李建平,謝應(yīng)忠,2
(1.寧夏大學(xué)農(nóng)學(xué)院,寧夏 銀川 750021;2.西北退化生態(tài)系統(tǒng)恢復(fù)與重建國(guó)家重點(diǎn)實(shí)驗(yàn)室培育基地,寧夏 銀川 750021)
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封育對(duì)黃土高原天然草地深層土壤碳、氮儲(chǔ)量的影響
李建平1,謝應(yīng)忠1,2
(1.寧夏大學(xué)農(nóng)學(xué)院,寧夏 銀川 750021;2.西北退化生態(tài)系統(tǒng)恢復(fù)與重建國(guó)家重點(diǎn)實(shí)驗(yàn)室培育基地,寧夏 銀川 750021)
封育是當(dāng)前恢復(fù)和改良退化草地的重要措施,準(zhǔn)確評(píng)估植被變化過(guò)程中土壤碳、氮儲(chǔ)量動(dòng)態(tài),具有重要的生態(tài)意義。本研究以寧夏云霧山封育30年天然草地和未封育草地土壤為研究對(duì)象,對(duì)深層(0-500 cm)土壤有機(jī)碳(SOC)、全氮(STN)固持動(dòng)態(tài)進(jìn)行分析。結(jié)果表明,封育30年草地土壤碳含量(0-440 cm)顯著高于未封育草地(P<0.05),但是草地SOC變化臨界層不明顯,未封育草地SOC含量變化臨界層大約位于130 cm處;封育與未封育草地STN含量在130 cm以下顯著降低,STN的累積均發(fā)生在60-320 cm土壤中,而表層(0-60 cm)和較深層(>320 cm)中STN積累較少;封育30年草地340-500 cm土壤SOC儲(chǔ)量趨于穩(wěn)定,每20 cm土層約為11.5 mg·hm-2,未封育草地140-500 cm土壤SOC儲(chǔ)量趨于穩(wěn)定,每20 cm土層維持在8 mg·hm-2;封育30年草地0-100 cm土壤SOC、STN儲(chǔ)量分別占0-500 cm土層的44.8%和27.2%,0-200 cm分別占72.2%和38.5%,0-300 cm分別占83.5%和64%,0-400 cm分別占92.2%和80.1%。因此,深層土壤有著大量的碳、氮儲(chǔ)量信息,對(duì)于100-500 cm土壤碳、氮?jiǎng)討B(tài)研究非常必要。
土壤有機(jī)質(zhì);土壤全氮;封育;天然草地;深層土壤
草地是世界上分布最廣的植被類(lèi)型之一,也是目前受人類(lèi)活動(dòng)影響最為嚴(yán)重的區(qū)域,作為陸地生態(tài)系統(tǒng)的重要組成部分[1],覆蓋了全球陸地面積的26%,參與了全球碳、氮源、匯及其循環(huán)過(guò)程,在區(qū)域和全球氣候變化及碳、氮循環(huán)中扮演著重要的角色[2-3]。另外,草地90%的碳和氮貯存于土壤中,土壤有機(jī)碳(SOC)、全氮含量(STN)與分布對(duì)地表覆蓋變化,大氣組成及氣候變化有著重要的影響[4-7]。SOC和STN及其動(dòng)態(tài)平衡是反映土壤質(zhì)量和草地健康的重要指標(biāo),直接影響著草地土壤肥力和草地生產(chǎn)力[8-9]。草地生態(tài)系統(tǒng)中,碳、氮循環(huán)密切相關(guān),它們?cè)谙嗷プ饔煤拖嗷ビ绊懙倪^(guò)程中發(fā)生著復(fù)雜的變化[10],研究草地碳氮循環(huán)耦合特征、氣候變化和人類(lèi)擾動(dòng)關(guān)系,直接影響著人類(lèi)發(fā)展和全球變化[11-12]。因此,無(wú)偏估計(jì)草地土壤碳、氮固持對(duì)生態(tài)評(píng)估、尋找碳匯丟失之謎和應(yīng)對(duì)全球氣候變暖具有重要意義。
