DOI: 10.5061/dryad.34tmpg4j4
title: Phytolith-occluded carbon sequestration potential in three major steppe types along a precipitation gradient in Northern China

description: 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
 phytolith carbon 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 the desert steppe, the dry typical
 steppe and the 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 the 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 86.44, 58.73 and
 76.94 g kg-1 in the belowground biomass of the desert steppe, the
 dry typical steppe and the 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 fluxes of steppe communities mainly through altering plant
 production, whereas their effects on phytolith and PhytOC contents were
 relatively small. Our study provides more evidence on the importance of
 incorporating belowground PhytOC production for estimating phytolith
 carbon sequestration potential, and suggests it crucial to quantify
 belowground PhytOC production taking into account of plant perenniality
 and PhytOC deposition over multiple years.
description: <b>Sampling sites</b>
 This study was conducted at three sites in the steppe region of
 central Inner Mongolia along a climatic gradient of increasing annual
 precipitation, that is, at a desert steppe site (within Sunite Right
 Banner, at 43°511'N, 113°42'E), a dry typical steppe site
 (within Maodeng farm of Xilinhot city, at 44°50'N, 116°36'E),
 and a wet typical steppe site (within West Ujimqin Banner, at
 45°43'N, 118°30'E). The region experiences a temperate semiarid
 climate. The mean annual temperature (MAT) were 0.19, 2.67 and 3.32℃, and
 mean annual precipitation (MAP) were 182, 278 and 342 mm, respectively at
 the desert steppe, the dry typical steppe and the wet typical steppe
 sites; and 75%-85% of annual precipitation falls in the plant growing
 seasons from May to September (average of the 1960-2016 period). In the
 year for field study (2016), the annual temperature (TEMP) were 0.98, 3.49
 and 4.50℃ and annual precipitation were 189, 309 and 299 mm, and the plant
 growing-season precipitation were 129, 215 and 237 mm, respectively in the
 desert steppe, the dry typical steppe and the wet typical steppe sites.
 The desert steppe site is on a calcic brown soil, whereas the other two
 steppe sites are on chestnut soil. The humus layer is 15-30 cm in the soil
 profile, and the calcic horizon (mostly CaCO<sub>3</sub>) is
 30-60 cm in the soil profile, both increasing from the desert steppe to
 the dry typical steppe and the wet typical steppe. The dominant species of
 the vegetation are <i>Stipa klemenzii, Cleistogenes songorica, Allium
 bidentatum </i>and <i>Salsola
 collina</i> in the desert steppe, <i>Leymus chinensis, Stipa krylovii
 </i>and <i>Cleistogenes squarrosa</i> in the dry
 typical steppe, and <i>Leymus chinensis, Stipa grandis
 </i>and <i>Cleistogenes squarrosa</i> in the wet
 typical steppe (Table 2). <b>Field
 sampling</b> In each of the three
 steppe sites along the precipitation gradient, three delineated plots of
 20m × 20m located in three separate farms were selected for plant and soil
 sampling. These plots were on flat ground and covered with representative
 native steppe communities, and fenced to exclude animal grazing at the
 beginning of the plant growing season in 2016. Plant and soil samples were
 collected from these plots at the end of August. Five quadrates of 1m ×1m
 were set up at the center and four corners of each delineated plots, and
 all standing live and dead vascular plants (that was produced during the
 current season) in these quadrats were harvested at ground level species
 by species, dried to a constant weight at 65℃ and weighed. The dry mass of
 all plant species per quadrat averaged over five replicates was used to
 determine the aboveground plant biomass at peak plant biomass time, and
 this was also used to approximate ANPP of the grassland (Scurlock et al.,
 2002). The belowground biomass and its distribution profile (0-70 cm) were
 measured using the soil coring method, and BNPP (during the plant growing
 season from May to October) of the studied grassland was obtained from
 previous studies (Chai et al., 2014; Hou et al., 2014).
