Transport fluxes of nitrogen, phosphorus, cadmium and arsenic at farmland plot scale in the typical areas of Pearl River Delta region

The transport of nitrogen, phosphorus, cadmium and arsenic is the main factor affecting agricultural production as well as a significant non-point source of pollution of farmland. Taking the experimental farmland of the Foshan Institute of Agricultural Science and Technology (FIAST) in the Pearl River Delta as a typical research area, and based on the soil apparent balance model, a soil apparent balance model of nitrogen, phosphorus, cadmium and arsenic was constructed at the farmland plot scale, to analyze the transport structure and balance of nitrogen, phosphorus, cadmium and arsenic in the soil. The results showed that fertilization was the main input of the four elements in the study area, but the main outputs were different. The main output of nitrogen and phosphorus was crop enrichment, accounting for 57.5% and 39.0%, respectively, whilst the main outputs of cadmium and arsenic were surface runoff and crop enrichment, accounting for 66.7% and 10.7%, respectively. The enrichment was observed for all four elements in the study area to different degrees. On the plot scale, the balance of nitrogen and phosphorus was in surplus, while cadmium and arsenic were in deficit, with their intensity being 37.40, 8.88, −1.35 and −20.50 kg/(hm²·a), respectively. Nitrogen and phosphorus were within the local soil environmental safety thresholds. The analysis of cadmium and arsenic enrichment in five types of vegetables showed that the enrichment coefficient of arsenic was greater than 70.0% in all the types of vegetables, while the enrichment of cadmium was only found in pepper leaves, with a coefficient of 57.5%. Therefore, more attention should be paid to the effective utilization of fertilizer input and the monitoring of heavy metals in irrigation water, crops and surface runoff to ensure regional food security and water quality.