黄土高原及周边地区土壤有机质对现代土壤磁化率的影响

张 博，刘卫国

（1.中国科学院地球环境研究所，黄土与第四纪地质国家重点实验室，西安 710061；2.中国科学院大学，北京 100049）

黄土高原及周边地区土壤有机质对现代土壤磁化率的影响

张 博1,2，刘卫国1

（1.中国科学院地球环境研究所，黄土与第四纪地质国家重点实验室，西安 710061；2.中国科学院大学，北京 100049）

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—61.4×10-8m3· kg-1、68.6×10-8—107.5×10-8m3· kg-1、8.5×10-8—44.4×10-8m3· 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。

土壤磁化率；有机质；C / N比值；黄土高原

磁化率指示物质被磁化的难易程度，是黄土-古土壤序列古气候研究的一个重要指标（Heller and Liu，1982；王俊达等，1987；Liu et al，1988；An et al，1991，1995；刘秀铭和刘东生，1993；朱日祥等，1994；强小科等，2004；刘青松，2009；贾佳等，2011；赵国永等，2012；赵辉等，2012；安芷生等，2015）。在黄土高原地区，一般认为磁化率指示了季风强度的变化（An et al，1991）。黄土古土壤中磁化率信号的主要载体是细颗粒部分，其中磁铁矿和赤铁矿对磁化率有着主要的贡献（安芷生等，1990）。在黄土高原，古土壤磁化率要比黄土高，对于磁化率增强的原因，前人做了很多研究，提出了多种解释。Heller and Liu（1984）认为沉积物压实和碳酸盐淋滤作用使古土壤磁化率增强。Zhou et al（1990）认为是成壤过程中细粒磁性矿物的生成使土壤磁化率增强。Kukla et al（1988）认为是由物源不同导致黄土、古土壤磁化率的差异。Meng et al（1997）连续4年每月采集中国北方的降尘，通过分析这些样品，发现土壤中极细的磁性颗粒很有可能是来自于降解的植被枯落物。一般认为，成壤作用是导致黄土磁化率增强的主要原因（Zhou et al，1990；Hellar and Evans，1995；邓成龙等，2007；刘青松，2009）。

土壤有机质是土壤的重要组成部分，其含量是表征土壤肥力的一个重要指标。土壤有机质会影响其物理、化学、生物等多种性质。有机质和磁化率间存在何种关系是一个值得关注的问题。有学者在城市街道灰尘和河流阶地样品中发现样品磁化率和有机质含量成正相关关系 （Xie et al，2000；Shilton et al，2005；Torrent et al，2010）；Maher（1998）曾报道，英国雏形土中土壤磁化率和有机质含量成正相关关系。Grimley et al（2004）通过研究美国伊利诺伊州东北部的黑土，发现在土壤有机质含量最高的区域其磁化率也最高。张普和刘卫国（2008）对黄土高原中部竖井剖面的磁化率和总有机碳含量（TOC）进行了对比，发现这两种指标有着良好的对应关系，成壤作用较强的暖湿时期磁化率和TOC均出现高值，成壤作用较弱的干冷时期也同时出现低值；其他学者的研究也得到了相似的结果（胡雪峰，2004；李明启等，2005；谢巧勤等，2012）。

本文主要研究黄土高原地区土壤磁化率与土壤有机质间的相关关系。测试了黄土高原地区现代土壤的磁化率、有机碳含量、有机碳同位素组成和碳氮比值（C / N）指标，以期能得到土壤磁化率与相关指标间的关系，以及有机质对土壤磁化率的影响。

1 材料与方法

在黄土高原及周边地区的森林地区、黄土塬面和黄土-沙漠过渡区共采集了50份土壤样品。黄土塬面采样点位于陕北榆林、延安等地，森林采样点位于黄陵山区和黄龙山区，黄土-沙漠过渡区在腾格里沙漠周边，采样点分布见图1。

