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Impact of grassland degradation on soil phytolith carbon sequestration in Inner Mongolian steppe of China
Geoderma 308 (2017) 86–92
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Impact of grassland degradation on soil phytolith carbon sequestration in Inner Mongolian steppe of China
MARK
Wenjie Pana,b, Zhaoliang Songa,⁎, Hongyan Liuc, Karin Müellerd, Xiaomin Yanga, Xiaodong Zhanga, Zimin Lie, Xu Liuc, Shuang Qiuc, Qian Haoa, Hailong Wangf,⁎ a
Institute of the Surface-Earth System Science Research, Tianjin University, Tianjin 300072, China School of Environment and Resources, Zhejiang Agricultural and Forestry University, Lin'an, Zhejiang 311300, China c College of Urban and Environmental Sciences, Peking University, Peking 100871, China d The NZ Institute for Plant and Food Research Limited, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand e Earth and Life Institute-Environmental Sciences, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium f School of Environment and Chemical Engineering, Foshan University, Foshan, Guangdong 528000, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Phytolith-occluded carbon Degradation Grassland management Northern China
Grasslands play an important role in the terrestrial biogeochemical carbon (C) cycle and partly mitigate climate change through C occlusion within phytoliths. Grassland degradation has a significant influence on the coupled biogeochemical cycles of C and silicon in the Inner Mongolian steppe of China, but there are few reports about the impact of grassland degradation on phytolith C sequestration in the steppe, the main grassland in northern China. Twelve sampling sites were chosen in the Xilingol League. Soil samples (0–50 cm) were collected from grasslands of four different degradation degrees to investigate the impact of grassland degradation on the soil phytolith and phytolith-occluded C (PhytOC) accumulation using a mass-balance approach. Soil phytolith storages were 12.97 ± 2.15, 15.90 ± 0.65, 14.35 ± 0.79 and 13.22 ± 1.07 t ha− 1 in non-degraded, lightly degraded, moderately degraded and seriously degraded grasslands, respectively. The corresponding storages of soil PhytOC were 0.11 ± 0.02, 0.16 ± 0.02, 0.12 ± 0.01 and 0.07 ± 0.01 t ha− 1, respectively. The observed significant differences in soil phytoliths and PhytOC among grasslands of different degradation degrees indicate that grassland degradation influenced the phytolith and PhytOC accumulation in grassland soils. Grazing and harvesting are likely the major factors affecting soil phytolith and PhytOC storages through reducing the litterfall returning fluxes. Our preliminary findings imply that grassland restoration could be a promising way to increase long-term phytolith C sequestration through maximizing plant PhytOC production fluxes and soil PhytOC accumulation in degraded grasslands.
1. Introduction Bioavailable silicon (Si) is absorbed by plant roots from soil solution and deposited as phytoliths in cell wall and cell lumen of the plants (Parr and Sullivan, 2005; Ma and Yamaji, 2006; Song et al., 2012a) or as other siliceous forms in intercellular spaces or in an extracellular (cuticular) layer (Sangster et al., 2001; Ma and Yamaji, 2006). A small amount (about 0.2–5.8%) of organic carbon (C) is occluded within phytoliths during their formation (Wilding, 1967; Parr et al., 2010). Phytolith-occluded C (PhytOC) can be preserved in soils or sediments for hundreds to thousands of years due to silica (SiO2) protection (Parr and Sullivan, 2005; Zuo et al., 2014) and phytolith C sequestration is well-known as an important mechanism for long-term terrestrial biogeochemical C sequestration (Parr and Sullivan, 2005; Song et al.,
⁎
2012b; Song et al., 2016). For example, Parr et al. (2010) estimated that the annual worldwide median PhytOC production flux of bamboo forests is 98.18 kg ha− 1. Soil PhytOC storage in soils to a depth of 100 cm under bamboo can reach 3.91 t ha− 1 (X. Zhang et al., 2016). Previous studies highlighted that fertilizer applications could enhance PhytOC production flux through increased phytolith accumulation and aboveground net primary productivity (ANPP) rates (Song et al., 2012a, 2013a; Li et al., 2013; Song et al., 2016). These results have been verified in crop (Guo et al., 2015) and grassland ecosystems (Zhao et al., 2016). Grassland ecosystems play an important role in phytolith production because of their large distribution area, high ANPP and high Si concentration of plants (Poaceae as Si accumulators) (Carnelli et al., 2001; Blecker et al., 2006; Song et al., 2012a). Grassland degradation is
Corresponding authors at: Institute of the Surface-Earth System Science Research, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China. E-mail addresses: [email protected] (Z. Song), [email protected] (H. Wang).
http://dx.doi.org/10.1016/j.geoderma.2017.08.037 Received 10 April 2017; Received in revised form 18 August 2017; Accepted 23 August 2017 0016-7061/ © 2017 Published by Elsevier B.V.
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communities in lightly degraded grasslands are Agropyron cristatum and Cleistogenes squarosa. In moderately degraded grasslands Artemisia desertorum is dominant, and in seriously degraded grasslands Psammochloa villosa becomes dominant.
an urgent ecological and economic problem in the entire world, but particularly in China (Le Houérou, 1996; J. Zhang et al., 2016). Grassland degradation commonly occurs in the steppe driven by wind erosion, drought, pest damage and human disturbances in the semi-arid region of China (Zhao et al., 2005). In Inner Mongolia, China, nearly 90% of the grasslands are suffering from degradation with varying degrees (Jiang et al., 2009). The steppe ecosystems are fragile and easily affected by human disturbance. For example, human disturbance may significantly reduce vegetation cover, biodiversity and underground biomass, damage the soil macro-arthropod community (Zhou et al., 2008; Zhao et al., 2014) and significantly alter soil physical and chemical properties (Li et al., 2005; Jin et al., 2013; Xiong et al., 2017). In grassland ecosystems, studies about phytolith C sequestration mainly focused on plant parts. Song et al. (2012a) estimated that aboveground phytolith and PhytOC production rates of global grasslands were 7.52 × 108 and 1.13 × 107 t a− 1, respectively, and reported that phytolith and PhytOC production rates of grasslands could be significantly influenced by their ANPP. Recently, Qi et al. (2017) reported that PhytOC accumulation in underground roots was higher than those in aboveground plant parts, and suggested that the belowground productivity of plants may play a dominant role in PhytOC production in grassland ecosystems. Although the accumulations of soil phytoliths and PhytOC in different grasslands have also been studied in eastern Inner Mongolia, China (Pan et al., 2017), the impact of grassland degradation on soil phytolith and PhytOC storages remains unclear. Therefore, this study aims to (1) investigate distribution and accumulation of soil phytoliths and PhytOC in grasslands of different degradation degrees, (2) explore the factors influencing soil phytolith and PhytOC storage, and (3) evaluate the significance of grassland restoration for phytolith C sequestration. These results will provide scientific evidence for developing management practices for grasslands with the focus on long-term biogeochemical C sequestration.
2.2. Field sampling Three experimental sites were randomly chosen for each degradation gradient, non-degraded, lightly degraded, moderately degraded and seriously degraded. In order to address the spatial heterogeneity of soil properties, we randomly selected three plots (2 m × 2 m) at each site. We collected a soil sample of about 500 g from 0 to 10, 10–30, and 30–50 cm depth of each plot. The samples collected from the same layer of the three plots were mixed thoroughly into a composite sample, which was air-dried, ground and sieved (< 2 mm). Soil bulk density was determined on undisturbed soil samples from each layer using bulk density rings with a volume of 200 cm3, and three repetitions were done for every degradation degree. 2.3. Sample analysis Soil pH was determined in a mass ratio of soil to water of 1:5, and soil organic C (SOC) was measured using the potassium dichromate method (Lu, 1999). As primary source of plant Si, the total soil SiO2 content was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 2000, PerkinElmer Co., USA) after the soil samples were fused with lithium metaborate and dissolved in dilute nitric acid (4%) (Lu, 1999). Additionally, the soil bioavailable Si was extracted according to Song et al. (2013b). In order to extract phytoliths, soil samples were treated by a wet oxidation method followed by a heavy liquid suspension method (ZnBr2, 2.36 g cm− 3) (Zuo et al., 2014). All extracted phytoliths were dried at 65 °C for 24 h before weight determination. The potassium dichromate method was applied to determine the organic C content of phytoliths after phytolith dissolution with hydrofluoric acid (1 mol L− 1) (Li et al., 2013). All organic C content determinations were monitored using GBW07405 standard soil reference samples. The precision was better than 5%.
