| The paper gains the bankroll of special research of forestry public welfare industry (item 201304317).Secondary forest in sutropical area is a natural forest which formed after the evergreen broad-leaved forest was damaged and restored. It is the main forest types in the subtropical area. It plays an important role on biodiversity conservation, regional ecological balance maintaining, ecological security and sustainable development. In order to meet the needs of social and economic rapid development for wood production, many artificial forest was planted in subtropical region, and made a major contribution in the social and economic development. There are many studies about soil physical and chemical properties, biomass and soil nutrient content under different forest types in subtropical region at present. But there are few reports on the impact of natural secondary forests protection and the difference of forest tree species on the evolution of soil fertility. In addition, the secondary forest and plantation has a big difference in stand age and parenting. The comparative study about secondary forest and plantation on soil physical and chemical properties is more rarely in the subtropical area. We selected three kinds of secondary forest (Pinus massoniana+ Lithocarpus glaber mixed forest, Choerospondias axillaries deciduous broad leaved forest, Choerospondias axillaries deciduous broad leaved forest) and Cunninghamia lanceolata plantation forest as the research object, analysised the fractal dimension of soil particle size and the variation trend of nutrient content and their relationship, revealed the influence mechanism of forest vegetation restoration on soil fertility and evolution process, to provid scientific basis about forest ecosystem restoration and management in this area. It has a great significance on forest ecological system updates, recovery and reconstruction. The main results are as follows:(1) Soil clay (<0.005 mm) content (44.38%) in four forest types was highest, in the range of 35.75%～56.19%. The silt content (32.67%) was second, in the range of 28.05%-39.48%. The sand content (22.39%) was lowest, in the range of 12.88%～ 30.49%. The composition of soil particle displayed the features of the typical texture soil in hilly red soil region of southern China. The soil of four subtropical forests was clayey loam. The soil fractal dimension in the four subtropical forests was in the range of 2.737-2.835, belonged to the scope of soil fractal dimension (2.359-2.852) in China, and ranked in the order of C. lanceolata plantation> C. axillaries deciduous broadleaved forest>L. glaber +C. glauca evergreen broad leaved forest>P. massoniana+ L. glaber mixed forest. Soil fractal dimension in C. lanceolata plantation had significant different with it in other three forest (P<0.05). And the difference of soil fractal dimension among three kinds of secondary forests was not significant (P>0.05). Soil fractal dimension was very significantlyh negatively correlated with tree species diversity, The content of different soil particle size was reflected in soil fractal dimension, and the highest was clay, the second was sand and coarse clay. Soil fractal dimension was very significantly positively correlated with soil bulk density, while not correlated with soil moisture content, soil bulk density in four forest was 1.02-1.48g-cm-3 in 0-15 cm soil layer, was 1.14～1.53g·cm-3 in 15-30 cm soil layer. Soil bulk density of C. lanceolata plantation was higher than that in other three secondary forests. Soil moisture content was highest in C. lanceolata plantation(26.62%～26.44%), gradually decreased in C. axillaries deciduous broadleaved forest (22.26%-25.53%) and C. glauca evergreen broad leaved forest (21.61%～22.53%), and was lowest in P. massoniana+L. glaber mixed forest (18.33%～20.27%).(2) The soil pH value of four kinds of forest was 4.55～4.69. The soil was acidic. Soil pH decreased with soil depth increasing. The seasonal variation patterns of soil pH value in four forest types showed as followed:soil pH was lowest in spring, gradually increased in summer and autumn, and was highest in winter, but the differences among seasons were small. Contents of soil total nitrogen, total phosphorus, total potassium, alkali-hydrolyzable nitrogen, available phosphorus and available potassium in 0-15 cm soil layer were higher than them in 15-30 cm soil layer. Average content of total nitrogen in 0-15cm soil layer in forest was 1.12～1.65g·kg-1 and was 0.96～1.33g·kg-1 in 15-30cm soil layer. The difference of total nitrogen content between two soil layers was significant (P<0.05) (except in Cunninghamia lanceolata planted forest). Average content of total phosphorus in 