黃土高原及周邊地區(qū)土壤有機(jī)質(zhì)對(duì)現(xiàn)代土壤磁化率的影響

張 博，劉衛(wèi)國(guó)

（1.中國(guó)科學(xué)院地球環(huán)境研究所，黃土與第四紀(jì)地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室，西安 710061；2.中國(guó)科學(xué)院大學(xué)，北京 100049）

黃土高原及周邊地區(qū)土壤有機(jī)質(zhì)對(duì)現(xiàn)代土壤磁化率的影響

張 博1,2，劉衛(wèi)國(guó)1

（1.中國(guó)科學(xué)院地球環(huán)境研究所，黃土與第四紀(jì)地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室，西安 710061；2.中國(guó)科學(xué)院大學(xué)，北京 100049）

磁化率是黃土-古土壤序列古氣候研究的一個(gè)重要指標(biāo)。本文調(diào)查了黃土高原及周邊地區(qū)三種類(lèi)型土壤的磁化率、土壤有機(jī)碳含量、有機(jī)碳同位素組成和碳氮比值等指標(biāo)。樣品采集自黃土-沙漠過(guò)渡區(qū)、黃土塬面和森林地區(qū)，代表了黃土高原地區(qū)主要的土壤類(lèi)型。結(jié)果顯示：黃土塬面、林區(qū)、黃土-沙漠過(guò)渡區(qū)土壤的磁化率變化區(qū)間分別為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。黃土塬面土壤有機(jī)碳含量在0.05%到0.62%之間變化，而林區(qū)土壤的有機(jī)碳含量在1.19%到3.35%間變化。黃土塬面的土壤C / N比值也較低，在0.6到6.1之間變化，林區(qū)樣品C / N比值在6.2到11.83之間變化。黃土-沙漠過(guò)渡區(qū)土壤磁化率較低，森林地區(qū)土壤磁化率較高，土壤磁化率與有機(jī)碳含量、C / N比值呈正相關(guān)關(guān)系。筆者認(rèn)為有機(jī)質(zhì)含量增加對(duì)土壤的磁化率增強(qiáng)有明顯貢獻(xiàn)。有機(jī)質(zhì)含量較高時(shí)，更適宜土壤中磁性細(xì)菌的生長(zhǎng)。同時(shí)，較高的有機(jī)質(zhì)含量指示著較高植被覆蓋，這也對(duì)土壤中磁性礦物增加有一定貢獻(xiàn)。燃燒有機(jī)質(zhì)還會(huì)使非磁性礦物更易轉(zhuǎn)化為磁性礦物。這些因素都會(huì)增強(qiáng)土壤的磁化率。

土壤磁化率；有機(jī)質(zhì)；C / N比值；黃土高原

磁化率指示物質(zhì)被磁化的難易程度，是黃土-古土壤序列古氣候研究的一個(gè)重要指標(biāo)（Heller and Liu，1982；王俊達(dá)等，1987；Liu et al，1988；An et al，1991，1995；劉秀銘和劉東生，1993；朱日祥等，1994；強(qiáng)小科等，2004；劉青松，2009；賈佳等，2011；趙國(guó)永等，2012；趙輝等，2012；安芷生等，2015）。在黃土高原地區(qū)，一般認(rèn)為磁化率指示了季風(fēng)強(qiáng)度的變化（An et al，1991）。黃土古土壤中磁化率信號(hào)的主要載體是細(xì)顆粒部分，其中磁鐵礦和赤鐵礦對(duì)磁化率有著主要的貢獻(xiàn)（安芷生等，1990）。在黃土高原，古土壤磁化率要比黃土高，對(duì)于磁化率增強(qiáng)的原因，前人做了很多研究，提出了多種解釋。Heller and Liu（1984）認(rèn)為沉積物壓實(shí)和碳酸鹽淋濾作用使古土壤磁化率增強(qiáng)。Zhou et al（1990）認(rèn)為是成壤過(guò)程中細(xì)粒磁性礦物的生成使土壤磁化率增強(qiáng)。Kukla et al（1988）認(rèn)為是由物源不同導(dǎo)致黃土、古土壤磁化率的差異。Meng et al（1997）連續(xù)4年每月采集中國(guó)北方的降塵，通過(guò)分析這些樣品，發(fā)現(xiàn)土壤中極細(xì)的磁性顆粒很有可能是來(lái)自于降解的植被枯落物。一般認(rèn)為，成壤作用是導(dǎo)致黃土磁化率增強(qiáng)的主要原因（Zhou et al，1990；Hellar and Evans，1995；鄧成龍等，2007；劉青松，2009）。

