Vegetation and soil covariation, not grazing exclusion, control soil organic carbon and nitrogen in density fractions of alpine meadows in a Tibetan permafrost region
Introduction
Permafrost ecosystem, which mainly occurs at high-latitude and high-altitude regions, stores large amounts of soil organic carbon (SOC) and nitrogen (N) and is highly sensitive to climate change (Ding et al., 2016, Hugelius et al., 2014, Tarnocai et al., 2009). The changes in SOC and N in a permafrost ecosystem pique interest because of their potential effects on global climate change and consequently on the functioning of the ecosystem (Natali et al., 2014, Schadel et al., 2016, Schuur et al., 2008, Schuur et al., 2015). Climate change will have potential impacts on the development of vegetation and the decomposition of organic matter in the permafrost ecosystem, thereby affecting SOC and N stocks (Schuur et al., 2015, Yang et al., 2010). Although experimental studies have explored the storage and drivers of SOC and N in the permafrost region (Mu et al., 2015, Jiang et al., 2018, Siewert et al., 2016, Wu et al., 2016, Wu et al., 2017, Nie et al., 2019, Zhao et al., 2018b), the underlying driving factors for SOC and N fractions remain poorly understood (Eze et al., 2018, Gentsch et al., 2015, Gentsch et al., 2018, Mueller et al., 2015, Shang et al., 2016, Wu et al., 2018, Zhong et al., 2019).
Soil organic matter is generally composed of several functional fractions with different physical or chemical protective mechanisms (Sollins et al., 2006, Swanston et al., 2002, Whalen et al., 2000). On the basis of decomposition degrees and turnover rates, soil organic matter can be divided into labile (e.g. light fractions) and nonlabile or slow-decomposing fractions (e.g. heavy fractions) (Swanston et al., 2002, Whalen et al., 2000). Labile fractions have faster turnover rates to environmental change compared with the total soil organic matter (Mueller et al., 2015, Swanston et al., 2002, Whalen et al., 2000). Meanwhile, nonlabile fractions are the main contributors to total soil organic matter due to their high absolute storage (e.g. Mueller et al., 2015, Tan et al., 2007). Nevertheless, labile and nonlabile fractions are potential indicators of responses to environmental changes, such as climate and land use (Gentsch et al., 2015, Song et al., 2014, Tan et al., 2007). Therefore, further understanding of their responses to environmental change is needed (Eze et al., 2018, Shang et al., 2016, Tan et al., 2007). However, such understanding has been mostly confined to agricultural ecosystems, and that of permafrost ecosystems, especially in the Tibetan Plateau, remains less explored (Dorfer et al., 2013, Liu et al., 2018, Liu et al., 2019, Shang et al., 2016, Wu et al., 2018, Zhong et al., 2019).
As the ‘roof of the world’ and ‘water tower of China’, the Tibetan Plateau includes a large area of highly sensitive permafrost (Cheng and Wu, 2007, Wu et al., 2015). This permafrost ecosystem stores an average of 15 Pg of C at the 0–3 m soil layer (Ding et al., 2016). Alpine meadow, which occupies approximately 40% of the Tibetan Plateau, is one of the most important grassland types for herdsmen and wildlife in this region (Hopping et al., 2018, Liu et al., 2018, Yang et al., 2018). Compared with an alpine steppe, an alpine meadow has higher vegetation productivity and more soil carbon and N storage (Ding et al., 2016, Zhao et al., 2018b, Mu et al., 2015). However, large areas of alpine meadows have been degraded due to the potential influence of natural factors (such as climate change) and human activities (Babel et al., 2014, Hopping et al., 2016, Hopping et al., 2018). Meadow degradation inevitably has far-reaching effects on vegetation and soil carbon and N storage (Dong et al., 2012, Peng et al., 2018, Liu et al., 2018, Zhao et al., 2019). Therefore, studies should explore and quantify the site characteristics and driving factors of SOC and N stocks in alpine meadows under the contexts of meadow degradation and climate change (Guan et al., 2018, Peng et al., 2018, Wu et al., 2016, Wu et al., 2017).
