Life cycle greenhouse gases emission of rice production with pig manure application

Environmental problems due to livestock and poultry waste releases have worsened with increasing scale of breeding industries. Utilization of livestock and poultry wastes in farmlands could limit the production and application of chemical fertilizers and the corresponding pollution. However, this could as well lead to greenhouse gases emissions in the agro-ecological systems. Studies on greenhouse gases emissions from farmland with partial replacements of chemical nitrogen (N) fertilizers by livestock and poultry manure are critical for exploring farmland environmental carrying capacity of livestock and poultry wastes. This study aimed to assess greenhouse gases emission per ton of produced rice with different fertilization practices. Life Cycle Assessment (LCA) was used to evaluate greenhouse gases emission of a rice production system under different substitution ratios of chemical N fertilizer with pig manure. The life cycle of one ton of produced rice was divided into three phases — raw material mining, agricultural materials production and crop planting. Inventory analysis and calculation were performed for the three stages. Pure chemical fertilizer (N1), half manure N plus half inorganic fertilizer N (NM1) and manure N (NM2) treatments were set up in field plots. Greenhouse gases (carbon dioxide, methane and nitrous oxide) from paddy fields were collected and measured by the static chamber-gas chromatography method, while CO₂ emissions from other sources were calculated using the amount and emission factors of input materials as documented in relevant literatures. The results showed that gross greenhouse gases emissions under N1, MN1 and NM2 treatments were 1.760 t(CO₂-eq)·t^-1, 1.997 t(CO₂-eq)·t^-1 and 2.550 t(CO₂-eq)·t^-1, of which 0.145 t(CO₂-eq)·t^-1, 0.085 t(CO₂-eq)·t^-1 and 0.047 t(CO₂-eq)·t^-1 were for raw material mining, 0.032 t(CO₂-eq)·t^-1, 0.014 t(CO₂-eq)·t^-1 and 0 t(CO₂-eq)·t^-1 for agricultural materials production, and 1.583 t(CO₂-eq)·t^-1, 1.898 t(CO₂-eq)·t^-1and 2.503 t(CO₂-eq)·t^-1 for crop planting. Under NM1 and NM2 treatments, crop yields increased respectively by 15.87% and 9.14% and the total greenhouse gases emission during the whole life cycle increased by 13.63% and 44.89%. Compared with N1, greenhouse gases emissions under NM1 and NM2 treatments decreased respectively by 41.37% and 61.58% for raw material mining, 56.25% and 100.00% for agricultural materials production, but then increased by 20.25% and 58.23% for crop planting. Greenhouse gases emission under the three treatments that was due to resource consumption during field operations was 0.069t(CO₂-eq)·t^-1, 0.065t(CO₂-eq)·t^-1 and 0.075 t(CO₂-eq)·t^-1. The CO₂ emissions were divided into direct and indirect emissions. Direct emissions came from the use of diesel oil, electricity, pesticides and field emissions during rice growth period at crop planting stage. Then indirect emissions were from the use of raw coal and crude oil (at raw material mining stage), and coal and electricity (at agricultural materials production stage). Irrigation was the main sources of greenhouse gases emission during field management, followed by tillage, cutting stubble and reaping. NM1 and NM2 treatments decreased greenhouse gases emission at raw material mining and agricultural materials production stages, but significantly increased greenhouse gases emissions at crop planting stage. This subsequently increased greenhouse gases emissions in rice production. Advanced fertilization technologies should be explored to improve N utilization efficiency so as to reduce environmental impact. Also livestock and poultry manures should be harmlessly processed before farmland utilization.