土壤有機(jī)質(zhì)是土壤的重要組成部分，其含量是表征土壤肥力的一個(gè)重要指標(biāo)。土壤有機(jī)質(zhì)會(huì)影響其物理、化學(xué)、生物等多種性質(zhì)。有機(jī)質(zhì)和磁化率間存在何種關(guān)系是一個(gè)值得關(guān)注的問(wèn)題。有學(xué)者在城市街道灰塵和河流階地樣品中發(fā)現(xiàn)樣品磁化率和有機(jī)質(zhì)含量成正相關(guān)關(guān)系 （Xie et al，2000；Shilton et al，2005；Torrent et al，2010）；Maher（1998）曾報(bào)道，英國(guó)雛形土中土壤磁化率和有機(jī)質(zhì)含量成正相關(guān)關(guān)系。Grimley et al（2004）通過(guò)研究美國(guó)伊利諾伊州東北部的黑土，發(fā)現(xiàn)在土壤有機(jī)質(zhì)含量最高的區(qū)域其磁化率也最高。張普和劉衛(wèi)國(guó)（2008）對(duì)黃土高原中部豎井剖面的磁化率和總有機(jī)碳含量（TOC）進(jìn)行了對(duì)比，發(fā)現(xiàn)這兩種指標(biāo)有著良好的對(duì)應(yīng)關(guān)系，成壤作用較強(qiáng)的暖濕時(shí)期磁化率和TOC均出現(xiàn)高值，成壤作用較弱的干冷時(shí)期也同時(shí)出現(xiàn)低值；其他學(xué)者的研究也得到了相似的結(jié)果（胡雪峰，2004；李明啟等，2005；謝巧勤等，2012）。

本文主要研究黃土高原地區(qū)土壤磁化率與土壤有機(jī)質(zhì)間的相關(guān)關(guān)系。測(cè)試了黃土高原地區(qū)現(xiàn)代土壤的磁化率、有機(jī)碳含量、有機(jī)碳同位素組成和碳氮比值（C / N）指標(biāo)，以期能得到土壤磁化率與相關(guān)指標(biāo)間的關(guān)系，以及有機(jī)質(zhì)對(duì)土壤磁化率的影響。

1 材料與方法

在黃土高原及周邊地區(qū)的森林地區(qū)、黃土塬面和黃土-沙漠過(guò)渡區(qū)共采集了50份土壤樣品。黃土塬面采樣點(diǎn)位于陜北榆林、延安等地，森林采樣點(diǎn)位于黃陵山區(qū)和黃龍山區(qū)，黃土-沙漠過(guò)渡區(qū)在騰格里沙漠周邊，采樣點(diǎn)分布見(jiàn)圖1。

所采集到的土壤樣品代表了黃土高原地區(qū)幾種典型的土壤類(lèi)型。為避免人類(lèi)和牲畜活動(dòng)的影響，采集距離地表2—3 cm深度的樣品，這一深度的土壤有機(jī)質(zhì)已經(jīng)處于穩(wěn)定的狀態(tài)。采樣點(diǎn)距離工業(yè)區(qū)在40 km以上，避免了人造磁性物質(zhì)經(jīng)空氣傳播對(duì)樣品造成污染，這樣將外部因素對(duì)磁化率的影響最小化。

土壤樣品在50℃的條件下烘干。將可見(jiàn)的植物根系從土壤樣品中挑出，再稱(chēng)量10 g土壤樣品，利用Bartington公司MS2磁化率儀測(cè)得樣品的低頻磁化率（χlf，0.465 kHz）。有機(jī)碳和有機(jī)氮的含量通過(guò)美國(guó)Leco公司CS-344元素分析儀測(cè)得，精度為4%。

