生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的研究進(jìn)展*

董心亮, 林啟美

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生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的研究進(jìn)展*

董心亮1,2, 林啟美2**

(1. 中國(guó)科學(xué)院遺傳與發(fā)育生物學(xué)研究所農(nóng)業(yè)資源研究中心/中國(guó)科學(xué)院農(nóng)業(yè)水資源重點(diǎn)實(shí)驗(yàn)室/河北省節(jié)水農(nóng)業(yè)重點(diǎn)實(shí)驗(yàn)室 石家莊 050022; 2. 中國(guó)農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院 北京 100193)

生物質(zhì)炭在農(nóng)業(yè)與環(huán)境中的應(yīng)用已成為近期國(guó)內(nèi)外研究熱點(diǎn), 有關(guān)生物質(zhì)炭特性以及生物質(zhì)炭對(duì)土壤化學(xué)、生物學(xué)性質(zhì)和作物產(chǎn)量的影響, 已經(jīng)有一些綜述, 但是生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的相關(guān)綜述很少。本文對(duì)近10年生物質(zhì)炭對(duì)土壤物理性質(zhì)影響相關(guān)的研究成果進(jìn)行了整理分析。研究結(jié)果發(fā)現(xiàn)生物質(zhì)炭可以降低土壤容重, 提高土壤團(tuán)聚體穩(wěn)定性, 增加田間持水量和土壤有效水含量, 降低飽和導(dǎo)水率等。生物質(zhì)炭影響土壤物理性質(zhì)的主要原因是生物質(zhì)炭具有較大的比表面積和孔隙度。此外, 生物質(zhì)炭與土壤礦質(zhì)顆粒結(jié)合, 并通過(guò)對(duì)土壤微生物活性和植物生長(zhǎng)的影響間接影響土壤物理性質(zhì)。生物質(zhì)炭對(duì)土壤物理性質(zhì)的影響與多種因素有關(guān), 如生物質(zhì)炭原料、裂解溫度、施用量和顆粒大小, 土壤質(zhì)地和處理時(shí)間等。關(guān)于生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的長(zhǎng)期研究很少, 且缺乏田間試驗(yàn)。因此, 將來(lái)的研究應(yīng)更加傾向于長(zhǎng)期田間條件下生物質(zhì)炭對(duì)土壤物理性質(zhì)的影響, 并逐漸發(fā)現(xiàn)生物質(zhì)炭的作用機(jī)理, 為實(shí)際的農(nóng)業(yè)生產(chǎn)和生態(tài)治理提供科學(xué)依據(jù)。

生物質(zhì)炭; 土壤容重; 土壤水分; 土壤團(tuán)聚作用; 土壤熱性質(zhì)

生物質(zhì)炭是由生物質(zhì)在高溫低氧或無(wú)氧條件下裂解而成的固體物質(zhì), 具有發(fā)達(dá)的孔隙結(jié)構(gòu), 表面積比較大, 具有親水和疏水雙重特性; 表面有多種官能團(tuán), 帶有大量的正負(fù)電荷, 具有吸附極性和非極性分子及陰陽(yáng)離子的能力; 生物質(zhì)炭富含芳香碳, 化學(xué)性質(zhì)十分穩(wěn)定, 在土壤及環(huán)境中分解非常緩慢, 半衰期幾百至千年以上[1-3]。越來(lái)越多的研究結(jié)果表明, 生物質(zhì)炭是良好的土壤調(diào)理劑, 不僅可以提高土壤肥力和農(nóng)作物產(chǎn)量, 而且可以改善農(nóng)產(chǎn)品品質(zhì)[4-7]; 施用生物質(zhì)炭還可以大幅度擴(kuò)大土壤碳庫(kù), 是一種可行的碳封存與減少溫室氣體排放的舉措, 也是未來(lái)可持續(xù)綠色農(nóng)業(yè)的發(fā)展方向之一[8-9]。生物質(zhì)炭在農(nóng)業(yè)與環(huán)境中的應(yīng)用已經(jīng)成為當(dāng)前國(guó)內(nèi)外研究的熱點(diǎn), 已有不少學(xué)者總結(jié)分析了近10~20年來(lái)生物質(zhì)炭特性[10-12]、生物質(zhì)炭對(duì)土壤化學(xué)性質(zhì)[13-14]、生物學(xué)性質(zhì)[15]、作物生長(zhǎng)發(fā)育的影響[16-17]及生物質(zhì)炭的固碳減排作用等[8,18-19], 但目前關(guān)于生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的綜述十分有限[20]。

