| Following such a sampling strategy that 15 to 20 subsamples were collected at locations with different vegetation and topography types within an area of ca. 1 to 2 km2, and then combined into one sample, for each specific sampling site, surface soil samples and dung samples of herbivore have been collected from107 sites in Inner Mongolia Plateau, northern edge of the Chinese Loess Plateau, Qinghai-Tibet Plateau and arid Xinjiang, in China that spanning 28 °N to 51 °N. The plant residues in the surface soil samples have been carefully selected out. The concentrations of total organic carbon(TOC) and nitrogen(TON) of the surface soil samples were measured, as well as the stable carbon isotopic compositions(δ13C) of total organic matter of the surface soil samples, herbivore dung samples and plant residue samples. In addition,α-cellulose has been extracted from the plant residue samples and herbivore dung samples, for δ13C measurements. Finally, based on the analyses of the obtained data and corresponding climatic data(temperature and precipitation amount) of each sampling site, following major conclusions have been obtained.The δ13C of surface soil samples, plant residue samples and herbivore dung samples demonstrated that the vegetation overlying the sampling sites are mainly predominated by C3 plants, with only limited contributions from C4 plants. Therefore, the vegetation that overlying the sampling sites can be divided into two groups, for most sampling sites, the overlying vegetation can be treated as composed by pure C3 plants, and for the remain sampling sites is C3/C4 mixed vegetation. The δ13C data from the sites with C3/C4 mixed vegetation have a moderately significant positive correlation with the corresponding mean annual temperature, implying the increased C4 relative abundance with increased temperature. The δ13C data from the sites with pure C3 vegetation have a moderately significant negative correlation with the corresponding annual precipitation amount(-0.3‰/100 mm for δ13C data of surface soil samples and herbivore dung samples), implying more negative δ13C values of C3 plants under more humid conditions,generally consistent with other study results of modern C3 plants. A large number of δ13C data for surface soil samples and modern plant samples that distributed globally have been collected out from many literatures, and the analyses of the collected δ13C data also demonstrated the extremely positive correlation between the C4 relative abundance and annual mean temperature, as well as the extremely negative correlation between the δ13C data of C3 plants and the surface soils under pure C3 vegetation and annualprecipitation amount, supporting the analysis results of our original data.For the detecting of the fractionation between δ13C of surface soils and corresponding plant residues(Δ13Csoil-litter), the δ13C data of surface soils, plant residues and herbivore dungs have been further compared.The results indicated that the Δ13Csoil-litter can also been divided into two groups. One group is, when the surface soil δ13C data are relatively positive as a whole, the surface soil δ13C are more negative than that of plant residues, probably resulting from the lignin content in the surface soils and relatively more quick degradation rate of C4 plants. In the Chinese loess/paleosol sequences, generally, it has been accepted that relatively higher C4 relative abundances occurred in paleosol layers. However, considering the relatively faster decomposition rate of C4 plants, which will resulted in the more negative δ13C values, and therefore the subsequently underestimated C4 relative abundance, such a situation should be seriously considered in the Chinese loess δ13C study. The other group is, when the surface soil δ13C data are relatively negative as a whole, the surface soil δ13C are more positive than that of plant residues, with an average positive magnitude of ca. 1.4‰. Under such a situation, when the fractionation between δ13C data of herbivore dungs and plant residues has been deduced, the average fractionation magnitude between surface soil δ13C and corresponding source plants is ca. 1.5‰. Such a more positive surface soil δ13C relative to the overlying plants is mainly resulted from the microbial degradation of the plant residues which will resulted in the preferential release of the 12 C. The analysis results of different vegetation types suggested that precipitation amount is the main factor that influences the fractionation magnitude, i.e. the positive magnitude of fractionation of surface soil δ13C relative to the δ13C of overlying plants will decreased under higher precipitation amount. Due to the different climatic background of loess and paleosol layers(normally more wetter background for paleosol layers and more arid background for loess layers), different fractionation magnitudes need to be take into account for the reconstruction of past C3/C4 relative abundances. The further analyses of the collected surface soil and modern plant δ13C data from the literatures demonstrated that the average value of Δ13Csoil-plant is ca. 1.8‰ in the middle and East Asia, and ca. 3.8‰ for the global. The result of average 1.4‰ in our study region is close to the result for the middle and East Asia, and within the result for the global. Along the latitudes, it seems the climatic factors(temperature and precipitation amount) don’t show significant impact on the Δ13Csoil-plant all over the world.When the overlying vegetation is C3/C4 mixed vegetation, the α-cellulose δ13C data from plant residues and herbivore dungs all shown more positive trends with increased annual mean temperature,similar as the δ13C data of total organic matter in surface soils, plant residues and herbivore dungs, further reinforced that temperature is the major factor that control the C4 relative abundance. When the overlying vegetation is pure C3 vegetation, the α-cellulose δ13C data from plant residues and herbivore dungs all shown more negative trends with increased annual precipitation amount, also similar as the δ13C data of total organic matter in surface soils, plant residues and herbivore dungs, further indicating the negativeresponse of δ13C of C3 plants to more humid conditions. It is worth to note that the linear correlations between plant residue α-cellulose δ13C data and corresponding climatic data are not significant, implying the potential influence from other factors on the plant residue α-cellulose δ13C data.Our study results can provide modern references from several aspects for the studies of past C3/C4 variations and relevant paleoecology, paleoenvironment and paleoclimate studies in inland China.Potentially, the samples obtained in this study can be further used in other relevant studies, such as, theα-cellulose samples extracted from plant residues and herbivore dungs can be used for the studies of hydrogen and oxygen isotopes in the future. Similarly, the collected large number of surface soil and modern plant δ13C data from many literatures can also be analyzed and used by other studies. |
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