## 4 Discussion
### Coresponse of Canopy COS and CO\({}_{2}\) Fluxes to Variations in Environmental Conditions  
At timescales under diurnal cycling, \(\rm F_{C,COS}\) and \(\rm F_{C,CO_{2}}\) both decreased in FA vs. DD and D vs. N under the changes in environmental conditions. However, \(\rm F_{C,COS}\) increased in the MXF, \(\rm CPL_{C3}\), and \(\rm CPL_{C4}\) biomes, which was also found in most studies of NLF. In addition, in the N vs. ME group, \(\rm F_{C,CO_{2}}\) in MXF and MSH increased in the FA vs. DD and D vs. N groups, and \(\rm F_{C,CO_{2}}\) decreased in NLF, BLF, GRA, and \(\rm CPL_{C3}\). In this study, when the light intensity was reduced by 300-700 umol \(\rm m^{-2}\) s\({}^{-1}\), \(\rm F_{C,COS}\) was composed of continuing assimilation by the leaves. For instance, stomata open in the dark and cause COS assimilation [135], which makes up 17% of daily \(\rm F_{C,COS}\)[118]. Under a minor increase in light intensity, \(\rm F_{C,COS}\) significantly decreased. However, \(\rm F_{C,CO_{2}}\) increased under cooler conditions. In croplands and mid-latitude forests, \(\rm F_{C,COS}\) also decreased under water shortage [65; 101; 136]. Water shortage, an increase in temperature, an increase in \(\rm C_{a,COS}\) and a decrease in \(\rm C_{a,CO_{2}}\) were all main factors resulting in a decrease in \(\rm F_{C,CO_{2}}\), as the tissue water content is important in canopy \(\rm CO_{2}\) assimilation [137]. However, an slightly increase of \(\rm C_{a,CO_{2}}\) value also indicated the lowest \(\rm F_{C,COS}\), as there was a hysteresis between \(\rm F_{C,COS}\) and \(\rm F_{C,CO_{2}}\) fluctuations within diurnal cycles [96]. Simultaneous increases in \(\rm F_{C,COS}\) and \(\rm F_{C,CO_{2}}\) were found only in MXF. In contrast, in NLF and \(\rm CPL_{C3}\), only \(\rm F_{C,CO_{2}}\) decreased. In the nonstomataal structures, the COS assimilation rate increased with water shortage [138; 139; 140]. For example, in some bryophytes, mosses, liverworts and lichens, as the hydrolysis process is catalyzed by the light-independent CA enzyme [37] and is affected by reverse hydrolysis, the net absorption rate of COS is independent of light conditions [85]. Bidirectional hydrolysis has also been observed in some fungi and bacteria [141; 142; 143]. This retrohydrolysis does not cause COS emissions and influence the \(\rm F_{C,COS}\) of the higher plants overall [51; 56; 144]. For higher plants, although there is a reverse hydrolysis process in the stomata opening process, resistance and osmotic pressure stopped COS from being emitted; the light-independent CA enzyme is still the key to the absorption rate of COS during photosynthesis [145; 146]. Enhanced frictional wind at sunrise caused COS emission from the canopy gap; furthermore, soil microorganism breakdown boosted by light seems to be the reason for the significantly increased \(\rm F_{C,COS}\). Recent studies also discussed whether \(\rm F_{C,COS}\) would be affected by the diversity of the enzyme CA between different species [147; 148]. When the COS concentration in the subcavities of the mesophyll (\(\rm C_{m,COS}\)) is close to zero, \(\rm F_{C,COS}\) is strongly affected by \(\rm C_{m,COS}\)[144; 149]; however, no correlation was found between the changes in \(\rm C_{a,COS}\) and \(\rm F_{C,COS}\). The LRU Variation Mediated by the Canopy COS and CO\({}_{2}\) Fluxes under the Regulation of Stomata  
Most of the gas exchange in leaflets is controlled by the stomata [73; 150]. In this study, at the canopy scale, no evidence was found regarding the correlation between the LRU and \(\rm F_{C,COS}\) or \(\rm F_{C,CO_{2}}\). In the diurnal cycling groups, increased \(\rm g_{s}\) and decreased \(\rm F_{C,COS}\) were found to occur concurrently, except in the case of N vs. ME. This difference in N vs. ME could have been due to the stomatal closure that occurs at midday [130]. However, in non-wetting vegetation, such as NLF, MXF, \(\rm CPL_{C3}\) and \(\rm CPL_{C4}\), moderate stomatal closure and increased enzyme CA activity caused an increase in the \(\rm F_{C,COS}\) of N vs. ME.