李英臣,侯翠翠,李勇,過治軍
河南師范大學(xué),河南 新鄉(xiāng) 453007
人類活動引起的溫室氣體濃度增加是全球變暖的主要原因(IPCC,2007)。作為三種主要的溫室氣體,CO2、CH4和N2O在大氣中的濃度不斷增加,它們的體積分?jǐn)?shù)分別由工業(yè)革命前的約280×10-6、700×10-9、270×10-9增加至 2006 年的381.2×10-6、1782×10-9和 320.1×10-9,分別增加了136%、255%和119%(WMO,2006)。農(nóng)田土壤是重要的溫室氣體排放源,由于人類農(nóng)業(yè)活動導(dǎo)致的溫室氣體排放占溫室氣體排放量的 10%~12%(IPCC,2007;Lenka和 Lal,2013)。各種農(nóng)業(yè)措施對溫室氣體排放都有重要的影響(Almaraz等,2009;Kallenbach等,2010;鄒曉霞等,2011)。
以減耕或免耕,作物秸稈覆蓋和作物輪作為特點的保護(hù)性農(nóng)業(yè),與上世紀(jì) 70年代在美國興起,由于其在提高作物產(chǎn)量、減少機(jī)械燃料耗費(fèi),緩解土地退化等方面的作用得到大力推廣(Govaert等,2009)。目前主要在美國、巴西、阿根廷、加拿大和澳大利亞種植面積較大,在歐洲以及非洲也慢慢被接受而進(jìn)行推廣。作為兩種主要的保護(hù)性農(nóng)業(yè)措施,免耕和秸稈覆蓋被越來越多的應(yīng)用于農(nóng)業(yè)生產(chǎn)中,在減少水土流失、增加作物產(chǎn)量、增加土壤質(zhì)量等方面發(fā)揮了巨大作用(Blaver等,2009;Duiker和Lal,1999;Kassam等,2012)。截至2009年,全球免耕土地面積已經(jīng)達(dá)到 111×106hm-2,以美國最多,占23.9%,巴西次之(23.0%),再者為阿根廷(17.8%)和加拿大(12.2%),我國目前有1.33×106hm-2,僅為全球的1.2%,不過與2003年相比已增加了12倍(Derpsch等,2010)。但是,相比傳統(tǒng)耕作方式,免耕和秸稈覆蓋對溫室氣體排放的綜合影響尚不清楚,結(jié)果存在爭議(Ahamad等,2009;Dendooven等,2012;Regina和Alakukku,2013;Abdalla等,2013)。因此,本文對免耕和秸稈覆蓋處理下,土壤3種溫室氣體CO2、CH4和N2O的排放影響進(jìn)行綜述,揭示2種保護(hù)性農(nóng)業(yè)措施對3種溫室氣體排放的影響機(jī)制,并為制定合理的耕作和覆蓋措施提供理論支持。
土壤CO2排放主要是由土壤有機(jī)質(zhì)分解、凋落物分解和植物根系呼吸3部分組成(Oorts等,2007)。影響土壤有機(jī)質(zhì)分解的主要因素有土壤溫度、土壤濕度、土壤有機(jī)碳含量等(Rodrigo等,1997),枯落物分解同時還受枯落物質(zhì)量和分布位置以及土壤養(yǎng)分含量的影響(Gamier等,2003)。農(nóng)田CO2排放受多因素影響,耕作和秸稈覆蓋是影響農(nóng)田溫室氣體排放的重要管理措施(Pandy等,2012)。
耕作方式主要是通過改變土壤有機(jī)碳分解環(huán)境,如土壤充氣環(huán)境、破壞土壤團(tuán)聚體結(jié)構(gòu),以及改變土壤與秸稈的接觸面積等影響土壤 CO2排放(Ussiri和Lal,2009)。大部分研究表明,相對于常規(guī)耕作,免耕會降低土壤CO2釋放量(表1),原因為頻繁的耕作會導(dǎo)致土壤有機(jī)碳大量損失,CO2排放量增加;而免耕則有效的控制土壤有機(jī)碳損失,減少土壤干擾,降低土壤微生物對活性碳的利用,進(jìn)而抑制 CO2排放(Almaraz 等,2009;Ussiri和 Lal,2009)。