| Phosphorus is the limiting factor of water eutrophication, which has become one of the most severe crises that threaten water quality of China. Migration and leaching of the non-point source phosphorus in the purple soil area are an important source of nutri-ents/contamination to the upper reaches of Yangtze River. The parent rock of purple soil is porous, thus leading to huge soil erosion under the washing effect of rainfall. The eroded soil particles with adsorbed nutrients/contamination transfer into local ponds and reservoirs through ditches, then to tributaries of Yangtze River, and eventually converging to Yangtze River and the Three Gorges Reservoir, which increases the risk of huge area of water pollu-tion. Agricultural ponds and reservoirs serve as the transition zone between non-point pollu-tion sources and large water bodies. The soil sedimentation could absorb the phosphorus in water and reduce phosphorus concentration. However, when the sediments reach their maxi-mum adsorption capacity, this phosphorus accumulation process only increases the danger of releasing and results in agricultural non-point source phosphorus pollution. Therefore, in or-der to develop agriculture-and environment-friendly soil phosphorus management measures, identifying the mechanism of phosphorus release for purple soils is necessary. The sediments are collected from the upper reaches of Yangtze River and the middle area of Three Gorges Reservoir, which are the ersion product of representative purple soils developed from Shaxi- miao Formation of Jurassic. The sediments are marked with various sources and sampling reservoirs of different usages. The method of laboratory simulation and analysis was used to study the phosphorus adsorption/desorption characteristics of the eroded sediments, the pre-sent forms of phosphorus and their distribution and transition.The main findings are as follows:(1) The erosion sediments of Purple soils were mainly composed of sand and silt, de-pending on the land use patterns and sources. There were more sand in Beibei Reservoir and more silt in Zhongxian Reservoir; the pH of erosion sediments was neutral or alkaline, except for several individual reservoirs (ZX001) being acid. The content of CaCO3, organic matter and amorphous Fe and A1oxides in the erosion sediments were in correlation with particle size distribution; they decreased with increasing particle size. pH was also an influencing factor; the content of CaCO3was positively proportional with pH while that of Fe and Al ox-ides were negatively proportional with it.(2) The phosphorus adsorption capacity of purple soil sediments ranged from263to2000mg/kg. The number for sediments from Beibei Reservoir was between263to926mg/kg, compared with417to2000mg/kg in Zhongxian Reservoir. The sediments composition was closely related with their phosphorus adsorption capacity; the ordering of adsorption capacity for different particle size was sand (263to417mg/kg), silt (333to1111mg/kg), clay (411to2000mg/kg). pH and CaCO3had an indirect effect on the adsorption capacity of the sediments. The amorphous Fe and A1oxides were the main matrix that adsorbed phosphorus and their contents increased with decreasing particle size. There were no significant differences in phosphorus adsorption among various size for Neutral and calcareous purple soil sediments, for CaCO3(also rich in coarse particles) as well had important influence on phosphorus ad-sorption, besides amorphous Fe and A1oxides.(3)The desorption of purple soil erosion sediments increased linearly with the amount of adsorption. The desorption rate for sediments with low adsorption energy was a constant; while the desorption curve with high adsorption energy followed a power function (Qt=k.ta,0<a<1). The desorption process could be divided into three stages of fast desorption, slow adsorption and dynamic equilibrium. The phosphorus desorption rate was positively corre- lated with pH, CaCO3and sand content, and was significantly negatively correlated with clay content, organic matter and amorphous Fe, A1oxide content.(4) The present forms of phosphorus (TP) in sediments were non-apatite inorganic phosphorus (NAIP), apatite inorganic phosphorus (AP) and organic phosphorus (OP). NAIP was biologically-available and water-soluble and capable of releasing. It was rich in acid pur-ple soils and accounted for21.34%-38.26%of TP. AP and OP were stable and unlikely to be released; they accounted for37.40%～68.78%and9.88%～25.75%respectively of TP. The present forms of phosphorus had a close relationship with the physical and chemical char-acteristics of sediments, of which NAIP content was significantly correlated with amorphous Fe and Al oxides (r=0.56, p<0.05), AP was significantly correlated with CaCO3content (r=0.53, p<0.05), in the meantime affected by pH and water content. The content of OP was sig-nificantly in proportion with organic matter content (r=0.93, p<0.01). For total phosphorous content in different particle size, there were sand (285.53-511.27mg/kg)<the silt (294.68～525.13mg/kg)<clay (308.36～632.95mg/kg).(5) The contents of NAIP and AP increased after adsorption of phosphorus, while that of OP was nearly unchanged. The phosphorus adsorption capacity approximately equaled the IP addition. It demonstrated that phosphorus adsorbed by amorphous Fe, Al oxide was mainly present in form of NAIP and phosphorus adsorbed by calcium compound was in form of AP. During the desorption process, the adsorbed NAIP was released by amorphous Fe the Al ox-ides, while the AP and OP were barely changed. That was to say, the desorbed phosphorus was mainly NAIP. That the main factor leading to water euthrophication in the upstream (Beibei) and middle stream (Zhongxian) of Yangtze River was NAIP adsorbed with amor-phous Fe and A1oxides could be inferred. It is recommended that water eutrophication in Three Gorges Reservoir area could be alleviated by controlling the content and activity of amorphous Fe and A1through agricultural practices. |
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