目前對(duì)于草地土壤碳、氮?jiǎng)討B(tài)評(píng)估的研究主要集中在表層土壤(0-100 cm),而對(duì)于深層(>100 cm)土壤碳、氮儲(chǔ)量研究報(bào)道較少,已有報(bào)道證實(shí)土地利用變化對(duì)0-200 cm土壤碳、氮有著重要影響[13-14],植被恢復(fù)對(duì)深層(0-400 cm)土壤有機(jī)碳、氮的積累至關(guān)重要[15-16]。但是有關(guān)封育草地土壤碳、氮含量的確定及其碳、氮固持發(fā)生土層的研究仍不足?;诖耍狙芯恳渣S土高原半干旱區(qū)寧夏固原云霧山封育草原為研究對(duì)象,假設(shè)不同時(shí)期封育草地空間信息能夠代替草地時(shí)間演替序列,通過(guò)野外調(diào)查和實(shí)驗(yàn)室分析相結(jié)合的方法,對(duì)封育30年草地和未封育草地0-500 cm SOC、STN動(dòng)態(tài)特征進(jìn)行研究,探討SOC、STN固持動(dòng)態(tài)及碳、氮流失或封存的發(fā)生土層,以期對(duì)黃土高原半干旱區(qū)草地土壤固碳、氮潛力與效應(yīng)評(píng)價(jià)及碳、氮垂直積累動(dòng)態(tài)的認(rèn)知提供科學(xué)依據(jù)。
1.1 試驗(yàn)樣地
試驗(yàn)區(qū)設(shè)于寧夏固原東北部45 km處的云霧山草原,處于黃土高原腹地,地理位置為106°21′-106°27′ E,36°10′-36°17′ N,海拔1 700-2 148 m,溫帶半干旱氣候區(qū),為典型的半干旱氣候,植被類(lèi)型為典型草原。年平均氣溫7 ℃,年平均降水量425 mm(1980-2014年平均值),60%~75%的降水集中分布于夏季7月-9月,土壤類(lèi)型以山地灰褐土和黑壚土為主,水資源補(bǔ)給主要來(lái)源于大氣降水。主要優(yōu)勢(shì)植物有長(zhǎng)芒草(Stipabungeana)、大針茅(S.grandis)、百里香(Thymusmongolicus)、鐵桿蒿(Artemisiasacrorum)、星毛委陵菜(Potentillaacaulis)等[17]。自1980年至今,實(shí)行了數(shù)次嚴(yán)格的封山禁牧措施,具有典型的封育年限梯度。
1.2 野外取樣和數(shù)據(jù)測(cè)定
首先,選取具有代表性未封育樣地和封育30年樣地各3塊,每塊樣地間隔10 m設(shè)置1 m×1 m樣方(每塊樣地設(shè)置3個(gè)樣方),土壤樣品采樣深度為0-500 cm,其中0-40 cm土層分0-10、10-20、20-30、30-40 cm共4層依次取樣,40-500 cm,每20 cm為一層,共計(jì)27層,使用直徑為6 cm的土鉆取樣,每個(gè)樣方3次重復(fù),將取好的樣品按層分裝在自封袋中,并用標(biāo)簽寫(xiě)好樣地號(hào),帶回實(shí)驗(yàn)室備用。其次,去除混入的凋落物和根系后,將土樣風(fēng)干后過(guò)2 mm篩備用。最后,將各個(gè)樣地3個(gè)樣方同一土層的土壤均勻混合,取1/4土壤作為對(duì)應(yīng)土層樣品,進(jìn)行SOC、STN等土壤理化性質(zhì)測(cè)定。土壤容重采用直徑6 cm的根鉆和“環(huán)刀鉆”交替使用的方式采集。SOC采用重鉻酸鉀容量法測(cè)量,STN采用半微量凱氏蒸餾法測(cè)量[18]。
1.3 數(shù)據(jù)分析與處理
SOC儲(chǔ)量可以根據(jù)一定土壤土層中的SOC含量和土壤容重計(jì)算。通常,去除所采土壤中直徑>2 mm的土壤顆粒后,測(cè)定土壤容重和SOC含量,然后,通過(guò)公式計(jì)算土壤碳儲(chǔ)量(CS)[19]:
(1)
