| It’s all known that urban areas are easily disturbed by intense human activities. The spatial expansion and population accumulation bring in a series of environmental issues such as excessive nutrient discharge, heavy metal pollution, and other organic pollutants and so on. Meanwhile, the high intensity of human activities in the urban area can alter the macro elements distribution of soils and river sediments. Calcium is one of the macro elements greatly affected by urbanization. The geochemical processes of aquatic phosphorous, which are affected directly and indirectly by natural calcium, have often been studied. However, their influences by artificial calcium are still unknown. The existing forms and physical-chemical properties of artificial calcium are different from those of physical calcium. The questions about whether artificial calcium affects the aquatic phosphorous, the way, and degree are significant to the urban aquatic environment. We chose two rivers which are located in the urbanized area as typical cases and took thirty-three and twenty surface sediments at the depth of 0-5 cm, respectively. Several indexes of water quality together with some basic physical and chemical properties of sediments were analyzed. In addition, we interpreted the land use types at river basin areas. The data of soil CaO content in the surface layer (0-20 cm,693 samples) and deep layer (150-200 cm,174 samples) in the Qinhuai River Basin were collected. The objectives of this research are to reveal the relationship between urbanization and calcium enrichment in surface soils and river sediments; to elucidate the spatial distribution of phosphorus along the urbanization gradient and the mechanism of P fixation with this anthropogenic calcium; and to investigate the effects of anthropogenic calcium on the roles of sediments acting as P sources or sink.The results and conclusions were listed below:1. The contents of CaO in the surface and deep soils of the Qinhuai River Basin decreased from northwest to southeast, with the average contents of 0.88% and 0.77%, respectively. Compared the contents of CaO between the surface and deep soils, the results showed that the calcium accumulation caused by urbanization mainly concentrated in the urban proper of Nanjing city. Based on the data of land use types in Qinhuai River Basin, the results showed that the average contribution of anthropogenic Ca caused by urban activities was 5.4 g kg-1.2. The content of total phosphorus (TP) in sediments of the Qinhuai River increased with the increasing urbanization gradient along the river, and the average value was 1612.9 mg kg-1. The spatial distribution of TP contents is complicated along the rural to urban gradient of the Nanfei River, which is mainly affected by downstream point source discharge and tributary influx. And the average TP contents of the Nanfei River sediments was 3008.4 mg kg-1. With regard to the composition of all P forms, the results showed that FeP accounted for the largest proportion, followed by CaP, and LP minimum in the sediments of both the Qinhuai River and the Nanfei River. On the rural to urban gradient belt, the contents of all the P forms in the Qinhuai River sediments increased with the increasing urbanization gradient along the river, except the DeP. However, the contents of LP and OP in the Nanfei River sediment increased with the increasing urbanization gradient. The FeP content decreased with the increase of urbanization, but the contents of CaP and DeP had no significant relationship with the coastal urbanization gradient along the Nanfei River.3. On the rural to urban gradient belt, the residential land along the Nanfei River directly improves the content of OP and total nitrogen (TN) in the sediments due to the discharge of domestic sewage. The farmland along the Nanfei River promoted the conversion of sediment phosphorus from other P to FeP due to the transportation of large amounts of Fe/Al oxides particles into the river. However, the contents of CaP and DeP in sediments showed no significantly correlation with the land use types along the Nanfei River. Because of the large amount of P input from Hefei city, the land use types along the Nanfei River also have no significant effect on the TP content of sediments.4. Compared with the rural upper reaches of the Qinhuai River, the results show that the proportion of FeP in sediments of the urbanized downstream was decreased, while the proportion of TCaP (CaP+DeP) was increased. Meanwhile, the contents of Ca showed significantly positive correlation with the contents of LP, CaP DeP and TP in the sediments of the Qinhuai River. These results suggested that the accumulation of anthropogenic Ca could promote the conversion of sediment P from other P forms to calcium bound P. Based on the relative data of water quality, it is found that the possible mechanism of P fixation by the elevated Ca is:in the interstitial water of the sediments, the phosphate ion first binds to the calcium ion, released from the anthropogenic Ca, to form weakly stable calcium phosphates (LP). Under certain conditions, these weakly stable calcium phosphates were partially converted to metastable calcium-bound phosphorus (CaP). On a long-term scale, a portion of this metastable calcium-bound phosphorus can be further converted to more stable phosphorus (DeP).5. On the rural to urban gradient belt, the roles of sediments in the Nanfei River and the Qinhuai River were that the sediments released P to the overlying water in the upstream of the rivers, while the sediments equilibrated with or adsorbed the P in the overlying water in the downstream of the rivers. On the whole, when the concentration of soluble reactive P (SRP) in the overlying water was lower than 0.50 mg L-1, the sediments began to release phosphorus to the water. There was a positive correlation between the calcium content of sediments and the values of the maximum P adsorption capacity (Qm) and the native absorbed P (NAP). These results indicated that the enrichment of calcium in the Nanfei River and Qinhuai River could increase the sediment adsorption capacity. In addition, a significant positive correlation was also observed between the Ca contents and zero equilibrium adsorption concentration (EPCO) in the two studied rivers. It indicated that the elevated calcium can not effectively reduce the P release in the two studied rivers sediments. However, the contents of Fe/Al oxides in the sediments were negatively correlated with the EPCO, which suggested that they can effectively reduce the risk of P release in the sediments of the studied rivers. Meanwhile, the results showed that when the mass ratio of Fe and P (Fe:P) of the sediments is greater than 50.4, the risk of P release from sediments decreased rapidly. |
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