潘軍, 宋乃平*, 吳旭東, 楊新國, 陳林, 曲文杰, 翟德蘋

(1.寧夏大學(xué)西北土地退化與生態(tài)恢復(fù)國家重點(diǎn)實(shí)驗(yàn)室培育基地,銀川 750021；2.寧夏大學(xué)西部生態(tài)與生物資源開發(fā)聯(lián)合研究中心,銀川 750021；3.寧夏大學(xué)農(nóng)學(xué)院,銀川 750021)

荒漠草原不同種植年限人工檸條林土壤碳氮磷化學(xué)計(jì)量特征

潘軍1,2, 宋乃平1,2*, 吳旭東1,3, 楊新國1,2, 陳林1,2, 曲文杰1,2, 翟德蘋1,2

(1.寧夏大學(xué)西北土地退化與生態(tài)恢復(fù)國家重點(diǎn)實(shí)驗(yàn)室培育基地,銀川 750021；2.寧夏大學(xué)西部生態(tài)與生物資源開發(fā)聯(lián)合研究中心,銀川 750021；3.寧夏大學(xué)農(nóng)學(xué)院,銀川 750021)

以荒漠草原不同林齡(9 a,17 a,27 a)人工檸條林為對象,研究不同土層深度以及檸條灌叢不同部位的土壤有機(jī)碳(soil organic carbon,SOC)、全氮(total nitrogen,TN)和全磷(total phosphorus,TP)含量及其生態(tài)化學(xué)計(jì)量特征變化規(guī)律.結(jié)果表明：不同林齡檸條灌叢各部位SOC和TN含量在垂直剖面上的分布具有統(tǒng)計(jì)學(xué)上的極顯著差異(P<0.01)；灌叢各部位SOC、TN含量均隨林齡增加逐漸增大,且隨土層加深逐漸減小,具有明顯的表聚性,灌叢各部位均表現(xiàn)為內(nèi)部>邊緣>外部；隨著檸條林齡的增長,灌叢各部位土壤TP含量呈逐漸增加的趨勢.隨著林齡的增長,灌叢各部位土壤C/N均呈現(xiàn)逐漸降低的趨勢；而土壤C/P在灌叢內(nèi)部和邊緣呈現(xiàn)先降低后增加的趨勢,而在灌叢外部呈現(xiàn)逐漸增加的趨勢；土壤N/P在灌叢內(nèi)部和外部呈現(xiàn)逐漸增加的趨勢,而在灌叢邊緣呈現(xiàn)先降低后增加的趨勢.隨著土層深度的增加,土壤C/N呈逐漸增加的趨勢,而C/P和N/P整體上呈先降低后增加的趨勢.N/P與TP呈極顯著負(fù)相關(guān),而與TN和C/P呈極顯著正相關(guān).可見,人工檸條林在生長過程中土壤養(yǎng)分呈現(xiàn)明顯的表聚性,檸條灌叢出現(xiàn)“肥島效應(yīng)”;由于檸條在生長過程中受到諸多因素的影響,還需加強(qiáng)植被與土壤生態(tài)系統(tǒng)N/P化學(xué)計(jì)量特征,土壤N、P養(yǎng)分限制和植被養(yǎng)分適應(yīng)特征相互關(guān)系的研究.

荒漠草原； 人工檸條灌叢； 種植年限； 土壤； 碳氮磷化學(xué)計(jì)量特征

Summary The thicketization of the shrub has become a common phenomenon in the arid and semi-arid steppe worldwide. TheCaraganaintermediahas been widely used to prevent grassland degradation because of its low transpiration rate, strong resistance and wide adaptability to the environment. The life activity ofC.intermediawas not only restricted by water shortage, but also limited by soil nutrients and environmental stress. This paper revealed the balance and cycle process of soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) in the soil, in which artificial shrub and grass were mixed planting. Therefore, the results can provide basic data for research on the balance and circulation of SOC, TN, TP in the soil of desert grassland ecosystems.

The artificialC.intermediashrubs in different planting-years (9 a, 17 a, 27 a) were taken as research objects, to analyze the contents and the ecological stoichiometry of SOC, TN, TP in the depth of 0-40 cm.