式中:CS為土壤有機(jī)碳儲(chǔ)量(mg·hm-2);BD為土壤容重(g·cm-3),SOC為土壤有機(jī)碳含量(g·kg-1),D為土層厚度(cm),由于樣地土壤中幾乎無(wú)礫石,所以公式中略去礫石含量參數(shù)??傮w土壤碳、氮儲(chǔ)量,將各層碳、氮儲(chǔ)量累加。土壤全氮儲(chǔ)量MS(mg·hm-2)參照公式(1),將SOC更換為STN(g·kg-1)即可。采用SAS9.1.3軟件對(duì)不同土層間有機(jī)碳含量進(jìn)行了方差分析和多重比較。對(duì)封育30年和未封育草地間有機(jī)碳含量進(jìn)行了t檢驗(yàn)。利用Origin9.0作圖。
2.1 土壤SOC和TN含量
封育30年和未封育草地的SOC含量均隨剖面深度的增加呈逐漸降低趨勢(shì)(表1)。從表層往下至最底層,封育草地0-140cm各土層間(除40-80cm與100-140cm無(wú)顯著差異)差異極顯著(P<0.01),未封育草地0-140cm各土層SOC含量(除20-40cm間無(wú)顯著差異)差異極顯著(P<0.01);140-300cm土層,封育草地土壤各層間差異極顯著(P<0.01),未封育土層差異不明顯(P>0.01);320-500cm土層,封育與未封育各層土壤SOC含量差異均不明顯(P>0.01)。因此,封育對(duì)于SOC累積的影響主要發(fā)生在140-300cm土層。t測(cè)驗(yàn)表明,0-440cm各土層SOC差異較明顯,440-500cm土層SOC含量差異較小,除400-420和480-500cm土層外,封育各土層SOC含量均大于未封育土層,說(shuō)明封育措施對(duì)于草地土壤有機(jī)碳的積累至關(guān)重要。因此,土壤中有機(jī)碳的累積趨勢(shì)從表層到深層逐漸增加,最后趨于穩(wěn)定。
封育30年草地和未封育草地STN含量隨著剖面深度的增加呈降低趨勢(shì)(表2)。而且封育草地各土層STN含量均高于未封育草地;140-500cm土層,隨著土層深度增加,封育草地和未封育草地各土層的STN含量降低但差異不明顯(P>0.01);封育與未封育草地的STN含量穩(wěn)定層大約位于130cm處。t檢驗(yàn)顯示,封育和未封育草地10-20cm土層STN含量差異極其顯著(P<0.01),20-30cm土層差異不顯著(P>0.05),0-10cm和30-60cm土層差異顯著(P<0.05);60-320cm土層封育草地土壤各層SNT含量極顯著高于未封育草地(P<0.01),320-500cm土層各層封育與未封育含量差異相對(duì)較小。因此,封育對(duì)于STN的累積動(dòng)態(tài)發(fā)生在60-320cm土層中。
表1 封育30年和未封育草地土壤有機(jī)碳 含量(g·kg-1)的剖面分布Table 1 Vertical distribution of soil organic carbon (g·kg-1)in the soil profiles under grassland of enclosed30 years and natural grassland
注:同列不同大寫(xiě)字母表示差異極顯著(P<0.01)。**表示同一土層封育草地間差異極顯著(P<0.01),*表示同一土層封育草地間差異顯著(P<0.05),ns表示同一土層封育草地間差異不顯著(P>0.05)。下同。
Note:Different captital letters within the same column means significantly difference at 0.01 level; **,* indicated signifcantly difference at 0.01 and 0.05 level, respectively; ns indicated no signficant difference. The same below.