2. Materials and methods 2.1. Study area The study area (42–46°N, 115–118°E) is located in Xilingol League in northern China and has a temperate continental climate. The mean annual temperature is 1.5 °C and the mean annual precipitation is 295 mm. The average elevation of the sampling sites is about 1300 m. The soils are mainly Arenosols based on the FAO soil classification system (IUSS Working Group WRB, 2007). Light yellow or light brown fine sand particles are the dominant particles in all soils (Fig. 1). It is very difficult to develop a unified degradation index system because the scope of grassland degradation is very broad, and reasons causing degradation can vary and be complex. According to the National Standards of “Parameters for degradation, sandification and salification of rangelands” (GB19377-2003) and previous studies (Li, 1997), we choose plant community structure to quantify the grassland degradation degrees because it can indicate the status of grassland degradation better than other features in our study area. Precipitation data from 1980 to 2010 were collected firstly from National Meteorological Information Center of China (http://data.cma. cn). We systematically sampled 120 plots along a rainfall gradient in July and August of 2014. Every plot had a size of 2 m × 2 m. Plant species richness and vegetation coverage were investigated and recorded for each plot. According to our field ecological survey data, grasslands in our study area were divided into four degradation gradients, namely non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands. The information on vegetation status and soil profiles among non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands is shown in Table 1 and Fig. 1. The non-degraded grasslands are dominated by Leymus chinensis, Stipa baicalensis and Filifolium sibiricum. The dominant species of grassland
2.4. Calculations and statistics The storages (t ha− 1) of soil phytoliths and PhytOC in grasslands of different degradation degree were estimated using the following equations: n
Soil phytolith storage =
∑ Ti × BDi × (phytolith content in soils)i i=1
(1)
n
Soil phytolith storage =
∑ Ti × BDi × (PhytOC content in soils)i i=1
(2)
where i (i = 1, 2 and 3) is the soil profile layer (0–10, 10–30, and 30–50 cm from upper to lower, respectively), Ti is the thickness of each soil layer in different soil profiles (cm) and BDi represents soil bulk density for each layer (g cm− 3). The equations were multiplied by 0.1 to transform results from mg cm− 2 to t ha− 1. All data presented are the average of three replicates. One-way analysis of variance (ANOVA) and Duncan's test were carried out using SPSS 20.0 statistical package program (SPSS Inc., USA). 3. Results 3.1. Soil physico-chemical characteristics Soil bulk density was in the range of 1.41–1.55 g cm− 3 and increased with depth at all grassland sites (Table 2). Soil pH was 87
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Fig. 1. Comparison of vegetation status and soil profiles between non-degraded grasslands (NDG, a), lightly degraded grasslands (LDG, b), moderately degraded grasslands (MDG, c) and seriously degraded grasslands (SDG, d). More details see Table 1.
2.63 ± 0.48 g kg− 1; Fig. 2a). Soil phytolith content decreased with depth. The soil phytolith contents in 0–10 and 30–50 cm layers were significantly higher under lightly degraded grasslands than those under moderately and seriously degraded grasslands. Similarly, the soil PhytOC contents in the soil layers varied significantly (p < 0.05) among grasslands of different degradation degrees, from 0.008 ± 0.001 to 0.045 ± 0.004 g kg− 1 (Fig. 2b), with the highest value in the topsoil (0–10 cm) of the lightly degraded grasslands. The storage of soil phytoliths was highest in the lightly degraded grasslands, medium in the moderately degraded grasslands and lowest in the non-degraded and seriously degraded grasslands (15.90 ± 0.65, 14.35 ± 0.79, 12.97 ± 2.15 and 13.22 ± 1.07 t ha− 1, respectively) (Fig. 3a). Similarly, storage of soil PhytOC decreased in the order: lightly degraded grasslands > moderately degraded grasslands ≈ non-degraded grasslands > seriously degraded grasslands (0.16 ± 0.02, 0.12 ± 0.01, 0.11 ± 0.02 and 0.07 ± 0.01 t ha− 1, respectively) (Fig. 3b).
significantly (p < 0.05) lower in the non-degraded grasslands than in the three other grassland types for the 0–10 cm layer (Table 2). There were no significant differences in the soil pH between the lightly, moderately and seriously degraded grasslands for the top (0–10 cm) and bottom layers (30–50 cm) (Table 2). The soil SOC contents of the 0–10 and 10–30 cm layers were significantly (p < 0.05) lower for the seriously degraded grasslands than for the non-degraded, lightly and moderately degraded grasslands (Table 2). The SOC content in the topsoil of the moderately degraded grasslands was significantly (p < 0.05) higher than the SOC contents of the other grasslands in the same depth (Table 2). The total soil SiO2 content in different degraded grasslands ranged from 708.4 to 783.3 g kg− 1, and the average total soil SiO2 contents across the three soil layers of the non-degraded, lightly, moderately and seriously degraded grasslands were 760.0, 760.9, 764.9 and 741.1 g kg− 1, respectively (Table 2). The soil bioavailable Si content was significantly (p < 0.05) lower in the top layer (0–10 cm) for non-degraded grasslands than for the lightly and moderately degraded grasslands, and soil bioavailable Si contents increased with depth (Table 2).
4. Discussion 3.2. Contents and storages of soil phytoliths and PhytOC in grasslands of different degradation degree
4.1. Factors controlling accumulation of soil phytoliths and PhytOC in degraded grasslands
The phytolith content of each soil depth varied significantly for grasslands of different degradation degrees (from 1.52 ± 0.13 to
The aboveground phytolith production fluxes of non-degraded, lightly degraded, moderately degraded, and seriously degraded
Table 1 Vegetation coverage and species richness for grasslands of different degradation degrees and soil texture of the associated soils. Degradation degreea
NDG LDG MDG SDG a b c
Vegetation coverage (%)b
75 65 55 40
Species richnessb
33 30 31 21
Soil particle size distribution (%)c Coarse sand (> 0.1 mm)
Fine sand (0.1–0.05 mm)
Silt and clay (< 0.05 mm)
42–81 88–90 88–93 91–94
9–51 4–33 2–28 2–25
4–9 2–6 2–4 1–4
NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively. Data based on field investigations. Data taken from grassland soils investigated by Li et al. (2005), Jin et al. (2013) and Xiong et al. (2017), which are located close to our study areas.
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Table 2 Soil physico-chemical characteristics under grasslands of different degradation degrees (means ± standard deviations). Degradation degree
Depth (cm)
Bulk density (g cm− 3)
pH
NDG
0–10 10–30 30–50 0–10 10–30 30–50 0–10 10–30 30–50 0–10 10–30 30–50
1.41 1.46 1.52 1.49 1.51 1.54 1.39 1.49 1.54 1.51 1.54 1.55
6.33 6.48 6.68 6.74 6.60 6.73 6.63 6.87 7.23 6.76 7.03 6.56
LDG
MDG
SDG
± ± ± ± ± ± ± ± ± ± ± ±
0.09bA 0.03bA 0.03aA 0.02abB 0.03abAB 0.01aA 0.02bB 0.05abA 0.01aA 0.02aB 0.01aA 0.02aA
SiO2 (g kg− 1) ± ± ± ± ± ± ± ± ± ± ± ±
0.26bB 0.18bA 0.18bA 0.11aB 0.10bC 0.32aA 0.09aB 0.44abA 0.29abA 0.11aC 0.31aA 0.19abB
745.6 771.4 762.9 736.1 763.4 783.3 745.9 771.3 777.4 708.4 763.8 751.0
± ± ± ± ± ± ± ± ± ± ± ±
SOC (g kg− 1) 1.3abA 1.0aA 3.2aA 3.0aA 1.4aA 2.8aA 3.2abA 1.2aA 4.5aA 5.1bB 1.4aA 2.7aA
8.82 6.16 3.20 4.54 3.57 2.12 9.59 4.45 2.25 3.52 1.95 1.77
± ± ± ± ± ± ± ± ± ± ± ±
Bioavailable Si (mg kg− 1) 4.93aA 1.74aA 1.44aA 0.96abA 1.28abAB 0.16aB 1.00aA 2.22abB 0.34aB 1.06bA 0.25bB 0.52aB
95.5 ± 16.3b 93.2 ± 13.8b 102.9 ± 19.4a 117.4 ± 9.1a 105.0 ± 10.6b 118.8 ± 1.1a 115.23 ± 5.6a 138.6 ± 8.6a 126.4 ± 25.9a 107.3 ± 2.4ab 97.67 ± 4.6b 118.6 ± 22.0a
Different lowercase letters indicate significant differences between grasslands of different degradation degrees at the same soil layer, while different uppercase letters show significant differences between the different soil depths at the same degradation degree, at p < 0.05 level based on Duncan's test. NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
Fig. 3. Storages of soil phytoliths (a) and PhytOC (b) in grasslands of different degradation degrees. Different lowercase letters indicate significant differences between grasslands of different degradation degrees (p < 0.05 level based on Duncan's test). Error bars represent the standard deviations of the means (n = 3). NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
Fig. 2. Contents of phytoliths (a) and PhytOC (b) in soils under grasslands of different degradation degrees. Different lowercase letters indicate significant differences between grasslands of different degradation degrees in the same depth, while different uppercase letters show significant differences between different soil depths for grasslands with the same degradation degree (p < 0.05 level based on Duncan's test). Error bars represent the standard deviations of the means (n = 3). NDG, LDG, MDG and SDG stand for nondegraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
heterogeneities of herbivore manure and harvesting might have resulted in lower phytolith return fluxes in non-degraded grasslands. However, storage of both soil phytolith and PhytOC decreased in the order of lightly degraded grasslands > moderately degraded grasslands > seriously degraded grasslands (Fig. 3). These trends were consistent with the observed phytolith and PhytOC production fluxes (Table 3).