The result showed that the SOC and TN contents of the artificialC.intermediashrubs with different planting-years had significant differences in the vertical section (P<0.01). The contents of SOC and TN in shrubs increased with the planting-years, but decreased with soil depth from 0 to 40 cm, indicating surface accumulation of SOC and TN in the soil. For SOC and TN contents in any part of the shrubs, the order was inside>edge>outside. The contents of TP increased in all the parts of the shrubs with the planting-years, but the soil C/N ratio of each shrub decreased gradually. Furthermore, the soil C/P ratio first decreased and then increased on the edge and the inside of the shrubs, while gradually increased in the outside of the shrubs. The soil N/P ratio in the inside and outside of shrubs both showed increasing trends, and first decreased and then increased in the edge of the shurbs. The soil C/N ratio increased gradually with the increasing soil depth, while the C/P and N/P ratios first decreased and then increased as a whole. The N/P ratio had a significant negative correlation with the TP content (P<0.01), and significant positive correlations with the TN content and the C/P ratio (P<0.01).

In conclusion, the soil nutrients in artificialC.intermediashrubs show a significant surface accumulation and “fertile island” effect. As the growth ofC.intermediashrubs was affected by various factors, further research should be considered on the N/P stoichiometry characteristics of vegetation and soil ecosystem, and the relations between the limiting nutrients of soil N, P and the nutrient adaptability characteristics of vegetation.

生態(tài)化學(xué)計(jì)量學(xué)是研究生物系統(tǒng)能量和化學(xué)元素平衡的科學(xué)[1],是目前研究元素在生物地球化學(xué)循環(huán)和生態(tài)過程中計(jì)量關(guān)系和規(guī)律的綜合有效的方法.碳(C)、氮(N)、磷(P)化學(xué)計(jì)量特征在生態(tài)系統(tǒng)中具有重要意義[2].國內(nèi)外關(guān)于植物生態(tài)化學(xué)計(jì)量已展開了大量研究,但多集中于濕地、森林和草地生態(tài)系統(tǒng)[3-6],而對干旱的人工荒漠生態(tài)系統(tǒng)[7-8]的研究甚少.干旱區(qū)植物的生命活動(dòng)不僅受水分短缺的限制,還受到土壤養(yǎng)分貧瘠以及脅迫環(huán)境的限制,其中,土壤養(yǎng)分限制將會(huì)對植物葉片中的營養(yǎng)元素含量產(chǎn)生影響[9].從土壤化學(xué)計(jì)量學(xué)角度揭示人工灌草結(jié)合的生態(tài)系統(tǒng)土壤內(nèi)部C、N、P平衡和循環(huán)過程[8,10],能為我國荒漠草原生態(tài)系統(tǒng)土壤養(yǎng)分平衡和循環(huán)研究提供基礎(chǔ)數(shù)據(jù).

荒漠草原是草原向荒漠過渡的旱生化草原生態(tài)系統(tǒng),是旱生性最強(qiáng)的一類草原[11].灌叢因其獨(dú)特的形態(tài)和生理特性,對荒漠草原較為適應(yīng),成了這些區(qū)域普遍分布的主要群落類型之一[12].草原灌叢化的自然過程和為防風(fēng)治沙建植的人工灌叢已經(jīng)成為全球范圍干旱、半干旱地區(qū)草原普遍發(fā)生的現(xiàn)象[13-14].檸條因蒸騰速率低、抗逆性和適應(yīng)性強(qiáng)等特點(diǎn)已經(jīng)被廣泛用于遏制草地退化[15-16].針對荒漠草原檸條灌叢土壤養(yǎng)分[15,17-18]、物種多樣性[19-20]、生態(tài)水文及固沙效應(yīng)[21]方面已經(jīng)進(jìn)行了大量研究,但是在檸條林植被恢復(fù)及演替過程中對于土壤化學(xué)計(jì)量特征的報(bào)道不多,而且缺乏灌叢空間尺度上的深入研究.本文以不同種植年限的檸條為研究對象,比較不同種植年限檸條灌叢土壤有機(jī)碳、全氮和全磷含量,分析不同種植年限檸條林對荒漠草原土壤化學(xué)計(jì)量特征影響,以及將計(jì)量學(xué)特征與環(huán)境因子進(jìn)行偶聯(lián)研究,進(jìn)而揭示荒漠草原區(qū)土壤C、N、P平衡和循環(huán),為荒漠草原可持續(xù)發(fā)展、環(huán)境保護(hù)和生態(tài)恢復(fù)等提供理論依據(jù)以及參考資料.