所采集到的土壤样品代表了黄土高原地区几种典型的土壤类型。为避免人类和牲畜活动的影响，采集距离地表2—3 cm深度的样品，这一深度的土壤有机质已经处于稳定的状态。采样点距离工业区在40 km以上，避免了人造磁性物质经空气传播对样品造成污染，这样将外部因素对磁化率的影响最小化。

土壤样品在50℃的条件下烘干。将可见的植物根系从土壤样品中挑出，再称量10 g土壤样品，利用Bartington公司MS2磁化率仪测得样品的低频磁化率（χlf，0.465 kHz）。有机碳和有机氮的含量通过美国Leco公司CS-344元素分析仪测得，精度为4%。

为测量样品的δ13C值，将3 g土壤样品在玛瑙研钵中研磨并过筛，使样品粒径小于150 μm。过筛后的样品在室温下用2 mol · L-1盐酸浸泡24小时，以去除其中的碳酸盐和可溶于酸的有机质，将酸处理过的样品用蒸馏水洗至pH＞4后在60℃条件下烘干。烘干后的样品和银箔与氧化铜一起放入石英管中密封，在800—850℃条件下加热至少4小时，通过液氮将制得的CO2气体分离纯化后进行碳同位素测试。利用MAT-251气相质谱进行分析，同位素比值表示为相对于PDB标准的千分偏差，精度为0.2‰，δ13C值通过下式计算得到：

图1 采样点分布Fig.1 The sampling sites

2 结果

表1列出了测得的黄土塬面和林区土壤样品的磁化率值、有机碳同位素组成、有机碳含量和C / N比值。黄土塬面土壤样品的磁化率值（χlf）在26.6×10-8m3· kg-1到61.4×10-8m3· kg-1之间变化，林区土壤样品的磁化率值在68.6×10-8m3· kg-1到107.5×10-8m3· kg-1之间变化，林区现代土壤样品的磁化率值显著高于黄土塬面上样品的值。由表2可见，腾格里沙漠周边地区样品的磁化率值在8.5×10-8m3· kg-1到44.4×10-8m3· kg-1之间变化。黄土塬面上土壤样品的δ13C值在-22‰到-24.4‰之间变化；腾格里沙漠周边土壤样品的δ13C值在-20.66‰到-24.69‰间变化，这两地区样品值的变化区间大致相同。而从黄陵山区和黄龙山区森林采集的土壤样品δ13C值在-24.5‰到-26.9‰间变化，相比黄土塬面和黄土-沙漠过渡区的样品值显著偏负。

黄土塬面土壤样品的有机碳含量较低，在0.05%到0.62%之间变化，而林区土壤样品的有机碳含量变化区间为1.19%到3.35%。C / N比值也显示了相似的变化特征，黄土塬面的C / N比值较低，在0.6到6.1之间变化，而林区样品值在6.2到11.83之间变化。

总体来看，不同采样地区（黄土-沙漠过渡带、黄土区、林区）的指标有不同的变化范围。在黄土塬面和黄土-沙漠过渡带，土壤样品的磁化率、有机碳含量、C / N比值较低，δ13C值偏正；林区土壤样品的情况相反，有较高的磁化率值、有机碳含量、C / N比值，而δ13C值更偏负。已有学者认为有机质含量增加会使土壤的磁化率值上升（贾蓉芬等，1992），此次在黄土高原得到的结果支持了这一观点。如图2所示，土壤磁化率随着土壤有机碳含量的增加而上升，林区的土壤碳含量较高，它们的磁化率值也比黄土塬面和沙漠地区的土壤磁化率高。

表1 黄土塬面和森林地区土壤样品磁化率值、有机碳同位素、有机碳含量和C/N比值Tab.1 The magnetic susceptibility (χlf),δ13C, organic carbon content, and C/N ratio of modern soils from loess platform and forest areas

表2 腾格里沙漠周边黄土-沙漠过渡带样品磁化率值(χlf)与δ13CTab.2 The magnetic susceptibility (χlf) andδ13C of modern soil from loess-desert area near the Tengger Desert