grasslands in the same area have been studied by Ru (2015), indicating that the aboveground phytolith production flux of non-degraded grasslands was the highest among grasslands of different degradation degrees (Table 3). However, our study showed that the storage of soil phytoliths in non-degraded grasslands was lowest (Fig. 3a). Spatial 89
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Table 3 Contents and production fluxes of phytoliths and PhytOC in aboveground plant parts (means ± standard deviations) grown in grasslands of different degradation degrees. Degradation degree
Phytolith content (g kg− 1)
PhytOC content (g kg− 1)
ANPP (kg ha− 1 a− 1)
Phytolith production flux (kg ha− 1 a− 1)
PhytOC production flux (kg ha− 1 a− 1)
NDG LDG MDG SDG
22.37 20.37 24.78 28.15
0.19 0.21 0.20 0.16
1202.67 720.92 596.00 660.70
26.91 14.68 14.77 18.60
0.86 0.56 0.43 0.38
± ± ± ±
0.60 1.26 1.29 2.62
± ± ± ±
0.03 0.05 0.05 0.03
± ± ± ±
0.73 0.91 0.77 1.73
± ± ± ±
0.12 0.14 0.11 0.08
Data taken from Ru (2015). NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
degraded grasslands is likely to be higher than that of degraded grasslands. Blecker et al. (2006) reported that soil phytoliths were preserved better in soils of lower water content because soil phytolith dissolution increased with increasing soil water contents. Therefore, it can be assumed that more soil phytoliths are dissolved in non-degraded grasslands soils than in degraded grassland soils. The bioavailable Si content was significantly (p < 0.05) lower for the non-degraded grasslands than for the lightly, moderately and seriously degraded grasslands in the 0–10 cm layer (Table 2), with a similar trend for soil phytolith content (Fig. 2a). Since the phytolith production flux was the highest for the non-degraded grasslands among the four grassland types (Table 3), more bioavailable Si might be absorbed from these soils. Generally, grassland degradation affects underground biomass production of grasslands, for example, grass root biomass significantly decreased with increased degradation degree (Jin et al., 2013). Consequently, uptake of bioavailable Si might be influenced by reduced plant root biomass, and more bioavailable Si might be retained in degraded grasslands than in non-degraded grasslands. In general, the soil total SiO2 content is mainly comprised of the SiO2 fraction in silicate minerals (Liu et al., 2001). Cornelis and Delvaux (2016) reviewed the dissolution of soil silicate minerals, and concluded that it could govern the concentration of soil bioavailable Si. However, our results showed that the total soil SiO2 contents did not differ significantly between grasslands of different degradation degrees at the same soil depth, except for the 0–10 cm soil depth in seriously degraded grasslands (Table 2). Therefore, the variation of bioavailable Si in different degraded grasslands might be mainly caused by root uptake and dissolution of soil phytoliths. The mechanisms controlling phytolith dissolution are multiple and complex. As different non-crystalline Si forms can be mutually transformed (Liu et al., 2001; Borrelli et al., 2008; Song et al., 2016), the decrease of bioavailable Si may accelerate the transformation of soil phytoliths (one of the non-crystalline Si) to bioavailable Si under certain soil conditions (Liu et al., 2001; Zhang et al., 2017). Differences in soil properties (e.g., SOC and pH) can influence soil phytolith dissolution (Hart and Humphreys, 2003; Fraysse et al., 2006, 2009). Previous studies have demonstrated that the dissolution rate of phytoliths significantly increased from acid solution (pH = 3) to alkaline solution (pH = 10) (Fraysse et al., 2006, 2009). At short-term scale, the complexation of phytoliths with SOC can protect phytoliths from dissolution, because phytoliths are generally surrounded by extraneous organic materials (Li et al., 2013; Parr and Sullivan, 2014). However, at long-term scale, the prosperous root systems in non-degraded grassland soils excrete large amounts of rhizospheric organic acids (Brolsma et al., 2017), which may indirectly contribute to the dissolution of phytoliths by enhancing the decomposition of phytolith extraneous organic materials (Zhu et al., 2014). Furthermore, the dissolution of soil phytoliths is also depended on the plant origin, morphotypes, and ion (e.g., Al3 + and Fe3 +) concentration of the phytoliths in the soils (Bartoli and Wilding, 1980; Fraysse et al., 2009; Cabanes et al., 2011; Li et al., 2014; Song et al., 2016), which were not considered in this study. Therefore, the physico-chemical properties of phytoliths in different degraded grasslands should be further studied. In our study area, Xilingol League, the total number of livestock
In general, phytoliths formed in plant tissues are released into soil through decomposition of plant materials. The phytolith production flux of aboveground plants is the original source for soil phytoliths when little or no human disturbance occurs (Hodson et al., 2005; Song et al., 2012a). In our study, grassland degradation was mainly caused by overgrazing (more details see below). Although the observed variations of phytolith and PhytOC contents were small in the aboveground parts of the vegetation, the phytolith and PhytOC production fluxes and ANPP all tended to decrease from non-degraded to seriously degraded grasslands (Table 3). This indicates that the phytolith and PhytOC returning fluxes from plants to soils decreased with increasing degree of grassland degradation. For grassland ecosystems, phytolith and PhytOC return fluxes are assumed to be far lower than their production fluxes because of human disturbances such as grazing and harvesting (Wang, 1998). Generally, the return of manure from herbivores compensates some of the phytolith loss in biomass (Vandevenne et al., 2013). Thus, spatial heterogeneities of herbivore manure and harvesting may play a crucial role in returning phytoliths and PhytOC to soils. However, the storages of soil phytolith and PhytOC depend not only on phytolith input flux from plants, but also on the phytolith output fluxes due to harvesting loss, translocation, dissolution and erosion of phytoliths (Meunier et al., 1999; Blecker et al., 2006; Fishkis et al., 2009; Song et al., 2016). The main pathways of phytolith output fluxes are wind and water erosion (Zuo et al., 2014). Reduced vegetation cover has been found to be positively correlated with grassland degradation (Li et al., 2005; Jin et al., 2013; Xiong et al., 2017) and can enhance wind and water erosion. Wind erosion affects the size distribution of soil particles in the topsoil because small soil particles are easily transported by wind (J. Zhang et al., 2016). The size of phytoliths is mostly within 5–200 μm, which is close to the size of silt and clay (Ge et al., 2010; Table 1). Latorre et al. (2012) found that phytoliths were present in the air, highlighting the possibility of phytolith loss by wind. Therefore, phytoliths originating in degraded grassland soils may also be transported by wind. Heavy precipitation can cause water erosion in particular if the vegetation cover is decreased, and so phytoliths may also be lost via water erosion from degraded grasslands. Zuo et al. (2014) reported about 2.6 × 106 t of phytolith loss due to soil erosion in the Loess Plateau in China. In our study, the flux of soil phytolith loss from degraded grasslands was not estimated. But we recommend that the loss of soil phytoliths should be considered when investigating the impact of grassland degradation on soil phytolith and PhytOC storages. Soil permeability and structure of degraded grasslands may also affect soil water dynamics (Jin et al., 2013). Previous studies have reported that the content of clay particles decreased while the content of sand particles increased gradually with increasing grassland degradation, and consequently, the content of sand particles in non-degraded grasslands was significantly lower than in degraded grassland (Table 1). A coarser soil texture leads to higher hydraulic conductivities. In accordance with this, Jin et al. (2013) found significantly higher soil water contents in undisturbed grasslands compared to desertified grasslands in Hunshandake Sandy Land of Inner Mongolia, which is close to our study area. Therefore, the soil water content of non90
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increased from 0.16 × 107 in 1949 to 1.63 × 107 (including 1.45 × 107 sheep) in 2016; the grazing density increased significantly from 0.09 livestock ha− 1 to 1.10 livestock ha− 1, resulting in extensive grassland degradation (http://www.xlgl.gov.cn). Additionally, the current area harvested in this region reached up to 0.26 × 106 ha; and the harvests of grass or forage (mainly comprising of Leymus chinensis, Stipa baicalensis, Cleistogenes squarosa) were > 2.12 t ha− 1 year− 1 (http://www.nmagri.gov.cn). To meet the needs of economic development and reduce the destruction of degraded grasslands, rest-grazing or rotational-grazing was applied in most of lightly and moderately degraded grasslands, and grazing was forbidden in seriously degraded grasslands by the local government. However, extensive open-grazing and free harvesting usually occurred on non-degraded grasslands, which would also contribute to the lower storages of phytoliths and PhytOC in non-degraded grasslands compared with those in lightly degraded grasslands (Fig. 3) by decreasing the return flux of phytolihs and PhytOC. As the differences in physico-chemical soil characteristics, such as pH, bulk density and SiO2 content, observed between non-degraded grasslands and lightly, moderately and seriously degraded grasslands were mainly non-significant (Table 2), we conclude that, in our study, the observed changes in soil phytolith and PhytOC storage were mainly driven by human disturbances, especially grazing and harvesting.