1 研究地區(qū)與研究方法

1.1 研究區(qū)概況

研究區(qū)位于寧夏鹽池縣楊寨子社(37°43′—37°55′ N,107°21′—107°38′ E),海拔1 300～1 350 m.北部與毛烏素沙地相連,東南部與黃土高原相連,屬典型溫帶大陸性氣候,氣候特點(diǎn)為干旱少雨,蒸發(fā)量大,冬春2季風(fēng)大沙多.年平均氣溫8.2 ℃,年平均降水量292 mm且主要集中在7—9月,蒸發(fā)量2 710 mm,干燥度3.1,無霜期160 d,≥10 ℃積溫2 945 ℃.地帶性土壤為灰鈣土,非地帶性土壤主要是風(fēng)沙土和鹽堿土.

1.2 研究方法

1.2.1 樣地選取 于2013年9月上旬在研究區(qū)選擇行距均為8 m的9 a(2004年種植)、17 a(1996年種植)、27 a(1986年種植)檸條林作為研究對象,分別在研究地點(diǎn)的丘陵坡地中部的坡度平緩地帶建立90 m×150 m研究樣地,將其劃分為30 m×30 m的網(wǎng)格,在每一網(wǎng)格內(nèi)選取檸條灌叢斑塊1叢,共45叢,其形態(tài)特征見表1.試驗(yàn)區(qū)地勢平坦,表層明顯沙化,3個(gè)樣地在空間上相鄰,地形、植被及土壤類型基本一致.

表1 研究區(qū)檸條形態(tài)特征

1.2.2 土壤取樣與分析 分別在9 a、17 a、27 a灌叢內(nèi)部、邊緣、外部用剖面法按0～5、5～10、10～20、20～40 cm不同土層取樣,各土層3個(gè)重復(fù),分層取樣后相同層均勻混合,共取180個(gè)土樣,于室內(nèi)自然風(fēng)干,挑去活體根系,研磨過60目篩,采用元素分析儀(Elementar Vario MACRO)測定土壤有機(jī)碳(soil organic carbon,SOC)和全氮(total nitrogen,TN)含量,采用高氯酸-硫酸消化,鉬銻抗比色法[22](UV-2450)測定全磷(total phosphorus,TP)含量.

1.3 數(shù)據(jù)統(tǒng)計(jì)與分析

數(shù)據(jù)在 Microsoft Excel 2003軟件中進(jìn)行整理,利用SPSS 19.00軟件,采用單因素方差分析(one-way ANOVA)和最小顯著差法(least-significant difference, LSD)對數(shù)據(jù)進(jìn)行檢驗(yàn),用皮爾遜(Pearson)相關(guān)系數(shù)判定數(shù)據(jù)的相關(guān)性,同時(shí)結(jié)合Origin 8.0軟件進(jìn)行制圖.

2 結(jié)果與分析

2.1 檸條灌叢SOC、TN、TP含量特征

從表2及表3中可以看出,不同林齡檸條灌叢各部位SOC和TN含量在垂直剖面上的分布呈極顯著差異(P<0.01).灌叢各部位SOC、TN含量均隨林齡增加逐漸增大,且隨土層加深逐漸減小,具有明顯的表聚性,在剖面上呈現(xiàn)出“倒金字塔”的分布(表2),各部位均表現(xiàn)為內(nèi)部>邊緣>外部(表3).具體表現(xiàn)為灌叢內(nèi)部SOC質(zhì)量分?jǐn)?shù)為2.398～6.786 g/kg,灌叢邊緣SOC質(zhì)量分?jǐn)?shù)為2.262～6.226 g/kg,灌叢外部SOC質(zhì)量分?jǐn)?shù)為2.378～4.660 g/kg；土壤TN在灌叢內(nèi)外均為17 a時(shí)最高,除9 a外,17 a及27 a檸條灌叢內(nèi)部TN含量極顯著高于灌叢外部(P<0.01),可得出檸條灌叢存在明顯的“肥島效應(yīng)”；土壤TP含量在灌叢內(nèi)部隨著土層深度的增加呈現(xiàn)先增加后減小的趨勢,而在灌叢邊緣和外部隨著土層深度的增加整體上呈現(xiàn)遞減的趨勢,隨著檸條林齡的增長灌叢各部位土壤TP含量呈逐漸增加的趨勢,在9 a時(shí)檸條灌叢土壤TP含量為外部>邊緣>內(nèi)部,而在17 a和27 a時(shí)檸條灌叢土壤TP含量為內(nèi)部>邊緣>外部,隨著土層深度的增加,TP含量呈波動(dòng)性變化,在土壤剖面上表現(xiàn)為“圓柱體”分布模式.