土壤物理性狀是土壤功能的基礎(chǔ), 主要通過(guò)土壤容重、孔隙度、團(tuán)聚體組成、水分特征和熱特性等體現(xiàn), 不同土壤物理特征參數(shù)并不是相互獨(dú)立的, 彼此存在復(fù)雜的關(guān)系[21]。如土壤容重的改變將影響土壤孔隙度, 進(jìn)而影響土壤持水能力和導(dǎo)水特征, 而團(tuán)聚體穩(wěn)定性高的土壤通常具有較低的土壤容重, 具有較高的孔隙度[22]。生物質(zhì)炭對(duì)土壤物理性質(zhì)的影響包括直接影響和間接影響[23]。直接影響是指生物質(zhì)炭本身特性對(duì)土壤物理性質(zhì)的影響, 如生物質(zhì)炭容重低, 孔隙度高, 具有較強(qiáng)的持水能力, 因此影響土壤相應(yīng)的物理性質(zhì)[24-25]。而間接影響是指生物質(zhì)炭通過(guò)促進(jìn)作物生長(zhǎng)、提高土壤微生物活性等進(jìn)而促進(jìn)土壤團(tuán)聚體形成等影響土壤物理結(jié)構(gòu)[26-27]。

本文通過(guò)分析生物質(zhì)炭對(duì)土壤容重、土壤團(tuán)聚體、土壤水分特征和土壤熱性質(zhì)的影響, 討論生物質(zhì)炭對(duì)土壤物理性質(zhì)影響及差異的原因, 并提出亟待解決的問(wèn)題與下一步應(yīng)開(kāi)展的研究方向。

1 生物質(zhì)炭對(duì)土壤容重的影響及其機(jī)理

土壤物理性質(zhì)惡化是導(dǎo)致土壤肥力降低的重要原因之一, 其中土壤壓實(shí)最為常見(jiàn), 最直接的結(jié)果就是土壤容重提高, 孔隙度降低, 土壤通透性下降[28-30]。生物質(zhì)炭是多孔材料, 盡管其容重因原料和裂解條件而異, 但一般較低, 為0.09~0.74 g?cm-3[31-33]。越來(lái)越多的研究結(jié)果表明, 施用生物質(zhì)炭可以降低土壤容重[34-37], 但其效果隨生物質(zhì)炭種類(lèi)和用量而異, 也與土壤類(lèi)型有關(guān), 其作用機(jī)理也不完全一樣[33,35]。

生物質(zhì)炭降低土壤容重的直接原因是稀釋效應(yīng)[23], 即由于生物質(zhì)炭容重遠(yuǎn)低于土壤容重, 因此生物質(zhì)炭容重越低、施用量越大, 土壤容重越低。Githinji[36]報(bào)道在500 ℃下裂解1 h制備的花生殼生物質(zhì)炭施用量每增加1%(/), 砂質(zhì)壤土容重降低2.4×10-3g?cm-3; Alburquerque等[25]發(fā)現(xiàn)向壤質(zhì)砂土中添加小麥秸稈和橄欖樹(shù)枝條制備的生物質(zhì)炭7.5%(/)(容重分別為0.19 g?cm-3和0.66 g?cm-3), 培養(yǎng)2個(gè)月后, 小麥秸稈和橄欖枝條生物質(zhì)炭處理的土壤容重分別為1.25 g?cm-3和1.45 g?cm-3。