图2 土壤磁化率与有机碳含量、C/N比值间的关系Fig.2 Relationship between magnetic susceptibility (χlf) and organic carbon content and C/N ratios of modern soils from loess platform and forest areas

3 讨论

土壤的有机质主要来自陆生植物，因此土壤的有机碳同位素反映了植被生长状况（Schwartz et al，1986；Cerling et al，1989；Ambrose and Sikes，1991；Nordt et al，1994；Boutton and Yamasaki，1996；Hatté et al，1998；刘卫国等，2002；Liu et al，2005；宁有丰，2010；匡欢传等，2013）。黄土塬面上是C4植物和C3植物混合生长的，而在林区则是C3植物占主导地位，本研究测得的黄土塬面和林区土壤有机碳同位素值与这一情况是相符的，即林区值更偏负，黄土塬面值偏正。图3显示，在三种采样地，土壤磁化率值和δ13C值的相关性均较弱。可见植被类型并不会对土壤磁化率产生很大的影响。

图3 土壤磁化率与土壤碳同位素值间的关系Fig.3 Relationship between magnetic susceptibility (χlf) and organic carbon isotopic composition of modern soils

现代土壤的C / N比值反映了生长在土壤上植被的生物量（张普和刘卫国，2008），植被覆盖度高的情况下，土壤的碳含量和C / N比都会增加。植物的C / N比值通常都大于20（Lynch and Hobbie，1988；王晶苑等，2011；付珊等，2015），而土壤的C / N相比而言较低，因为在植物降解为土壤有机质的过程中，C / N比值会随之下降（Hedges and Oades，1997）。王维奇等人研究发现，土壤碳分解速率与土壤C / N存在显著的负相关关系，所以，C / N可以作为预测有机质分解速率的一个很好的指标（王维奇等，2011）。本结果证明现代土壤的磁化率和有机碳含量、C / N比值均呈正相关关系。高碳含量和高C / N比值指示着高有机质含量，有机质可能从以下几个方面导致磁化率的增强。

一部分有机质自身具有磁性。磁化率测定结果表明，有机质随着时间推移会逐渐从顺磁性向抗磁性转变。黄土在地质历史中属于近代地表沉降物，其中的有机质演变程度相当于泥炭阶段，所以具有较高的顺磁性（贾蓉芬等，1992）。有机质存在的条件下，无定形铁老化为氧化铁时更易于形成磁性较强的磁赤铁矿；有机质也可阻碍磁性矿物的老化，例如无定形水合氧化铁在吸附有机质后，氧化铁晶核的生长受到阻碍,使得无定形铁不易老化为针铁矿，针铁矿和磁赤铁矿不易老化为赤铁矿（胡雪峰，2004）。

土壤有机质主要来自植物残体，前人研究表明，植物体可从多方面引起土壤磁化率的增强。植物残体分解或燃烧会使土壤中磁性矿物增多（Meng et al，1997），从而增强土壤的磁性。植物在生长过程中，根系会分泌多种物质如H+，HCO2-等，这些物质会改变根系土壤的pH值，使矿物发生转化；根系的活动还会消耗或提供氧，这样也会改变根系周围的氧化还原环境，活化矿物中的铁，这有助于磁性矿物的形成（Lü and Liu，2001）。