2.2 SOC儲(chǔ)量
從封育30年草地不同土層儲(chǔ)量來(lái)看(圖 1),0-500 cm土層中SOC儲(chǔ)量先上升后下降再趨于平緩。20-40 cm土層SOC儲(chǔ)量最高,為87.6 mg·hm-2,之后隨著土層的加深逐漸降低。100-160 cm各土層中,SOC儲(chǔ)量維持在50 mg·hm-2左右,180-200 cmSOC儲(chǔ)量下降至24.17 mg·hm-2,200-340 cm SOC儲(chǔ)量緩慢下降,從200-220 cm的21.41 mg·hm-2下降到320-340 cm的10.17 mg·hm-2,340-500 cm各層SOC儲(chǔ)量波動(dòng)較小,維持在11.5 mg·hm-2左右。
未封育草地SOC儲(chǔ)量變化趨勢(shì)隨著土層深度增加呈現(xiàn)先迅速下降后變化平緩的趨勢(shì)(圖 2)。0-20 cm SOC儲(chǔ)量最高為65.8 mg·hm-2,20-40 cm SOC儲(chǔ)量為47.2 mg·hm-2,40-60 cm SOC為40 mg·hm-2,隨著土壤深度增加,各層SOC儲(chǔ)量逐步降低,140-500 cm土層,每20 cm土層SOC儲(chǔ)量均維持在8 mg·hm-2左右。因此,封育30年和未封育草地各層SOC儲(chǔ)量的垂直分布有較大的差異,封育30年草地0-500 cm各層土壤比未封育土層SOC儲(chǔ)量高,表明封育對(duì)于SOC儲(chǔ)量的積累有重要影響,而且封育草地SOC積累發(fā)生在0-500 cm各個(gè)土層,僅研究土壤淺表層(如0-100 cm)土壤SOC積累會(huì)遺失大量碳積累信息。
2.3 土壤SOC和STN儲(chǔ)量累積百分率
天然草地封育30年后0-30 cm土層SOC、 STN儲(chǔ)量分別占到0-500 cm土層的14.0%和8.5%;0-100 cm土層SOC、STN儲(chǔ)量分別占土層44.8%和27.2%;0-200cm土層分別占層72.2%和38.5%;0-300 cm土層SOC、STN儲(chǔ)量分別占層83.5%和64%;0-400 cm土層分別占到92.2%和80.1%;封育草地SOC儲(chǔ)量垂直累積速率比STN累積快(圖3、4)。
圖2 未封育草地不同土層有機(jī)碳儲(chǔ)量Fig.2 SOC storage of different soil layers of natural grassland
2.4 土壤碳、氮垂直積累動(dòng)態(tài)
SOC和STN垂直積累在封育30年后呈現(xiàn)不同程度的累積(圖5)。封育30年草地土壤各層有機(jī)碳含量均高于未封育草地,封育30年,草地0-10 cm土壤有機(jī)碳含量為57 g·kg-1,是未封育草地有機(jī)碳含量的近兩倍,封育草地180-200 cm土壤有機(jī)碳含量為17 g·kg-1,是未封育草地有機(jī)碳含量的4倍,以Y=10 g·kg-1土壤有機(jī)碳含量和X=180-200 cm(第12層)為參照坐標(biāo)系可以看出,隨著封育年限的增加,200 cm以下各層SOC含量均靠近第一象限,即封育對(duì)于草地深層SOC積累有著較大的影響。同樣STN含量在封育30年后均靠近第一象限,即深層STN含量逐步提高,封育30年與未封育草地0-200 cm各層STN含量差異并不大,但是200-500cm土層STN含量封育30年明顯高于未封育草地。
在估算土壤有機(jī)碳儲(chǔ)量時(shí),目前通常使用的土壤深度均≤1 m[20-24],這對(duì)于土層較淺的地區(qū)比較適用,而黃土高原區(qū)域土層深厚是否適用,需要驗(yàn)證,對(duì)深層(0-400 cm)土壤有機(jī)碳已有的研究[14-16]表明,深層SOC碳儲(chǔ)量不容忽視,本研究驗(yàn)證了這一觀點(diǎn),深層土壤有機(jī)碳、氮儲(chǔ)量較大。
本研究表明,草地封育30年后土壤中SOC的含量從表層到深層逐漸降低,最后趨于穩(wěn)定,封育30年草地SOC變化臨界層不明顯。長(zhǎng)期放牧?xí)档屯寥辣韺覵OC含量,而對(duì)土壤深層SOC影響較小[25-27]。而且封育對(duì)于STN影響主要是硝態(tài)氮[28]。這與本研究結(jié)果一致,未封育草地土壤SOC和STN均低于封育草地,封育草地SOC含量變化臨界層大約位于130 cm處,因此未封育草地土壤碳的積累或流失應(yīng)該發(fā)生在0-130 cm表層土壤,本研究認(rèn)為分析未封育或放牧草地土壤物理化學(xué)特性時(shí),可取土深度130 cm代表
圖3 草地封育30年不同深度土壤有機(jī)碳百分比Fig.3 Percentage of SOC storage in the soil profiles under grassland of enclosed 30 years