2.2 不同林齡SOC、TN、TP化學(xué)計(jì)量特征

從表3可知,不同林齡檸條灌叢各部位土壤C/N、C/P和N/P呈現(xiàn)規(guī)律性的變化,具體表現(xiàn)為：不同林齡檸條灌叢從內(nèi)部、邊緣到外部其C/N均表現(xiàn)為先增大后減小的變化規(guī)律；C/P在 9 a及27 a表現(xiàn)為邊緣>外部>內(nèi)部,而在17 a表現(xiàn)為外部>邊緣>內(nèi)部；N/P在9 a和27 a時(shí)從灌叢內(nèi)部到外部表現(xiàn)為逐漸降低的趨勢,而在17 a表現(xiàn)為先減小后增大的規(guī)律.另外,隨著林齡的增加,灌叢各部位土壤C/N均呈現(xiàn)逐漸降低的趨勢；而土壤C/P在灌叢內(nèi)部和邊緣呈現(xiàn)先減小后增加的趨勢,而在灌叢外部呈現(xiàn)逐漸增加的趨勢；土壤N/P在灌叢內(nèi)部和外部呈現(xiàn)逐漸增加的趨勢,而在灌叢邊緣呈現(xiàn)先減小后增加的趨勢.9 a檸條灌叢土壤的C∶N∶P (61.87∶4.85∶1)顯著高于17 a檸條灌叢土壤的C∶N∶P(41.83∶3.63∶1),同時(shí)也高于27 a檸條灌叢土壤的C∶N∶P(36.88∶3.32∶1).

表2 灌叢斑塊不同部位的SOC、TN、TP質(zhì)量分?jǐn)?shù)

SOC:土壤有機(jī)碳；TN:全氮；TP:全磷.同列數(shù)據(jù)后的不同大寫字母表示在同一種植年限灌叢的不同土層間在P<0.01水平差異有統(tǒng)計(jì)學(xué)意義.

SOC: Soil organic carbon; TN: Total nitrogen; TP: Total phosphorus. Different capital letters in the same column indicate significant differences at the different soil layers of the same planting-year shrub at the 0.01 probability level.

表3 不同林齡各部位SOC、TN、TP化學(xué)計(jì)量特征

SOC:土壤有機(jī)碳；TN:全氮；TP:全磷.同列數(shù)據(jù)后的不同大寫字母表示在P<0.01水平差異有統(tǒng)計(jì)學(xué)意義.

SOC: Soil organic carbon; TN: Total nitrogen; TP: Total phosphorus. Different capital letters in the same column indicate significant differences at the 0.01 probability level.

2.3 不同深度土層SOC、TN、TP化學(xué)計(jì)量特征

如圖1所示：隨著土層深度的增加,土壤C/N呈逐漸增加的趨勢,而C/P和N/P整體上呈先降低后增加的趨勢.隨著土層深度的增加,在各年限中土壤C/N在灌叢內(nèi)部、邊緣、外部均呈現(xiàn)逐漸增大的趨勢；土壤C/P在各年限灌叢的內(nèi)部和邊緣先降低后增加,而在灌叢外部,9 a和17 a時(shí)先降低后升高,在27 a時(shí)呈逐漸降低的趨勢；各年限土壤N/P在灌叢內(nèi)部均逐漸降低,而在灌叢邊緣和外部在9 a和17 a時(shí)先降低后升高,在27 a時(shí)呈逐漸降低的趨勢.

