THE EFFECT OF NITROGEN AND PHOSPHORUS ON THE AGGREGATION OF SOIL COLLOIDAL PARTICLES IN THE THREE GORGES RESERVOIR AREA

Nitrogen (N) and phosphorus (P) concentrations in tributaries and bays of the Three Gorges Reservoir area increase significantly after impoundment. It will affect processes such as coalescence/dispersion of soil colloidal particles, which in turn will affect the ecological safety of the reservoir water bodies. We analyzed the aggregation process of purple soil colloidal particles, and found that aggregation was controlled by the interaction of N and P. With the increase of N and P concentration in the water body, purple soil colloidal particles transformed from slow aggregation (represented by linear growth) to fast aggregation (represented with a power function). We chose three forms of purple soil (acidic, calcareous, and alkaline) to check how interactions between nutrients and physical aggregation processes may vary across soil types. Average aggregation rate (TAA) of the three purple soils differed significantly, and the critical coalescence concentration (CCC) of neutral, alkaline, and acidic purple soils was 220.14, 117.49, and 47.20mmolL(-1) and 507.49, 437.15, 328.30mmolL(-1), respectively. Compared to phosphorus, the nitrogen system has higher TAA and lower CCC, indicating that nitrogen is more effective in triggering colloidal aggregation of purple soils. In the N and P systems, the surface potentials of neutral, alkaline and acidic purple soils decreased from -172.85mV to -70.28mV and -187.65mV to -81.98mV, respectively. With the increase of N and P concentrations, the surface charge density and the absolute surface potential values of the three purple soil colloids decreased, the surface potentials of the three purple soil colloids (absolute values) at the same concentration showed that P was greater than N. Meanwhile, the activation energy of interaction of all three purple soil colloids decreased continuously with the increase of N and P concentrations, and the activation energy of interaction in the N system was significantly lower than that in the P system under the same concentration conditions. Theoretical calculations showed that N and P changed the surface charge properties of the soil and increased the net gravitational force between colloidal particles, which made the net force behave as attraction and colloidal particles were more likely to agglomerate. When the N and P concentrations increased to 0.2, 0.1, 0.05molL(-1), and 0.5, 0.4, 0.3molL(-1), respectively, they were basically consistent with their CCC values. The net force between the three purple soil particles was negative and behaved as attraction. The surface potentials at the corresponding concentrations were all about -125mV. This study showed that N and P in the water body ultimately affected the aggregation process of soil colloids by changing the surface charge properties of particles, which in turn causes changes in the interaction forces and activation energy between colloidal particles.