| Alnus species (spp.), which are used as fast-growing timber and water and soil conservation tree species, are widely cultivated in Hunan province. Despite its important role in water and soil conservation, systematic researches of their capabilities are so hard to be retrieved that objective evaluations are unreachable. The experiment took the Alnus cremastogyne forest, the Alnus formosana forest and the wasteland as investigation objects, focused on comparison and analysis of the dynamic changes of soil water content (SWC). This paper quantitatively analyzed the characters of the dynamic changes of SWC in the Alnus forest, studied the rules and factors behind the changes, and provided the theory of spreading and management of Alnus spp. The results are:1. With the soil depth goes deeper, the water content of the soil from Ocm to 75cm showed a gradually increasing trend. The average soil water content varied in as follow:A. cremastogyne forest (19.86%)>A formosana forest (17.59%)>wasteland (17.18%). In the 15-75cm soil layer, water content levels were A cremastogyne forest > A. formosana forest> wasteland. Moreover, comparing within the layer, there had obviously difference (p<0.05). The coefficient of variation of the SWC decreased along with the soil depth emboldening. Compared to the wasteland, SWC of A.cremastogyne forest and A. formosana forest were more stably.2. Though by the seasons changed, there had differences among the SWC of the 3 types of forest land, they were the same trend basically. The dynamic changes of soil water curve of A. cremastogyne forest was always above that of the A. formosana forest and the wasteland. The variation of SWC was not distinct between A. formosana forest and the wasteland within seasons (p>0.05). SWCs of three lands were affected by the seasonal changes greatly.3. Because of the different characters of the sites, the three types of forestland had different SWCs. Due to a greater stand density of 2115 plant·hm-2 and large canopy density (0.80), which could result in better water conservation performance, the A. cremastogyne forest had a higher SWC compared to A. formosana forest.4. Among the three types of forestland, the biomass of<1cm diameter root decreased by the soil depth emboldening. Within the layer, weight of the root biomass were A. cremastogyne forest> A. formosana forest> wasteland. Examine the root biomass only, there showed a negative correlation between its effect and SWC.5. With the rainfall increasing, the SWC of A. cremastogyne forest and A. formosana forest became bigger, which was significantly positive correlated. Under soil depth of 30cm, there was no such significant association between the SWC and precipitation of two types of A. forestland. The daily average temperature had no significant effect (p>0.05) on the SWC of the A. cremastogyne forest and the A. formosana forest. Furthermore, the daily average air relative humidity had no significant effect on the SWC of A. cremastogyne forest, but significantly positive associated with the SWC in the 0-75 cm soil of the A. formosana forest.6. When seasons changed, the gross litter and the reserved amount of undecomposed litter of the A. formosana forest were gradually increasing. And basically, the semi-decomposed litter increased along with the season change. The gross litter and the reserved amount of undecomposed litter were A. cremastogyne forest> A. formosana forest; the SWC of semi-decomposed litter and litter were A. formosana forest> A. cremastogyne forest. However, the largest water content of the semi-decomposed litter layer was found in A. cremastogyne forest. There had a negative correlation between the litter biomass and the SWC, and the water content of all litter layers had a positive correlation with the SWC.7. In soil depth of 0-75cm, soil bulk densities were:wasteland (1.43g·cm-3)> A. cremastogyne forest (1.38g·cm3)> A. formosana forest (1.37g·cm-3), which showed a significantly positive correlation with water content. The biggest soil capillary porosity was found in A. cremastogyne forestland (39.23%), and the smallest one in A. formosana forestland (37.54%). In the A. cremastogyne forestland, the soil capillary porosity and the SWC had a positive relationship, but there was no such correlation could be found in A. formosana forestland. There was an extremely negative relationship between non-capillary porosity and SWC. The initial and stable water infiltration speeds were A. formosana forest> A. cremastogyne forest> wasteland. The SWC and infiltration speed had a distinctly negative association. The correlation coefficient were:A. cremastogyne forest-0.586, A. formosana forest-0.425, and wasteland-0.548.8. In forestland, the content of soil organic matter and total nitrogen decreased along with the soil depth went deeper. The order was A. cremastogyne forest> A. formosana forest> wasteland. The average pH value of the A. cremastogyne forestland was 5.11, the A. formosana forest was 5.03, and the wasteland was 5.00, showedthat planting A. could improve soil acidity. In A. formosana forest, the SWC and soil organic matter had a significant relationship in 15-30cm layer only.9. For A cremastogyne forest, the regression equation of changes of SWC, which was established using the significant factors, had 9 main factors, and the upper end three factors were the soil deepth, the total nitrogen conteng, the soil bulk density. There were 10 main factors which were selected by the regression equation for A. formosana forest; the upper end of three factors were water content of undecomposed litter layer, the soil depth, the water content of semi-decomposed litter layer. We can regulate the SWC using those main factors in practice.In the red deceloped that derive from quaternary red clay area of northern Hunan, plant A. cremastogyne and A. formosana has many ecological benefits. For examples: amend the physicochemical properties of soil, improve water holding capacity and osmosis, and protect water sources. It showed preferable water and soil conservation benefits.
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