引用本文: 冯雍, 陶贞, 高全洲, 邓浩俊, 姚玲, 李银花, 丁健, 王振刚, 黎坤, 徐鹏. 雅砻江下游干热河谷表土化学风化及其控制因子[J]. 第四纪研究, 2020, 40(4): 1083-1094. doi: 10.11928/j.issn.1001-7410.2020.04.21 冯雍, 陶贞, 高全洲, 邓浩俊, 姚玲, 李银花, 丁健, 王振刚, 黎坤, 徐鹏. 雅砻江下游干热河谷表土化学风化及其控制因子[J]. 第四纪研究, 2020, 40(4): 1083-1094. doi: 10.11928/j.issn.1001-7410.2020.04.21 Feng Yong, Tao Zhen, Gao Quanzhou, Deng Haojun, Yao Ling, Li Yinhua, Ding Jian, Wang Zhengang, Li Kun, Xu Peng. The chemical weathering and its controlling factors of topsoil in the hot-dry valley of the lower Yalongjiang River[J]. Quaternary Sciences, 2020, 40(4): 1083-1094. doi: 10.11928/j.issn.1001-7410.2020.04.21 Citation: Feng Yong, Tao Zhen, Gao Quanzhou, Deng Haojun, Yao Ling, Li Yinhua, Ding Jian, Wang Zhengang, Li Kun, Xu Peng. The chemical weathering and its controlling factors of topsoil in the hot-dry valley of the lower Yalongjiang River[J]. Quaternary Sciences , 2020, 40(4): 1083-1094. doi: 10.11928/j.issn.1001-7410.2020.04.21

化学风化是地表岩石矿物向土壤释放营养元素同时形成土壤粘粒组分的地球化学过程，这一过程使土壤具有生态环境功能。本文选择采集青藏高原东南缘的雅砻江下游不同地貌部位和植物群落的表土样品并分析其粒度组成和地球化学特征。结果表明：研究区表土粒度组成以粉砂为主（46.68%），其次是砂粒（34.05%）和粘粒（19.28%）；元素组成以Si、Al和Fe为主；K、P和Si相对于上陆壳亏损，与区内沉积岩为主的岩石分布特征一致。研究区表土粘粒含量和化学蚀变指数（CIA）存在明显的空间差异：海拔 < 1300 m、坡度较大的南部谷坡地表土粘粒平均含量为6.51%，CIA平均为65，处于脱Ca、Na的中等风化阶段早期；海拔>2400 m、坡度平缓的西部坡地和宽谷地表土粘粒含量达39.21%，CIA平均达86，风化程度较高。母岩、海拔、坡度和土壤总氮含量对表土CIA值的贡献依次是57.34%、23.46%、10.33%和6.87%。显然，母岩性质是控制研究区表土化学风化过程的主要因素，地貌条件（海拔和坡度）是驱动化学风化过程最重要的外部因素，且海拔高度的影响大于坡度；生物作用对CIA值有一定的贡献。本研究可为深入探讨干热河谷地区土壤生物地球化学过程提供基础数据。

化学蚀变指数 Abstract:

Rocks/minerals chemical weathering is a superficial geochemical process in which releasing nutrient elements to soil and forming clay minerals in the soil, and simultaneously developing eco-environmental functions of soil. In this study, we selected 10 community plots, located respectively in the different slopes and mountain rift basins (26°32'~34°05'N, 96°52'~102°48'E) with elevation ranging from 1000 m to 3000 m in the lower reaches of the Yalongjiang River on the southeastern Qinghai-Tibetan Plateau, including grassland, savanna, mixed woodland, Leucaena leucocephala woodland, coniferous and broad-leaved mixed forest land, Pinus yunnanensis woodland, wide-valley grassland, corn field, mango orchard and apple orchard, etc .,  and collected its topsoil samples, respectively. The grain size composition and mineral elemental composition were analyzed, the proxies of grain size and geochemical parameters and the methods of correlation analysis and ridge regression analysis were employed. The objectives of this study were to explore the evolution trend of the topsoil chemical weathering; to quantify the contributions of major controlling factors to the topsoil chemical weathering; and to evaluate the carbon sink potential of the topsoil chemical weathering in the hot-dry valleys of the southwestern China. The results show that the spatial differences of the topsoil chemical weathering intensity resulted in significant spatial heterogeneity of the topsoil grain size composition in the study area. In the south of the study area with lower altitude (below 1300 m) and steeper slope, including grassland, savanna, mixed woodland, Leucaena leucocephala woodland, corn field and mango orchard, the grain size of the topsoil occurs more coarse (mean grain size ranging from 3.94 φ to 6.35 φ) and merely 6.51% of clay content. The topsoil is characterized by relatively weak eco-environmental function. However, in the western study area with higher elevation (above 2400 m) and gentle slope, including wide-valley grassland, Pinus yunnanensis woodland, coniferous and broad-leaved mixed forest land and apple orchard, the grain size of the topsoil exists more finer (mean grain size ranging from 7.91 φ to 8.57 φ) and higher clay content (39.21%). Hence, the topsoil is characterized by stronger eco-environmental function.

Comparing with the upper continental crust, terrestrial shale and Chinese soil geochemical baseline values, the content of element K, P, and Si is lower, and the content of element Ti, Mn, and Fe is higher in the topsoil of the study area. The contents of mobile elements (Mg, Ca, and Na) varies widely in the topsoil among the different communities, which is consistent with the rocks features of widespread distribution of carbonate rocks, sand shales and mudstones, and sparse silicate rocks as well as the intensive slope erosion in the study area. For the purpose of ensuring the yield and quality of crops and the soil resources in the study area, it is necessary to rational application of K and P-containing fertilizers and organic fertilizers, and at the same time strengthen field management to reduce the pollution of rainwater runoff carrying nutrients flushing into the cascade reservoirs in the lower Yalongjiang River.

The spatial variation of the Chemical Index of Alteration (CIA) and the A-CN-K triangle diagrams show that the topsoil in the western study area is undergoing a moderate-intensity weathering process (the averaged CIA=86), and is in the stage of desilicating and ferrallitic weathering process. The chemical weathering of the topsoil on the steeper slopes in the southern study area is in the lower to medium development stage (the averaged CIA=65), and is in the leaching stage of element Ca and Na. Hence, the topsoil chemical weathering process in the study area may still consume atmospheric CO ₂ and serve as carbon sinks. The Ridge regression analysis suggested that the spatial differences of the topsoil chemical weathering intensity in the study area were mainly predominated by parent rocks, altitude, slope, and soil TN content, which contribution to the topsoil CIA value is 57.34%, 23.46%, 10.33%, and 6.87%, respectively. Obviously, the nature of the parent rocks is the major factor controlling the topsoil chemical weathering process in the study area. The geomorphological factors (elevation and slope) are the main external factors driving the chemical weathering process, and the impact of the elevation on the CIA value is greater than that of the slope; The biological action contribute limitedly to the CIA value too. This study can provide basic data for deeply discussing soil biogeochemical processes in hot-dry valleys.

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Vegetation types and distribution of sampling sites in the study area. Vegetation data from reference[ 27 ]

Figure 2.

Frequency distribution and cumulative frequency distribution curve of soil particles.

Figure 3.

Chemical composition and loss on ignition(LOI)of topsoil

Figure 4.

UCC standardized curve of the average content of major elements in topsoil

Figure 5.

Comparison of weathering intensity indexes

Figure 6.

A-CN-K triangular diagram

Figure 7.

Relationships between CIA value and environmental factors

Figure 8.

Relationship between measured CIA and calculated CIA