Reicosky和Archer(2007)研究指出隨著耕作層深度增加,CO2排放量增加,并明顯高于免耕處理(Reicosky和Archer, 2007)。也有研究指出耕作加速土壤有機(jī)質(zhì)氧化,并在耕作后短期內(nèi)釋放大量的 CO2(La Scala等,2006;Omonode等,2007)。同時,常規(guī)耕作增加土壤團(tuán)聚體周轉(zhuǎn)率,加速團(tuán)聚體有機(jī)質(zhì)分解,而免耕增加團(tuán)聚體穩(wěn)定性,有利于難分解有機(jī)質(zhì)的形成(Paustian等,2000)。有學(xué)者利用13C自然豐度法研究發(fā)現(xiàn),免耕土壤和常規(guī)耕作土壤有機(jī)質(zhì)平均周轉(zhuǎn)時間分別為 73年和 44年(Paustian等,2000)。同時也有研究表明耕作對CO2排放影響不大(Ahamad等, 2009; Elder和Lal, 2008;Regina和 Alakukku,2010)。另外也有部分研究者得出相反的結(jié)論,Hendrix等(1988)通過對喬治亞州高粱和大豆地的監(jiān)測發(fā)現(xiàn),免耕地有更高的CO2排放(Hendrix等, 1988)。這可能與不同農(nóng)田的土壤溫度、土壤濕度以及土壤理化性質(zhì)不同有關(guān)(Rodrigo等, 1997)。氣候條件對土壤CO2排放產(chǎn)生一定的影響。在溫帶季風(fēng)氣候區(qū),季節(jié)變化也會對不同耕作方式下溫室氣體排放產(chǎn)生影響(Al-Kaisi和Yin, 2005; Alvarez 等, 2001; Ussiri和 Lal, 2009)。Ussiri和Lal (2009)研究表明,耕作處理對夏季和秋季平均每天的CO2排放通量影響顯著,但是對冬季和春季的影響不大(Ussiri和Lal,2009)。在趨向于使土壤濕潤的氣候條件下(高的降水量或者低的蒸散發(fā)),免耕更有利于降低土壤 CO2排放(Alvarez等,2001; La Scala等,2006; Ussiri和Lal,2009)(表1)。在干燥的氣候條件下,耕作對土壤CO2排放的結(jié)論不一致(表 1),這表明土壤質(zhì)地對土壤CO2排放有重要的影響。耕作措施的短期和長期影響的機(jī)理有所不同,耕作措施造成的短期影響主要是對土壤的物理干擾作用而產(chǎn)生的;長期影響則可通過改變土壤的物理、化學(xué)以及生物性質(zhì)進(jìn)而改變土壤CO2排放(Ussiri和Lal,2009)。所以耕作制度對土壤CO2排放的影響需要短期和長期定位實驗相結(jié)合的方式共同研究。
表1 不同氣候條件下免耕和常規(guī)耕作對土壤CO2排放的影響Table 1 The effect of no-till and conventional tillage on CO2 emissions in different climates
秸稈覆蓋作為另一種重要的保護(hù)性耕作措施也會明顯影響CO2排放。秸稈本身分解會釋放一部分CO2(Bavin等,2009; Oorts等,2007),秸稈覆蓋一般增加CO2排放,并隨著秸稈覆蓋量的增加CO2排放量增大(Lenka和Lal, 2013)。同時秸稈覆蓋通過影響土壤理化性質(zhì)進(jìn)而影響CO2排放(Kallenbach等,2010)。有研究者認(rèn)為,長期秸稈覆蓋措施下會增加土壤表層 0~10 cm水穩(wěn)性團(tuán)聚體含量(>250 μm),這種團(tuán)聚體的增加與CO2排放有很好的正相關(guān)關(guān)系(Lenka和 Lal,2013)。Bavin等(2009)通過對玉米-大豆輪作地為期 2年的研究發(fā)現(xiàn),CO2排放量的增加主要是由于覆蓋枯落的分解導(dǎo)致的(Bavin等,2009)。經(jīng)過長時間的秸稈覆蓋,0~30 cm的土壤有機(jī)碳68%~74%來自于秸稈(Ussiri和Lal,2009)。