Tracing the sources of phosphorus in lake at watershed scale using phosphate oxygen isotope (δ¹⁸O_P)

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 (δ¹⁸O_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 δ¹⁸O_P values ranged from +15.23 to +21.92‰ in agricultural soil, +16.53 to +24.10‰ in estuary sediments, +18.90 to +20.90‰ in pond sediments, and +17.42 to +19.70‰ in lake sediments. Isotopic signatures indicated that chemical fertilizers with heavier δ¹⁸O_P values (+20.70 to +26.50‰) 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 δ¹⁸O_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, δ¹⁸O_P from different sources can provide indirect and important evidences for the identification and management of P sources in the lake catchment.