劉 爽 ,王 雅 ,徐志超
(1.山西大學(xué)黃土高原研究所,山西太原030006;2.山西省農(nóng)業(yè)科學(xué)院農(nóng)業(yè)環(huán)境與資源研究所,山西省土壤環(huán)境與養(yǎng)分資源重點實驗室,山西太原030031;3.青海省水利水電勘測設(shè)計研究院,青海西寧810001)
降低土壤壓實、經(jīng)濟適用的作物輪作以及地表殘茬覆蓋是保護性耕作的3個核心內(nèi)容[1-2]。其不但可以維持長期作物生產(chǎn)力而且能降低環(huán)境風(fēng)險[3-4]。其中,地表殘茬覆蓋是指作物收獲后至少將30%的作物殘茬覆蓋在地表[5]。許多學(xué)者研究了不同類型的殘茬及其覆蓋量對土壤及作物的影響[6-8]。作物殘茬覆蓋的優(yōu)點和不足主要取決于作物殘茬的種類、質(zhì)量及其與不同土壤氣候條件的相互作用[9]。與傳統(tǒng)耕作系統(tǒng)相比,保護性耕作可以降低土壤風(fēng)蝕、水蝕以及土壤水分蒸發(fā)[10-12],增加降雨入滲量[13],提高土壤表層有機碳含量[14-15],增強土壤團聚體的數(shù)量及穩(wěn)定性[16-19],降低人力及能源輸入[20-22]。因此,保護性耕作多應(yīng)用于高侵蝕及半干旱土壤區(qū)[23-25]。目前,保護性耕作約達1.25億hm2,約占全球農(nóng)地面積的9%,其中,半干旱草原區(qū)保護性耕作阿根廷占70%的農(nóng)地面積,美國占全部農(nóng)地面積的16%[26];而我國的保護性耕作面積占全部農(nóng)業(yè)面積的比例非常小。
在過去的幾十年內(nèi),保護性耕作系統(tǒng)已在全球范圍內(nèi)的不同氣候區(qū)域,如在熱帶、亞熱帶以及溫帶氣候區(qū)進行評估及采用。保護性耕作下的作物類型、氣候及土壤滲漏狀況等都影響作物產(chǎn)量。當氣候為冷濕條件或土壤幾乎不滲漏的情況下,保護性耕作下的玉米產(chǎn)量較傳統(tǒng)耕作低[17,27-30];而當氣候為干燥溫暖或土壤滲漏狀況較好時,保護性耕作下的玉米產(chǎn)量較傳統(tǒng)耕作高[26,31-38];此外,在不考慮氣候及土壤條件時,某些土壤類型保護性耕作下的大豆和冬小麥產(chǎn)量較傳統(tǒng)耕作高或與傳統(tǒng)耕作下的產(chǎn)量無明顯差別[29,33,39-43]。
在熱帶地區(qū),許多學(xué)者研究了保護性耕作對作物產(chǎn)量及土壤理化性狀的影響,結(jié)果發(fā)現(xiàn),與傳統(tǒng)的耕作方式相比,保護性耕作,如免耕或少耕可提高土壤含水量及玉米產(chǎn)量[31,34,44-46]。在熱帶地區(qū),保護性耕作在農(nóng)業(yè)及經(jīng)濟上的作用主要受降水量及其分布的影響[47]。因此,相應(yīng)的研究結(jié)果存在區(qū)域性差異,例如THIAGALINGAM等[32]研究發(fā)現(xiàn),免耕能提高大豆和玉米的產(chǎn)量,尤其在干旱年份效果更明顯;但ZOTARELLI等[48]研究表明,免耕能提高大豆產(chǎn)量,而傳統(tǒng)耕作方式下的玉米產(chǎn)量則更高;同樣,WILHELM等[49]研究發(fā)現(xiàn),玉米產(chǎn)量隨降雨量及其分布而變化。其他研究結(jié)果顯示,由于相應(yīng)水分利用效率(WUE)的提高,地表作物殘茬覆蓋可顯著提高作物產(chǎn)量,尤其在干旱年份當?shù)乇砀采w有作物殘茬時,WUE 會更高[8,47,50]。VERHULST等[51]研究發(fā)現(xiàn),在作物生長季,免耕條件下,作物雖在初始階段生長緩慢,但后期生長較快,從而提高了最終的籽粒產(chǎn)量。此外,免耕還可提高土壤有機質(zhì)含量及表層土壤的孔隙度[45],但對0~10 cm土層的土壤容重?zé)o明顯的影響[52]。
研究表明,在亞熱帶地區(qū),免耕可提高總的土壤有機碳含量[53-54]及0~5 cm土層的水穩(wěn)性團聚體含量,但使得表層土壤的孔隙度降低[55-56],土壤容重增大[57]。許多研究表明,與傳統(tǒng)耕作相比,免耕可保持更多的水分,且顯著提高玉米籽粒的產(chǎn)量及水分利用率[41,58-62];SARKAR等[21]研究發(fā)現(xiàn),淺層耕作可以提高土壤水分、大麥產(chǎn)量以及水分利用率。