在土壤有机质含量较高的区域，丰富的有机质和发育较好的土壤可以为土壤中的微生物提供养分，支持微生物的活动。一些种类的微生物可发生矿化，导致土壤磁性增强。微生物的矿化分为两种形式（Lowenstam，1981），一种是细胞外的生物诱导矿化，即微生物改变周边的环境，使磁性矿物易聚集在生物体周围。另一种是发生在细胞体内的生物有机矿化，在这种方式中，细胞内部存在有铁元素。这类细胞内含有铁元素的细菌是由Blakemore首次在盐沼中发现的（Blakemore，1975），称为趋磁细菌。趋磁细菌里铁的含量可高达干重的3%—4%，这一值约是一般生物铁含量的100倍（李志文等，2008）。土壤有机质、植物枯落物和土壤微生物相互作用使土壤中的极细的趋磁细菌增多（Vali et al，1987；Fassbinder et al, 1990）。前人的研究已证明，这些具有磁性的微体细菌可能与土壤中一些细粒磁铁矿的产生有关（Jia et al，1996；Maher，1998；贾蓉芬等，2001）。趋磁细菌死亡以后，体内的磁小体会保存下来，当趋磁细菌在土壤中数量较大时，很可能在一定程度上影响土壤磁化率的大小。

有学者通过燃烧实验证明，对同一研究点来说，表面覆盖有植被灰烬的土壤磁化率比天然土壤磁化率要更高（Lü et al，2000）；在有机质存在且缺氧的条件下，加热针铁矿和其他非铁磁性矿物到400—500℃时，可以将这些矿物转化为磁铁矿（Schwertmann and Fechter，1984）。黄土沉积物中的有机质在高温加热的条件下会消耗氧，使周边形成相对还原的局部环境，含硅酸盐矿物或粘土矿物等就可能会转化成磁铁矿（邓成龙等，2007）。因此自然火灾对黄土高原土壤磁化率的增强也有一定的积极贡献。

4 结论

对黄土塬面、森林地区、黄土-沙漠过渡带的土壤磁化率、有机碳含量等指标进行了研究，结果显示黄土塬面的土壤样品的磁化率值、有机碳含量、C / N比值较低，δ13C值偏正；林区土壤样品的情况相反，有较高的磁化率值、有机碳含量、C / N比值，而δ13C值更偏负。有机质可能从以下几方面影响土壤磁化率：高有机质含量指示着高植被覆盖度，植被对磁化率的增强有积极贡献；有机质丰富的土壤更适合磁性细菌的生长，当磁性细菌死亡时，体内的磁小体会保存在土壤中从而增强土壤的磁化率；燃烧有机质时非磁性矿物更易转化为磁性矿物。综合以上这些因素，可以认为有机质含量增加对土壤磁化率的增强有明显贡献。

安芷生, Porter S, Kukla G, et al. 1990. 最近 13 万年黄土高原季风变迁的磁化率证据[J].科学通报, 35(7): 529 – 532. [An Z S, Porter S, Kukla G, et al. 1990. The magnetic evidence of monsoon changes in Loess Plateau during the past 130 ka [J].Science Bulletin, 35(7): 529 – 532. ]

安芷生, 吴国雄, 李建平, 等. 2015. 全球季风动力学与气候变化[J].地球环境学报, 6(6): 341 – 381. [An Z S, Wu G X, Li J P,et al. 2015. Global monsoon dynamics and climate change [J].Journal of Earth Environment, 6(6): 341 – 381.]

邓成龙, 刘青松, 潘永信, 等. 2007. 中国黄土环境磁学[J].第四纪研究, 27(2): 193 – 208. [Deng C L, Liu Q S, Pan Y X, et al. 2007. Environmental magnetism of Chinese loess-paleosol sequences [J].Quaternary Sciences, 27(2): 193 – 208.]

付 姗, 吴 琴, 郑艳明, 等. 2015. 鄱阳湖沙山植物叶片与土壤C:N, C:P沿沙化梯度分布特征[J].长江流域资源与环境, 24(3): 447 – 454. [Fu S, Wu Q, Zheng Y M, et al. 2015. C:N and C:P stoichiometry of leaf and soil in response to deserti fi cation gradient in a sandy hill along Poyang Lake [J].Resources and Environment in the Yangtze Basin, 24(3): 447 – 454.]

胡雪峰. 2004. “黄土-古土壤”序列中氧化铁和有机质对磁化率的影响[J].土壤学报, 41(1): 8 – 12. [ Hu X F. 2004. Influence of iron oxides and organic matter on magnetic susceptibility in the loess-paleosol sequence [J].Acta Pedologica Sinica, 41(1): 8 – 12.]