為測(cè)量樣品的δ13C值，將3 g土壤樣品在瑪瑙研缽中研磨并過(guò)篩，使樣品粒徑小于150 μm。過(guò)篩后的樣品在室溫下用2 mol · L-1鹽酸浸泡24小時(shí)，以去除其中的碳酸鹽和可溶于酸的有機(jī)質(zhì)，將酸處理過(guò)的樣品用蒸餾水洗至pH＞4后在60℃條件下烘干。烘干后的樣品和銀箔與氧化銅一起放入石英管中密封，在800—850℃條件下加熱至少4小時(shí)，通過(guò)液氮將制得的CO2氣體分離純化后進(jìn)行碳同位素測(cè)試。利用MAT-251氣相質(zhì)譜進(jìn)行分析，同位素比值表示為相對(duì)于PDB標(biāo)準(zhǔn)的千分偏差，精度為0.2‰，δ13C值通過(guò)下式計(jì)算得到：

圖1 采樣點(diǎn)分布Fig.1 The sampling sites

2 結(jié)果

表1列出了測(cè)得的黃土塬面和林區(qū)土壤樣品的磁化率值、有機(jī)碳同位素組成、有機(jī)碳含量和C / N比值。黃土塬面土壤樣品的磁化率值（χlf）在26.6×10-8m3· kg-1到61.4×10-8m3· kg-1之間變化，林區(qū)土壤樣品的磁化率值在68.6×10-8m3· kg-1到107.5×10-8m3· kg-1之間變化，林區(qū)現(xiàn)代土壤樣品的磁化率值顯著高于黃土塬面上樣品的值。由表2可見(jiàn)，騰格里沙漠周邊地區(qū)樣品的磁化率值在8.5×10-8m3· kg-1到44.4×10-8m3· kg-1之間變化。黃土塬面上土壤樣品的δ13C值在-22‰到-24.4‰之間變化；騰格里沙漠周邊土壤樣品的δ13C值在-20.66‰到-24.69‰間變化，這兩地區(qū)樣品值的變化區(qū)間大致相同。而從黃陵山區(qū)和黃龍山區(qū)森林采集的土壤樣品δ13C值在-24.5‰到-26.9‰間變化，相比黃土塬面和黃土-沙漠過(guò)渡區(qū)的樣品值顯著偏負(fù)。

黃土塬面土壤樣品的有機(jī)碳含量較低，在0.05%到0.62%之間變化，而林區(qū)土壤樣品的有機(jī)碳含量變化區(qū)間為1.19%到3.35%。C / N比值也顯示了相似的變化特征，黃土塬面的C / N比值較低，在0.6到6.1之間變化，而林區(qū)樣品值在6.2到11.83之間變化。

總體來(lái)看，不同采樣地區(qū)（黃土-沙漠過(guò)渡帶、黃土區(qū)、林區(qū)）的指標(biāo)有不同的變化范圍。在黃土塬面和黃土-沙漠過(guò)渡帶，土壤樣品的磁化率、有機(jī)碳含量、C / N比值較低，δ13C值偏正；林區(qū)土壤樣品的情況相反，有較高的磁化率值、有機(jī)碳含量、C / N比值，而δ13C值更偏負(fù)。已有學(xué)者認(rèn)為有機(jī)質(zhì)含量增加會(huì)使土壤的磁化率值上升（賈蓉芬等，1992），此次在黃土高原得到的結(jié)果支持了這一觀點(diǎn)。如圖2所示，土壤磁化率隨著土壤有機(jī)碳含量的增加而上升，林區(qū)的土壤碳含量較高，它們的磁化率值也比黃土塬面和沙漠地區(qū)的土壤磁化率高。

表1 黃土塬面和森林地區(qū)土壤樣品磁化率值、有機(jī)碳同位素、有機(jī)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 騰格里沙漠周邊黃土-沙漠過(guò)渡帶樣品磁化率值(χlf)與δ13CTab.2 The magnetic susceptibility (χlf) andδ13C of modern soil from loess-desert area near the Tengger Desert

圖2 土壤磁化率與有機(jī)碳含量、C/N比值間的關(guān)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 討論

