Soil Organic Matter Carbon Isotope In Sand Fields Of Northern China And Vegetation Succession

Ninety surface soil samples were collected from three sand fields of Northern Chi-na(Mu Us sand field,Otindag sand field and Hulu Buir sand field),and then measured in the labrotory to obtain their organic carbon isotope.The relationship between organ-ic matter isotope and modern climate was analyzed using statistical methods.Besides,paleoclimatic indexes including organic carbon isotope of several dune sequences and loess profiles in this area were also measured.Some new knowledge has been gained from our new data.TOC of the surface samples were low,between 2.89%and 0.03%,averaged 0.90%.By comparison of TOC in these three dune fields,it is suggested that aridity rather than precipitation is the main factor in controlling total organic content of the surface sam-ple.?13C of surface samples were between-26.56%?and-19.41‰,averaged-23.54%‰.?13C of the samples from the Otindag sand field were highest among these three dune fields,while ?13C of the samples from the Hulun Buir were relatively low,and their values in Mu Us were lowest.The relationships between the ?13C of soil organic matter and climatic parameters such as mean annual temperature(MAT),mean annual preeipitation(MAP),growing-season precipitation,aridity and growing-season aridity were investigated by regres?sion analysis.A weak linear correlation was observed(R2=0.14;n=90)between ?13C and the MAT in all these dune fields.The soil organics ?13C is significantly correlated with MAP:every 100 mm increase in the MAP results in a 1.9‰decrease in the value of surface soil ?13C,with the equation expressed as:Y=-0.019X-17.97(Y is soil organic matter ?13C and X is the MAP in mm).The organic ?13C and the mean growing-season(April-September)precipitation are weakly correlated(R2=0.46;n=90;p<0.05).Third-ly,we calculated the correlation coefficient between the surface soil organic ?13C val-ues and the aridity and growing-season aridity,respectively.Results shows that the organic ?13C values increase with increasing aridity or increasing growing-season ariu-iLy,though their correlatin Ou weaker than those between the organic ?13C values and precipitation.Where the correlation betweene growing-season aridity and ?13C values is more significant(R2=0.40,n=90,p<0.05).Based on analysis of ?13C,modern temperature condition,growth condition of C4 plant and results in other area,by comprehensive analysis of our new data with published data from surrounding area,we suggest that the soil organic matter content is mostly supplied by C3 plant contribute,rather than C4 plant.Thus,we suggest that variation in the ?13C of soil organic matter is largely determined by ?13C of C3 plants.The proportion of C4 plant increase with aridity may also contribute to the changes in?13C of soil organic matter.The C3/C4 biomass of four sedimentary sections in this area was calculated by the ?13C.The results shows that C3/C4 biomass in all of the 4 profiles is consistently responded to climatic variations,that is,C4 decrease in glacial period,but increase in interglacial period.This result is consistent with other studies in Chinese Loess Plateau that C4 biomass is mainly growth in interglacial period.In addition,C4 plant in low latitude were higher than high latitude in interglacial period,which is most likely due to their temperature differences.In glacial period,C4 proportion in NYZG profile were higher than ZBT profile,this may attribute to the influence of east Asia summer monsoon when the sea level was declined.Finally,the ?13C-precipitation transfer function obtained from modem observa-tions is tried to apply to sedimentary sequences,but failed.The reason may be that vegetation type in the past during glacial-interglacial cycle probably is not as same as modem vegetation.The modern vegetation is mainly C3 plant,the proportion of C4 plant is low,while during glacial period,C3 plant is predominated,and both C3 and C4 could dominate during interglacial period.Therefore,it should be cautious when the vegetationrecordedby Profile was mixing Plants.By the way,modem landscape has been greatly influenced by human activities.the transfer function based on modem processes should be carefully apply to paleocli-matic reconstructions.