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.