土壤的有機(jī)質(zhì)主要來(lái)自陸生植物，因此土壤的有機(jī)碳同位素反映了植被生長(zhǎng)狀況（Schwartz et al，1986；Cerling et al，1989；Ambrose and Sikes，1991；Nordt et al，1994；Boutton and Yamasaki，1996；Hatté et al，1998；劉衛(wèi)國(guó)等，2002；Liu et al，2005；寧有豐，2010；匡歡傳等，2013）。黃土塬面上是C4植物和C3植物混合生長(zhǎng)的，而在林區(qū)則是C3植物占主導(dǎo)地位，本研究測(cè)得的黃土塬面和林區(qū)土壤有機(jī)碳同位素值與這一情況是相符的，即林區(qū)值更偏負(fù)，黃土塬面值偏正。圖3顯示，在三種采樣地，土壤磁化率值和δ13C值的相關(guān)性均較弱。可見(jiàn)植被類(lèi)型并不會(huì)對(duì)土壤磁化率產(chǎn)生很大的影響。

圖3 土壤磁化率與土壤碳同位素值間的關(guān)系Fig.3 Relationship between magnetic susceptibility (χlf) and organic carbon isotopic composition of modern soils

現(xiàn)代土壤的C / N比值反映了生長(zhǎng)在土壤上植被的生物量（張普和劉衛(wèi)國(guó)，2008），植被覆蓋度高的情況下，土壤的碳含量和C / N比都會(huì)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相比而言較低，因?yàn)樵谥参锝到鉃橥寥烙袡C(jī)質(zhì)的過(guò)程中，C / N比值會(huì)隨之下降（Hedges and Oades，1997）。王維奇等人研究發(fā)現(xiàn)，土壤碳分解速率與土壤C / N存在顯著的負(fù)相關(guān)關(guān)系，所以，C / N可以作為預(yù)測(cè)有機(jī)質(zhì)分解速率的一個(gè)很好的指標(biāo)（王維奇等，2011）。本結(jié)果證明現(xiàn)代土壤的磁化率和有機(jī)碳含量、C / N比值均呈正相關(guān)關(guān)系。高碳含量和高C / N比值指示著高有機(jī)質(zhì)含量，有機(jī)質(zhì)可能從以下幾個(gè)方面導(dǎo)致磁化率的增強(qiáng)。

一部分有機(jī)質(zhì)自身具有磁性。磁化率測(cè)定結(jié)果表明，有機(jī)質(zhì)隨著時(shí)間推移會(huì)逐漸從順磁性向抗磁性轉(zhuǎn)變。黃土在地質(zhì)歷史中屬于近代地表沉降物，其中的有機(jī)質(zhì)演變程度相當(dāng)于泥炭階段，所以具有較高的順磁性（賈蓉芬等，1992）。有機(jī)質(zhì)存在的條件下，無(wú)定形鐵老化為氧化鐵時(shí)更易于形成磁性較強(qiáng)的磁赤鐵礦；有機(jī)質(zhì)也可阻礙磁性礦物的老化，例如無(wú)定形水合氧化鐵在吸附有機(jī)質(zhì)后，氧化鐵晶核的生長(zhǎng)受到阻礙,使得無(wú)定形鐵不易老化為針鐵礦，針鐵礦和磁赤鐵礦不易老化為赤鐵礦（胡雪峰，2004）。

土壤有機(jī)質(zhì)主要來(lái)自植物殘?bào)w，前人研究表明，植物體可從多方面引起土壤磁化率的增強(qiáng)。植物殘?bào)w分解或燃燒會(huì)使土壤中磁性礦物增多（Meng et al，1997），從而增強(qiáng)土壤的磁性。植物在生長(zhǎng)過(guò)程中，根系會(huì)分泌多種物質(zhì)如H+，HCO2-等，這些物質(zhì)會(huì)改變根系土壤的pH值，使礦物發(fā)生轉(zhuǎn)化；根系的活動(dòng)還會(huì)消耗或提供氧，這樣也會(huì)改變根系周?chē)难趸€原環(huán)境，活化礦物中的鐵，這有助于磁性礦物的形成（Lü and Liu，2001）。

