Eight types of phytoliths were recorded in Larix gmelinii forest soils (Fig. 1), which similar to the SEM images of epidermal cell wall phytoliths in Klein and Geis’s studies(Klein and Geis 1978). Elongate was the dominant phytolith shape in Larix gmelinii forest soils, while the dendritic-elongate, papillate-elongate phytoliths maybe reflect that isolated phytoliths from Larix gmelinii forest soils not only come from the Larix gmelinii trees but also affected by understory grasses and shrubs.
The carbon sequestration potential of phytoliths not only depends on the encapsulating efficiency of organic matter during the formation of phytoliths (Parr and Sullivan 2011; Parr et al. 2010), but also on the phytolith yield during plant growth (Li et al. 2013a). Accordingly, the significant positive correlations between soil phytolith contents and carbon content occluded in phytolith, phytolith carbon content in soil were also found in Larix gmelinii forest (p < 0.01, Fig. 7), indicating that the phytolith content increasing would promote the carbon sequestration capacity of phytolith. Almost all of the silica in larix needles is located in the epidermal and hypodermal cell walls (Sangster et al. 2001). Therefore, the Larix phytoliths all seem to be cell wall types and deposited on a carbohydrate matrix, which will result in potentially high in carbon. However, the specific influence mechanism on carbon sequestration of Larix gmelinii remains to be studied further.
The fate of phytolith in soil is important because of its effects on agronomical nutrient resources and carbon sequestration (Nguyen et al. 2019). Under different environmental conditions, phytolith accumulation is mainly controlled by the return flux of phytolith and the stability of phytolith in soil. Phytoliths are released into the soil after plant death and decay. Phytolith accumulation occurs when phytolith input exceeds its translocation, dissolution and leaching in soil profile (Zhang et al. 2016). Therefore, phytolith content in soil is largely controlled by aboveground plant yield and forest types. Generally, different forest types are characterized as different return flux of phytoliths. The stability of phytoliths is mainly controlled by its physicochemical properties and environmental conditions (Nguyen et al. 2019). Differences in soil properties, such as soil texture (Hart and Humphreys 2003) and pH (Fraysse et al. 2009; Fraysse et al. 2008) may affect the retention and the loss of phytoliths in soil. Soil conditions, especially soil moisture and pH, can not only affect phytolith accumulation in soil due to their relations with the stability of soil phytolith (Chen et al. 2018), but also influence the bioavailability of silicon in soil, in turn, the silicon absorption from soil solution by plants (Li et al. 2014; Parr and Sullivan 2005; Yang et al. 2018). Previous studies have shown that soil with low pH and high organic matter can induce more absorption and accumulation of silicon by plants and more accumulation of phytolith carbon in plants (Song et al. 2012b). Our results showed that there was a significant positive correlation between phytolith content and SOC in Larix gmelinii forest soil (Fig. 3), indicating the effect of litter input on phytolith accumulation (Zhang et al. 2016). Meanwhile, the occluded effects of organic matter on phytolith may be another factor in phytolith preserving since its shield protection against the hydrolysis of silica (Nguyen et al. 2014; Parr and Sullivan 2005; Trinh et al. 2017). Furthermore, a significant negative correlation was found between soil phytolith and pH in Larix gmelinii forest soil (p < 0.05). Such strong pH dependency, a result of increasing pH deprotonation of Si–OH groups resulting in a H-bonded H2O adsorption on the negatively charged Si–O−surface (Ehrlich et al. 2010), was also observed in other studies (Fraysse et al. 2009; Li and Delvaux 2019; Nguyen et al. 2014). The pH is well understood as a crucial factor driving phytolith dissolution kinetics via protonation or deprotonation reactions (Fraysse et al. 2009; Nguyen et al. 2014; Trinh et al. 2017), which can either enhance surface resistance or make the surface more vulnerable from nucleophilic attacks (Dove and Crerar 1990). At higher pH, Si–O bonds are weakened and