Biotic and abiotic factors control SOC and N stocks through their effects on organic matter input and mineralisation (Stockmann et al., 2013, Wiesmeier et al., 2019). At the regional scale, factors, such as climate, vegetation, topography and soil texture, shape the development of SOC and N (Doetterl et al., 2015, Wiesmeier et al., 2019). At the local scale, vegetation characteristics (such as biomass, cover and species composition) and soil properties (such as soil moisture, temperature, texture and nutrient) affect SOC and N stocks significantly (O'Brien et al., 2010, Wiesmeier et al., 2019). The SOC and N and their fractions vary greatly under different grassland types in the alpine grasslands of the Tibetan Plateau (Chen et al., 2019, Shang et al., 2016, Wu et al., 2016). However, whether site-level SOC and N stocks depend on vegetation characteristics or soil properties remains unclear (Qin et al., 2018, Shen et al., 2018, Tian et al., 2018). In addition, grazing exclusion (GE) is often adopted to restore degraded alpine grasslands on the Tibetan Plateau (Bi et al., 2018, Hu et al., 2016). However, results regarding the effects of GE on SOC and N stocks are contradictory (Bi et al., 2018, Zhao et al., 2018a, Lu et al., 2017, Xiong et al., 2016). Several studies have suggested that GE increases SOC and N stocks in alpine grasslands (Hu et al., 2016, Ma et al., 2016, Wu et al., 2010), whereas others have reported zero or negative effects (Bi et al., 2018, Shang et al., 2014, Shi et al., 2013). Therefore, the relative roles of these factors in influencing SOC and N fractions in alpine meadows due to the complex interaction between biotic and abiotic factors must be quantified.
In this study, three long-term (10 years) fenced alpine meadows with different vegetation covers were used to investigate the relative effects of abiotic and biotic factors on SOC and N fractions in the permafrost region of the Tibetan Plateau. The objectives were (1) to evaluate the effects of long-term GE on SOC and N fractions, and (2) determine the relative influence of vegetation traits, soil properties and GE on SOC and N fractions. We assume that the effect of GE (10 years) on SOC and N contents in this high-altitude permafrost region may be weak due to the low annual temperature, and the effect could be modified by abiotic and biotic factors.
Section snippets
Study area
This study was conducted at the Kaixinling site in the continuous permafrost region of the northern Tibetan Plateau, Qinghai Province, China (Latitude 33.96° N, Longitude 92.35° E and 4627 m a.s.l.; Fig. 1). The site has a mean annual temperature of –3.8 °C. The mean annual precipitation is 350 mm and mainly occurs from May to September. The active layer thickness approximates 2.4–3.4 m and shows an evident increasing trend (Wu et al., 2015). The soils are classified as Cambosol (Chinese Soil
Effects of site and GE on vegetation and soil variables
GE had significant effects on aboveground biomass and cover, but none on the diversity index (Table 1). Site had significant effects on aboveground biomass, cover, and diversity index (Table 1). The interaction of site and GE had no significant effect on the vegetation variables (Table 1). Site had significant effects on soil physicochemical properties, whereas GE had none (Table 2). Soil sampling depth had significant effects on soil physicochemical properties, except for pH and soil moisture
Discussion
GE is considered as an effective management measure for degraded grassland in many areas because it can reduce disturbance and increase vegetation and soil carbon and N contents (Abdalla et al., 2018, Hu et al., 2016). However, whether GE can increase SOC and N contents in alpine meadows in this high-altitude permafrost ecosystem, which has low average annual temperature and short growing season, remains unclear. Results from the present study suggest that GE for 10 years significantly affected
Conclusions
The relative importance of abiotic and biotic factors in driving the SOC and N fractions of alpine meadows was investigated in the permafrost region of the Tibetan Plateau. Site had significant effects on the SOC and N contents, whereas GE had none. A strong interaction existed between soil and vegetation variables in their influence on SOC and N contents. SOC and N contents significantly increased with aboveground biomass, cover and soil moisture content and significantly decreased with soil
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the Natural Science Foundation of China (41701066, U1703244 and 41601073) and the Fund of State Key Laboratory of Frozen Soil Engineering (52Y552J71 and 52Y652J31).
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