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Among the most promising approaches of long-term atmospheric CO2 sequestration is terrestrial biogeochemical carbon sequestration. One of the most promising terrestrial biogeochemical carbon sequestration mechanisms is the occlusion of carbon within phytoliths, the silicified features that deposit within plant tissues. Using phytolith content-biogenic silica content transfer function obtained from our investigation, in combination with published silica content and above-ground net primary productivity (ANPP) data of China's grasslands, we estimated the production of phytoliths and phytolith-occluded carbon (PhytOC) in grasslands. The results show that the average above-ground phytolith production rates of China's grasslands (10.9 106 t yr−1 or 1.45% of world grasslands) are much lower than those of other grasslands (e.g. North American nonwoody grasslands) mainly because of much lower ANPP. Assuming a median content of PhytOC of 1.5%, the average above-ground PhytOC production rates of China's grasslands and world grasslands are estimated to be 0.6 106 t CO2 yr−1 and 41.4 106 t CO2 yr−1, respectively. The management of grasslands to maximize ANPP has the potential to result in considerable quantities of phytoliths and securely bio-sequestered carbon.
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The rapid increase in atmospheric CO2 and global surface temperature since the advent of the Industrial Revolution has motivated many scientists to investigate methods that can securely sequester atmospheric CO2 (Luyssaert et al., 2008; Street-Perrott & Barker, 2008). Among the most promising approaches of long-term atmospheric CO2 sequestration are terrestrial biogeochemical carbon sequestration (Parr et al., 2010; Song et al., 2011).
Phytoliths, also referred to as plant opal, are silicified features that form as a result of biomineralization within plants (Parr & Sullivan, 2005). Phytoliths are present in most plants and range in concentration from 0.5% or less in most dicotyledons, 1–3% in typical dryland grasses, and may comprise 10–15% in Cyperaceae and wetland species of Poaceae (Epstein, 1994; Parr et al., 2010). Recent studies revealed that phytoliths contain 0.2–5.8% of phytolith-occluded carbon (PhytOC) (Parr & Sullivan, 2005, 2011; Parr et al., 2009, 2010; Zuo & Lv, 2011), are highly resistant against decomposition and may accumulate in soil for several thousands of years after plant decomposition (Parr & Sullivan, 2005), demonstrating the potential of phytoliths in the long-term biogeochemical sequestration of atmospheric CO2.
Grassland is one of the most widespread vegetation types worldwide, occupying more than one fifth of the world's land surface (Hall et al., 1995; Scurlock & Hall, 1998). Grassland ecosystems may play an important role in the global terrestrial production of phytoliths due to their large area, high net primary productivity (NPP), and high Si concentration (Carnelli et al., 2001; Blecker et al., 2006). As grassland ecosystems may contribute as much as 20% of the total terrestrial NPP (Scurlock & Hall, 1998) and phytolith content in grassland communities is as high as or higher than that in other ecosystems (e.g. woodlands) (Carnelli et al., 2001), it can be inferred that they may contribute at least 20% of the total terrestrial phytolith production rate. However, large uncertainties still exist in estimates of grassland phytolith production, and its contribution to global terrestrial phytolith production (Carnelli et al., 2001; Blecker et al., 2006). Moreover, to the best of our knowledge, the potential of grassland phytoliths in the long-term biogeochemical sequestration of atmospheric CO2 has not been quantified globally and even regionally.
China's grasslands cover nearly one third of the country's area and are experiencing notable effects of anthropogenic activities (Department of Animal Husbandry & Veterinary, 1996; Chen & Wang, 2000). However, our knowledge of phytolith production in China's grasslands remains limited, partly because of a lack of direct measurements and large spatial heterogeneity in grassland NPP. In this study, we conducted a field sampling campaign in September of 2011 across the northern part of China. Using phytolith content-biogenic silica content transfer function obtained from the above sites, in combination with published silica content and above-ground net primary productivity (ANPP) data of grasslands, we estimated the production of phytoliths and PhytOC in China's grasslands.
Materials and methods
General characteristics of the grasslands
As an important component of China's grasslands, temperate grasslands are continuously distributed in arid and semiarid regions, whereas alpine grasslands are continuously located on the Tibetan Plateau. Based on China's vegetation classification system, the continuously distributed China's grasslands can be divided into five types: desert steppe, typical steppe, meadow steppe, alpine steppe, and alpine meadow (Fig. 1). The ANPP, plant composition and other characteristics vary greatly among the five types of grasslands (Table 1).