Si release is facilitated, thus phytoliths are more easily dissolved under alkaline conditions (Song et al. 2012a); In an acidic environment, lower pH value can reduce the nucleophilic attack of OH− on ≡ Si-O-Si ≡ bond (Dove and Crerar 1990), and make the phytolith stable for a long time (Fraysse et al. 2009; Fraysse et al. 2006; Nguyen et al. 2014). In addition, low pH may convert Alox and Feox into soluble Al3+, Fe3+, thus preventing the dissolution of phytoliths (Bartoli and Wilding 1980; Nguyen et al. 2014; Nguyen et al. 2019; Van Bennekom et al. 1991). It has been reported that Al3+ is preferentially adsorbed on the deprotonated sites on the surface of soil phytoliths to slow down the electron density transfer, inherently inhibiting the desilication effect (Nguyen et al. 2019). There was also a significant correlation between soil phytolith and NH4+-N and TP in Larix gmelinii forest soil. The elevated soil nutrients levels enhance the absorption of soluble silicon by plant roots and the accumulation of silicon in phytoliths (Huang et al. 2015; Zhao et al. 2016), then presenting indirect effects on promoting the formation of phytoliths in plants and the input amount of phytoliths into soils by litter. Additionally, the hydrolysis of NH4+ also results in soil acidification, which can increase the preservation of phytoliths in soil. In our studies, the mineral element contents of Na, Mg and K were dominant factors affecting phytolith content, which can neutralize soil acidity and increase pH, and then the phytolith dissolution were promoted. Oxides, such as Alox and Feox, have many OH−, OH2, OH3+ and other groups on their surface and possess strong adsorption capacity for of Al3+, K+ and other cations, which weakens the depressing effect of metal cations on Si release (Nguyen, et al., 2014).
Combined with previous studies (He 2016; Lin 2015), phytolith storages in soil from different climatic zones of China were calculated (Fig. 8). An obvious latitude trend from south to north in China was found: tropical zone (15.9 t ha− 1) < subtropical zone (21.6 t ha− 1) < cold temperate zone (41.0 t ha− 1). Phytolith contents in soil distributed in different regions are closely related to the regional geographical conditions and vegetation (Wang and Lv 1993). Theoretically, the soil is rapidly desalinated and desiliconized in the high temperature and humidity area (Alexandre et al. 1997). Thus, the biogeochemical stability of soil phytoliths in tropical areas is lower than that in subtropical areas (Zhang et al. 2017). As a result of the strong desalination and desiliconization, phytolith storage in tropical soils is lower than in subtropical soils. In addition to the effects of species and soil environment (pH, water, nutrients, etc.), climate and human activities (Parr and Sullivan 2005; Yang et al. 2016; Zhang et al. 2016; Zhao et al. 2016; Zuo et al. 2014) can also affect the stability of soil phytoliths by affecting biogeochemical activities. Comparatively, the Larix gmelinii forest is distributed in the cold temperate zone with a low temperature and weak desalination and desiliconization effects, which is beneficial to the preservation of soil phytolith (Yang et al. 2016). Phytolith mainly comes from soil silicon. Sufficient available silicon in soil can provide favorable conditions for vegetation to form phytoliths (Guo et al. 2015). As tropical soils are mostly highly weathered, where silicon supply for plants is much lower than in other ecosystem soils. Therefore, our results showed that the phytolith content in soil from Larix gmelinii forest was obviously higher than those in other areas.
However, some recent studies have shown that phytoliths most probably reflect only the minor part of phytogenic silica in plants and soils (Hodson 2019; Kaczorek et al. 2019; Puppe and Leue 2018) and a combination of microscopic analyses and silicon extraction techniques should be applied in the silicon cycling examinations. Puppe et al. (2017) concluded that about 84% of small-scale and/or fragile phytogenic silicon was not quantified by the used phytolith extraction method. Analyses of small-scale and fragile phytogenic silicon structures are urgently needed in future work as they are the most important drivers of silicon cycling in terrestrial biogeosystems.