). Unabsorbed nutrients from chemical fertilizers used in agriculture leach into groundwater and surface water due to rain and irrigation, and this causes surface water eutrophication and groundwater nitrate pollution [30 ,31 ]. Incineration of farmland crop residue or crop residue dumping will cause organic matter and microorganisms to enter the water body and cause water pollution [32 ,33 ,34 ]. According to the second survey report of pollution sources in China in 2016, livestock and poultry manure ranked first among all pollution sources from the agricultural sector. The feces, urine, and sewage generated during the livestock and poultry breeding process lead to a large amount of loss of nitrogen and phosphorus entering waterways [35 ,36 ]. Aquaculture also results in pollution due to excessive inputs of bait, the production of excrement, and the use of chemicals and antibiotics [37 ]. Finally, with the improvement of the living standards in China’s rural areas, the discharge of domestic sewage is increasing which has become one of the main sources of water pollution [38 ]. All of these sources are accounted for in our analysis.
ANPS pollution can be calculated for different sources with differences in the geographic characteristics of pollutants [39 ,40 ,41 ]. The equations used to calculate the Total Nitrogen (TN), Total Phosphorus (TP), and COD emissions from agricultural production units in each province every year are provided in Table 2.
In each equation, the loading coefficient, $\mathsf{\lambda }$ , is for N, P, and COD for each pollutant (details of the loading coefficients for all sources are in the Supplementary Materials). For fertilizer, aquaculture, and rural domestic sewage, the pollution is estimated by multiplying the variable from Table 1 by the respective loading coefficient. For example, the total amount of the kth fertilizer (1 = nitrogen fertilizer, 2 = phosphate fertilizer, 2 = compound fertilizer) input is ${\mathrm{T}}_{\mathrm{k}}$ ; the loading coefficient s of N and P in the kth fertilizer are ${\mathsf{\lambda }}_{{\mathrm{kF}}_{\mathrm{n}}}$ and ${\mathsf{\lambda }}_{{\mathrm{kF}}_{\mathrm{p}}}$ . The emissions of nitrogen and phosphorus, ${\mathrm{F}}_{\mathrm{n}}$ and ${\mathrm{F}}_{\mathrm{P}}$ , equal ${\mathrm{T}}_{\mathrm{k}}$ multiplies ${\mathrm{T}}_{\mathrm{n}}\times {\mathsf{\lambda }}_{{\mathrm{kF}}_{\mathrm{n}}}$ and ${\mathrm{T}}_{\mathrm{p}}\times {\mathsf{\lambda }}_{{\mathrm{kF}}_{\mathrm{p}}}$ .
More coefficients need to be included for livestock poultry and crop residue. When calculating the emissions from livestock and poultry farming, ${\mathrm{T}}_{\mathrm{z}}$ is the end-of-period or slaughtering quantity of the zth livestock and poultry (1 = poultry, 2 = pigs, and 3-cattle) and ${\mathsf{\theta }}_{\mathrm{zL}}$ is the growth cycle of the zth livestock and poultry. The growth cycle of ${\mathsf{\theta }}_{\mathrm{zL}}$ is 140, 180, and 365 days for poultry, pigs, and cattle respectively. The loading coefficient s, ${\mathsf{\lambda }}_{{\mathrm{zL}}_{\mathrm{n}}},{\mathsf{\lambda }}_{{\mathrm{zL}}_{\mathrm{p}}},{\mathsf{\lambda }}_{{\mathrm{zL}}_{\mathrm{n}}}$ , are multiplied by ${\mathrm{T}}_{\mathrm{z}}$ and ${\mathsf{\theta }}_{\mathrm{zL}}$ to get ${\mathrm{L}}_{\mathrm{n}}$ , ${\mathrm{L}}_{\mathrm{p}}$ , ${\mathrm{L}}_{\mathrm{cod}}$ , the emissions of N, P, and COD from livestock and poultry farming. For crop residue, the yield of the mth crop is ${\mathrm{T}}_{\mathrm{m}}$ ; the crop residue production coefficient of the mth crop is ${\mathsf{\phi }}_{\mathrm{m}}$ ; the loading coefficient s of N, P, and COD in the crop residue are ${\mathsf{\lambda }}_{{\mathrm{S}}_{\mathrm{n}}}$ , ${\mathsf{\lambda }}_{{\mathrm{S}}_{\mathrm{p}}}$ , ${\mathsf{\lambda }}_{{\mathrm{S}}_{\mathrm{c}}}$ ; ${\mathrm{S}}_{\mathrm{n}}$ , ${\mathrm{S}}_{\mathrm{p}}$ , ${\mathrm{S}}_{\mathrm{c}}$ are the total amount of N, P, COD emissions from crop residue which come from ${\mathrm{T}}_{\mathrm{m}}$ multiplied by ${\mathsf{\phi }}_{\mathrm{k}}$ and ${\mathsf{\lambda }}_{{\mathrm{S}}_{\mathrm{n}}}$ , ${\mathsf{\lambda }}_{{\mathrm{S}}_{\mathrm{p}}}$ , ${\mathsf{\lambda }}_{{\mathrm{S}}_{\mathrm{c}}}$ .
Finally, we add up the emissions from all pollution sources to obtain the ANPS pollution emissions of i = 1, …, 31 provinces (municipalities and autonomous regions) from t = 2010 to 2019 (Details of the calculation process are in the Supplementary Materials): $${\mathrm{ANPS}}_{it}={\mathrm{wTN}}_{\mathrm{it}}\left({\mathrm{F}}_{\mathrm{nit}}+{\mathrm{S}}_{\mathrm{nit}}+{\mathrm{L}}_{\mathrm{nit}}+{\mathrm{E}}_{\mathrm{nit}}+{\mathrm{A}}_{\mathrm{nit}}\right)+{\mathrm{wTP}}_{\mathrm{it}}\left({\mathrm{F}}_{\mathrm{pit}}+{\mathrm{S}}_{\mathrm{pit}}+{\mathrm{L}}_{\mathrm{pit}}+{\mathrm{E}}_{\mathrm{pit}}+{\mathrm{A}}_{\mathrm{pit}}\right)+{\mathrm{wCOD}}_{\mathrm{it}}\left({\mathrm{S}}_{\mathrm{CODit}}+{\mathrm{L}}_{\mathrm{CODit}}+{\mathrm{E}}_{\mathrm{CODit}}+{\mathrm{A}}_{\mathrm{CODit}}\right)$$
where ${\mathrm{ANPS}}_{it}$ is the ANPS pollution emissions in the ith province in the tth year; the weights of TN, TP, and COD emissions, ${\mathrm{wTN}}_{\mathrm{i},\mathrm{t}}$ , ${\mathrm{wTP}}_{\mathrm{i},\mathrm{t}}$ , and ${\mathrm{wCOD}}_{\mathrm{i},\mathrm{t}}$ , are calculated by the entropy method [42 ,43 ] (Entropy method is one of the common methods to determine weight and is a relatively objective and widely used than the analytic hierarchy process and coefficient of variation method. This paper draws on the improvement of entropy method made by Yang and Sun (2015), and adds time variable for analysis, so as to realize the comparison between different years.). It is worth emphasizing that we consider the differences in topography, climate, farming methods, crops, or breeding types of each province (municipalities and autonomous regions), and we use different loading coefficients for each area. However, the loading coefficients are held constant. Hence, if policies affect practices that reduce the amount of pollution generated by an activity, this will not be captured in our analysis.