4.1 Phytolith and PhytOC contents in grassland ecosystems
The dominant plant species of grasslands, belonging to Poaceae and Cyperaceae, are Si accumulators (Conley, 2002; Epstein, 1994; Strömberg et al., 2016), and deposit Si mostly in phytoliths (Hodson et al., 2005; Schaller et al., 2013). In the present study, the highest phytolith content in the aboveground parts of plants was detected in Carex duriuscula , a perennial Cyperaceae, whereas the lowest content was inSalsola collina , an annual Chenopodiaceae, consistent with results from other studies (Ru et al., 2018). The highest phytolith content in the belowground parts was detected in Stipa grandis , a perennial bunchgrass, whilst the lowest was also in Salsola collina . Even though a high Si content is not a general feature of monocots (Hodson et al., 2005), many monocots accumulate more Si than non-monocots (Epstein, 1994). In the present study, phytolith contents averaged 22.40 and 57.25 g kg-1 respectively in the above- and belowground parts of monocots, higher than that in the above- and belowground parts of dicots, respectively 9.14 and 35.88 g kg-1.
Both species composition and NPP of plant communities vary substantially along climatic gradient in semi-arid regions (Bai et al., 2004; Dai et al., 2012; Hou et al., 2014), whilst the biomass-weighted mean phytolith or PhytOC contents does not show a significant difference among steppe types (Fig. 3). This is because that the dominant species in the studied steppe communities at the three sites are phylogenetically close (i.e., belong to the same genera), thus have similar phytolith or PhytOC contents, and contribute to a large proportion of community biomass. For example, Stipa species are dominants in all the three steppe sites, with their AGB (either of S. klemenzii, S. grandis orS. krylovii ) contributing to 45.79%, 53.25% and 48.42% of the community AGB respectively in the desert steppe, dry typical steppe and wet typical steppe. Also, dominant Cleistogenes songorica orC. squarrosa accounts for 15.59%, 9.35% and 25.04% of the community AGB respectively in these three sites. As a result, the variation in community phytolith or PhytOC contents has much smaller effects on its production flux (= phytolith or PhytOC content × NPP) across the steppe sites, and the variation in phytolith or PhytOC production flux is mainly associated with the changes in NPP. This indicates a predominant role played by plant above- and belowground biomass production along the climate gradient for the phytolith and PhytOC production, which overshadows the effects of the variation in plant PhytOC content at community level.
The significantly higher PhytOC content in the below- than the aboveground biomass, is most likely associated with the perenniality of plants, the PhytOC in the aboveground part is deposited in the current year, whereas that in the belowground part may be deposited in multiple years (Qi et al. 2016). The higher PhytOC content in the aboveground part of desert steppe community than that of typical steppe communities is consistent with our hypothesis that the intense transpiration required for Si deposition in plant tissues in the desert steppe contributed to the higher PhytOC content. However, the PhytOC production flux is significantly lower in desert steppe community than the typical steppes due to its lower NPP.
The PhytOC content in plants is associated with the silica deposition, thus with the amount of plant Si uptake (Parr et al., 2009; Li et al., 2013a). The environment factors, such as climate and soil conditions, may influence plant Si uptake, thus the efficiency of PhytOC accumulation in plants (Pan et al., 2017; Li et al., 2013a; Yang et al., 2018). In our study, low pH and high SOC can account for the higher phytolith accumulation in wet than dry typical steppe, which is consistent with the results in previous report (Song et al., 2012b). A higher pH and moisture content accelerate Si to dissolve in soil solutions (Fraysse et al., 2009; Yang et al., 2018), and to be taken up by plants and transpired in vascular bundles in plants (Epstein, 2009; Parr et al., 2009). Pan et al. (2017) reports that soil SiO2 content shows no significant difference among the grassland sites with different degradation levels, while bioavaliable Si content in the top soil of non-degraded grassland was relatively lower due to more bioavailable Si being absorbed by plants at the site. In studied steppes, the bioavailable Si content in the soil was higher in dry steppe site than in other two sites, which is probably resulted from the less uptake of bioavailable Si from the soil at this site, as its phytolith accumulation was lower than in other two steppe sites.