Phytolith-occluded carbon sequestration potential in three major steppe types along a precipitation gradient in Northern China

Phytolith-occluded carbon (PhytOC) is an important long-term stable carbon fraction in grassland ecosystems, and plays a promising role in global carbon sequestration. Determination of the PhytOC traits of different plants in major grassland types is crucial for precisely assessing their PhytOC sequestration potential. Precipitation is the predominant factor in controlling net primary productivity (NPP) and species composition of the semiarid steppe grasslands. We selected three representative steppe communities of desert steppe, dry typical steppe and wet typical steppe in Northern Grasslands of China along a precipitation gradient, to investigate their species composition, biomass production and PhytOC content for quantifying its long-term carbon sequestration potential. Our results showed that (i) the phytolith and PhytOC contents in plants differed significantly among species, with dominant grass and sedge species having relatively high contents, and the contents are significantly higher in below- than the aboveground parts. (ii) The phytolith contents of plant communities were 16.68, 17.94 and 15.85 g kg-1 in the above- and 85.44, 58.73 and 76.94 g kg-1 in the belowground biomass of desert steppe, dry typical steppe and wet typical steppe, respectively; and the PhytOC contents were 0.68, 0.48 and 0.59 g kg-1 in the above- and 1.11, 0.72 and 1.02 g kg-1 in the belowground biomass of the three steppe types. (iii) Climatic factors affected phytolith and PhytOC production of steppe communities mainly through altering plant production, whereas their effects on phytolith and PhytOC contents were relatively small. Plant aboveground biomass and PhytOC content were strongly associated with the current-year climate and soil bio-available Si content; while plant belowground biomass and PhytOC content were relatively stable, and their variation across the sites is in accordance with the spatial variation in the long-term means of climatic and soil factors, reflecting the perenniality of plant belowground part.