Agricultural non-point source pollution in Guangdong Province of China from 1991 to 2021

Guangdong Province is one of the largest agricultural regions in China, in addition to the robust economy. Agricultural development has been accelerated to ensure the food security in recent years. The agricultural non-point source pollution has been triggered by the large-scale investments in chemical fertilizers, pesticides, plastic mulch, and the rapid development of the livestock and poultry breeding industry. The purpose of this study is to unveil the emission quantity and sources of agricultural non-point source pollutants in Guangdong Province, elucidate the main characteristics and development trends of agricultural non-point source pollution, and thereby propose control measures targeting the primary pollutant sources. In this study, the emission coefficient was proposed to estimate the number of various pollutants that emitted by agriculture, livestock and poultry breeding, and aquaculture in Guangdong Province of China from 1991 to 2021. The data was also collected to better understand the agricultural development in the fertilization, arable land area, pig, cattle, poultry and aquaculture. The main causes were then determined for the agricultural non-point source pollution and their changing trends over time. The results show that: 1) An upward trend was found in the total discharges of chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total nitrogen (TN), and total phosphorus (TP) from the agricultural sources. The discharges of various pollutants decreased slightly since 2018, due to the reducing use of chemical fertilizer. The amount of fertilizer was applied per unit area more than twice the international average level. It is very necessary to reduce the chemical fertilizer use for the high efficiency. The discharges of COD, NH₃-N, TN, and TP from the agricultural sources increased to 1.9, 1.9, 1.7, and 2.2 times, respectively, in 2021, compared with 1991. 2) The primary contributors to the agricultural non-point source pollution were the planting, livestock and poultry breeding industries. The planting industry was contributed the most to the nitrogen discharges, accounting for 48% of NH₃-N losses, and 52% of total nitrogen losses from the agricultural sources, respectively. In addition, the water pollution that caused by the planting industry was mainly from the application of chemical fertilizers. The livestock and poultry breeding industry was the primary source of COD and TP discharge from the agricultural sources. Specifically, 90% and 51% of the total COD and TP discharges from agricultural sources were from the livestock and poultry breeding sector, respectively. The number of pollutants that discharged by the large-scale farming were accounted for more than 70% of the total pollutants discharged by the livestock and poultry breeding industry. Among different types of livestock and poultry, pigs were contributed the most to the emission of various pollutants, followed by poultry. Different pollutants that emitted by the aquaculture industry were accounted for between 10% and 16% of total emissions from agricultural sources. Aquaculture was ever increasing every year. Moreover, the effluent that produced by aquaculture (particularly mariculture) was challenging to clean up. Consequently, the number of pollutants that discharged into water from aquaculture sources will increase over time, indicating the increasing share of the total emissions from agricultural sources. Therefore, it is urgent to find the solutions for the waste disposal of large-scale pig farms. Also, the pollutant discharges from aquaculture needs to be considered. The finding can also provide a strong reference for the decision-making in the agricultural environment.