0-15cm soil layer in forest was 0.20-0.29g·kg-1, and was 0.19-0.27g·kg-1 in 15-30cm soil layer. But there was no significant difference between soil layers (P>0.05). Average content of total potassium in 0-15cm soil layer in forest was 4.91～5.82g～kg-1, and was 4.66～5.76g·kg-1in 15-30cm soil layer. There was no significant difference between soil layers (P>0.05). Average content of alkali-hydrolyzable nitrogen in 0-15cm soil layer in forest was54.39～77.93mg·kg-1, and was 37.95～64.36mg·kg-1 in 15-30cm soil layer. The difference of alkali-hydrolyzable nitrogen content between soil layers was significant (P<0.05). Average content of available phosphorus in 0-15cm soil layer in forest was 1.96～2.73mg·kg-1, and was 1.35-2.15mg·kg-1 in 15-30cm soil layer. Average content of available potassium in 0-15cm soil layer in forest was 52.55～69.30mg·kg-1, and was 42.13-57.75mg-kg"1 in 15-30cm soil layer. The difference of available potassium content between soil layers was significant (P <0.05). Nitrogen, phosphorus and potassium supplying intensy in 0-15cm soil layer was higher than that in 15-30cm soil layer, but the difference was not significant (P>0.05). Soil nutrient content and the supplying intensity of nitrogen, phosphorus and potassium among different forest types had obvious different. Soil nutrient content in Cunninghamia lanceolata planted forest was lower than that in other three kinds of secondary forests, but its intensity of nitrogen, phosphorus and potassium was higher. It indicated that soil nutrient accumulation, storage and conversion among different forest types had significant differences. The seasonal variation characterstics of alkali-hydrolyzable nitrogen, available phosphorus, and available potassium content of two soil layers in four forest types was obvious. The comprehensive index of soil nutrient pools in the study area was 2.51～3.57, and ranked in the order of Choerospondia axillaris broadleaved decidural forest (3.57)> Lithocarpus glaber+ Cyclobalanopsis evergreen broad-leaved forest (3.35)>Pinus massonana+Lithocarpus glaber mixed forest (3.03)>Cunninghamia lanceolata planted forest (2.51). the difference of Cunninghamia lanceolata planted forest and three kinds of secondary forest was significant (P>0.05), But the difference among three kinds of secondary forest was not significant (P>0.05). Soil nutrient content in four forest types increased with tree species diversity, but was not significantly correlated with stand density (P>0.05).(3) Soil fractal dimension was not significantly correlated with soil pH, but was siginificantly negatively correlated with soil orgabic matter. Soil fractal dimension had correlations with soil total nitrogen, total phosphorus, total potassium, alkali-hydrolyzable nitrogen, available phosphorus and available potassium. The fractal dimension had significantly positive correlations with total nitrogen and phosphorus content in Cunninghamia lanceolata planted forest, has sigificantly negative correlation with total potassium content in Pinus massonana+Lithocarpus glaber mixed forest. Soil fractal dimension was significantly positively correlated with total phosphorus and alkali-hydrolyzable nitrogen and negatively correlated with available potassium in Choerospondia axillaris broadleaved decidural forest. There was a significantly negative correlation between soil fractal dimension and total phosphorus, available phosphorus and available potassium content in Lithocarpus glaber+ Cyclobalanopsis evergreen broad-leaved forest. There had correlations among different soil nutrients in different forest types. Soil pH had an obvious positive correlation or extremely obvious positive correlation with total phosphorus content, has an obvious negative correlation or extremely obvious negative correlation with total potassium content. Total nitrogen content was extremely obviously positively correlated with alkali-hydrolyzable nitogen. Total phosphorus content had an obvious negative correlation or extremely obvious negative correlation with total potassium content. In the whole research area, total potassium content had an extremely obvious positive correlation with available potassium content. Except in Pinus massonana+Lithocarpus glaber mixed forest, alkali-hydrolyzabl nitrogen content had an obvious positive correlation or extremely obvious positive correlation with available phosphorus content in the whole research aera and other three forest types. It indicated that soil alkali-hydrolyzabl nitrogen and available phosphorus contents in four forest types depended on soil total nitrogen content, while available potassium content depended on total potassium content. |
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