在土壤有機(jī)質(zhì)含量較高的區(qū)域，豐富的有機(jī)質(zhì)和發(fā)育較好的土壤可以為土壤中的微生物提供養(yǎng)分，支持微生物的活動(dòng)。一些種類(lèi)的微生物可發(fā)生礦化，導(dǎo)致土壤磁性增強(qiáng)。微生物的礦化分為兩種形式（Lowenstam，1981），一種是細(xì)胞外的生物誘導(dǎo)礦化，即微生物改變周邊的環(huán)境，使磁性礦物易聚集在生物體周?chē)Ａ硪环N是發(fā)生在細(xì)胞體內(nèi)的生物有機(jī)礦化，在這種方式中，細(xì)胞內(nèi)部存在有鐵元素。這類(lèi)細(xì)胞內(nèi)含有鐵元素的細(xì)菌是由Blakemore首次在鹽沼中發(fā)現(xiàn)的（Blakemore，1975），稱(chēng)為趨磁細(xì)菌。趨磁細(xì)菌里鐵的含量可高達(dá)干重的3%—4%，這一值約是一般生物鐵含量的100倍（李志文等，2008）。土壤有機(jī)質(zhì)、植物枯落物和土壤微生物相互作用使土壤中的極細(xì)的趨磁細(xì)菌增多（Vali et al，1987；Fassbinder et al, 1990）。前人的研究已證明，這些具有磁性的微體細(xì)菌可能與土壤中一些細(xì)粒磁鐵礦的產(chǎn)生有關(guān)（Jia et al，1996；Maher，1998；賈蓉芬等，2001）。趨磁細(xì)菌死亡以后，體內(nèi)的磁小體會(huì)保存下來(lái)，當(dāng)趨磁細(xì)菌在土壤中數(shù)量較大時(shí)，很可能在一定程度上影響土壤磁化率的大小。

有學(xué)者通過(guò)燃燒實(shí)驗(yàn)證明，對(duì)同一研究點(diǎn)來(lái)說(shuō)，表面覆蓋有植被灰燼的土壤磁化率比天然土壤磁化率要更高（Lü et al，2000）；在有機(jī)質(zhì)存在且缺氧的條件下，加熱針鐵礦和其他非鐵磁性礦物到400—500℃時(shí)，可以將這些礦物轉(zhuǎn)化為磁鐵礦（Schwertmann and Fechter，1984）。黃土沉積物中的有機(jī)質(zhì)在高溫加熱的條件下會(huì)消耗氧，使周邊形成相對(duì)還原的局部環(huán)境，含硅酸鹽礦物或粘土礦物等就可能會(huì)轉(zhuǎn)化成磁鐵礦（鄧成龍等，2007）。因此自然火災(zāi)對(duì)黃土高原土壤磁化率的增強(qiáng)也有一定的積極貢獻(xiàn)。

4 結(jié)論

對(duì)黃土塬面、森林地區(qū)、黃土-沙漠過(guò)渡帶的土壤磁化率、有機(jī)碳含量等指標(biāo)進(jìn)行了研究，結(jié)果顯示黃土塬面的土壤樣品的磁化率值、有機(jī)碳含量、C / N比值較低，δ13C值偏正；林區(qū)土壤樣品的情況相反，有較高的磁化率值、有機(jī)碳含量、C / N比值，而δ13C值更偏負(fù)。有機(jī)質(zhì)可能從以下幾方面影響土壤磁化率：高有機(jī)質(zhì)含量指示著高植被覆蓋度，植被對(duì)磁化率的增強(qiáng)有積極貢獻(xiàn)；有機(jī)質(zhì)豐富的土壤更適合磁性細(xì)菌的生長(zhǎng)，當(dāng)磁性細(xì)菌死亡時(shí)，體內(nèi)的磁小體會(huì)保存在土壤中從而增強(qiáng)土壤的磁化率；燃燒有機(jī)質(zhì)時(shí)非磁性礦物更易轉(zhuǎn)化為磁性礦物。綜合以上這些因素，可以認(rèn)為有機(jī)質(zhì)含量增加對(duì)土壤磁化率的增強(qiáng)有明顯貢獻(xiàn)。

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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；錄用日期：2016-02-29

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

國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃（2013CB955900）

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

劉衛(wèi)國(guó)，E-mail: liuwg@loess.llqg.ac.cn