圖4 草地封育30年不同深度STN百分率Fig.4 Percentage of STN storage in the soil profiles under grassland of enclosed 30 years
圖5 不同封育年限不同土層SOC和STN含量垂直分布Fig.5 Thevertical distribution of SOC and STN content in different closed grassland
注:橫坐標(biāo)軸上的3,20-30;6,60-180;9,120-140;12,180-200;15,240-260;18,300-320;21,360-380;24,420-440;27,480-500。
總體參數(shù),但是研究長(zhǎng)期封育草地土壤特性時(shí),封育對(duì)于SOC的累積動(dòng)態(tài)可能發(fā)生在140-320 cm土層,表層土壤數(shù)據(jù)特性不能完全反映土壤總體參數(shù)特性(表1)。土壤中SOC積累和STN的累積動(dòng)態(tài)并非同步,STN的累積發(fā)生在60-320 cm土壤中,而非表層(0-60 cm)和較深層(>320 cm)中,封育與未封育STN含量差異較小(表2 )。因此,研究長(zhǎng)期封育草地STN含量,應(yīng)該集中在0-320 cm深度,表層(0-100 cm)樣本數(shù)據(jù)不能代表土壤總體參數(shù)。
封育30年和未封育草地各層土壤碳儲(chǔ)量的垂直分布有較大的差異。研究表明,草地植被生物量與土壤碳呈密切正相關(guān)關(guān)系,隨著生態(tài)系統(tǒng)的發(fā)育(或恢復(fù)) 進(jìn)程,其碳固持能力會(huì)逐漸增加,并且土壤碳氮積累會(huì)向深層土壤延伸,最終達(dá)到一個(gè)相對(duì)平衡狀態(tài)[29-30]。本研究指出,封育30年草地0-500 cm各層土壤儲(chǔ)量比未封育土層土壤碳儲(chǔ)量高,表明封育是碳匯過(guò)程,并且碳匯發(fā)生在0-500 cm各層土壤中。因此,研究土壤有機(jī)碳的積累應(yīng)該考慮更深層次土壤碳儲(chǔ)量。從不同深度草地土壤碳、氮儲(chǔ)量所占百分比看,0-100 cm土壤有機(jī)碳、全氮儲(chǔ)量分別占封育30年草地0-500 cm土壤有機(jī)碳、全氮儲(chǔ)量的44.8%和27.2%,0-200 cm為72.2%和38.5%,0-300 cm為83.5%和64.0%,0-400 cm為92.2%和80.1%(圖3,4),所以非常有必要對(duì)深層土壤有機(jī)碳動(dòng)態(tài)進(jìn)行深入的研究,經(jīng)研究表層土壤有機(jī)碳,在碳、氮評(píng)估過(guò)程中勢(shì)必會(huì)導(dǎo)致數(shù)據(jù)偏小,或遺漏碳氮儲(chǔ)量信息。封育后土壤碳、氮積累在深層(200-500 cm)發(fā)生著較大變化。所以,封育草地土壤碳、氮積累不僅發(fā)生在表層,而且發(fā)生在土壤深層,故對(duì)深層土壤碳、氮研究似乎才能揭示氮、碳的積累動(dòng)態(tài)。
綜上所述,封育草地土壤中碳、氮含量,碳、氮儲(chǔ)量,碳、氮垂直分布等與未封育草地土壤相比有較大差異,并且土壤碳、氮含量和儲(chǔ)量在深層土壤發(fā)生著較大的變化。因此研究封育草地土壤碳、氮?jiǎng)討B(tài)及固持,至少應(yīng)該挖掘0-500 cm土壤信息,僅研究0-100 cm土壤信息會(huì)導(dǎo)致碳、氮儲(chǔ)量及固定速率偏小。
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(責(zé)任編輯 茍燕妮)
Effects of natural grassland enclosure on carbon and nitrogen storage in deep soil in the Loess Plateau of China
Li Jian-ping1, Xie Ying-zhong1,2
(1.School of Agriculture, Ningxia University, Yinchuan 750021, China;2.Breeding Base of State Key Laboratory for Preventing Land Degradation and Ecological Restoration,Ningxia University, Yinchuan 750021, China)