秸稈分解提供的礦質(zhì)氮可以為硝化作用提供基質(zhì),秸稈也是可利用性碳的主要的來源,碳分解消耗 O2,造成土壤 O2缺乏,造成厭氧環(huán)境(Chantigny等,2002)。秸稈覆蓋通過降低土壤溫度、增加土壤濕度等條件間接影響土壤CO2排放。通常來說,在寒冷干燥的地區(qū),秸稈覆蓋會通過增加土壤溫度,增加土壤濕度間接增加土壤CO2排放,而在濕潤溫暖的地區(qū),秸稈覆蓋則對土壤CO2排放影響不一致,且與土壤理化性質(zhì)有關(guān)。Al-Kaisi和Yin(2005)通過對比研究免耕條件下,發(fā)現(xiàn)無秸稈覆蓋處理CO2排放量明顯高于秸稈覆蓋處理,原因主要有以下幾方面:秸稈覆蓋在表層降低了土壤溫度;覆蓋的秸稈阻礙了土壤向大氣排放CO2的通道;表層覆蓋的秸稈由于與土壤接觸面小而導(dǎo)致分解速率低(Al-Kaisi和Yin,2005)。
CH4增溫潛勢是CO2的25倍,對全球變暖的貢獻(xiàn)率僅次于CO2,占23%(IPCC, 2007)。農(nóng)業(yè)土壤對CH4排放也有重要的作用,土壤CH4排放主要是通過在厭氧土壤中的產(chǎn)甲烷菌釋放,在往大氣傳輸?shù)倪^程中受到甲烷氧化菌的作用,只有部分能夠進(jìn)入大氣(孫曉新,2009)。耕作措施和秸稈覆蓋主要是通過影響土壤濕度、土壤硬度及土壤營養(yǎng)含量等來影響CH4排放(Omonode等,2007;Regina和Alakukku,2010)。大部分研究認(rèn)為免耕降低CH4排放量或者對 CH4排放量沒有顯著影響(Ahamad等,2009;Lidaming等,2011;Ussiri和Lal,2009。表2)。原因可能為免耕土壤更穩(wěn)定,通透性好,有利于甲烷氧化(Ball等,1999),增強(qiáng)甲烷氧化菌活性。農(nóng)業(yè)土壤 CH4的主要排放源為水稻田(Fengjinfei等,2013;Pandey等,2012;鄒建文等,2003),有研究者通過對水稻田研究發(fā)現(xiàn)免耕降低CH4排放量(Ahamad等,2009)。Feng等(2013)通過對水稻田的研究得出相同的研究結(jié)論(Fengjinfei等,2013)。Pandey等(2012)通過對小麥-水稻田輪作地研究發(fā)現(xiàn),減少耕作頻率明顯降低CH4排放速率(Pandy等,2012)。Li等(2011)通過對中國南方雙季水稻田研究發(fā)現(xiàn)免耕處理下早稻和晚稻的甲烷排放量分別降低了 29%和 68%(Lidaming等,2011)。干旱地區(qū)土壤CH4排放排放量較低,有時表現(xiàn)為 CH4的源,有時表現(xiàn)為 CH4的匯(Bavin等,2009;Elder和Lal,2008;Regina和 Alakukku,2010)。大部分研究認(rèn)為,在干旱土壤,耕作將增加CH4氧化能力,原因為耕作增加土壤擾動,增加土壤通氣條件,使CH4吸收增加(Ball等,1999;Omonode等,2007)。Omonode等(2007)通過對玉米地和玉米-大豆輪作地研究發(fā)現(xiàn)犁耕和鑿耕的農(nóng)田為弱的 CH4排放源,免耕農(nóng)田為弱的CH4的匯(Omonode等,2007)。同時,Kessavalou等(1997)研究發(fā)現(xiàn)免耕休閑地比耕作的冬小麥-休閑輪作地CH4排放高,且在春季差異顯著,但也有學(xué)者研究表明 CH4通量不受耕作方式的影響(Kessavalou等,1997)。同時,有很多研究表明免耕降低 CH4排放(Feng等,2013;Pandey等,2012)。造成這種不同結(jié)果的原因可能為在旱地土壤CH4排放量很低,很多時候都無法監(jiān)測到有效數(shù)據(jù),而且受外界干擾因素較多(Bavin等,2009)。
