1 INTRODUCTION
Climate warming and the increase of extreme climatic events are mostly
attributed to the increasing concentrations of greenhouse gases in the
atmosphere, especially carbon dioxide (CO2). The coupled
biogeochemical cycles of carbon (C) and silicon
(Si) in the terrestrial system are
deemed a mechanism that impacts the long-term regulation of atmospheric
CO2 (Berner, 1992; Parr et al., 2010; Parr & Sullivan,
2011; Song et al., 2012a; Song et al., 2012b). Plants absorb
mono-silicic acid from soil solution via their roots (Epstein, 1994; Ma
& Yamaji, 2006) and deposit silica (SiO2) in plant
tissues as phytoliths (Hodson et al., 2005;
Schaller et al., 2013).
Approximately 0.1%-6.0% organic C in plants is reported to be
incorporated in phytoliths during their formation, and is referred to as
phytolith-occluded carbon (PhyOC)
(Bartoli & Wilding, 1989; Jones & Milne, 1965; Zuo & Lü, 2011).
Phytoliths exist in most plants, and their content in plant tissues
varies among plant species, ranging from less than 0.5% in most
dicotyledons, 1%-3% in dryland grasses, and up to 10%-15% in the
Cyperaceae and wetland Poaceae species (Epstein, 1994). Phytoliths are
mainly deposited in the cell wall, cell lumen and intercellular spaces
or the extracellular layer in plant issues (Epstein, 2009; Hodson et
al., 2005; Ma & Yamaji, 2006; Schaller et al., 2013); after plants’
death, the phytoliths can be incorporated into soil or sediments after
the decomposition of plant litter (Blecker et al., 2006). Since
phytoliths are resistant to decomposition, PhytOC can be preserved in
soil or sediments with the Si-coat protection for several hundred or
thousand years, and may account for 82% of the total organic carbon in
some old soils (Parr & Sullivan, 2005; Song et al., 2017). The
formation and stability of PhytOC in the coupled biogeochemical cycling
of C and Si has been increasingly recognized as a promising mechanism of
terrestrial ecosystems to sequester atmospheric CO2,
which has motivated many researchers to quantify the PhytOC
sequestration potential of various systems (Parr et al., 2010; Parr &
Sullivan, 2011; Song et al., 2012a).
Grasslands are an important terrestrial ecosystem covering more than
one-fifth of the world’s land surface (Scurlock & Hall, 2010). The
large distribution area and the high PhytOC concentration in grassland
plants, especially Poaceae and Cyperaceae species (Epstein, 1994;
Clarkson & Hanson, 1980), make grassland a particularly important
long-term C sequestration process (Blecker et al., 2006; Song et al.,
2017). Several studies assessed the PhytOC sequestration potential of
grasslands (Pan et al., 2017; Qi et al., 2016; Song et al., 2012a). Song
et al. (2012a) reported that phytolith and PhytOC production rates in
aboveground biomass of grassland were significantly influenced by their
aboveground net primary productivity (ANPP); Qi et al. (2016) suggested
that the belowground productivity of plants could play a dominant role
in PhytOC production in grassland ecosystems; and Ji et al. (2018)
reported that Si distribution in the aboveground parts, thus the PhytOC
sequestration potential, of grassland plants, varies markedly among
plant species and across grassland types. However, the effects of
grassland type and species composition on the production rate of
phytoliths
and PhytOC (including the above- and belowground parts) are still not
fully understood. Precipitation is the predominant climatic factor that
controls plant species composition and net primary productivity (NPP) of
grassland ecosystems in semi-arid steppe region (Bai et al., 2004; Dai
et al., 2012; Hou et al., 2014), thus affecting the PhytOC sequestration
potential. As such, it is necessary to investigate the PhytOC production
rate in different grassland types along climatic gradient, for accurate
estimation of PhytOC sequestration potential and prediction of their
response to climate changes. In present study, we selected three
representative vegetation types in climatically different regions alone
a precipitation gradient in Inner Mongolia, that is, the desert steppe,
the dry typical steppe, and the wet typical steppe regions, to study
their PhytOC production. Specifically, we aimed to investigate the
species composition, and measure the phytolith and PhytOC contents of
the major plant species, of these grassland types, so as to increase the
accuracy in estimating phytoliths and PhytOC production and storage in
plant communities, and analyze their relations with climatic factors. We
hypothesized that along the gradient of climate aridity increase (that
is, precipitation decrease) from the wet dry steppe site, via the dry
typical steppe site to the desert steppe site, the grassland NPP would
decrease, while the contents of phytoliths and PhytOC in the steppe
plants would increase due to the intense plant transpiration but low
biomass production.