1 INTRODUCTION
Grassland is one the most widespread types of vegetation worldwide, covering one-fifths of the earth’s land surface and among the world’s most widespread biomes (Parton et al. 1995, Scurlock and Hall 1998, He et al. 2009, Corona et al. 2016). Grasslands in China occupy vast contiguous areas of the country’s northern temperate regions with continental arid climates, the cold and arid climate of the Tibetan Plateau at very high elevations, and small, scattered patches with a warm climate (Hou 1982, Ni 2002). These vast and widely distributed grasslands are responsible for many important ecosystem functions and offer such services as livestock production, biodiversity maintenance, and soil and water conservation in addition to carbon (C) sequestration and mitigation of the adverse effects of climate change. The grasslands store about 10% of the organic C (Scurlock et al. 2002) and 34% of the total C stored in global terrestrial ecosystems and are also the largest source of uncertainty in estimating the quantities of terrestrial C from biomass accumulation (Cheng et al. 2011), It has been estimated that C storage in Chinese grasslands accounts for 8% of C storage in global grasslands and 16.7% of C storage in Chinese terrestrial ecosystems (Jian 2001, Ni 2002, Ma et al. 2016). Hence, C storage by grassland ecosystems is valuable in maintaining a stable climate. However, global climate change and extensive socio-economic development have rapidly degraded grasslands, and the desertification of grasslands in particular has had significant impacts on soil degradation and the global C cycle (Zhou et al. 2008). The loss of even relatively small patches of grassland through either climate change or human activity represents a significant loss of terrestrial C stocks, which, in turn can affect plant diversity and ecosystem functions. Lal (2001) estimated that global desertification led to a total loss of 19–29 Pg of C from the plant–soil continuum, and Zhou et al. (2008) estimated that the total losses from the plant–soil system of the Horqin sandy lands due to desertification in the last century amounted to 107.53 Mt, as compared to the total C present in the grasslands before. Restoring these degraded ecosystems through appropriate land use could increase the uptake and storage of C in the ecosystems. Therefore, it is important to the sustainability of regional ecosystems to study the effects of restoration strategies on C storage in grassland ecosystems as part of the efforts to mitigate the adverse effects of climate change.
The sandy grassland in Horqin, which is part of the semi-arid agro-pastoral transition zone between the Inner Mongolian Plateau and the North-East China Plain, is considered one of the four great sandy lands. Historically, the Horqin sandy land was a grassland with many lakes and lush vegetation dominated by palatable grass species, along with sparsely scattered woody species. However, almost 80% of this region has suffered desertification since the 1950s (Li et al. 2013, Li et al. 2017). Desertification is driven by many factors including overexploitation by continued cultivation, over-grazing, unsustainable collection of fuelwood, irresponsible use of water resources, and extreme climatic conditions (Zhang et al. 2004, Li et al. 2013). To mitigate the degradation of the Horqin sandy grasslands, various restoration measures have been implemented in the area, including the establishment of grazing enclosures to protect the surviving vegetation from livestock and afforestation using indigenous and introduced tree and shrub species (Yan et al. 2011). The efforts to restore degraded and overexploited grasslands, which aim to recover biodiversity and ecosystem functions services and to strike a balance between ecological protection and socio-economic development, also increase C sequestration in soil and improve soil structure. Most of the studies so far related to the Horqin sandy lands have focused on restoration of plant diversity, benefits in the form of soil and water conservation, productivity, soil fertility, and climatic conditions (Su and Zhao 2003, Shirato et al. 2004, Su et al. 2004, Zhang et al. 2005, Zhao et al. 2007, Zuo et al. 2009, Hu et al. 2015). However, the effects of afforestation on ecosystem C storage in this area remain poorly understood, and only a few studies have examined the impacts of different approaches to grassland restoration on C storage and the C cycle in south-eastern Horqin sandy lands in northern China; even fewer have attempted to quantify C pools of the total ecosystem—a knowledge gap the present study seeks to bridge.
Accordingly, we explored in quantitative terms the impacts of three typical restoration protocols (and a control) on C storage in the ecosystem. More specifically, the main objective of the present study was to estimate the amounts of C stored in different components of the ecosystem, namely trees, herbs, standing litter, and soil (to a depth of 100 cm) and thus to arrive at the total C. Such estimations will provide suitable benchmarks in devising appropriate strategies to restore vegetation and to mitigate the adverse effects of climate change in the south-eastern Horqin sandy lands in northern China.