贾 佳, 夏敦胜, 魏海涛, 等. 2011. 阿西克剖面记录的西天山地区黄土磁学性质及古气候意义初探[J].中国沙漠, 31(6): 1406 – 1415. [Jia J, Xia D S, Wei H T, et al. 2011.Magnetic property and paleoclimatic implication recordedby AXK Sequence in West Tianshan area [J].Journal of Desert Research, 31(6): 1406 – 1415.]

贾蓉芬, 刘东生, 林本海. 1992. 陕西兰田段家坡黄土剖面有机质磁性的初步研究[J].地球化学, 3: 234 – 242.[Jia R F, Liu T S, Lin B H. 1992. A preliminary study on magnetism of organic matters in Duanjiapo loess section at Lantian, Shaanxi Province [J].Geochemistry, 3: 234 – 242.]

贾蓉芬, 彭先芝, 高梅影, 等. 2001. 中国黄土剖面趋磁细菌的组成特征与生态意义[J].岩石矿物学杂志, 20(4): 428 – 432. [Jia R F, Peng X Z, Gao M Y, et al. 2001. Compositional features of magnetotactic bacteria in loess of China and their ecological signi fi cance [J].Acta Petrologica et Mineralogica, 20(4): 428 – 432.]

匡欢传, 周浩达, 胡建芳, 等. 2013. 末次盛冰期和全新世大暖期湖光岩玛珥湖沉积记录的正构烷烃和单体稳定碳同位素分布特征及其古植被意义[J].第四纪研究, 33(6): 1222 – 1233. [Kuang H C, Zhou H D, Hu J F, et al. 2013. Variations ofn-alkanes and compoundspecific carbon isotopes in sediment from huguangyan maar lake during the last glacial maximum and Holocene optimum:implications for paleovegetation [J].Quaternary Sciences, 33(6): 1222 – 1233.]

李明启，靳鹤龄，张 洪，等. 2005. 浑善达克沙地磁化率和有机质揭示的全新世气候变化[J].沉积学报, 23(4): 683 – 689. [Li M Q, Jin H L, Zhang H, et al. 2005. Climate change revealed by magnetic susceptibility and organic matter during the Holocene in Hunshandak Desert [J].Acta Sedimentologica Sinica, 23(4): 683 – 689.]

李志文, 李保生, 孙 丽, 等. 2008. 影响中国黄土磁化率差异的多因素评述[J].中国沙漠, 28(2): 231 – 237. [ Li Z W, Li B S, Sun L, et al. 2008. Evaluation of multifactors that affect the Susceptibility of China's loess [J].Journal of Desert Research, 28(2): 231 – 237.]

刘青松. 2009. 磁化率及其环境意义[J].地球物理学报, 52(4): 1041 – 1048. [Liu Q S. 2009.Magnetic susceptibility and its environmental significances [J].Chinese Journal of Geophysics, 52(4): 1041 – 1048.]

刘卫国, 宁有丰, 安芷生, 等. 2002. 黄土高原现代土壤和古土壤有机碳同位素对植被的响应[J].中国科学(D辑)地球科学, 32(10): 830 – 836. [Liu W G, Ning Y F, An Z S. 2002. The relationship of carbon isotope compostion and plants in modern soil and paleosol of Loess Plateau [J].Science in China Series D: Earth Science, 32(10): 830 – 836.]

刘秀铭, 刘东生. 1993. 中国黄土磁性矿物特征及其古气候意义[J].第四纪研究, 13(3): 281 – 287. [Liu X M, Liu T S. 1993. Characteristics of Chinese loess magnetic mineral and its implication in paleoclimate [J].Quaternary Sciences, 13(3): 281 – 287.]

