Tracing the sources of phosphorus in lake at watershed scale using phosphate oxygen isotope (d18OP)

Phosphorus (P) is normally considered as the limited nutrient for shallow freshwater lakes and can potentially trigger eutrophication on account of high concentrations. Due to the various transportation and transformation processes, P source apportionment and management in lake ecosystems have become more and more difficult. Combining with sequential extraction of P fractions and mineralogical analysis, the isotopic compositions of oxygen in phosphate (delta O-18(P)) of resin-extractable P from the different samples including soil, estuary sediments, pond sediments, and lake sediments in the Shijiuhu Lake catchment, China, were investigated. The results showed that delta O-18(P) values ranged from +15.23 to +21.92 parts per thousand in agricultural soil, +16.53 to +24.10 parts per thousand in estuary sediments, +18.90 to +20.90 parts per thousand in pond sediments, and +17.42 to +19.70 parts per thousand in lake sediments. Isotopic signatures indicated that chemical fertilizers with heavier delta O-18(P) values (+20.70 to +26.50 parts per thousand) were the predominant contributors of P in the soil. The river transportation together with Fe/Al-P desorption on anaerobic condition simultaneously stimulated the enrichment of P in the lake sediments, even though the biotic activity regulated the isotope values moving toward the equilibrium. Eroded soil was the important source of P in lake and pond sediments via drainage and runoff, and conserved the source isotope signal in the samples. Stronger biotic activity in the aquatic environments dragged delta O-18(P) values toward the equilibrium. However, conspicuous off-equilibrium isotope signature suggested the terrestrial sources in the aquatic ecosystems. The calculation of two end-member linear mixing models suggested that soils also predominantly controlled the P occurrence in the lake sediments with contribution higher than 80%, indicating that decreasing inputs from the agricultural activities is important in P reduction on catchment scale. Generally, delta O-18(P) from different sources can provide indirect and important evidences for the identification and management of P sources in the lake catchment.

Automated summary

This study investigates the sources of phosphorus in the Shijiuhu Lake catchment in China by analyzing the oxygen isotopic composition. The results show that agricultural activities are the main source of phosphorus in the soil, while eroded soil contributes to phosphorus in lake and pond sediments through drainage and runoff. The study suggests that reducing agricultural inputs is crucial for phosphorus reduction at the catchment scale.