Grassland enclosure and establishing are important measures to recovery and reclamation of degraded grassland vegetation. Accurate assessment of dynamics of soil carbon and nitrogen storage has important ecological significance. The present study focuses on dynamic of organic carbon (SOC) and total nitrogen (STN) in deep soil (0-500 cm) in 30 years enclosure grassland and overgrazing grassland in semi-arid region of Guyuan city, Ningxia Province. The results showed that SOC content in 0-440 cm soil of 30 years enclosure grassland was significantly higher (P<0.05) than that in overgrazing grassland. There was no critical change layer of SOC in enclosure grassland, however, there was critical change layer of SOC located at 130 cm soil layer in overgrazing grassland. There was critical change layer of STN located at 130 cm soil layer in both enclosure and overgrazing grassland. The accumulation of STN occurred in 60-320 cm soil layers with very less accumulation topsoil layers (0-60 cm) and deeper layers (>320 cm). SOC storage in 30 years enclosure grassland was stable in 340-500 cm depth soil with 11.5 mg·hm-2of each 20 cm soil layers, however, SOC storage in overgrazing grassland was stable in 140-500 cm depth soil with 8 mg·hm-2of each 20 cm soil layers. SOC and STN storage in 0-100 cm soil in 30 years enclosure grassland accounted for 44.8% and 27.2% of total SOC and STN storage in 0-500 cm soil, respectively, those in 0-200 cm soil accounted for 72.2% and 38.5%, respectively, those in 0-300 cm accounted for 83.5% and 64%, respectively, those in 0-400 cm accounted for 92.2% and 80.1%, respectively. These results showed that there was much carbon and nitrogen information in the deep soil which was necessary to study dynamic of SOC and STN in the deep soil.
soil organic carbon; soil total nitrogen; fencing; natural grassland; deep soil layer
Li Jian-ping E-mail:lijianpingsas@163.com
10.11829/j.issn.1001-0629.2016-0200
2016-04-15 接受日期:2016-06-22
寧夏自然科學(xué)基金(NZ15049)
李建平(1982-),陜西延安人,講師,博士,研究方向?yàn)椴莸厣鷳B(tài)學(xué)。E-mail:lijianpingsas@163.com
S812.2
A
1001-0629(2016)10-1981-08*
李建平,謝應(yīng)忠.封育對(duì)黃土高原天然草地深層土壤碳、氮儲(chǔ)量的影響.草業(yè)科學(xué),2016,33(10):1981-1988.
Li J P,Xie Y Z.Effects of natural grassland enclosure on carbon and nitrogen storage in deep soil in the Loess Plateau of China.Pratacultural Science,2016,33(10):1981-1988.