2.4 SOC、TN、TP含量與化學(xué)計(jì)量比之間的皮爾遜(Pearson)相關(guān)分析

由表4可以看出,研究區(qū)SOC與TN含量呈極顯著相關(guān)(P<0.01).SOC與C/N呈極顯著負(fù)相關(guān),而與C/P之間呈極顯著正相關(guān)(P<0.01)；TN與C/N和N/P呈極顯著正相關(guān)(P<0.01)；TP與C/N和N/P均呈極顯著負(fù)相關(guān),而與C/P呈極顯著正相關(guān)(P<0.01).C/N值與TN的相關(guān)性大于與SOC的相關(guān)性,表明研究區(qū)土壤中C/N值主要受土壤TN的影響；C/P值與SOC的相關(guān)性大于與TP的相關(guān)性,說明土壤中的C/P值主要受SOC的影響；N/P值與TP呈極顯著負(fù)相關(guān),而與TN含量呈極顯著正相關(guān)；C/P與N/P呈極顯著正相關(guān).

圖1 不同深度土層SOC、TN、TP化學(xué)計(jì)量特征

表4 SOC、TN、TP含量與化學(xué)計(jì)量比之間的皮爾遜相關(guān)分析

SOC:土壤有機(jī)碳；TN:全氮；TP:全磷.**表示在0.01水平(雙尾檢驗(yàn))顯著相關(guān).

SOC: Soil organic carbon; TN: Total nitrogen; TP: Total phosphorus. **Correlation is significant at the 0.01 probability level (2-tailed).

3 討論

本研究不同林齡檸條灌叢各部位SOC和TN含量在垂直剖面上的分布具有統(tǒng)計(jì)學(xué)上的極顯著差異(P<0.01).SOC的垂直分布規(guī)律是由于根系分布深度及其分泌物不同導(dǎo)致的,檸條灌叢根系入土較深,檸條根系死亡會(huì)增加土壤中的有機(jī)質(zhì),因此SOC在其根系分布密集處含量最高并出現(xiàn)峰值[23-24].此外,檸條灌叢枯枝落葉的分解和礦化也會(huì)導(dǎo)致土壤有機(jī)質(zhì)含量提高[25].檸條林地的土壤氮素主要來源于大氣氮沉降、根系吸收與有機(jī)質(zhì)分解和根瘤固氮作用的平衡[26],檸條根系的生長改變了土壤TN含量的垂直分布狀況,從而使SOC和TN具有明顯的表聚性,在剖面上呈現(xiàn)出“倒金字塔”的分布.土壤P素來源相對固定,主要通過巖石的風(fēng)化.由于巖石風(fēng)化是一個(gè)漫長的過程,風(fēng)化程度在0～40 cm土壤層中差異不大,這就使得TP在土壤中的垂直分布呈現(xiàn)上下差異不大的“圓柱體”形狀,而在灌叢邊緣和外部隨著土層深度增加呈現(xiàn)遞減的趨勢,說明土壤微生物垂直方向的固氮作用和養(yǎng)分歸還效應(yīng)高于其根系分布及分泌物等影響[27],同時(shí)也說明荒漠草原引入檸條灌叢后改變了土壤可利用碳源和氮源的微生物群落的垂直分布[28],從而導(dǎo)致SOC與TN和TP在土壤垂直方向分布規(guī)律表現(xiàn)出一定的偏差.本研究不同林齡檸條灌叢不同部位SOC和TN含量均呈內(nèi)部>邊緣>外部,隨著檸條林齡的增長灌叢內(nèi)部和邊緣TP含量逐漸增加,這與其他學(xué)者的研究結(jié)果[29]一致,通過比較可以得出檸條灌叢存在明顯的“肥島效應(yīng)”.