accumulation of manure on the soil surface (He et al., 2009). Applying additional organic manure amendments can provide a slow release source of nutrients for the vegetation of degraded grasslands to recover while grazing and harvesting is needed to be restricted or completely forbidden (He et al., 2009; Xiong et al., 2016). Therefore, management practices to recover degraded grasslands such as organic manure amendment, N fertilization, and reducing grazing and harvesting will be helpful to increase the storages of soil phytoliths and PhytOC. The litterfall returning flux is nearly equal to the production flux of aboveground plants in natural grasslands (Song et al., 2012a). Assuming that the aboveground biomass of the degraded grasslands could be restored to non-degraded grasslands, the soil PhytOC accumulation would also be improved correspondingly. For example, the restoration of degraded grasslands has the potential to reduce wind and water erosion, reducing the transport of soil phytoliths. Although the soil PhytOC stock is probably quite small compared with the soil SOC stock of the grassland ecosystems (Yang et al., 2010), PhytOC can be conserved in grassland soils for millennia and plays an important role in the long-term terrestrial biogeochemical C sink due to physical protection by phytoliths (Parr and Sullivan, 2005; Song et al., 2016). Therefore, grassland restoration might play an important role in reducing the global atmospheric carbon dioxide concentrations at a centennial–millennial scale through increasing soil phytolith and PhytOC storages in degraded grasslands. Further research is needed to improve our understanding of the mechanisms of increasing phytolith and PhytOC storages in degraded grasslands.
4.2. Implications for grassland management of long-term phytolith C sequestration There are > 74.4 × 106 ha of degraded grasslands in northern China (Li et al., 2011). Grassland degradation will lead to the loss of soil C and nutrients, decreased aboveground and belowground biomass production, increased soil erosion and eventually reduced livestock productivity (Zhou et al., 2008; Jiang et al., 2009). In our study area, the plant PhytOC production fluxes of the lightly, moderately and seriously degraded grasslands were 35, 50 and 56% lower compared to those in the non-degraded grasslands (Table 3). The corresponding litterfall phytolith returning fluxes and soil PhytOC storages are assumed to be lower, too. Assuming that all degraded grasslands could be restored to non-degraded status, the potential production fluxes of phytoliths and PhytOC should at least be 26.91 ± 0.73 kg ha− 1 a− 1 and 0.86 ± 0.12 kg ha− 1 a− 1, respectively (Table 3). Thus, approximate 2.00 × 106 t a− 1 of phytoliths and 0.06 × 106 t a− 1 of PhytOC could be sequestrated by the restored grasslands in northern China, respectively. In recent years, the PhytOC sequestration rate of forests (143 × 106 ha, Song et al., 2013b), wetlands (13.7 × 106 ha, Li et al., 2013), and croplands (160 × 106 ha, Song et al., 2014) have been reported to be 0.15 × 106 t a− 1, 0.46 × 106 t a− 1, and 1.20 × 106 t a− 1 in China. Compared with wetlands, the lower PhytOC sequestration rate of the restored grasslands was mainly due to the lower PhytOC production flux. However, the larger distribution areas of forests and croplands should be the main reasons that the PhytOC sequestration rate of the restored grasslands was conspicuously lower compared to those ecosystems. Notwithstanding, the potential of phytolith C sequestration in degraded grasslands would be huge because of the extensive areas. Therefore, grassland protection and restoration are important and necessary for grassland managers (Jiang et al., 2009), which can not only reverse grassland degradation processes (J. Zhang et al., 2016), but also increase the phytolith production flux controlled by increasing ANPP (Song et al., 2012a). Recent studies have indicated that nitrogen (N) application can not only significantly increase aboveground biomass and diversity (Xu et al., 2015), but also significantly enhance the aboveground PhytOC production flux (Zhao et al., 2016). Moreover, livestock manures are important nutrient reservoirs in grassland ecosystems, and contain C, N, Si, phosphorus and other nutrients (Schröder et al., 2007; Vandevenne et al., 2013; Sileshi et al., 2017). Generally, low ambient temperature and rainfall, characteristic of the semi-arid climate in China, will lead to
5. Conclusions Human disturbances, especially grazing and harvesting, were identified as major factors for soil phytolith and PhytOC storages in degraded grasslands of China. Grassland degradation decreased plant PhytOC production fluxes by 35, 50 and 56% in lightly, moderately and seriously degraded grasslands, respectively, and corresponding soil PhytOC storages decreased, too. Our study indicates that grassland restoration has the potential to increase long-term phytolith C sequestration. However, it has to be noted that in this study, grassland degradation was classified into four gradients based only on the status of the aboveground plants. Other indicators such as soil nutrients and soil texture should be equally considered in future studies. Results and conclusions of this local study may not be applicable to other regions. Further research on degraded grassland should be carried out to better understand the impact of degradation on soil phytolith and PhytOC accumulation at larger scales, including national and global scales, and should be conducted with a more complex and generalized classification system for degraded grasslands. Acknowledgements We thank Yutong Li, Ning Ru and Zhuo Ji for their assistance with sampling and analysis. The research was supported by National Natural Science Foundation of China (41522207, 41571130042) and State's Key Project of Research and Development Plan of China (2016YFA0601002). The authors declare no conflict of interest. References Bartoli, F., Wilding, L.P., 1980. Dissolution of biogenic opal as a function of its physical and chemical properties. Soil Sci. Soc. Am. J. 44, 873–878. Blecker, S.W., Mcculley, R.L., Chadwick, O.A., Kelly, E.F., 2006. Biologic cycling of silica across a grassland bioclimosequence. Glob. Biogeochem. Cycles 20, 1–11. Borrelli, N., Osterrieth, M., Marcovecchio, J.E., 2008. Interrelations of vegetal cover, silicophytolith content and pedogenesis of Typical Argiudolls of the Pampean Plain, Argentina. Catena 75, 146–153. Brolsma, K.M., Vonk, J.A., Mommer, L., Van Ruijven, J., Hoffland, E., De Goede, R.G., 2017. Microbial catabolic diversity in and beyond the rhizosphere of plant species and plant genotypes. Pedobiologia 61, 43–49. Cabanes, D., Weiner, S., Shahack-Gross, R., 2011. Stability of phytoliths in the
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Impact of grassland degradation on soil phytolith carbon sequestration in Inner Mongolian steppe of China
MARK
Wenjie Pana,b, Zhaoliang Songa,⁎, Hongyan Liuc, Karin Müellerd, Xiaomin Yanga, Xiaodong Zhanga, Zimin Lie, Xu Liuc, Shuang Qiuc, Qian Haoa, Hailong Wangf,⁎ a
Institute of the Surface-Earth System Science Research, Tianjin University, Tianjin 300072, China School of Environment and Resources, Zhejiang Agricultural and Forestry University, Lin'an, Zhejiang 311300, China c College of Urban and Environmental Sciences, Peking University, Peking 100871, China d The NZ Institute for Plant and Food Research Limited, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand e Earth and Life Institute-Environmental Sciences, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium f School of Environment and Chemical Engineering, Foshan University, Foshan, Guangdong 528000, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Phytolith-occluded carbon Degradation Grassland management Northern China
Grasslands play an important role in the terrestrial biogeochemical carbon (C) cycle and partly mitigate climate change through C occlusion within phytoliths. Grassland degradation has a significant influence on the coupled biogeochemical cycles of C and silicon in the Inner Mongolian steppe of China, but there are few reports about the impact of grassland degradation on phytolith C sequestration in the steppe, the main grassland in northern China. Twelve sampling sites were chosen in the Xilingol League. Soil samples (0–50 cm) were collected from grasslands of four different degradation degrees to investigate the impact of grassland degradation on the soil phytolith and phytolith-occluded C (PhytOC) accumulation using a mass-balance approach. Soil phytolith storages were 12.97 ± 2.15, 15.90 ± 0.65, 14.35 ± 0.79 and 13.22 ± 1.07 t ha− 1 in non-degraded, lightly degraded, moderately degraded and seriously degraded grasslands, respectively. The corresponding storages of soil PhytOC were 0.11 ± 0.02, 0.16 ± 0.02, 0.12 ± 0.01 and 0.07 ± 0.01 t ha− 1, respectively. The observed significant differences in soil phytoliths and PhytOC among grasslands of different degradation degrees indicate that grassland degradation influenced the phytolith and PhytOC accumulation in grassland soils. Grazing and harvesting are likely the major factors affecting soil phytolith and PhytOC storages through reducing the litterfall returning fluxes. Our preliminary findings imply that grassland restoration could be a promising way to increase long-term phytolith C sequestration through maximizing plant PhytOC production fluxes and soil PhytOC accumulation in degraded grasslands.
