Introduction
Grassland ecosystems cover a large proportion of the arid and semi-arid
regions of the world, playing an important role in the global carbon
cycle (Scurlock and Hall 1998; Schuman et al. 2002; Zhou et
al. 2019; Lei et al. 2020).
The degree to which terrestrial
ecosystems serve as net carbon sinks or sources depends on the balance
between the carbon fixed by plant photosynthesis and the carbon released
into the atmosphere by plant and soil respiration (Peng et al .
2014; Li et al. 2017b; Jin et al. 2023). While numerous
evidence has addressed the degree to which forested systems serve as net
carbon sinks or sources (Martens et al. 2004; Rebane et
al. 2020), the status of grassland ecosystems as sources or sinks can
be highly variable (Dai et al. 2014; Smith 2014; Chang et
al. 2021). Grasslands can be an important carbon sink in some places or
times (Hafner et al. 2012; Sha et al. 2020), but in other
places and times, grassland carbon fluxes can be in equilibrium (neither
a source or sink) (Hao et al. 2017), a net carbon source
(Kuzyakov and Gavrichkova 2010), or fluctuate between states (Daiet al. 2014; Zhang et al. 2020).
Livestock grazing is a major land-use category by which human activities
can influence the structure and function of grassland ecosystems,
profoundly altering the carbon cycle and stability of grassland
productivity (Zhang et al. 2023). Grazing directly affects plant
productivity and respiration because livestock foraging removes leaves
while promoting compensatory growth, and their trampling and excretion
redistributes soil organic matter and alters soil respiration (Caoet al. 2004; Chen et al. 2015; Barthelemy et al.2018; Veldhuis et al. 2018); Grazing also alters soil nitrogen
content and other processes important to the carbon cycle, such as
litter decomposition and photosynthate distribution (Xia and Wan 2008).
As a result, grazing can moderate the net ecosystem exchange of
grasslands and whether they serve as a net carbon sink or source. In
some cases, light to moderate levels of grazing can facilitate
grasslands being net carbon sinks (Derner et al. 2006; Shaet al. 2020; Chang et al. 2021), while high levels of
grazing can accelerate the release of carbon and switch the ecosystem
from a carbon sink to a carbon source (Liang et al. 2017; Tanget al. 2018); In other cases, grazing appears to have little
influence on the carbon budget of grassland ecosystems (Fang et
al. 2010; PiƱeiro et al. 2010).
Desert steppe is particularly vulnerable to degradation due to livestock
grazing and to carbon sinks transitioning to carbon sources (Zhanget al. 2020). We assessed ecosystem carbon balances over a
sustained 10-year period and explored the influencing factors. We
concluded that precipitation patterns and grazing combine to cause
changes in the carbon sink function of grasslands (Jin et al.2023; Wang et al. 2023), but that ecosystem carbon exchange is
disturbed by a combination of environmental (soil, climate) and
biological (grazing) factors. How environmental and biological factors
influence net ecosystem carbon exchange depends on the relationship
between carbon uptake via primary productivity and carbon release via
plant and soil respiration. Furthermore, there is considerable
variability and uncertainty regarding the factors influencing carbon
exchange in grassland ecosystems (Liu et al. 2015; Sha et
al. 2020). This is likely because the variability in grassland types is
mediated by climate, vegetation and soil (Helfter et al. 2015;
Hussain et al. 2015; Liang et al. 2020), as well as by
grazing practices (Fang et al. 2010; Dai et al. 2014).
Thus, simply measuring net ecosystem exchange and aboveground biomass is
not enough to fully understand the influence of biotic and abiotic
factors on these rates (Li et al. 2017a; Bajgain et al.2018). It is necessary to more fully identify how carbon exchange and
soil respiration are influenced by grazing and background environmental
factors, in particular. This information will not only help us better
understand the factors influencing the carbon dynamics of these
important ecosystems, but it will also help inform the formulation of
policies for the sustainable management and conservation of grassland
resources. In this study, we measured ecosystem carbon fluxes and their
associations in respond to a long-term (16-year) grazer manipulation
experiment in a desert steppe grassland in Inner Mongolia, China. We
specifically asked (1) how does grazing influence features of the plant
community and soil conditions and (2) how those effects influence the
parameters of net ecosystem carbon exchange, including gross ecosystem
productivity and respiration. On the basis of our previous research, we
further measured aboveground and belowground biomass, plant nutrients
(carbon and nitrogen content of plant communities) and soil nutrient
indexes to analyze the main drivers that influence the changes of
CO2 fluxes in desert steppe ecosystems and their
responses to grazing disturbances, and to provide theoretical basis for
the adaptive management of desert steppe.