 Whole plants of dominant species in each sampling plot (within a
 separate farm) were collected by digging up each individual to a depth of
 20 cm below ground level, and then each individual was cut into two parts:
 aboveground part (shoots) and belowground part (noted as roots, but it
 includes plant roots and rhizomes as well as shoot stumps buried below the
 soil surface). A sample of about 300g dry matter of the shoots and the
 roots of each plant species was collected in each of the three
 sampling plots at each site. The samples were washed with deionized water,
 dried at 65℃ and then cut into pieces (&lt;5mm) for phytolith
 analysis. The soil bulk density and moisture content of
 soil profile (0-70 cm) were obtained by the cutting-ring method and the
 oven drying method (at 105℃). The soil samples were collected using soil
 cores (diameter=7cm) and air dried at ambient temperature in the
 laboratory. <b>Sample analysis</b>
 The phytoliths within plant parts were extracted using a
 microwave digestion process (Parr et al., 2001) followed by a Walk-Black
 type digestion to ensure the purity of the phytoliths (Parr
 & Sullivan, 2014; Walkley & Black, 1934). Two duplicates
 were analyzed for each plant sample. The exacted phytoliths were dried at
 65℃ to a constant weight. The PhytOC was determined using the PhytOC
 alkalidis solution spectrophotometer method (Yang et al., 2014). In this
 method, sodium hydroxide solution was used to dissolve the Si compound in
 order to release the occluded organic carbon from the phytoliths, then
 potassium dichromate (K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>)-sulfuric acid
 (H<sub>2</sub>SO<sub>4</sub>)
 solution was used to oxidize the released organic carbon, and the
 concentration of Gr<sup>3+</sup> produced in this oxidation was
 determined by spectrophotometer with its absorbance at 590 mm wavelength.
 The organic carbon concentration was calculated based on the amounts of
 potassium dichromate consumed, and the accuracy and repeatability of the
 method were well verified against the results obtained with acid
 dissolution-Elementar Vario MAX CN method (Germany) (Yang et al., 2014).
 The phytoliths and PhytOC contents of the two parts of each plant species
 were calculated as the average of the three replicate plots. For each
 species, the ratio of aboveground to belowground biomass (shoots/roots)
 was calculated based on the sampled plant individuals. The ratio was used
 to calculate the belowground biomass of the species in 1
 m<sup>2</sup> based on the measured
 aboveground biomass of the species. The air dried soil
 samples was separated into the 100 mesh soil samples and the 10 mesh soil
 samples. The soil organic carbon (SOC) was determined with the 100 mesh
 soil samples using the method of classical potassium dichromate (Walkley
 & Black, 1934), and the soil pH and bioavailable Si content were
 analyzed with 10 mesh soil samples using a pH meter and silicomolybdic
 acid method (Yang et al., 2018), respectively.
 <b>Data
 calculations and statistics</b> The
 formula for calculating phytolith and PhytOC contents in plant species
 were as follows:<br>     phytolith content (g kg<sup>-1</sup>) =
 phytolith weight (g) / dry biomass (kg) [1]<br>     PhytOC content
 (g kg<sup>-1</sup>) = PhytOC weight (g) / dry
 biomass (kg)         [2] The community weighted mean
 contents of phytolith (Phytolith<sub>CWM</sub>)
 and PhytOC (PhytOC<sub>CWM</sub>) were also calculated using the
 contents of phytolith and PhytOC in the species of the community and the
 relative biomass of the species (as weight), similar to the calculation of
 other community-weighted mean plant traits (Ricotta and Moretti,
 2011).     phytolith<sub>CWM</sub> (g
 kg<sup>-1</sup>)=∑<i><sub>i</sub></i><i> </i>phytolith content<i><sub>i</sub></i> (g kg<sup>-1</sup>)×biomass<i><sub>i</sub></i> (%)            [3]<br>    PhytOC<sub>CWM </sub>(g kg<sup>-1</sup>)=∑<i><sub>i</sub></i><i> </i>PhytOC content<i><sub>i</sub></i> (gkg<sup>-1</sup>)×biomass<i><sub>i</sub></i> (%)               [4] i enumerates each species.     PhytOC stock (kg ha<sup>-1</sup>)= PhytOC<sub>CWM </sub>(g kg<sup>-1</sup>)×biomass (kg ha<sup>-1</sup>)×10<sup>-3</sup><sup> </sup><sup>       </sup><sup> </sup>[5]<br>     PhytOC production flux (kg ha<sup>-1</sup> yr<sup>-1</sup>) = PhytOC<sub>CWM</sub> (gkg<sup>-1</sup>)×NPP (kg ha<sup>-1</sup> yr<sup>-1</sup>)×10<sup>-3</sup>           [6] One-way ANOVA and Duncan's multiple range test were performed to examine the difference in phytolith and PhytOC contents among different parts of plant species. A principal component analysis (PCA) of PhytOC content and production parameters and environmental factors were performed to show their interrelations. R 3.3.3 was used for all the statistics and Sigma Plot 12.0 was used for figures.