Effect of grazing intensity on net ecosystem CO2exchange
Grazing can shift the balance between vegetation being a carbon source
or sink in steppe grasslands (de la Motte et al. 2018; Ondieret al. 2021). Our finding that NEE, ER, and GEP decreased with
increasing grazing intensity is consistent with previous results from
desert steppe (Jin et al. 2023; Wang et al. 2023). Our
finding that heavy grazing had a stronger inhibitory effect on ER than
GEP is consistent with the results of Peng et al. (2007). This may have
resulted because livestock feeding reduces the aboveground biomass which
cannot be compensated by regrowth (Zhang et al. 2018; Zhanget al. 2023), such that the effective amount of leaf area
available for both photosynthesis and respiration is reduced so that the
net CO2 exchange rate decreases (Oba et al. 2000;
Shi et al. 2022).
Although our finding of the positive correlation between productivity
and NEE is consistent with many previous studies (Danielewska et
al. 2015; Xu et al. 2022), we also found a positive correlation
between aboveground biomass and NEE, primarily driven by shrubs and
semi-shrubs and perennial forbs, which is consistent with previous work
(Zhang et al. 2023). Our finding that shrubs and semi-shrubs and
perennial forbs were strongly influenced by grazing, while grasses were
less so is consistent with the idea that shrubs and semi-shrubs and
perennial forbs are more palatable and have higher nutritional value
than grasses, mainly stipa breviflora , which are not preferred by
livestock. Shrub roots can reach up to 70 cm deep into the soil layer,
allowing them to better utilize deeper water and nutrients to maintain a
high carbon fixation capacity and a high net carbon uptake capacity (Niuet al. 2023). which can explain why their loss dramatically
influenced NEE.
Our finding that plant N content is negatively correlated with net
ecosystem CO2 exchange is inconsistent with previous
findings that loss of leaf N attenuates ecosystem carbon cycling (Wang
et al. 2014 in Chinese)(Gong et al. 2021), This may be because
altered plants allocated more N to non-photosynthetic proteins to
increase their compensatory growth in response to grazing, but with
reduced photosynthetic capacity (Onoda et al. 2004), resulting in
a decrease in net CO2 exchange rate (Zhang et al.2006). A study by Wu et al. (2021) showed that N addition in desert
steppe increased the net CO2 exchange rate, while You et
al. (2016) showed that high levels of N addition inhibited NEE, but
moderate levels promoted NEE. This suggests that the changes of nitrogen
absorbed and used by plants are complex and require further
investigation (Schimel et al. 2001).
Although NEE decreased in response to increasing grazing intensity, it
is of interest that there was no significant difference in NEE rates
between the LG and MG treatments in our study (Fig. 4a, bars), because
although short-flowered needlegrass was a well-established species and
widely distributed in our experimental sample plots, livestock did not
prefer it, resulting in no significant difference in vegetation stock
and cover between the LG and MGC treatments and the non-grazed areas, so
their net CO2 exchange rates were not significantly
different from those of ck. The net CO2 exchange rate
was not significantly different from that of CK (p >
0.05, Fig. S1a).
Although NEE decreased in response to increasing grazing intensity, we
found no difference in NEE rates between the light and moderate grazing
treatments. This was likely because less preferred grasses dominated
both treatments.