表2 不同氣候條件下免耕和常規(guī)耕作對土壤CH4排放的影響Table 2 The effect of no-till and conventional tillage on CH4 emissions in different climates
秸稈覆蓋增加土壤厭氧條件(Jacinthe和Lal,2005),改變土壤物理性質(zhì),增加土壤養(yǎng)分條件等過程間接影響CH4排放(Lenka和Lal,2013)。通常條件下秸稈覆蓋增加 CH4排放(Lenka和 Lal,2013;Majing等,2009),但是通過改變覆蓋措施會降低CH4排放量(Majing等,2008;Majing等,2009)。Ma等(2008)和Ma等(2009)認(rèn)為帶狀覆蓋有利于減少CH4排放(Majing等,2008;Majing等,2009)。不同的覆蓋作物由于其化學(xué)性質(zhì)不同對CH4排放產(chǎn)生的影響也有所不同。Seneviratne和Van Holm(1998)通過室內(nèi)模擬實驗研究發(fā)現(xiàn),不通的秸稈覆蓋處理下CH4排放量都有所增加,但是秸稈覆蓋處理之間CH4排放量也有所差異,與秸稈中氮含量有很好的相關(guān)關(guān)系(Seneviratne和 Van Holm,1998)。
N2O作為一種長效溫室氣體,增溫潛勢是CO2的298倍(IPCC,2007)。農(nóng)業(yè)N2O排放量占每年N2O釋放入大氣總量的 40%~60%(Jarecki等,2009)。土壤N2O氣體主要是由土壤微生物的硝化-反硝化作用產(chǎn)生的(Hénault等,2005)。影響N2O排放的主要環(huán)境因子為土壤溫度、土壤孔隙充水率、土壤礦質(zhì)氮含量以及可利用碳含量等(Kroeze等,1999)。免耕對N2O排放的影響主要通過對溫度,土壤濕度和土壤性質(zhì)等過程來實現(xiàn)(Flechard等,2007)。免耕對N2O排放的影響有增大,無影響或降低等不同的結(jié)論(Ahamad等,2009;Baggs等,2003;Chatskikh和 Olesen,2007;Ussiri和 Lal,2009)(表3)。有研究表明免耕提高N2O排放,尤其是在實施免耕早期(Ball等,1999;Six等,2004),原因為免耕處理下高的土壤密度降低氣體擴(kuò)散能力,增加表層水保持能力,進(jìn)而增加土壤厭氧環(huán)境,使表層土壤潛在的反硝化速率增加(Ball等,2008)。但是也有研究發(fā)現(xiàn)傳統(tǒng)耕作有更高的 N2O排放(Almaraz等,2009;Elder和 Lal,2008;Mutegi等,2010;Passianoto等,2003),或者兩者的排放量沒有明顯差異(Choudhary等,2002)。Rochette等(2008)通過大量的對比研究發(fā)現(xiàn)免耕增加通氣條件差土壤的 N2O排放,對通氣好的土壤影響不大(Rochette,2008)。Abdalla(2013)通過綜述相關(guān)文獻(xiàn)認(rèn)為氣候條件與土壤對 N2O排放存在著交互作用(Abdalla等,2013)。在干燥的氣候條件下,免耕增加通氣條件差土壤的N2O排放(Lemke等,2007;Liuxuejun等,2006;Rochette,2008)(表 3),降低通氣性好土壤的 N2O排放或者影響不顯著(Choudhary等,2002;Haoxiying等,2001;Malhi和 Lemke,2007)。在濕潤的氣候條件下,不同的土壤性質(zhì)有不同的結(jié)論,并且與是否覆蓋秸稈有一定的交互作用(表3)。
