Phosphorus dynamics influenced by anthropogenic calcium in an urban stream flowing along an increasing urbanization gradient

Calcium (Ca) concentrations are commonly elevated in urban soils and sediments mainly due to the degradation of artificial Ca-rich materials. However, the influence of anthropogenic Ca on pollutant behaviour remains largely unknown. In this study, we aim to contribute to a better understanding of the accumulation of anthropogenic Ca and its mechanism for regulating the phosphorus (P) behaviour in the sediments of an urban stream flowing along an increasing urban land use in eastern China. The CaO content in sediments increased with the increasing urban land use and was significantly associated with the sampling distance from suburban to urban areas of the river (p < 0.05). Calcium carbonate contributed 34.4% to 58.9% of the total Ca. The results show that iron-bound P (FeP) and authigenic Ca-bound P (CaP) were the dominant P fractions in the river sediment and accounted for 41.7% and 40.7% of the total P (TP) on average, respectively. Compared to the rural upstream areas of the river, the proportion of FeP in TP decreased significantly in the urban downstream areas, whereas the proportions of total Ca-bound P increased in the downstream areas. The results of the correlation analyses between the elevated Ca and P fractions suggest that the anthropogenic Ca that was enriched in the sediments could facilitate the formation of Ca-bound P. Combined with the water quality parameters, our results suggests that the elevated Ca in the sediments might first precipitate with phosphate as metastable minerals in the interstitial water and then transform to thermodynamically stable apatite.