黄土高原自然植被的土壤有机碳同位素证据

根据黄土高原现代表土有机碳同位素(δ13CSOM)数据,结合前人关于末次间冰期和全新世中期的分析结果,研究这3个特征时期C4相对生物量(C4％)的空间变化特征。结果表明,3个时期黄土高原C4％均由东南(33％,25％和12％)向西北(0％)递减。10％等值线在末次间冰期大致位于交道-泾川西北、西峰东南一带; 到全新世西部向南推进,大致位于交道-洛川-旬邑以西北、西峰-灵台以东南一带; 而现今比全新世时期向东南推移约60km,退至黄陵以西北、彬县-延安以东南一带。全新世与末次间冰期温度和降水的不同可能是植被差异的主要原因。现代C4％较末次间冰期和全新世中期有较明显的降低,可能与较高的大气CO2浓度、较低的温度和干旱化有关。由于上述3个特征时期黄土高原东南地区均以C4草本植物为主,西北地区C3植物比例的增加亦为C3草本植物比例的增加。

THE NATURAL VEGETATIONS ON THE CHINESE LOESS PLATEAU: THE EVIDENCE OF SOIL ORGANIC CARBON ISOTOPE

In this paper,we obtained the organic carbon isotopes(δ 13 CSOM)of 11modern surface soil samples on the Chinese Loess Plateau(CLP)(34.6°~36.6°N,103.3°~110.2°E),and combined with that of the middle of the last interglacial and Holocene from13 loess sections by previous studies, and then analyzed the relative abundance of C4 plants(C4 ％ )spatial change characteristics during these three periods through the calculating of δ 13 CSOM.The results show that the C4 ％ of the three periods all decreased from southeast(33 ％ ,25 ％ and 12 ％ )to the northwest(0 ％ ).The 10 ％ isoline was located in the northwest of Jiaodao-Jingchuan and the southeast of Xifeng during the last interglacial,and the west side moved to the south slightly during the Holocene,resulting in location ofthe isoline in the northwest of Jiaodao-Luochuan-Xunyi and the southeast of Xifeng-Lingtai. The isoline moved to southeast by almost 60km during the modern time.The annual mean temperature(T)and annual mean precipitation(P)during the Holocene were lower than during the last interglacial(the southwest area T 14.3~14.4℃,P 815~884mm; the middle area T 13.7~14.4℃,P 770~835mm; the northeast area T 10~14.4℃,P 603~827mm),and this trend in the northeast area(T decreased 1.3~3.6℃,P decreased 61~190mm)was stronger than the southwest area(T decreased 0.2~0.3℃,P decreased 22~71mm).The differences of two environmental conditions during these periods maybe the mostly affect factors that induced the vegetation landscape on the CLP changed.The C4 ％ decreased greatly during the modern time,the high atmosphere CO2 concentration(can reach 379ppm),and the low temperature(T 7.0~13.7℃)and dry(P 450~631mm)may cause it.During these three periods,the southeast area of the CLP has a large number of C4 grasses(12 ％ ~33 ％ ),and because the temperature and precipitation levels in the northwest area are less favorable for trees,the increase ratio of C3 plants in the northwest area are grass plants and shrubs.