1. Introduction Bioavailable silicon (Si) is absorbed by plant roots from soil solution and deposited as phytoliths in cell wall and cell lumen of the plants (Parr and Sullivan, 2005; Ma and Yamaji, 2006; Song et al., 2012a) or as other siliceous forms in intercellular spaces or in an extracellular (cuticular) layer (Sangster et al., 2001; Ma and Yamaji, 2006). A small amount (about 0.2–5.8%) of organic carbon (C) is occluded within phytoliths during their formation (Wilding, 1967; Parr et al., 2010). Phytolith-occluded C (PhytOC) can be preserved in soils or sediments for hundreds to thousands of years due to silica (SiO2) protection (Parr and Sullivan, 2005; Zuo et al., 2014) and phytolith C sequestration is well-known as an important mechanism for long-term terrestrial biogeochemical C sequestration (Parr and Sullivan, 2005; Song et al.,
⁎
2012b; Song et al., 2016). For example, Parr et al. (2010) estimated that the annual worldwide median PhytOC production flux of bamboo forests is 98.18 kg ha− 1. Soil PhytOC storage in soils to a depth of 100 cm under bamboo can reach 3.91 t ha− 1 (X. Zhang et al., 2016). Previous studies highlighted that fertilizer applications could enhance PhytOC production flux through increased phytolith accumulation and aboveground net primary productivity (ANPP) rates (Song et al., 2012a, 2013a; Li et al., 2013; Song et al., 2016). These results have been verified in crop (Guo et al., 2015) and grassland ecosystems (Zhao et al., 2016). Grassland ecosystems play an important role in phytolith production because of their large distribution area, high ANPP and high Si concentration of plants (Poaceae as Si accumulators) (Carnelli et al., 2001; Blecker et al., 2006; Song et al., 2012a). Grassland degradation is
Corresponding authors at: Institute of the Surface-Earth System Science Research, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China. E-mail addresses: [email protected] (Z. Song), [email protected] (H. Wang).
http://dx.doi.org/10.1016/j.geoderma.2017.08.037 Received 10 April 2017; Received in revised form 18 August 2017; Accepted 23 August 2017 0016-7061/ © 2017 Published by Elsevier B.V.
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communities in lightly degraded grasslands are Agropyron cristatum and Cleistogenes squarosa. In moderately degraded grasslands Artemisia desertorum is dominant, and in seriously degraded grasslands Psammochloa villosa becomes dominant.
an urgent ecological and economic problem in the entire world, but particularly in China (Le Houérou, 1996; J. Zhang et al., 2016). Grassland degradation commonly occurs in the steppe driven by wind erosion, drought, pest damage and human disturbances in the semi-arid region of China (Zhao et al., 2005). In Inner Mongolia, China, nearly 90% of the grasslands are suffering from degradation with varying degrees (Jiang et al., 2009). The steppe ecosystems are fragile and easily affected by human disturbance. For example, human disturbance may significantly reduce vegetation cover, biodiversity and underground biomass, damage the soil macro-arthropod community (Zhou et al., 2008; Zhao et al., 2014) and significantly alter soil physical and chemical properties (Li et al., 2005; Jin et al., 2013; Xiong et al., 2017). In grassland ecosystems, studies about phytolith C sequestration mainly focused on plant parts. Song et al. (2012a) estimated that aboveground phytolith and PhytOC production rates of global grasslands were 7.52 × 108 and 1.13 × 107 t a− 1, respectively, and reported that phytolith and PhytOC production rates of grasslands could be significantly influenced by their ANPP. Recently, Qi et al. (2017) reported that PhytOC accumulation in underground roots was higher than those in aboveground plant parts, and suggested that the belowground productivity of plants may play a dominant role in PhytOC production in grassland ecosystems. Although the accumulations of soil phytoliths and PhytOC in different grasslands have also been studied in eastern Inner Mongolia, China (Pan et al., 2017), the impact of grassland degradation on soil phytolith and PhytOC storages remains unclear. Therefore, this study aims to (1) investigate distribution and accumulation of soil phytoliths and PhytOC in grasslands of different degradation degrees, (2) explore the factors influencing soil phytolith and PhytOC storage, and (3) evaluate the significance of grassland restoration for phytolith C sequestration. These results will provide scientific evidence for developing management practices for grasslands with the focus on long-term biogeochemical C sequestration.
2.2. Field sampling Three experimental sites were randomly chosen for each degradation gradient, non-degraded, lightly degraded, moderately degraded and seriously degraded. In order to address the spatial heterogeneity of soil properties, we randomly selected three plots (2 m × 2 m) at each site. We collected a soil sample of about 500 g from 0 to 10, 10–30, and 30–50 cm depth of each plot. The samples collected from the same layer of the three plots were mixed thoroughly into a composite sample, which was air-dried, ground and sieved (< 2 mm). Soil bulk density was determined on undisturbed soil samples from each layer using bulk density rings with a volume of 200 cm3, and three repetitions were done for every degradation degree. 2.3. Sample analysis Soil pH was determined in a mass ratio of soil to water of 1:5, and soil organic C (SOC) was measured using the potassium dichromate method (Lu, 1999). As primary source of plant Si, the total soil SiO2 content was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 2000, PerkinElmer Co., USA) after the soil samples were fused with lithium metaborate and dissolved in dilute nitric acid (4%) (Lu, 1999). Additionally, the soil bioavailable Si was extracted according to Song et al. (2013b). In order to extract phytoliths, soil samples were treated by a wet oxidation method followed by a heavy liquid suspension method (ZnBr2, 2.36 g cm− 3) (Zuo et al., 2014). All extracted phytoliths were dried at 65 °C for 24 h before weight determination. The potassium dichromate method was applied to determine the organic C content of phytoliths after phytolith dissolution with hydrofluoric acid (1 mol L− 1) (Li et al., 2013). All organic C content determinations were monitored using GBW07405 standard soil reference samples. The precision was better than 5%.
