| The Three Gorges Dam of the Yangtze River has created a huge reservoir which is surrounded by a new riparian region with an annual fluctuation of30meters of the water level. This region has been called as the hydro-fluctuation zone of the Three Gorges Reservoir (TGR). Practicing reforestation and vegetation reconstruction in this region to prevent soil erosion and restore the newly-formed riparian zone has been the hotspot of the research on TGR. Since the soil in the hydro-fluctuation zone is the foundation and premise for its ecological service function, a study on spatial and temporal variability of soil properties in the hydro-fluctuation zone of the TGR after its artificial vegetation reconstruction will be helpful to reveal the effect of vegetation restoration on soil properties, and can also provide theoretical basis for the artificial vegetation reconstruction. Therefore, aiming at exploring the effect of hydrology and artificial vegetation construction on soil chemical properties, and the dynamics of these properties under the interaction of water change and vegetation, this research was conducted through common garden and field experiments, to study the spatial and temporal variability of soil chemical properties (including pH value, C/N ratio, nutrient content (especially nitrogen and phosphorus content) in the hydro-fluctuation zone of the TGR.To simulate the water change in the hydro-fluctuation belt of the TGR, three water treatment groups were applied in this research firstly, including control (CK), continuous flooding (CF), and periodic flooding-drought (PF). Each group contained soil samples planted with one-year-old slash pine (Pinus elliottii) seedlings, Chinese wingnut (Pterocarya stenoptera) seedlings or blank soils with no vegetation. Results showed that:(1) For the unplanted soils, water treatment had significant effects on the soil chemical properties except for phosphorus and potassium content, while time had no significant effects on soil phosphorus content in the CF condition. However, the phosphorus content of the unplanted soils in the PF group showed a trend of fluctuation.(2) For the soils of slash pine, except for the non-significant effect of water treatment on soil pH value, water treatment, time and their interaction had significant effects on all the tested soil chemical properties. To the contrary, in terms of the soils of Chinese wingnut, water treatment had significant effects on soil chemical properties except for total potassium content, and time significantly affected soil properties except for organ matter content, while the soil chemical properties were all significantly influenced by the interaction of water treatment and time.(3) Compared with the unplanted soil, the TN and TP content in the soils of slash pine and Chinese wingnut was respectively significantly increased.Then, the chemical properties of soils in the artificial vegetation demonstration bases in the hydro-fluctuation zone of the TGR (respectively belongs to urban hydro-fluctuation zone and rural hydro-fluctuation zone) were studied. The dynamics of soil chemical properties under different hydrological condition, soil type, altitude and artificial vegetation types were analyzed. This research tested soil pH value, the ratio of C to N, and the content of organic matter (OM), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3-N), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), available potassium (AK), total nitrogen (TN), total phosphorus (TP) and total potassium (TK) in the soils of unplanted soil and artificial vegetations. The type of artificial vegetations in the urban hydro-fluctuation zone included Hemarthria compressa, Cynodon dactylon, and Saccharum spontaneum, while that in the rural hydro-fluctuation zone included Taxodium distichum, Salix matsudana, H. compressa, C. dactylon, S. matsudana+H. compressa, and S. matsudana+C. dactylon. Soil samples were collected in the summer of2012, March2013, May2013, July2013, and September2013respectively. Due to the difference in hydrology, soil samples in the urban hydro-fluctuation zone were collected in the altitude of172-175m, while those in the rural hydro-fluctuation zone were collected in the altitude of170-175m,165-170m, and160-165m. The result showed that the soil properties in the hydro-fluctuation zone of the TGR had certain spatial and temporal variability. The soil chemical properties were influenced by both of the hydrological and vegetation factors. As influenced by hydrological and other factors, the soil nutrient content did not decline with the increase of soil depth. The soil pH value tended to be neutral after flooding. Compared to the unplanted soil, soils of artificial vegetations had lower C/N ratio. Besides, the results of the soil chemical properties in the urban hydro-fluctuation zone showed that:(1) In addition to the hydrology, other factors (such as sewage discharge, human disturbance, and surface runoff) may also affect the soil chemical properties in the urban hydro-fluctuation zone.(2) The AN content of unplanted soils was increased after the drop of the water level. The soil AP content was found to be the highest in June2012, while the TP and TK content were increased in the latter period of the study. Therefore, special attention should be paid to the release of soil phosphorus in the urban hydro-fluctuation zone, and to strengthen the monitoring of phosphorus content in water.(3) Although both of the Jialing River hydro-fluctuation zone and Yangtze River hydro-fluctuation zone were in the same urban hydro-fluctuation zone, they had different soil nutrient contents. Overall, the soil nutrient contents in the Yangtze River hydro-fluctuation zone were higher than those in the Jialing River hydro-fluctuation zone. (4) The construction of artificial vegetation increased most of the soil nutrient contents in this area. Among the artificial vegetations in the urban hydro-fluctuation zone, C. dactylon promoted soil OM content most greatly. In addition, the same plant species in different habitats had different impacts on soil nutrient contents.(5) The soil TN content in the urban hydro-fluctuation zone was very low, resulting in high soil C/N ratio. In the research area, soil TN content had a positive correlation with the content of OM, the soil nitrogen in the region thus might mainly exist in the form of organic nitrogen.In the meantime, the results of the soil chemical properties in the rural hydro-fluctuation zone showed that:(1) For the unplanted soil, the pH value increased with the decline of altitude. The soils in165-170m which has a higher frequency of flooding-drought alternating and soils in165-170m which has higher water content had higher OM content than that in the altitude of170-175m. In addition, the content of soil NH4+-N and AP was decreased with the increasing of altitude.(2) The C/N ratio of soils under artificial vegetations decreased after the impoundment of the TGR, and followed by an increasing, and then tended to be stable with the extension of the experiment. Moreover, the dynamics of soil AN content under artificial vegetations was less sensitive than NH4+-N and NO3--N. Therefore, in order to make further study on the soil nitrogen transformation process in the hydro-fluctuation zone, it is necessary to study the change of soil nitrate nitrogen and ammonium nitrogen.(3) Among the artificial vegetations in the rural hydro-fluctuation zone, S. matsudana promoted soil OM content most greatly. Different types of artificial vegetation altered the soil properties differently. Even for the same kind of plant species, it might impose distinct effects on the nutrient content of soils at different altitudes.(4) In addition to the organic nitrogen, the TN in the soils in the rural hydro-fluctuation zone had other nitrogenous constituents. Water eutrophication might be aggravated due to the increase of soil N and P contents (especially P), after the construction of artificial vegetations (especially T. distichum and S. matsudana), when the vegetation growing well. |
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