黄土塬区包气带土壤CO2的特征及成因

THE CHARACTERISTICS AND ORIGIN OF CO2 IN UNSATURATED ZONE AT LOESS TABLELAND OF NORTHWESTERN CHINA

本文对甘肃省灵台县秋射村黄土包气带土壤CO2浓度及其δ13C、土壤含水量及细菌总数等指标进行了观测。研究结果表明黄土剖面中土壤CO2浓度远高于大气，不同黄土-古土壤层位的浓度有所差异，但没有明显的规律性。包气带-饱水带界面附近S14层位的CO2浓度最高，达4180μmol/mol。土壤CO2的δ13C主要集中在-21.31‰^-15.37‰之间，与1/[CO2]成正相关（r=0.7411）。表明灵台秋射剖面黄土CO2除来源于微生物分解土壤有机碳外，也与碳酸盐矿物-H2O-CO2化学平衡中碳酸盐沉淀的脱气作用有关。利用两端元混合法估算得到微生物降解有机碳对土壤CO2的贡献比例为40%~78%（平均65%），碳酸盐矿物风化/沉积过程对土壤CO2的贡献比例为22%~60%（平均35%），有机碳的贡献随深度递减，碳酸盐矿物沉积过程的贡献随深度递增。研究结果进一步证明黄土深层碳库内部碳库之间发生着碳的转化迁移，与大气碳库之间也存在着碳迁移。因此现代碳循环过程给黄土古气候定量重建研究提出了新的挑战与机遇，未来黄土古气候研究需重新审视黄土中物质及能量的现代迁移转化过程。

The Chinese Loess Plateau, with an area of 640000km2 and a thickness up to 200m, is regarded as a huge carbon sink in China. However, the modern carbon cycle in deep loess is still not well understood. Soil CO2 as a key factor for the carbon cycle of deep loess has been inadequately studied and reported. In order to observe the CO2 characteristics and origin in unsaturated zone at loess tableland and further understand carbon cycle in deep loess, a series of tubes for gas monitoring and sampling were installed into horizontal boreholes in Qiushe loess section（35°10＇08.13″N, 107°41＇08.79″E） of Lingtai County, Northwestern China, where develops a set of typical loess-paleosol sequences. The soil samples were collected when drilling the holes and their water content and total bacterial count were measured, and the concentration of loess CO2 and their δ13C were determined. The observed results show that the soil water content varied from 6.62% to 31.63%, displaying a increasing trend with depth, while the total bacterial count had a large range from 1000CFU/g to 1450×104CFU/g with no distinct trend along depth. The CO2 concentration in loess is higher than in atmosphere and reaches the maximum of 4180μmol/mol in S14, and different loess/paleosol fails to display an instinct trend. The δ13C value of CO2 ranged from -21.31‰ to -15.37‰, and had a positive relationship with 1/[CO2]（r=0.7411）. The results showed that CO2 in loess is not only relative to decomposed organic carbon by microbe, and also to the degassing effect in the balance system among CaCO3-H2O-CO2 in the interface between saturated and unsaturated zone. According to the two end member mixing model, the contribution ratios of organic carbon and carbonate precipitation to loess CO2 were calculated. As the result, the contribution ratios of organic carbon to loess CO2 is up to 40%~78%（average 65%）and have a decreasing trend with depth, while the contribution of carbonate precipitation is 22%~60%（average 35%）, increasing with depth. These carbon fluxes of deep loess carbon stock such as transportation and transformation is posing a new challenge on the study of the quantitative paleoclimate reconstruction using some proxies related to carbon in loess. The modern processes on matter and energy cycle in deep loess need to be re-explored so as to advance the study on paleoclimate records in loess.