宁有丰. 2010. 黄土有机碳同位素与古气候植被定量化研究进展[J].地质论评, 56(6): 851 – 857. [Ning Y F. 2010. Review on the quantitatively reconstructing paleoclimate and paleovegetation based on carbon isotope of soil organic matter on Chinese Loess Plateau [J].Geological Review, 56(6): 851 – 857.]

强小科, 安芷生, 李华梅, 等. 2004. 佳县红粘土堆积的磁学性质及其古气候意义[J].中国科学D辑：地球科学, 34(7): 658 – 667. [Qiang X K, An Z S, Li H M, et al. 2004.Magnetic properties of the red clay from Jiaxian section and its paleoclimatic significance [J].Science in China Series D: Earth Science, 34(7): 658 – 667.]

王晶苑, 王绍强, 李纫兰, 等. 2011. 中国四种森林类型主要优势植物的 C:N:P 化学计量学特征[J].植物生态学报, 35(6): 587 – 595. [Wang J Y, Wang S Q, Li R L, et al. 2011. C:N:P stoichiometric characteristics of four forest types' dominant tree species in China [J].Chinese Journal of Plant Ecology, 35(6): 587 – 595.]

王俊达, 李华梅, 刘东生, 等. 1987. 洛川黄土磁性、时代和古气候记录[J].矿物岩石地球化学通报, 4: 207 – 209. [Wang J D, Li H M, Liu T S, et al. 1987. The magnetic, age and paleoclimate records of Luochuan loess [J].Bulletin of Mineralogy, Petrology and Geochemistry, 4: 207 – 209.]

王维奇, 徐玲琳, 曾从盛, 等. 2011. 河口湿地植物活体-枯落物-土壤的碳氮磷生态化学计量特征[J].生态学报, 31(23): 7119 – 7124. [Wang W Q, Xu S L, Zeng C S, et al. 2011. Carbon, nitrogen and phosphorus ecological stoichiometric ratios among live plant-litter-soil systems in estuarine wetland [J].Acta Ecologica Sinica, 31(23): 7119 – 7124.]

谢巧勤, 陈天虎, 徐晓春, 等. 2012. 西峰黄土-红粘土序列有机质记录及其对磁化率古气候意义启示[J].第四纪研究, 23(4): 709 – 718. [Xie Q Q, Chen T H, Xu X C, et al. 2012. Organic matter record of Xifeng eolian deposits and its decipherment for the paleoclimatic proxy of magnetic susceptibility of the loess-red clay sequences in Chinese Loess Plateau [J].Quaternary Sciences, 23(4):709 – 718.]

张 普, 刘卫国. 2008. 黄土高原中部黄土沉积有机质记录特征及C / N指示意义[J].海洋地质与第四纪地质, 28(6): 119 – 124. [Zhang P, Liu W G. 2008. Loess sedimentary organic matter records from the central Chinese Loess Plateau and the implication of C / N [J].Marine Geology & Quaternary Geology, 28(6): 119 – 124.]

赵国永, 刘秀铭, 吕 镔, 等. 2012. 全新世黄土记录的古气候演化及磁化率和粒度参数灵敏性探讨[J].第四纪研究, 32(4): 777 – 784. [Zhao G X, Liu X M, Lü B, et al. 2012. The paleoclimatic evolution recorded by Holocene loess and discussion on the parameter sensitivity of magnetic susceptibility and median particle diameter [J].Quaternary Sciences, 32(4): 777 – 784.]

赵 辉，强小科，敖 红, 等．2012. 黄土高原西部红粘土岩石磁学性质及其指示的亚洲内陆中新世气候变化特征[J].第四纪研究, 32(4): 690 – 699. [Zhao H, Qiang X K, Ao H, et al. 2012. Mid Miocene climate in the Asian interior, based on the mineral magnetic record of the red clay sequence on the western Chinese Loess Plateau [J].Quaternary Sciences, 32(4): 690 – 699.]

