热带地区玄武岩发育土壤中的生物硅及其发生学意义

BIOGENIC SILICON IN BASALT-DERIVED SOILS IN HAINAN ISLAND AND ITS IMPLICATIONS IN PEDOGENESIS

运用重液分离法对海南岛北部不同喷发期玄武岩发育的7个土壤剖面生物硅进行了分离和测定,发现生物硅的含量变幅为2.9～54.0 g kg-1,其中最小值出现在发育时间较长的HE11剖面的B2层,最大值出现在发育时间较短的HE09剖面的表层。生物硅的剖面分布特征和有机碳、总氮比较相似,呈现出随土壤深度的增加含量逐渐降低的趋势,在0～40 cm范围之内这种现象更明显。生物硅和总硅之间极显著相关性表明生物硅土壤硅循环中起着主要作用。随着风化强度的提高,黏粒含量的增加,土壤pH逐渐降低,土壤中的生物硅含量有逐渐下降的趋势。初步认为:在土壤发育的初期阶段,来自原生矿物的直接风化释放的溶解硅有利于生物硅在土壤中保存和积聚。而随着土壤的进一步发育,原生矿物的逐渐被分解,土壤的pH下降和黏粒含量进一步增加,土壤中的生物硅也会被溶解、利用,结果是其含量逐渐下降。至发育的高级阶段,土壤中的硅进一步淋失,pH和黏粒含量逐渐趋于稳定。在陆地生物"泵"作用下,生物成因硅会被植物循环利用并维持在一个含量相对稳定的状态。

As the second most abundant element in the Earth crust after oxygen, the transfer and transformation of silicon （Si） are one of the most important processes of soil formation and evolution. Biological processes are known to play significant roles in soil formation and mineral weathering. Especially, plants can take up dissolved silicon （DSi） and deposit it to produce silicified structure as phytolith, a kind of biogenic silicon （BSi）. In this paper, a chronosequence including seven basalt-derived soil profiles in northern Hainan Island was studied. The extraction of BSi was conducted following a wet extraction procedure, in which heavy liquid of ZnBr2 （2.35 g cm^-3） was used to separate phytoliths from other heavy mineral fractions. Results showed that contents of BSi in the soils varied widely from 2.9 to 54.0 g kg^-1. The highest concentration of BSi occurred in the surface horizon of profile HE09, a young soil on basalt, while the lowest was found in the B2 layer of HE11, an older soil in the sequence. In general, the contents of BSi, OC and TN are high in the surface horizon of soil profiles and decrease rapidly with depth. This phenomenon is prominent in topsoil, especially from surface to 40 cm. The finding implies that the accumulation of BSi in topsoil is mainly due to BSi release from organic matter during its decomposition. During soil development, these soils became progressively more weathered, with base cations depleting, clay fraction increasing and pH decreasing. Furthermore, a linear correlation between BSi and TSi was found for all soil samples, indicating that BSi determines to a certain extent the whole Si cycles during soil development. In the initial stage of soil development, DSi from weathering of primary minerals may contribute to the conservation of BSi, while with time, continuous high rainfall and depletion of cations leads to leaching or recycling of BSi faster than plants can bring it to the surface. As a result, the contents of BSi may maintain a relatively stable state with the function of biogenic pump in the soil-plant system. In conclusion, BSi is playing a very important role in tropical soil formation and evolution, especially in the biogeochemical cycles of Si.