Table 1. Characteristics for the five grassland types of China
*,†Data of area and ANPP are from Ma et al. (2010). Other data are from Hou (1982).
Consist mainly of Poaceae, Asteraceae, Liliaceae and Polygonaceae
Consist mainly of Poaceae
Consist mainly of Poaceae, Asteraceae and Farbaceae
Consist mainly of Poaceae, Cyperaceae and Thymelaeaceae
Consist mainly of Poaceae and Cyperaceae
Grassland phytolith content-silica content transfer function
As phytolith consists mainly of silica (about 90%) and about 90% of silica in plants is hosted in phytoliths (Wang, 1998), plant phytolith content can be estimated from data of plant silica content. To construct phytolith content-silica content transfer function, we collected samples of desert steppe, typical steppe and meadow steppe, and analyzed phytolith and silica contents. The above ground parts of different plant species (four replicates) in grasslands of Inner Mongolia and Hebei were sampled during the autumn of 2011, including three plant species in desert steppe (Stipa glareosa, Cleistogenes mutiea, Clinelymus nutans), five plant species in typical steppe (Agropyron cristatum, Koeleria gracilis, Elymus dahuricus, Alopecuruspratensis, Chloris virgata) and 10 plant species in meadow steppe (Leymus chinensis, Stipa baicalensis, Poa sphondvlodes, Carex korshinskyi, Cleistogenes songorica, Calamagrostis epigeios, Alopecurus pratensis, Roegneria ciliarisi, Poa attenuata, Gypsophila davurica var. angustifolia) (Fig. 1). Each plant sample was made up of about 300 g of composite plant materials consisting of leaves, stems and sheath.
Plant samples were oven-dried at 65 °C to a constant mass and cut into small pieces. Plant samples were ashed at 500 °C to remove organic matter, fused with Li-metaborate, dissolved in dilute nitric acid, and analyzed for silica content by inductively coupled plasma-optical emission spectroscopy. The method used in this study for the isolation of plant phytoliths is a microwave digestion process followed by a Walkley–Black type digest to ensure that extraneous organic materials in the samples were removed (Parr et al., 2010). The phytolith isolates were then thoroughly dried at 75 °C for 24 h in a fan-forced oven and weighed to obtain plant phytolith content. The content of PhytOC was also determined (Parr et al., 2010). Monitored with plant standards (GSV-1) and repetition analysis, the precision was better than 5% in phytolith and silica measurements, and better than 10% in PhyOC measurement.
Grassland phytolith content-silica content transfer function was constructed with regression analysis method based on the determined phytolith and silica contents of grass samples (Fig. 2). Thus, silica content could be converted to phytolith content using the following equation:
where phytolith content and silica content represent grass phytolith content (wt%) and grass silica content (wt%), respectively.
Chemical data collection and phytolith content estimation
Chemical data of silica content were obtained from the published monograghs (Hou, 1982; Chen & Wang, 2000) and from our own determination. The above-ground parts of plant species from five grassland types were used to estimate phytolith content with a conversion factor of 0.965 (Eqn (1)).
Estimation of phytolith production
Since the production of biogenic Si is primarily driven by plant Si concentration and ANPP (Blecker et al., 2006), the phytolith production flux of grassland above-ground biomass can be estimated from grassland data of phytolith content and ANPP as:
where phytolith production flux is phytolith production amount of grassland above-ground biomass per area per year (g m−2 yr−1), phytolith content is the content of phytoliths in grassland above-ground biomass (wt%), and ANPP is above-ground net primary productivity of grasslands (g m−2 yr−1).
Phytolith production rate of grassland above-ground biomass was estimated from data of phytolith production flux and grassland area as:
where phytolith production rate is total phytolith production of grassland above-ground biomass per year (106 t yr−1), phytolith production flux is estimated from Eqn (2), area is the area of grasslands (104 km2).