2. Materials and methods 2.1. Study area The study area (42–46°N, 115–118°E) is located in Xilingol League in northern China and has a temperate continental climate. The mean annual temperature is 1.5 °C and the mean annual precipitation is 295 mm. The average elevation of the sampling sites is about 1300 m. The soils are mainly Arenosols based on the FAO soil classification system (IUSS Working Group WRB, 2007). Light yellow or light brown fine sand particles are the dominant particles in all soils (Fig. 1). It is very difficult to develop a unified degradation index system because the scope of grassland degradation is very broad, and reasons causing degradation can vary and be complex. According to the National Standards of “Parameters for degradation, sandification and salification of rangelands” (GB19377-2003) and previous studies (Li, 1997), we choose plant community structure to quantify the grassland degradation degrees because it can indicate the status of grassland degradation better than other features in our study area. Precipitation data from 1980 to 2010 were collected firstly from National Meteorological Information Center of China (http://data.cma. cn). We systematically sampled 120 plots along a rainfall gradient in July and August of 2014. Every plot had a size of 2 m × 2 m. Plant species richness and vegetation coverage were investigated and recorded for each plot. According to our field ecological survey data, grasslands in our study area were divided into four degradation gradients, namely non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands. The information on vegetation status and soil profiles among non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands is shown in Table 1 and Fig. 1. The non-degraded grasslands are dominated by Leymus chinensis, Stipa baicalensis and Filifolium sibiricum. The dominant species of grassland
2.4. Calculations and statistics The storages (t ha− 1) of soil phytoliths and PhytOC in grasslands of different degradation degree were estimated using the following equations: n
Soil phytolith storage =
∑ Ti × BDi × (phytolith content in soils)i i=1
(1)
n
Soil phytolith storage =
∑ Ti × BDi × (PhytOC content in soils)i i=1
(2)
where i (i = 1, 2 and 3) is the soil profile layer (0–10, 10–30, and 30–50 cm from upper to lower, respectively), Ti is the thickness of each soil layer in different soil profiles (cm) and BDi represents soil bulk density for each layer (g cm− 3). The equations were multiplied by 0.1 to transform results from mg cm− 2 to t ha− 1. All data presented are the average of three replicates. One-way analysis of variance (ANOVA) and Duncan's test were carried out using SPSS 20.0 statistical package program (SPSS Inc., USA). 3. Results 3.1. Soil physico-chemical characteristics Soil bulk density was in the range of 1.41–1.55 g cm− 3 and increased with depth at all grassland sites (Table 2). Soil pH was 87
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Fig. 1. Comparison of vegetation status and soil profiles between non-degraded grasslands (NDG, a), lightly degraded grasslands (LDG, b), moderately degraded grasslands (MDG, c) and seriously degraded grasslands (SDG, d). More details see Table 1.
2.63 ± 0.48 g kg− 1; Fig. 2a). Soil phytolith content decreased with depth. The soil phytolith contents in 0–10 and 30–50 cm layers were significantly higher under lightly degraded grasslands than those under moderately and seriously degraded grasslands. Similarly, the soil PhytOC contents in the soil layers varied significantly (p < 0.05) among grasslands of different degradation degrees, from 0.008 ± 0.001 to 0.045 ± 0.004 g kg− 1 (Fig. 2b), with the highest value in the topsoil (0–10 cm) of the lightly degraded grasslands. The storage of soil phytoliths was highest in the lightly degraded grasslands, medium in the moderately degraded grasslands and lowest in the non-degraded and seriously degraded grasslands (15.90 ± 0.65, 14.35 ± 0.79, 12.97 ± 2.15 and 13.22 ± 1.07 t ha− 1, respectively) (Fig. 3a). Similarly, storage of soil PhytOC decreased in the order: lightly degraded grasslands > moderately degraded grasslands ≈ non-degraded grasslands > seriously degraded grasslands (0.16 ± 0.02, 0.12 ± 0.01, 0.11 ± 0.02 and 0.07 ± 0.01 t ha− 1, respectively) (Fig. 3b).
significantly (p < 0.05) lower in the non-degraded grasslands than in the three other grassland types for the 0–10 cm layer (Table 2). There were no significant differences in the soil pH between the lightly, moderately and seriously degraded grasslands for the top (0–10 cm) and bottom layers (30–50 cm) (Table 2). The soil SOC contents of the 0–10 and 10–30 cm layers were significantly (p < 0.05) lower for the seriously degraded grasslands than for the non-degraded, lightly and moderately degraded grasslands (Table 2). The SOC content in the topsoil of the moderately degraded grasslands was significantly (p < 0.05) higher than the SOC contents of the other grasslands in the same depth (Table 2). The total soil SiO2 content in different degraded grasslands ranged from 708.4 to 783.3 g kg− 1, and the average total soil SiO2 contents across the three soil layers of the non-degraded, lightly, moderately and seriously degraded grasslands were 760.0, 760.9, 764.9 and 741.1 g kg− 1, respectively (Table 2). The soil bioavailable Si content was significantly (p < 0.05) lower in the top layer (0–10 cm) for non-degraded grasslands than for the lightly and moderately degraded grasslands, and soil bioavailable Si contents increased with depth (Table 2).
4. Discussion 3.2. Contents and storages of soil phytoliths and PhytOC in grasslands of different degradation degree
4.1. Factors controlling accumulation of soil phytoliths and PhytOC in degraded grasslands
The phytolith content of each soil depth varied significantly for grasslands of different degradation degrees (from 1.52 ± 0.13 to
The aboveground phytolith production fluxes of non-degraded, lightly degraded, moderately degraded, and seriously degraded
Table 1 Vegetation coverage and species richness for grasslands of different degradation degrees and soil texture of the associated soils. Degradation degreea
NDG LDG MDG SDG a b c
Vegetation coverage (%)b
75 65 55 40
Species richnessb
33 30 31 21
Soil particle size distribution (%)c Coarse sand (> 0.1 mm)
Fine sand (0.1–0.05 mm)
Silt and clay (< 0.05 mm)
42–81 88–90 88–93 91–94
9–51 4–33 2–28 2–25
4–9 2–6 2–4 1–4
NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively. Data based on field investigations. Data taken from grassland soils investigated by Li et al. (2005), Jin et al. (2013) and Xiong et al. (2017), which are located close to our study areas.
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Table 2 Soil physico-chemical characteristics under grasslands of different degradation degrees (means ± standard deviations). Degradation degree
Depth (cm)
Bulk density (g cm− 3)
pH
NDG
0–10 10–30 30–50 0–10 10–30 30–50 0–10 10–30 30–50 0–10 10–30 30–50
1.41 1.46 1.52 1.49 1.51 1.54 1.39 1.49 1.54 1.51 1.54 1.55
6.33 6.48 6.68 6.74 6.60 6.73 6.63 6.87 7.23 6.76 7.03 6.56
LDG
MDG
SDG
± ± ± ± ± ± ± ± ± ± ± ±
0.09bA 0.03bA 0.03aA 0.02abB 0.03abAB 0.01aA 0.02bB 0.05abA 0.01aA 0.02aB 0.01aA 0.02aA
SiO2 (g kg− 1) ± ± ± ± ± ± ± ± ± ± ± ±
0.26bB 0.18bA 0.18bA 0.11aB 0.10bC 0.32aA 0.09aB 0.44abA 0.29abA 0.11aC 0.31aA 0.19abB
745.6 771.4 762.9 736.1 763.4 783.3 745.9 771.3 777.4 708.4 763.8 751.0
± ± ± ± ± ± ± ± ± ± ± ±
SOC (g kg− 1) 1.3abA 1.0aA 3.2aA 3.0aA 1.4aA 2.8aA 3.2abA 1.2aA 4.5aA 5.1bB 1.4aA 2.7aA
8.82 6.16 3.20 4.54 3.57 2.12 9.59 4.45 2.25 3.52 1.95 1.77
± ± ± ± ± ± ± ± ± ± ± ±
Bioavailable Si (mg kg− 1) 4.93aA 1.74aA 1.44aA 0.96abA 1.28abAB 0.16aB 1.00aA 2.22abB 0.34aB 1.06bA 0.25bB 0.52aB
95.5 ± 16.3b 93.2 ± 13.8b 102.9 ± 19.4a 117.4 ± 9.1a 105.0 ± 10.6b 118.8 ± 1.1a 115.23 ± 5.6a 138.6 ± 8.6a 126.4 ± 25.9a 107.3 ± 2.4ab 97.67 ± 4.6b 118.6 ± 22.0a
Different lowercase letters indicate significant differences between grasslands of different degradation degrees at the same soil layer, while different uppercase letters show significant differences between the different soil depths at the same degradation degree, at p < 0.05 level based on Duncan's test. NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
Fig. 3. Storages of soil phytoliths (a) and PhytOC (b) in grasslands of different degradation degrees. Different lowercase letters indicate significant differences between grasslands of different degradation degrees (p < 0.05 level based on Duncan's test). Error bars represent the standard deviations of the means (n = 3). NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
Fig. 2. Contents of phytoliths (a) and PhytOC (b) in soils under grasslands of different degradation degrees. Different lowercase letters indicate significant differences between grasslands of different degradation degrees in the same depth, while different uppercase letters show significant differences between different soil depths for grasslands with the same degradation degree (p < 0.05 level based on Duncan's test). Error bars represent the standard deviations of the means (n = 3). NDG, LDG, MDG and SDG stand for nondegraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
heterogeneities of herbivore manure and harvesting might have resulted in lower phytolith return fluxes in non-degraded grasslands. However, storage of both soil phytolith and PhytOC decreased in the order of lightly degraded grasslands > moderately degraded grasslands > seriously degraded grasslands (Fig. 3). These trends were consistent with the observed phytolith and PhytOC production fluxes (Table 3).