碳氮磷比(C/N/P)是土壤有機(jī)質(zhì)或其他成分中的C、N、P總質(zhì)量的比值,是衡量土壤有機(jī)質(zhì)組成和營養(yǎng)平衡的一個(gè)重要指標(biāo)[29],是確定土壤C、N、P平衡特征的重要參數(shù)[30].與全球不同生態(tài)系統(tǒng)土壤的平均水平(C∶N∶P=186∶13∶1)[30]相比,本研究不同林齡灌叢檸條的C/N、C/P和N/P均較低.Tian等[31]在對土壤C/N/P的研究中也指出,雖然SOC和TN含量具有較大的空間變異性,但C/N相對穩(wěn)定,受氣候的影響很小.而本研究隨著檸條林齡的增長,土壤C/N有降低的趨勢,這是由于檸條為豆科灌木,根部有大量可以固定空氣中的游離氮素的根瘤菌,從而使土壤含氮量增加[32].土壤C/P比值可作為衡量微生物礦化土壤有機(jī)物質(zhì)釋放磷或從環(huán)境中吸收固持磷素潛力的一種指標(biāo)[23].在本研究中隨檸條土層深度的增加土壤C/P在各年限灌叢的內(nèi)部和邊緣先降低后增加.研究區(qū)土壤的C/P值為46,低于我國平均值(105),表明研究區(qū)土壤微生物體有機(jī)磷出現(xiàn)了凈礦化現(xiàn)象[33].在檸條灌叢0～40 cm土壤中,C/N隨著土層深度的增加均呈現(xiàn)逐漸增加的趨勢,主要是檸條灌叢氮下降幅度大于碳直接造成的；隨著土層深度的增加土壤的C/P在各年限灌叢的內(nèi)部和邊緣先降低后增加,隨檸條林齡的增長,土壤C、N、P含量也逐漸增加,而表層C/N和C/P值呈下降趨勢,深層土壤C/N和C/P的降低幅度不大,說明在林齡增加過程中,表層SOC的增加導(dǎo)致了N素和P素的增加,在同等程度下表層N素和P素的增加比C素增加更為敏感,而深層土壤C/N和C/P變化幅度并不明顯,說明深層土壤中N素和P素的增加與C素增加保持一致.土壤TN、TP狀況對植物的影響不僅表現(xiàn)為植物N、P營養(yǎng)失衡作用,也表現(xiàn)在土壤TN、TP含量高低對植物造成的營養(yǎng)貧瘠或毒害作用[34].

本研究檸條灌叢土壤SOC、TN含量呈極顯著相關(guān)(P<0.01),這一結(jié)果與前人的研究結(jié)果[35]相似.土壤TP與C/N和N/P均呈現(xiàn)出極顯著負(fù)相關(guān),而與C/P呈極顯著正相關(guān),C/N值與TN的相關(guān)性大于與SOC的相關(guān)性,表明研究區(qū)土壤中C/N值主要受土壤TN的影響；C/P值與SOC的相關(guān)性大于與TP的相關(guān)性,說明土壤中C/P值主要受土壤SOC的影響；N/P值與TP呈極顯著負(fù)相關(guān),而與TN含量呈極顯著正相關(guān),說明TN增加對N/P有貢獻(xiàn)作用。由于在檸條生長過程中土壤養(yǎng)分限制與植物特性差異的相互作用難以區(qū)分,如果進(jìn)一步研究其限制因素,還需對其葉片N和P含量進(jìn)行綜合研究,從而確定檸條灌叢土壤N/P是受N限制還是P限制.

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Effects of different planting-years of artificialCaraganaintermediashrubs on soil organic carbon, nitrogen and phosphorus stoichiometry characteristics in desert steppe. Journal of Zhejiang University (Agric. & Life Sci.), 2015,41(2):160-168

Pan Jun1,2, Song Naiping1,2*, Wu Xudong1,3, Yang Xinguo1,2, Chen Lin1,2, Qu Wenjie1,2, Zhai Deping1,2

(1.BreedingBaseofStateKeyLaboratoryforPreventingLandDegradationandEcologicalRestoration,NingxiaUniversity,Yinchuan750021,China; 2.UnionResearchCenterforEcologyandExploitionofBiologicalResourcesinWesternChina,NingxiaUniversity,Yinchuan750021,China; 3.CollegeofAgriculture,NingxiaUniversity,Yinchuan750021,China)

desert steppe; artificialCaraganaintermediashrubs; planting-years; soil; C, N, P stoichiometry characteristics

國家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃(973計(jì)劃)前期專項(xiàng)(2012CB723206);國家自然科學(xué)基金(41461046;31460123;31460161).

聯(lián)系方式：潘軍,E-mail:pj19890101shiyan@163.com

2014-05-04；接受日期(Accepted)：2014-09-02；網(wǎng)絡(luò)出版日期(Published online)：2015-03-20

S 154.1； S 812.2

A

*通信作者(Corresponding author)：宋乃平,E-mail:songnp@163.com

URL:http://www.cnki.net/kcms/detail/33.1247.S.20150320.2111.012.html