| Most of volatile organic compounds (VOC) are emitted from terrestrial vegetation, which play an important role in shaping tropospheric chemistry, local or regional atmospheric quality and composition. VOC emitted from temperate grassland vegetation to some extent affect the total biogenic VOC emission in view of its distribution area or region, although its emission potential is relatively less than those of other vegetation types. Moreover, these VOC are mainly derived from the photosynthesis of plant communities, and emitted from vegetation as a loss of carbon fixed by photosynthesis, which might play an important role in balancing the local or regional carbon cycling to some extent, especially for some particular vegetation types or ecosystems. To address above issues, we have attempted to answer the following four questions associated with VOC emission. (1) How much is the VOC emission potential of temperate grassland vegetation at species level, and whether there exist any relationship between them and plant functional groups? (2) How much is the VOC emission potential of sandland vegetation at species level, and whether there is any relationship between them and plant functional groups? (3) How does VOC emission potential vary between grassland vegetation and sandland vegetation (same plant species), and why? (4) What are the isoprene and monoterpene emission rate of temperate typical grassland and degraded grassland? And how much their contribution is in relation to grassland ecosystem carbon cycles? We have randomly selected 175 plant species from temperate grassland vegetation and measured the emission rates of isoprene and monoterpene in situ. The results showed that most of plant species have low VOC emission potential at species level, especially for some dominant plant species in the temperate typical grassland, such as Leymus chinensis, Stipa grandis, and Agropyron cristatum. Most of genera and some families have consistent feature of their isoprene and monoterpene emission, especially for isoprene, which provides the basic premise of taxonomic methodology to develop VOC emission inventories for temperate grassland vegetation. Then, we have first introduced and founded the VOC emission inventories (277 species) for temperate grassland vegetation. Moreover, there are some relationship between the emission potential of isoprene and monoterpene with plant functional groups (plant life form groups and water ecological groups), especially for the plant life form groups. Totally, the dominant plant life form groups of temperate grassland, namely perennial rhizome grasses or perennial bunch grasses, have relatively low VOC emission potential. There is no significantly different for VOC emission potential among different water ecological groups, though intermediate xerads vegetation or intermediate mesophytes vegetation (dominant plant functional groups) have low emission potential. So the importance of plant species with relative higher VOC emission potential would be enhanced when temperate grassland was degraded or deteriorated due to long-time overgrazing. There are more plant species in sandland than in grassland in Xilin River Basin, Inner Mongolia, and the emission potential of isoprene and monoterpene also vary significantly among different species. Of course, the relationships, between functional groups and isoprene and monoterpene emission potential of sandland plant species, are obvious. It is significantly different among plant life form groups, but such relationship is relatively obscure to water ecological groups. To sum up, perennial rhizome grasses or perennial bunch grasses have relatively low isoprene and monoterpene emission potential, and trees have higher emission potential. Moreover, mesophyte vegetation of sandland has higher emission potential, and it is relatively lower for hygrophyte vegetation. Comparing the data of grassland vegetation and sandland vegetation (same plant species), the results showed that the emission potentials of sandland vegetation are significantly higher than those of grassland vegetation as a whole. Furthermore, such significant different also appear among different plant life form groups and different water ecological groups. Why there are such significant difference between grassland vegetation and sandland vegetation? It would be the longtime adaptation strategy of plant species to higher temperate at sandland habitat, for monoterpene can replace photorespiration in protection from photodamage at high temperatures. That is, higher isoprene and monoterpene emission potential would help them to reduce or depress the probability of heat stress or heat damage in face of this periodic high temperature. In this study, we also investigated the emission rates of isoprene and monoterpenes from temperate typical grassland (TG) and degraded grassland (DG) of Inner Mongolia, and assessed the contribution of VOC emission to carbon fluxes at ecosystem scale, through compared with the actual measurement of net ecosystem productivity (NEP) and net primary productivity (NPP). The results showed that, the standardized emission rates of isoprene (at 30℃ and 1000 μmol m-2 s-1) and monoterpene (at 30℃) are 0.50 μgC g-1 h-1 and 0.69 μgC g-1 h-1 for TG, respectively; And they are 0.32μg C g-1 h-1 and 1.59 μgC g-1 h-1 for DG, respectively. During entire growth period, total isoprene and monoterpenes emission are 31.6 mgC m-2 and 70.4 mgC m-2 for temperate typical grassland, respectively, and 20.8 mgC m-2 and 168.8 mgC m-2 for degraded grassland, respectively.Total VOC emission of degraded grassland is far higher than those of typical grassland, especially for monoterpenes emission. Therefore, grassland degradation due to longtime overgrazing, caused the change of species composition, would obviously influence and enhance the terpene emission of temperate grassland as a whole. Carbon loss as VOC emission in temperate grassland ecosystem is very small in relation to NPP, however, these VOC emission in August are accounted for 5.32 % and 0.23 % of net ecosyststem productivity in TG and in DG, respectively. So the amount of carbon lost as VOC emission is to some extent relative to net ecosystem productivity in temperate typical grassland ecosystem, while VOC emission is small in relation to NPP. And it was importance of vegetation composition change (or vegetation degradation) not only to the VOC emission capacity but also to its role for carbon fluxes of grassland ecosystems. VOC emitted from terrestrial vegetation would play an important role in carbon cycles at some particular ecosystems or vegetation types, considering their global emission capacity, although they are little or neglectable in relation to NPP or NEP at temperate grassland ecosystems.
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