| 摘要: |
| 磁化率是黄土-古土壤序列古气候研究的一个重要指标。本文调查了黄土高原及周边地区三种类型土壤的磁化率、土壤有机碳含量、有机碳同位素组成和碳氮比值等指标。样品采集自黄土-沙漠过渡区、黄土塬面和森林地区，代表了黄土高原地区主要的土壤类型。结果显示：黄土塬面、林区、黄土-沙漠过渡区土壤的磁化率变化区间分别为26.6×10−8 — 61.4×10−8 m3 ∙ kg−1、68.6×10−8 — 107.5×10−8 m3 ∙ kg−1、8.5×10−8 — 44.4×10−8 m3 ∙ kg−1。黄土塬面土壤有机碳含量在0.05%到0.62%之间变化，而林区土壤的有机碳含量在1.19%到3.35%间变化。黄土塬面的土壤C / N比值也较低，在0.6到6.1之间变化，林区样品C / N比值在6.2到11.83之间变化。黄土-沙漠过渡区土壤磁化率较低，森林地区土壤磁化率较高，土壤磁化率与有机碳含量、C / N比值呈正相关关系。笔者认为有机质含量增加对土壤的磁化率增强有明显贡献。有机质含量较高时，更适宜土壤中磁性细菌的生长。同时，较高的有机质含量指示着较高植被覆盖，这也对土壤中磁性矿物增加有一定贡献。燃烧有机质还会使非磁性矿物更易转化为磁性矿物。这些因素都会增强土壤的磁化率。 |
| 关键词: 土壤磁化率 有机质 C / N比值 黄土高原 |
| DOI：10.7515/JEE201602005 |
| CSTR：32259.14.JEE201602005 |
| 分类号: |
| 基金项目:国家重点基础研究发展计划（2013CB955900） |
| 英文基金项目:National Basic Research Program of China?(2013CB955900) |
|
| Impact of soil organic matter on modern soil magnetic susceptibility in Loess Plateau and its surrounding areas |
|
ZHANG Bo, LIU Weiguo1,2
|
|
1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. University of Chinese Academy of Sciences, Beijing 100049, China
|
| Abstract: |
| Background, aim, and scope Magnetic susceptibility of soils can provide paleoclimatic information. In Chinese Loess Plateau, susceptibility enhancement is usually considered as a proxy of monsoon intensity. Several hypotheses were used to explain variations of this proxy. Here, we present a study on how soil magnetic susceptibility is related with soil organic matters. We analyzed magnetic susceptibility, organic carbon content, organic carbon isotopic composition, and C / N ratio of modern soils from Chinese Loess Plateau, in order to obtain the relationship between soil magnetic susceptibility and other parameters, as well as how soil organic matters affect soil magnetic susceptibility. Materials and methods Fifty modern soil samples were collected from the Loess Platform, forest areas at the Huangling and Huanglong Mount, and loess-desert area near the Tengger Desert. These soil samples represent modern soil types in the Loess Plateau. Samples were collected 2 — 3 cm below the surface. The sampling sites are at least 40 km away from any industrialized centers that could generate artificial, air-borne magnetic material. In this way, we minimized the effect of human and livestock activity. We tested magnetic susceptibility (χlf), organic carbon isotopic composition (δ13C), and organic carbon and nitrogen contents of these samples. Results The magnetic susceptibility varied from 26.6×10−8 m3 ∙ kg−1 to 61.4×10−8 m3 ∙ kg−1 for soils from the loess platform, and from 68.6×10−8 m3 ∙ kg−1 to 107.5×10−8 m3 ∙ kg−1 for soils from forest areas. The value of soil from forest areas is apparently higher than that from the loess platform. The magnetic susceptibility of soil samples from loess-desert area varied from 8.5×10−8 m3 ∙ kg−1 to 44.4×10−8 m3 ∙ kg−1. δ13C values of soil samples from the loess platform varied from − 22‰ to − 24.4‰. δ13C values of soil samples from loess-desert area varied from − 20.66‰ to − 24.69‰, whose range is similar to that from the platform. δ13C values of soil samples from forest areas in the Huangling Mount and the Huanglong Mount varied from − 24.5‰ to − 26.9‰ and thus are more negative than those from the loess platform and loess-desert areas. The organic carbon contents in soils from the loess platform area are relatively low, ranging from 0.05% to 0.62%, while the organic carbon contents in soils from the forest areas varied from 1.19% to 3.35%. C / N ratios show a similar pattern that the values for soils from the loess platform are relatively small, from 0.6 to 6.1, while they range from 6.2 to 11.83 for forest areas. In sum, soil samples from different areas showed different variations of measurements. Discussion Carbon isotopic composition of soil organic matter can provide us information about vegetation history since soil organic carbon is mainly derived from plant litter and thus records δ13C value of plants. In this study, δ13C measurements are in agreement with the fact that there is a mixture of C4 and C3 plants in the loess platform region, and forest areas are controlled by C3 plants. The data show that soil magnetic susceptibility is poorly correlated with δ13C values of modern soils. Our results also show that soil magnetic susceptibility increases with increasing soil C content. Magnetic susceptibility of soils from forest areas with higher organic carbon content is greater than that from loess platform and loess-desert areas. In general, soil organic matter is composed of plant residues and microorganism. In arid areas, vegetation is a major source of soil organic carbon. Higher C content is the result of enriched plant productivity. Soil C / N ratio is indicator of leaf litter content and extent of root decomposition. Based on our data, Soil magnetic susceptibility was positively related to C / N ratios of modern soils. High organic matter content, suggested by higher soil organic C content and C / N ratio, results in the increasing of magnetic susceptibility in several ways. Organic matter content indicates the amount of vegetation. Increasing plant productivity will enrich the fine magnetic minerals in surface soil. Also, interaction between plant litter and soil microorganisms during plants decomposition results in increasing magnetic bacteria in soil. Enough surface organic matter and well developed soil could sustain microbial activity, and thus more magnetic bacteria would thrive. The soil magnetic susceptibility will increase if a large number of magnetic bacteria accumulate in the soil. The combustion of soil organic matter may be another possible explanation. Burning experiment shows that the magnetic susceptibility of all modern soils with plant ashes on the surface are greater than that of the modern natural soils at the same site. Burning organic matter also helps nonmagnetic minerals turn to magnetic ones. Thus the contribution of organic matter on soil magnetic susceptibility should not be neglected when we take natural fire into account. Conclusions Our data show that both organic carbon contents and C / N ratios of modern soils are positively related to soil magnetic susceptibility. We conclude that organic matter contributes to the increase of soil magnetic susceptibility. Recommendations and perspectives This study showed that soil magnetic susceptibility is closely related to organic matter in the soil. Future work might be focused on the exact mechanism that results in the enhancement of soil magnetic susceptibility due to increasing organic matter content. |
| Key words: soil magnetic susceptibility organic matter C / N ratio Loess Plateau |