grasslands in the same area have been studied by Ru (2015), indicating that the aboveground phytolith production flux of non-degraded grasslands was the highest among grasslands of different degradation degrees (Table 3). However, our study showed that the storage of soil phytoliths in non-degraded grasslands was lowest (Fig. 3a). Spatial 89
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Table 3 Contents and production fluxes of phytoliths and PhytOC in aboveground plant parts (means ± standard deviations) grown in grasslands of different degradation degrees. Degradation degree
Phytolith content (g kg− 1)
PhytOC content (g kg− 1)
ANPP (kg ha− 1 a− 1)
Phytolith production flux (kg ha− 1 a− 1)
PhytOC production flux (kg ha− 1 a− 1)
NDG LDG MDG SDG
22.37 20.37 24.78 28.15
0.19 0.21 0.20 0.16
1202.67 720.92 596.00 660.70
26.91 14.68 14.77 18.60
0.86 0.56 0.43 0.38
± ± ± ±
0.60 1.26 1.29 2.62
± ± ± ±
0.03 0.05 0.05 0.03
± ± ± ±
0.73 0.91 0.77 1.73
± ± ± ±
0.12 0.14 0.11 0.08
Data taken from Ru (2015). NDG, LDG, MDG and SDG stand for non-degraded grasslands, lightly degraded grasslands, moderately degraded grasslands and seriously degraded grasslands, respectively.
degraded grasslands is likely to be higher than that of degraded grasslands. Blecker et al. (2006) reported that soil phytoliths were preserved better in soils of lower water content because soil phytolith dissolution increased with increasing soil water contents. Therefore, it can be assumed that more soil phytoliths are dissolved in non-degraded grasslands soils than in degraded grassland soils. The bioavailable Si content was significantly (p < 0.05) lower for the non-degraded grasslands than for the lightly, moderately and seriously degraded grasslands in the 0–10 cm layer (Table 2), with a similar trend for soil phytolith content (Fig. 2a). Since the phytolith production flux was the highest for the non-degraded grasslands among the four grassland types (Table 3), more bioavailable Si might be absorbed from these soils. Generally, grassland degradation affects underground biomass production of grasslands, for example, grass root biomass significantly decreased with increased degradation degree (Jin et al., 2013). Consequently, uptake of bioavailable Si might be influenced by reduced plant root biomass, and more bioavailable Si might be retained in degraded grasslands than in non-degraded grasslands. In general, the soil total SiO2 content is mainly comprised of the SiO2 fraction in silicate minerals (Liu et al., 2001). Cornelis and Delvaux (2016) reviewed the dissolution of soil silicate minerals, and concluded that it could govern the concentration of soil bioavailable Si. However, our results showed that the total soil SiO2 contents did not differ significantly between grasslands of different degradation degrees at the same soil depth, except for the 0–10 cm soil depth in seriously degraded grasslands (Table 2). Therefore, the variation of bioavailable Si in different degraded grasslands might be mainly caused by root uptake and dissolution of soil phytoliths. The mechanisms controlling phytolith dissolution are multiple and complex. As different non-crystalline Si forms can be mutually transformed (Liu et al., 2001; Borrelli et al., 2008; Song et al., 2016), the decrease of bioavailable Si may accelerate the transformation of soil phytoliths (one of the non-crystalline Si) to bioavailable Si under certain soil conditions (Liu et al., 2001; Zhang et al., 2017). Differences in soil properties (e.g., SOC and pH) can influence soil phytolith dissolution (Hart and Humphreys, 2003; Fraysse et al., 2006, 2009). Previous studies have demonstrated that the dissolution rate of phytoliths significantly increased from acid solution (pH = 3) to alkaline solution (pH = 10) (Fraysse et al., 2006, 2009). At short-term scale, the complexation of phytoliths with SOC can protect phytoliths from dissolution, because phytoliths are generally surrounded by extraneous organic materials (Li et al., 2013; Parr and Sullivan, 2014). However, at long-term scale, the prosperous root systems in non-degraded grassland soils excrete large amounts of rhizospheric organic acids (Brolsma et al., 2017), which may indirectly contribute to the dissolution of phytoliths by enhancing the decomposition of phytolith extraneous organic materials (Zhu et al., 2014). Furthermore, the dissolution of soil phytoliths is also depended on the plant origin, morphotypes, and ion (e.g., Al3 + and Fe3 +) concentration of the phytoliths in the soils (Bartoli and Wilding, 1980; Fraysse et al., 2009; Cabanes et al., 2011; Li et al., 2014; Song et al., 2016), which were not considered in this study. Therefore, the physico-chemical properties of phytoliths in different degraded grasslands should be further studied. In our study area, Xilingol League, the total number of livestock
In general, phytoliths formed in plant tissues are released into soil through decomposition of plant materials. The phytolith production flux of aboveground plants is the original source for soil phytoliths when little or no human disturbance occurs (Hodson et al., 2005; Song et al., 2012a). In our study, grassland degradation was mainly caused by overgrazing (more details see below). Although the observed variations of phytolith and PhytOC contents were small in the aboveground parts of the vegetation, the phytolith and PhytOC production fluxes and ANPP all tended to decrease from non-degraded to seriously degraded grasslands (Table 3). This indicates that the phytolith and PhytOC returning fluxes from plants to soils decreased with increasing degree of grassland degradation. For grassland ecosystems, phytolith and PhytOC return fluxes are assumed to be far lower than their production fluxes because of human disturbances such as grazing and harvesting (Wang, 1998). Generally, the return of manure from herbivores compensates some of the phytolith loss in biomass (Vandevenne et al., 2013). Thus, spatial heterogeneities of herbivore manure and harvesting may play a crucial role in returning phytoliths and PhytOC to soils. However, the storages of soil phytolith and PhytOC depend not only on phytolith input flux from plants, but also on the phytolith output fluxes due to harvesting loss, translocation, dissolution and erosion of phytoliths (Meunier et al., 1999; Blecker et al., 2006; Fishkis et al., 2009; Song et al., 2016). The main pathways of phytolith output fluxes are wind and water erosion (Zuo et al., 2014). Reduced vegetation cover has been found to be positively correlated with grassland degradation (Li et al., 2005; Jin et al., 2013; Xiong et al., 2017) and can enhance wind and water erosion. Wind erosion affects the size distribution of soil particles in the topsoil because small soil particles are easily transported by wind (J. Zhang et al., 2016). The size of phytoliths is mostly within 5–200 μm, which is close to the size of silt and clay (Ge et al., 2010; Table 1). Latorre et al. (2012) found that phytoliths were present in the air, highlighting the possibility of phytolith loss by wind. Therefore, phytoliths originating in degraded grassland soils may also be transported by wind. Heavy precipitation can cause water erosion in particular if the vegetation cover is decreased, and so phytoliths may also be lost via water erosion from degraded grasslands. Zuo et al. (2014) reported about 2.6 × 106 t of phytolith loss due to soil erosion in the Loess Plateau in China. In our study, the flux of soil phytolith loss from degraded grasslands was not estimated. But we recommend that the loss of soil phytoliths should be considered when investigating the impact of grassland degradation on soil phytolith and PhytOC storages. Soil permeability and structure of degraded grasslands may also affect soil water dynamics (Jin et al., 2013). Previous studies have reported that the content of clay particles decreased while the content of sand particles increased gradually with increasing grassland degradation, and consequently, the content of sand particles in non-degraded grasslands was significantly lower than in degraded grassland (Table 1). A coarser soil texture leads to higher hydraulic conductivities. In accordance with this, Jin et al. (2013) found significantly higher soil water contents in undisturbed grasslands compared to desertified grasslands in Hunshandake Sandy Land of Inner Mongolia, which is close to our study area. Therefore, the soil water content of non90
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increased from 0.16 × 107 in 1949 to 1.63 × 107 (including 1.45 × 107 sheep) in 2016; the grazing density increased significantly from 0.09 livestock ha− 1 to 1.10 livestock ha− 1, resulting in extensive grassland degradation (http://www.xlgl.gov.cn). Additionally, the current area harvested in this region reached up to 0.26 × 106 ha; and the harvests of grass or forage (mainly comprising of Leymus chinensis, Stipa baicalensis, Cleistogenes squarosa) were > 2.12 t ha− 1 year− 1 (http://www.nmagri.gov.cn). To meet the needs of economic development and reduce the destruction of degraded grasslands, rest-grazing or rotational-grazing was applied in most of lightly and moderately degraded grasslands, and grazing was forbidden in seriously degraded grasslands by the local government. However, extensive open-grazing and free harvesting usually occurred on non-degraded grasslands, which would also contribute to the lower storages of phytoliths and PhytOC in non-degraded grasslands compared with those in lightly degraded grasslands (Fig. 3) by decreasing the return flux of phytolihs and PhytOC. As the differences in physico-chemical soil characteristics, such as pH, bulk density and SiO2 content, observed between non-degraded grasslands and lightly, moderately and seriously degraded grasslands were mainly non-significant (Table 2), we conclude that, in our study, the observed changes in soil phytolith and PhytOC storage were mainly driven by human disturbances, especially grazing and harvesting.