秸稈覆蓋主要是通過影響土壤濕度和土壤養(yǎng)分狀況來改變土壤N2O排放速率(Lenka和Lal,2013)。大部分研究認(rèn)為秸稈覆蓋增加 N2O排放(Almaraz等,2009;Gomes等,2009;Liuchunyan等,2011),原因可能為秸稈覆蓋增加土壤中碳源,使本來受碳源限制的土壤硝化-反硝化細(xì)菌活性增加(Cuifeng等,2012)。與之研究結(jié)果類似,Gomes等(2009)認(rèn)為作物覆蓋提高N2O排放,增加的量與覆蓋作物的數(shù)量和質(zhì)量有關(guān)(Gomes等,2009),但是也有研究表明,秸稈覆蓋對N2O排放的影響不大,影響N2O的主要原因是氮肥施用(Ahamad等,2009;Jarecki等,2009)。同時,有研究者認(rèn)為,秸稈覆蓋對 N2O的影響與土壤濕度和灌溉制度有關(guān)(kallenbach等,2010)。Liu等(2011)研究發(fā)現(xiàn),秸稈覆蓋對N2O排放的影響與覆蓋作物種類有關(guān),小麥秸稈還田明顯增加N2O排放量,但是玉米秸稈還田對N2O排放的影響不顯著(Liuchunyan等,2011)。秸稈覆蓋還可與耕種方式產(chǎn)生相互作用。有研究表明,在無秸稈覆蓋的情況下,不同耕作處理N2O排放沒有明顯差異。但是秸稈覆蓋下,傳統(tǒng)耕作方式下農(nóng)田 N2O排放明顯高于條耕和少耕處理,秸稈還田使條耕和少耕 N2O排放減少 39%和9%,使傳統(tǒng)耕作方式N2O排放量增加35%。由此可見,秸稈覆蓋對N2O排放機(jī)理復(fù)雜,研究結(jié)果有很大的不確定性,針對典型區(qū)域及不同耕作措施影響下農(nóng)田土壤N2O排放需要進(jìn)一步研究。
3 不同氣候條件下免耕和常規(guī)耕作對土壤N2O排放的影響Table 3 The effect of no-till and conventional tillage on N2O emissions in different climates
通過以上對免耕和秸稈覆蓋措施對農(nóng)田三種主要的溫室氣體(CO2、CH4和 N2O)排放的影響研究發(fā)現(xiàn),與常規(guī)耕作相比,免耕總體上能有效減少土壤CO2排放,有利于土壤中CH4氧化,增強(qiáng)甲烷氧化菌活性,進(jìn)而降低CH4排放,免耕對N2O排放的影響因氣候類型和土壤類型等條件的不同而存在結(jié)果差異,在干燥的氣候條件下,免耕增加通氣條件差的土壤的N2O排放,對通氣好的土壤影響不大。而在濕潤的氣候條件下,不同的土壤性質(zhì)結(jié)論不一致。秸稈覆蓋促進(jìn)土壤CO2排放,并隨著秸稈覆蓋量的增加而增大,但對CH4排放的影響有很大的不確定性,與覆蓋方式和覆蓋秸稈性質(zhì)有密切聯(lián)系,大部分研究認(rèn)為秸稈覆蓋增加N2O排放,但也有研究認(rèn)為秸稈覆蓋對 N2O排放無影響或降低N2O排放量,具體的影響機(jī)理需進(jìn)一步深入研究。
目前,免耕和秸稈覆蓋等保護(hù)性農(nóng)業(yè)措施逐漸被接受而大面積推廣,大部分的研究主要集中在對土壤水分利用率,作物產(chǎn)量,土壤理化性質(zhì)等方面的研究,對保護(hù)性農(nóng)業(yè)措施下溫室氣體排放的研究相對較少,而且現(xiàn)有這方面的研究主要關(guān)注一種或兩種溫室氣體排放,對三種溫室氣體的綜合影響研究并不多見。因此需結(jié)合不同土地類型,開展不同氣候類型下免耕和秸稈覆蓋對三種主要溫室氣體排放影響的綜合研究,預(yù)測增溫潛勢,為不同氣候帶保護(hù)性農(nóng)業(yè)措施下溫室氣體排放提供基礎(chǔ)數(shù)據(jù),并為制定合理的耕作和秸稈覆蓋措施提供理論支持。加強(qiáng)秸稈覆蓋對溫室氣體的排放的影響研究,運(yùn)用同位素示蹤等技術(shù)明確秸稈對溫室氣體排放的直接和間接影響。根據(jù)不同研究區(qū)土壤類型和氣候條件,探索既可以減少溫室氣體排放,又可以保持作物產(chǎn)量的合適的秸稈覆蓋量和覆蓋措施。
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