朱日祥, 李春景, 吴汉宁, 等. 1994. 中国黄土磁学性质与古气候意义[J].中国科学 (B 辑), 24(9): 992 – 997. [Zhu R X, Li C J, Wu H N, et al. 1994.Magnetic properties and paleoclimate implication of the Chinese loess [J].Science in China (Series B), 24(9): 992 – 997.]

Ambrose S H, Sikes N E. 1991. Soil carbon isotope evidence for Holocene habitat change in the Kenya Rift Valley [J].Science, 253(5026): 1402 – 1405.

An Z S, Kukla G, Porter S C, et al. 1991.Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years [J].Quaternary Research, 36(1): 29 – 36.

An Z S, Porter S C, Chappell. 1995. Accumulation sequence of Chinese loess and climatic records of Greenland ice core during the last 130 ka [J].Chinese Science Bulletin, 40(15): 1272 – 1276.

Blakemore R. 1975.Magnetotactic bacteria [J].Science, 190(4212): 377 – 379.

Boutton T W, Yamasaki S I. 1996. Stable carbon isotope ratios of soil organic matter and their use as indicators of vegetation and climate change [J].Mass spectrometry of soils, 47 – 82.

Cerling T E, Quade J, Wang Y. 1989. Carbon isotopes in soil and paleosols as ecologic and paleoecologic indicators [J].Nature, 341: 138 – 139.

Fassbinder J W E, Stanjek H, Vali H. 1990. Occurrence of magnetic bacteria in soil [J].Nature, 343(6254): 161 – 163.

Grimley D A, Arruda N K, Bramstedt M W. 2004. Using magnetic susceptibility to facilitate more rapid, reproducible and precise delineation of hydric soils in the midwestern USA [J].Catena, 58(2): 183 – 213.

Hatté C, Fontugne M, Rousseau D D, et al. 1998.δ13C variations of loess organic matter as a record of the vegetation response to climatic changes during the Weichselian [J].Geology, 26(7): 583 – 586.

Hedges J I, Oades J M. 1997. Comparative organic geochemistries of soils and marine sediments [J].Organic Geochemistry, 27(7): 319 – 361.

Heller F, Evans M E. 1995. Loess magnetism [J].Reviews of Geophysics, 33(2): 211 – 240.

Heller F, Liu T S. 1982.Magnetostratigraphical dating of loess deposits in China [J].Nature, 300: 431 – 433.

Heller F, Liu T S. 1984.Magnetism of Chinese loess deposits [J].Geophysical Journal International, 77(1): 125 – 141.

Jia R, Yan B, Li R, et al. 1996. Characteristics of magnetotactic bacteria in Duanjiapo loess section, Shaanxi Province and their environment signi fi cance[J].Science in China Series D: Earth Science, 39(5): 478 – 485.

Kukla G, Heller F, Ming L X, et al. 1988. Pleistocene climates in China dated by magnetic susceptibility [J].Geology, 16(9): 811 – 814.

Liu W G, Ning Y F, An Z S, et al. 2005. Carbon isotopic composition of modern soil and paleosol as a response to vegetation change on the Chinese Loess Plateau [J].Science in China Series D: Earth Sciences, 48(1): 93 – 99.

Liu X M, Liu T S, Xu T C, et al. 1988. The Chinese loess in Xifeng, I. The primary study on magnet of stratigraphy of a loess profile in Xifeng area, Gansu Province [J].Geophysical Journal International, 92(3): 217 – 220.

Lowenstam H A. 1981. Minerals formed by organisms [J].Science, 211(4487): 1126 – 1131.

Lü H Y, Liu T S, Gu Z Y, et al. 2000. Effect of burning C3and C4plants on the magnetic susceptibility signal in soils [J].Geophysical Research Letters, 27(13): 2013 – 2016.

Lü H Y, Liu T S. 2001. The effect of C3and C4plants for themagnetic susceptibility signal in soils [J].Science in China Series D: Earth Sciences, 44(4): 318 – 326.

Lynch J M, Hobbie J E. 1988. Micro-organisms in action: concepts and applications in microbial ecology [M]// Lynch J M. The terrestrial environment. Oxford: Blackwell Scienti fi c Publications: 103 – 131.

