| As a typical silicon super-enrich gramineous plant, bamboo has a world distribution area of2.0×107hm-2, one-third of which is located in subtropical regions of China (e.g., Zhejiang, Jiangxi and Hunan) domianted by red soils. From the available information, biogenic Si accumulation rate in bamboo forest soil was much higher than that of tropic rain forest, grassland, temperate deciduous and coniferous forest soil. The investigation of the distribution and migration of silicon in soil-plant systems of bamboo forests is an important step to clarify the regulation and control-mechanism of biogeochemical cycling of silica in subtropical bamboo forest ecological system, and to improve the theory of terrestrial silica cycling.The study collected samples of soils, plants, rocks and stream water in forest watersheds of Phyllostachys pubescens and Phyllostachys praecox in Qingshan, Xi Tianmu and Hengfan town of Zhejiang province, main bamboo production areas of the world. We collected soil samples of the surface soil (0～2cm), the sub-surface (2～10cm), the middle soil (10～20cm) and the bottom soil (20～40cm) of Phyllostachys pubescens forest and the soil samples of the surface soil (0～10cm), the middle soil (10～20cm) and the bottom soil (20～40cm) of Phyllostachys praecox forest. We collected the different organ samples of leaf, branch, stem and root with different ages of Phyllostachys pubescens and Phyllostachys praecox forest. We also collected74water samples from small watersheds covered by bamboos and other forest types in the later winter and summer. We analyzed contents of total silicon, contents and plant-availability of silicon fractions in bamboo soils, and investigated the distribution of silicon in different bamboo organs, and Si distribution and output characteristics of stream water from bamboo forest watersheds.The results of the study showed that there were four soil extractable silicon fractions of amorphous silicon (Amorphous Si), Fe/Mn-oxide silicon (Fe/Mn-oxide Si), organic silicon (Organic Si) and available silicon (Available Si) in bamboo forest stands. Available silicon had an important significance on growth of plant in bamboo forest stands. The content of available silicon varied from37.52mg·kg-1to237.54mg·kg-1(average96.70mg·kg-1) and silicon bioavailability varied from1.42%to6.53%(average3.28%) in the soil profiles under Qingshan Phyllostachys pubescens forest. The content of available silicon and bioavailability of silicon decreased for the surface soil, increased for the sub-surface soil, increased and then decreased for the middle and bottom soil with the age of bamboo stands. The content of available silicon and bioavailability of silicon decreased with the depth of soil profiles.The content of available silicon varied from84.4mg·kg-1to317.9mg·kg-1(average172.93mg· kg-1) and silicon bioavailability varied from1.60%to12.3%(average5.20%) in the soil profiles under Xi Tianmu Phyllostachys Praecox forest. The content of available silicon and bioavailability of silicon decreased firstly and then increased for the surface and bottom soil, no obvious change for the middle soil. The content of available silicon and bioavailability of silicon increased with the depth of soil profiles before and after the coverage of mulching, respectively. The content of available silicon varied from66.8mg·kg-1to157.6mg·kg-1(average94.60mg·kg-1) and silicon bioavailability varied from0.47%to4.25%(average1.62%) in the soil profiles from Hengfan Phyllostachys Praecox forest. The content of available silicon and bioavailability of silicon decreased with the planting, and that decreased and then increased before and after the coverage of mulching, respectively.The storage (418.5kg·hm-2) of silicon in the organs of Phyllostachyspubescens, the flux (290kg· hm-2·yr-1) of weathering input and the flux (200kg·hm-2·yr-1) of litterfall decomposition account for60.1%,41.7%and28.7%of available silicon storage in Phyllostachyspubescens forest soil, respectively. The flux (280kg·hm-2·yr-1) of silicon absorbed by Phyllostachys pubescens forest and leached into river (70kg·hm-2·yr-1) account for40.2%and10.1%of available silicon storage in Phyllostachys pubescens forest soil, respectively. The flux (80kg·hm-2·yr-1) of silicon of Phyllostachys pubescens forest cutting and litterfall decomposition (200kg·hm-2·yr-1) account for19.1%and47.8%of silicon storage in Phyllostachys pubescens forest, respectively.The storage (656.9kg·hm-2) of silicon in the organs of Phyllostachys praecox and the flux (260kg· hm-2·yr-1) of litterfall decomposition account for60.8%and24.1%of available silicon storage (1080kg·hm-2) in Phyllostachys praecox forest soil, respectively. The flux (300kg·hm-2·yr-1) of weathering input before the coverage of mulching and the flux (208kg·hm-2·yr-1) of weathering input after the coverage of mulching account for27.8%,25.9%of available silicon storage in Phyllostachys praecox forest soil, respectively, that representing the flux of silicon decomposed from organic mulching took part from the silicon flux of weathering input from silicate minerals, thus proportion of weathering input could be declined.The flux (120kg·hm-2·yr-1) of litterfall decomposition and the flux (490.9kg·hm-2· yr-1) of silicon absorbed by Phyllostachys praecox forest and leached into river (70kg·hm-2·yr-1) account for11.1%,45.5%and6.5%of available silicon storage in Phyllostachys praecox forest soil, respectively. The flux (60kg·hm-2·yr-1) of silicon of Phyllostachys praecox forest cutting and litterfall decomposition (433.9kg·hm-2·yr1) account for9.1%and66.1%of the storage (656.9kg·hm-2) of silicon in the organs of Phyllostachyspubescens, respectively.Generally, silicate weathering intensity of bamboo watersheds in summer was higher than that in the end of winter. Concentrations of SiO2in stream water (average105μmol·L-1) and the ratio (average2.9) of Si/(Nasilicate+Ksilicate) from pure bamboo watersheds was obviously higher than that (60μmol·L-1) and (average2.2) from the other forest watersheds.The output fluxes of SiO2(average2.5×105mol· km-2·yr-1) by silicate weathering of bamboo watersheds was generally higher than that (average2.0×105mol·km-2·yr-1) from the other forest watersheds.Silica cycling in soil-plant system consists of processes of transformation and migration of silicon among soil mineral-soil water-plant-groundwater system of bamboo forest. It was a circulating process that free silicon (available Si) as a starting point of this process was absorbed by bamboo forest, then returned to the surface soil of bamboo forest as fading leaves and branches, and released into soil solution and groundwater underground of the bamboo forest during decomposition, finally absorbed by bamboo again. The silicate mineral weathering, litterfall decomposition were the main silicic sources of bamboo forest ecosystems. The silicon cycle in bamboo forest soil-plant system was driven by the absorption of silicon by bamboo. After the cycling, silicon was fixed as phytolith in bamboo forest soil or leaked to groundwater and poured into the stream water. |
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