Phytolith content in above-ground parts of grassland plants
Phytolith content in above-ground parts of plants varies greatly among plant species of the same grassland types and among different grassland types (0.10–8.75%, average 3.3%) (Tables 2 and 3). Generally, the content of phytolith is highest in meadow steppe (0.10–7.33%, average 4.12%), medium in typical steppe (1.57–8.75%, average 3.5%), and alpine steppe (1.04–7.30%, average 3.41%), lowest in desert steppe (1.18–5.65%, average 2.78%) and alpine meadow (1.64–6.57%, average 2.74%).
Table 2. Content of phytolith in above-ground parts of dominant plant species from five grassland types of China
Dominant plant species
Content of phytolith in above-ground parts of plant species is estimated from silica content data using grassland phytolith content-silica content transfer function of Eqn (1). (1) Estimated from chemical data of Hou (1982). (2) Estimated from chemical data of Chen & Wang (2000).
This study, 1
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This study, 2
This study, 1, 2
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This study, 2
This study, 1
This study, 1, 2
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This study, 2
Stipa subsessiliflora var.basiplumosa
Table 3. Estimated grassland phytolith production flux and grassland phytolith production rate
ANPP, above-ground net primary productivity is estimated from average of above-ground biomass densities of each grassland type in China from 1982 to 2006 (Ma et al., 2010).
Grassland phytolith production flux and rate are estimated using Eqns (2) and (3), respectively.
Above-ground grassland phytolith production
Grassland phytolith production flux and rate were estimated using Eqns (2) and (3) (Table 3). The average of phytolith production flux for the five types of China's grasslands is 2.8 g m−2 yr−1. Generally, phytolith production flux is the highest in meadow steppe (average 9.0 g m−2 yr−1), medium in typical steppe (average 3.8 g m−2 yr−1) and alpine meadow (average 2.9 g m−2 yr−1), the lowest in desert steppe (average 1.6 g m−2 yr−1) and alpine steppe (average 1.6 g m−2 yr−1). The total phytolith production rate for the five types of China's grasslands is 5.3 106 t yr−1. Phytolith production rate is the highest in alpine meadow (average 1.7 106 t yr−1) and typical steppe (average 1.5 106 t yr−1), followed by alpine steppe (average 1.1 106 t yr−1), meadow steppe (average 0.7 106 t yr−1), and desert steppe (average 0.3 106 t yr−1).
Factors controlling grassland phytolith production flux
The results demonstrate that for the five grassland types of China, a strong correlation (R2 = 0.97, P < 0.01) exists between above-ground grassland phytolith production (PP) flux and ANPP of grassland plants (Fig. 3a), but there was weaker relationship (R2=0.67, P > 0.5) between above-ground grassland PP flux and phytolith content (Fig. 3b). These data indicate that for grasslands, it is ANPP of grassland plants, rather than the actual quantity of silica taken up by the plants, that is most important in determining the above-ground grassland phytolith production flux.
Since climate (Piao et al., 2007; Ma et al., 2010) and human activities [e.g. grazing (Li et al., 2000; Su et al., 2005) and fertilization (Gough et al., 2000)] are the two main factors controlling ANPP of grassland plants, any change in future climate and human activities may influence the above-ground grassland phytolith production flux by controlling ANPP of grassland plants. Using measurements obtained from 341 sampling sites in China, together with a NDVI (normalized difference vegetation index) time series dataset over 1982–2006, Ma et al. (2010) observed an increasing trend of ANPP over the past 25 years due to rainfall and temperature change in spite of different responses of ANPP to climate variables among various grassland types. These results suggest that the above-ground grassland phytolith production flux may increase slightly in the near future although different grassland ecosystems in China may show diverse responses to future climate changes. Moreover, any practices to avoid overgrazing and to fertilize may enhance the above-ground grassland phytolith production flux by increasing ANPP of grassland plants.
The role of grasslands in the global phytolith production
On the basis of the data of grassland area, ANPP and phytolith content, we estimated the above-ground phytolith production rates for five grassland types of China, China's grasslands, world grasslands, and compared them with North American nonwoody grasslands and global terrestrial ecosystems (Table 4). The estimation uncertainties of grassland phytolith production rates exist mainly in the estimation of grassland area and great variations of ANPP, and partly in the fluctuation of phytolith content among different grassland ecosystems (Tables 2 and 3; Fig. 3).