Maher B A. 1998.Magnetic properties of modern soils and Quaternary loessic paleosols: paleoclimatic implications [J].Palaeogeography, Palaeoclimatology, Palaeoecology, 137(1 / 2): 25 – 54.

Meng X, Derbyshire E, Kemp R A. 1997. Origin of the magnetic susceptibility signal in Chinese loess [J].Quaternary Science Reviews, 16(8): 833 – 839.

Nordt L C, Boutton T W, Hallmark C T, et al. 1994. Late Quaternary vegetation and climate changes in central Texas based on the isotopic composition of organic carbon [J].Quaternary Research, 41(1): 109 – 120.

Schwartz D,Mariotti A, Lanfranchi R, et al. 1986.13C/12C ratios of soil organic matter as indicators of vegetation changes in the Congo [J].Geoderma, 39(2): 97 – 103.

Schwertmann U, Fechter H. 1984. The in fl uence of aluminum on iron oxides: Ⅺ. Aluminum-substituted maghemite in soils and its formation [J].Soil Science Society of America Journal, 48(6): 1462 – 1463.

Shilton V F, Booth C A, Smith J P, et al. 2005.Magnetic properties of urban street dust and their relationship with organic matter content in the West Midlands, UK [J].Atmospheric Environment, 39(20): 3651 – 3659.

Torrent J, Liu Q S, Barrón V. 2010.Magnetic susceptibility changes in relation to pedogenesis in a Xeralf chronosequence in northwestern Spain [J].European Journal of Soil Science, 61(2): 161 – 173.

Vali H, Förster O, Amarantidis G, et al. 1987.Magnetotactic bacteria and their magnetofossils in sediments [J].Earth and Planetary Science Letters, 86(2): 389 – 400.

Xie S, Dearing J A, Bloemendal J. 2000. The organic matter content of street dust in Liverpool, UK, and its association with dust magnetic properties [J].Atmospheric Environment, 34(2): 269 – 275.

Zhou L P, Old fi eld F, Wintle A G, et al. 1990. Partly pedogenic origin of magnetic variations in Chinese loess [J].Nature, 346: 737 – 739.

Impact of soil organic matter on modern soil magnetic susceptibility in Loess Plateau and its surrounding areas

ZHANG Bo1,2, LIU Weiguo1
（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）

Background, aim, and scopeMagnetic 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 methodsFifty modern soil samples were collected from the Loess Platform, forest areas at the Huangling and Huanglong Mount, and loess-desert area near the TenggerDesert. 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.ResultsThe magnetic susceptibility varied from 26.6×10-8m3· kg-1to 61.4×10-8m3· kg-1for soils from the loess platform, and from 68.6×10-8m3· kg-1to 107.5×10-8m3· kg-1for 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 8.5×10-8m3· kg-1to 44.4×10-8m3· 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.DiscussionCarbon 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 recordsδ13C value of plants. In this study,δ13C measurements are in agreement with the fact that there is a mixture of C4and C3plants in the loess platform region, and forest areas are controlled by C3plants. The data show that soil magnetic susceptibility is poorly correlated withδ13C 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 fi ne 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.ConclusionsOur 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 perspectivesThis study showed that soil magnetic susceptibility is closely related to organic matter in the soil. Future work might be focused on the exact mechanism thatresults in the enhancement of soil magnetic susceptibility due to increasing organic matter content.

soil magnetic susceptibility; organic matter; C / N ratio; Loess Plateau

LIU Weiguo, E-mail: liuwg@loess.llqg.ac.cn

10.7515/JEE201602005

2015-11-01；录用日期：2016-02-29

Received Date:2015-11-01;Accepted Date:2016-02-29

国家重点基础研究发展计划（2013CB955900）

Foundation Item:National Basic Research Program of China (2013CB955900)

刘卫国，E-mail: liuwg@loess.llqg.ac.cn