accumulation of manure on the soil surface (He et al., 2009). Applying additional organic manure amendments can provide a slow release source of nutrients for the vegetation of degraded grasslands to recover while grazing and harvesting is needed to be restricted or completely forbidden (He et al., 2009; Xiong et al., 2016). Therefore, management practices to recover degraded grasslands such as organic manure amendment, N fertilization, and reducing grazing and harvesting will be helpful to increase the storages of soil phytoliths and PhytOC. The litterfall returning flux is nearly equal to the production flux of aboveground plants in natural grasslands (Song et al., 2012a). Assuming that the aboveground biomass of the degraded grasslands could be restored to non-degraded grasslands, the soil PhytOC accumulation would also be improved correspondingly. For example, the restoration of degraded grasslands has the potential to reduce wind and water erosion, reducing the transport of soil phytoliths. Although the soil PhytOC stock is probably quite small compared with the soil SOC stock of the grassland ecosystems (Yang et al., 2010), PhytOC can be conserved in grassland soils for millennia and plays an important role in the long-term terrestrial biogeochemical C sink due to physical protection by phytoliths (Parr and Sullivan, 2005; Song et al., 2016). Therefore, grassland restoration might play an important role in reducing the global atmospheric carbon dioxide concentrations at a centennial–millennial scale through increasing soil phytolith and PhytOC storages in degraded grasslands. Further research is needed to improve our understanding of the mechanisms of increasing phytolith and PhytOC storages in degraded grasslands.
4.2. Implications for grassland management of long-term phytolith C sequestration There are > 74.4 × 106 ha of degraded grasslands in northern China (Li et al., 2011). Grassland degradation will lead to the loss of soil C and nutrients, decreased aboveground and belowground biomass production, increased soil erosion and eventually reduced livestock productivity (Zhou et al., 2008; Jiang et al., 2009). In our study area, the plant PhytOC production fluxes of the lightly, moderately and seriously degraded grasslands were 35, 50 and 56% lower compared to those in the non-degraded grasslands (Table 3). The corresponding litterfall phytolith returning fluxes and soil PhytOC storages are assumed to be lower, too. Assuming that all degraded grasslands could be restored to non-degraded status, the potential production fluxes of phytoliths and PhytOC should at least be 26.91 ± 0.73 kg ha− 1 a− 1 and 0.86 ± 0.12 kg ha− 1 a− 1, respectively (Table 3). Thus, approximate 2.00 × 106 t a− 1 of phytoliths and 0.06 × 106 t a− 1 of PhytOC could be sequestrated by the restored grasslands in northern China, respectively. In recent years, the PhytOC sequestration rate of forests (143 × 106 ha, Song et al., 2013b), wetlands (13.7 × 106 ha, Li et al., 2013), and croplands (160 × 106 ha, Song et al., 2014) have been reported to be 0.15 × 106 t a− 1, 0.46 × 106 t a− 1, and 1.20 × 106 t a− 1 in China. Compared with wetlands, the lower PhytOC sequestration rate of the restored grasslands was mainly due to the lower PhytOC production flux. However, the larger distribution areas of forests and croplands should be the main reasons that the PhytOC sequestration rate of the restored grasslands was conspicuously lower compared to those ecosystems. Notwithstanding, the potential of phytolith C sequestration in degraded grasslands would be huge because of the extensive areas. Therefore, grassland protection and restoration are important and necessary for grassland managers (Jiang et al., 2009), which can not only reverse grassland degradation processes (J. Zhang et al., 2016), but also increase the phytolith production flux controlled by increasing ANPP (Song et al., 2012a). Recent studies have indicated that nitrogen (N) application can not only significantly increase aboveground biomass and diversity (Xu et al., 2015), but also significantly enhance the aboveground PhytOC production flux (Zhao et al., 2016). Moreover, livestock manures are important nutrient reservoirs in grassland ecosystems, and contain C, N, Si, phosphorus and other nutrients (Schröder et al., 2007; Vandevenne et al., 2013; Sileshi et al., 2017). Generally, low ambient temperature and rainfall, characteristic of the semi-arid climate in China, will lead to
5. Conclusions Human disturbances, especially grazing and harvesting, were identified as major factors for soil phytolith and PhytOC storages in degraded grasslands of China. Grassland degradation decreased plant PhytOC production fluxes by 35, 50 and 56% in lightly, moderately and seriously degraded grasslands, respectively, and corresponding soil PhytOC storages decreased, too. Our study indicates that grassland restoration has the potential to increase long-term phytolith C sequestration. However, it has to be noted that in this study, grassland degradation was classified into four gradients based only on the status of the aboveground plants. Other indicators such as soil nutrients and soil texture should be equally considered in future studies. Results and conclusions of this local study may not be applicable to other regions. Further research on degraded grassland should be carried out to better understand the impact of degradation on soil phytolith and PhytOC accumulation at larger scales, including national and global scales, and should be conducted with a more complex and generalized classification system for degraded grasslands. Acknowledgements We thank Yutong Li, Ning Ru and Zhuo Ji for their assistance with sampling and analysis. The research was supported by National Natural Science Foundation of China (41522207, 41571130042) and State's Key Project of Research and Development Plan of China (2016YFA0601002). The authors declare no conflict of interest. References Bartoli, F., Wilding, L.P., 1980. Dissolution of biogenic opal as a function of its physical and chemical properties. Soil Sci. Soc. Am. J. 44, 873–878. Blecker, S.W., Mcculley, R.L., Chadwick, O.A., Kelly, E.F., 2006. Biologic cycling of silica across a grassland bioclimosequence. Glob. Biogeochem. Cycles 20, 1–11. Borrelli, N., Osterrieth, M., Marcovecchio, J.E., 2008. Interrelations of vegetal cover, silicophytolith content and pedogenesis of Typical Argiudolls of the Pampean Plain, Argentina. Catena 75, 146–153. Brolsma, K.M., Vonk, J.A., Mommer, L., Van Ruijven, J., Hoffland, E., De Goede, R.G., 2017. Microbial catabolic diversity in and beyond the rhizosphere of plant species and plant genotypes. Pedobiologia 61, 43–49. Cabanes, D., Weiner, S., Shahack-Gross, R., 2011. Stability of phytoliths in the
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