Table 4. A global comparison of above-ground grassland phytolith production together with phytolith production of global terrestrial ecosystems
Conley (2002) estimated above-ground phytolith production rate of global terrestrial ecosystems at 3600–12 000 106 t yr−1 (on the basis of a global annual C production rate of 60 Pg C yr−1 and a Si : C molar ratio of 0.012–0.040). However, this rate may be overestimated because an assumed global average Si content of 1–3% of total dry weight of all terrestrial primary production by Conley (2002) is probably too high (Hou, 1982; Epstein, 1994; Carnelli et al., 2001; Hodson et al., 2005).
The average above-ground phytolith production rate for world grasslands is 752.3 106 t yr−1 or 7–21% of global terrestrial ecosystems estimated by Conley (2002) (Table 4). The average above-ground phytolith production rates of five grassland types of China (5.3 106 t yr−1 or 0.7% of world grassland rate) and China's grasslands in total (10.9 106 t yr−1 or 1.45% of world grassland rate) are much lower than those of North American nonwoody grasslands (17.4 106 t yr−1 or 2.32% of world grasslands) (Table 4). The low above-ground phytolith production rates of five grassland types of China and China's grasslands in total compared to other grassland ecosystems (e.g. North American nonwoody grasslands) are mainly a result of extremely low grassland ANPP caused by arid climates and overgrazing (Table 4).
The potential of carbon occlusion within phytoliths of grasslands
Assuming a median content of occluded carbon in phytoliths (PhytOC) of 1.5% according to our determination and published data (Parr et al., 2010; Zuo & Lv, 2011), the average above-ground grassland PhytOC production rate is estimated (Table 4).
The average above-ground PhytOC production rate of world grasslands is 41.4 106 t CO2 yr−1, a rate much higher than that of world bamboo (15.6 106 t CO2 yr−1) (Parr et al., 2010), sugarcane (7.2 106 t CO2 yr−1) (Parr et al., 2009), and millet (2.7 106 t CO2 yr−1) (Zuo & Lv, 2011). Limited by their low above-ground phytolith production rates, the average above-ground PhytOC production rates of five grassland types of China (0.3 106 t yr−1 or 0.7% of world grassland rate) and China's grasslands in total (0.6 106 t yr−1 or 1.45% of world grassland rate) are much lower than those of North American nonwoody grasslands (1.0 106 t yr−1 or 2.32% of world grassland rate) (Table 4).
As ANPP of grassland plants is most important in determining the above-ground phytolith production flux and above-ground PhytOC production rate revealed from this study (Fig. 3a and b), any practices to avoid overgrazing (Li et al., 2000; Su et al., 2005) and to fertilize (Gough et al., 2000) may enhance ANPP and average above-ground PhytOC production rate of grasslands.
In this article we have demonstrated the role of grasslands in the global phytolith production and the potential of carbon occlusion within phytoliths of grasslands. The average above-ground phytolith production rate for world grasslands is 752.3 106 t yr−1 or 7–21% of global terrestrial ecosystems estimated by Conley (2002). The average above-ground phytolith production rates of China's grasslands (10.9 106 t yr−1 or 1.45% of world grasslands) are much lower than those of other grasslands (e.g. North American nonwoody grasslands) mainly because of much lower ANPP. Assuming a median content of occluded carbon in phytoliths of 1.5%, the average above-ground PhytOC production rates of China's grasslands and world grasslands are estimated to be 0.6 106 t CO2 yr−1 and 41.4 106 t CO2 yr−1, respectively. In spite of some uncertainties existing in estimates of grassland phytolith and PhtOC production mainly caused by temporal and spatial variation in plant composition, ANPP and phytolith content of different grassland types, the results of the study indicate that the management of grasslands to maximize ANPP has the potential to result in considerable quantities of phytoliths and securely bio-sequestered carbon. However, further work such as estimation of seasonal/interannual variation and worldwide distribution of grassland phytolith and PhtOC production, and belowground grassland production of phytolith and PhytOC should be done to enhance our knowledge of grassland phytolith production and to offer references for management of biogeochemical sequestration of atmospheric CO2.
We are grateful for support from National Natural Science Foundation of China (grant no. 41103042), Zhejiang Province Key Science and Technology Innovation Team (no. 2010R50030), Opening Project of State Key Laboratory of Environmental Geochemistry (SKLEG9011), and Opening Project of Ministry of Education Laboratory for Earth Surface Processes, Peking University.
The authors have declared no conflict of interest.