Research On Phosphorous Release At Water Fluctuation Zone Of The Three Gorges Reservoir Area

Researches on phosphorous release at water-soil interface have been primarily carried out at laboratory conditions, which cannot reflect the actual instance because of the differences between field condition and lab simulated ones. We carried out the control experiment for phosphorous release at both laboratory and in-situ flooding conditions at Wanzhou, the central of the Three Gorges Reservoir, with purple alluvial soil at alternately waterlogged and drained environments according to the spatial and temporal characteristics of the Water-Fluctuation-Zone (WFZ) at the Three Gorges Reservoir. This thesis dicussed the characteristics of phosphorous release from soil at the WFZ by the following researches: effects of periodic inundating-draining on phosphorous release from soil, effects of anthropogenic phosphorous added on soils quality and phorsphorous release, rules of phosphrous release along different temporal gradients (at simulated and in-situ conditions), rules of phosphrous release along different spatial gradients (at field conditions) , and effects of plant on phosphorous release. This research increased the knowledge of nutrient transport and fate process at water-soil interface and that of environmental influence of reservoirs.Results showed that the activity of each phosphorous fraction at the water-soil interface in alternately inundating-draining conditions followed the sequence of Ca₂-P, Fe-P>Ca₈-P, Al-P>0-P>Ca₁₀P. The primary impetus of phosphorous release at continuring inundating condition was the salvation of Ca₂-P and the transformation of Fe-P at alternately oxidating-deoxidizing conditions;and the main impetus of phosphorous release at alternately inundating-draining conditions was the latter. In addition, inundation cound not make O-P released from the soil, but it activated O-P which transformed to Fe-P when drained and Fe-P could be released when inundated again.Alternately inundating-draining condition held great influence on soil quality. pH of the soil dropped when inundated and recovered to the former level when drained. Inundation could make Olsen P increased. Oxalate-extractable iron (Fe_ox) insoil increased markedly in inundating condition, but dropped significantly to about the former level. Inundation could also make oxalate-extractable allumiu (Alox) increased, but not so evidently as Feox. The degree of phosphorous saturation dropped drastically in inundating condition, but increased when dried. The maximum adsorption of phosphorous (Qm) and the maximum buffer capacity (MBC) increased in inundating condition and dropped when drained, which indicted that adsorption ability of soil for phosphorous was more powerful when in inundating condition.Phosphorous concentration of overlying water executed an influence on phosphorous release. Time required to reach adsorption equilibrium was much shorter when phosphorous concentration in overlying water was low than that when phosphorous concentration in overlying water was high. In addition, microorganism contributed much to phosphorous release from soil.When small amount of anthropogenic phosphorous was added to soil, phosphorous concentration in overlying water continued to increase in inundating period. But when anthropogenic phosphorous added was high (>50 mg.kg'1), phosphorous release reached equilibrium at about 4 weeks, and phosphorous concentration dropped slowly. The amounts of Ca2-P, Fe-P, Cag-P, Al-P increased much with the increasing quantity of anthropogenic phosphorous added to soil, and O-P could also increase when the added anthropogenic phosphorous was enough (>50 mg.kg"1) ,but CaiO-P changed little.With the increasing amount of anthropogenic phosphorous added to soil, pH of the soil tended to neutral. Qm of both fresh and dried soil samples changed not markedly, but PSI and MBC of both fresh and dried soil samples decreased significantly, indicating that the phosphorous adsorption ability of soil decreased. Adding anthropogenic phosphorous into soil make available phosphorous increase and phosphorous adsorption ability of soil decrease, which heighten the risk of phosphorous release from the soil.Olsen P and Mehlich-PlII of dried soil fluctuated much along temporal gradient, but when in laboratory condition, both of them either in river water inundation or in distilled water inundation reached the peak at about 10 weeks, then dropped slowly. Alox of dried soil dropped slowly within the beginning 6 weeks, and kept the valuesince then, which was the same at any inundating ways. Feox of dried soil in river-water inundating condition was much higher than that in distilled water inundating condition, and did not change regularly along the temporal gradient. DPS of the dried soil at laboratory condition was much higher than that in field condition, and changed little at both overlying water inundation conditions.The available phosphorous of fresh soil inundated at deeper water level (>20m) was slightly higher than that at lower water level. Feox fluctuated much at each water level, but Alox changed little alone the spatial water level gradient. DPS also fluctuated at each water level. Research showed that there were differences of amounts of phosphorous release among each water level: phosphorous release was the highest at 15m below the water level, 81.lmg.kg'1, while that was 58.6mg.kg"1 at Om and 52 mg.Kg^'at 30m.pH of fresh soil cultivated with plants was much less than that without plants. Olsen P of soil with plants was more than that without plant—soils with Cynodon dactylon and Ficus tikoua were greater than that with plant by 21.5% and 12.7%, respectively. Alox of soil with plant was less than that without plant, but Feox went oppositely. DPS of soil with plant was slightly higher than that without plant, which indicated the more powerful release ability of soil with plants.