在溫帶地區(qū),當土壤處于滲漏性較差的條件時,保護性耕作(免耕或少耕)下的作物產(chǎn)量等于或低于傳統(tǒng)耕作[63-65];但也有些研究結(jié)果表明,保護性耕作提高了玉米產(chǎn)量[27]。免耕秸稈覆蓋可提高土壤有機碳含量[14,66],降低地表徑流,提高土壤含水量[67],并能降低N2O氣體的排放量。此外,研究結(jié)果還表明,免耕或少耕導(dǎo)致土壤表層容重增大,土壤入滲量降低[68-69]。盡管一些研究結(jié)果顯示,在溫帶氣候區(qū)保護性耕作的不足包括降低作物產(chǎn)量[70]及影響土壤物理特性(即降低土壤入滲量和增加土壤容重),但隨著免耕配套農(nóng)機具的逐步發(fā)展以及燃料費用的逐步降低,可能會使免耕成為較經(jīng)濟的耕作系統(tǒng)[71]。另外,保護性耕作具有較好的經(jīng)濟和生態(tài)效益[65]。
我國已有許多關(guān)于保護性耕作方面的研究,但這些研究主要集中于我國北方的華北平原(31°24′N~42°42′N,110°18′E~122°42′E)和西北地區(qū),尤其是黃土高原地區(qū)(34°30′N~38°6′N,103°47′E~113°26′E)。研究表明,傳統(tǒng)耕作方式可加速土壤退化及水資源的匱乏,降低作物生產(chǎn)力及水分利用效率[72-73]。而保護性耕作則可提高土壤物理化學(xué)及生物性狀,從而降低土壤侵蝕,提高土壤含水量、水分利用效率及作物產(chǎn)量[41,59,74-75],可起到儲水保墑、節(jié)能、節(jié)水、節(jié)肥的作用[76-78]。然而,在我國東北地區(qū)關(guān)于保護性耕作效益的研究較少,該區(qū)域位于溫帶氣候區(qū)且傳統(tǒng)耕作已被應(yīng)用了幾百年。CHEN等[79]研究了保護性耕作對于土壤表層(0~15 cm)含水量的影響;FAN等[80]研究了不同耕作方式及作物輪作對作物產(chǎn)量及效益的影響。而對于剖面土壤水分的分布,深層土壤水分儲量及水分利用效率方面的報道較少。
從國內(nèi)外對保護性耕作相關(guān)問題的研究看,保護性耕作在不同氣候區(qū)域都取得了長足發(fā)展,主要集中在保護性耕作對土壤質(zhì)量及作物生長和產(chǎn)量的影響等方面。就目前研究中存在的問題提出保護性耕作在國內(nèi)未來研究中的幾點建議。
從目前的研究來看,主要存在的問題為:保護性耕作多應(yīng)用于高侵蝕及半干旱土壤區(qū)域,對于濕潤及降水較多的區(qū)域,保護性耕作的應(yīng)用相對匱乏;在不同的區(qū)域,保護性耕作所對應(yīng)的技術(shù)體系不同,而保護性耕作技術(shù)因在不同區(qū)域推廣程度不同,其成熟程度差異較大,即使在推廣范圍大、技術(shù)成熟的區(qū)域,技術(shù)的適用區(qū)域也不十分明確;保護性耕作配套技術(shù)一直未能很好地解決;保護性耕作多應(yīng)用于玉米、大豆和小麥作物,對于其他作物應(yīng)用較少;與不同作物配套的保護性耕作機具匱乏,導(dǎo)致其增產(chǎn)效果以及對土壤的改良效果受到影響。
針對目前存在的問題,應(yīng)該根據(jù)不同氣候或土壤區(qū)域條件等實地情況,研發(fā)相應(yīng)的保護性耕作配套技術(shù)規(guī)程,例如,位于不同氣候區(qū)域的高原、丘陵、盆地以及平原地區(qū)其適宜的保護性耕作模式不同,應(yīng)根據(jù)具體區(qū)域進行研發(fā);應(yīng)該研發(fā)適用不同作物的保護性耕作技術(shù)體系和相應(yīng)的保護性耕作農(nóng)機具,如針對玉米、小麥、大豆等農(nóng)作物,其適宜保護性耕作技術(shù)體系不同,配套的農(nóng)機具也應(yīng)具體研發(fā);應(yīng)該普及保護性耕作的知識,定期進行保護性耕作知識的講座以及技術(shù)指導(dǎo),如秸稈還田、免耕播種、壟溝深松、淺層耕作、化學(xué)除草劑的使用等;進行保護性耕作的深入研究和推廣,應(yīng)在取得國家經(jīng)費支持的情況下,爭取地方政府的支持,使得研究成果及時得到示范推廣,比如對于響應(yīng)保護性耕作的農(nóng)民,地方政府應(yīng)該給予一定的優(yōu)惠政策,使保護性耕作得到廣泛有效的推廣和應(yīng)用。
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