Agricultural phosphorus pollution leads to a two-stage release of organic matter in sediments at the bottom of lake.

Agricultural non-point source phosphorus (P) pollution-primarily from phosphate fertilizers-significantly contributes to surface water P loading. Unlike existing findings that link P input to water eutrophication, this study identifies an additional critical pathway through which P threatens drinking water safety. The results of simulation experiments, described herein, showed that the phosphate entering surface water can lead to a two-stage, large-scale release of organic matter in the sediments. Phosphate rapidly displaces the organic matter in the upper layer of the sediments in the first stage, converting it into dissolved organic matter (DOM). Subsequently, phosphate can significantly alter the composition and function of the microbial community in the sediments during the second stage, thereby enhancing the decomposition of organic matter by microorganisms and causing a large release of organic matter from the sediment. Among them, the total organic carbon (TOC) of the supernatant increased by 1.5 ∼ 2 times, while that of the pore water decreased by 3.2 ∼ 4.3 times. Under the combined effects of chemical and microbial processes, the DOM concentration, aromaticity, average molecular weight, and humification degree in sediment-water systems all increased with rising initial P concentration and extended system operation time. In the context of drinking water supplies, these changes in surface water DOM led to an increase in the potential for the formation of disinfection by-products (DBPs) and the efficiency of DOM conversion into DBPs. After the addition of phosphate, the contents of trichloromethanes (THMs) and hydrofluoroacetic acids (HAAs) increased by 65% and 104% respectively. This study demonstrates that agricultural phosphate fertilizer pollution not only causes eutrophication but also leads to a decrease in water quality by releasing sediments organic matter. This process elevates DBPs formation risk, posing a threat to drinking water safety.