然而, 施用生物質(zhì)炭降低土壤容重, 并不能完全歸結(jié)于生物質(zhì)炭的稀釋效應(yīng)。Laird等[38]向粉壤土中添加5 g?kg-1、10 g?kg-1或20 g?kg-1木質(zhì)植物生物質(zhì)炭, 培養(yǎng)500 d后, 土壤容重顯著降低, 但不同用量之間沒(méi)有顯著性差異, 且土壤容重降低的幅度均大于生物質(zhì)炭稀釋效應(yīng)所能解釋的程度。Hardie等[39]也發(fā)現(xiàn)向白漿土(容重為1.27 g cm-3)中添加47 t?hm-2木質(zhì)植物生物質(zhì)炭(容重為0.51 g cm-3), 6個(gè)月后土壤容重降低到1.13 g?cm-3, 即使剔除土壤中的生物質(zhì)炭顆粒, 土壤容重仍為1.17 g?cm-3, 比原始土壤低近8%。上述研究結(jié)果說(shuō)明除了生物質(zhì)炭本身對(duì)土壤容重的稀釋效應(yīng)外, 生物質(zhì)炭還發(fā)揮著土壤調(diào)節(jié)劑的作用, 如生物質(zhì)炭通過(guò)改善土壤通氣狀況, 保持養(yǎng)分和水分, 從而提高土壤微生物的數(shù)量與活性, 增強(qiáng)菌體與礦物質(zhì)顆粒之間的相互作用, 也增強(qiáng)微生物代謝產(chǎn)物團(tuán)聚土壤礦物質(zhì)顆粒的作用, 從而降低土壤容重[23,40]。

此外, 生物質(zhì)炭對(duì)土壤容重的影響與土壤質(zhì)地、生物質(zhì)炭顆粒大小、時(shí)間效應(yīng)以及人為因素等有關(guān)。一般說(shuō)來(lái), 土壤砂粒含量越高, 容重越大, 生物質(zhì)炭對(duì)其影響也越大, 這主要取決于生物質(zhì)炭和土壤容重之差。如Herath等[24]向兩種容重不同的土壤(1.13 g?cm-3和0.75 g?cm-3)施入17.3 t?hm-2生物質(zhì)炭, 發(fā)現(xiàn)前者容重顯著降低, 而后者容重降低量很小。生物質(zhì)炭顆粒越大, 越容易降低土壤容重, 因?yàn)樾∮?.5 μm的生物質(zhì)炭顆?？梢蕴畛涞酵寥揽紫秲?nèi), 從而提高土壤容重; 而0.5~500 μm的生物質(zhì)炭則可以增加土壤孔隙, 從而降低土壤容重[41]。生物質(zhì)炭施用時(shí)間越長(zhǎng), 降低土壤容重的效果越弱, 甚至增加土壤容重。Madari等[42]發(fā)現(xiàn)施用木質(zhì)生物質(zhì)炭16 t?hm-2后立即測(cè)定土壤容重, 其降低了0.1 g?cm-3, 2年后生物質(zhì)炭對(duì)土壤容重沒(méi)有影響, 而5年后生物質(zhì)炭增加了土壤容重, 作者認(rèn)為是由于土壤黏粒(<2 μm)也可以進(jìn)入生物質(zhì)炭的大孔隙中。此外, 耕作會(huì)破碎生物質(zhì)炭, 減小生物質(zhì)炭顆粒; 還促進(jìn)生物質(zhì)炭及有機(jī)膠結(jié)物礦化, 破壞團(tuán)聚體, 尤其是大團(tuán)聚體, 從而削弱了生物質(zhì)炭對(duì)土壤容重的降低作用[43]。因此, 生物質(zhì)炭可以通過(guò)直接作用或間接作用降低土壤容重, 但是對(duì)土壤容重影響的效果與生物質(zhì)炭的裂解原料、溫度和施用量有關(guān), 也與土壤類(lèi)型和施用時(shí)間有關(guān)。

2 生物質(zhì)炭對(duì)土壤礦物質(zhì)顆粒團(tuán)聚作用及團(tuán)聚體穩(wěn)定性的影響與機(jī)理

穩(wěn)定的團(tuán)聚體是土壤這個(gè)“擬生命體”的細(xì)胞, 生物質(zhì)炭對(duì)土壤團(tuán)聚體的影響也是近期的研究重點(diǎn)之一。不少研究結(jié)果表明, 生物質(zhì)炭可促進(jìn)土壤礦物質(zhì)顆粒團(tuán)聚作用, 尤其是促進(jìn)大團(tuán)聚體的形成, 而且增強(qiáng)團(tuán)聚體的穩(wěn)定性[44-46]。如Liu等[46]向紅壤中施用小麥秸稈在350~550 ℃下裂解而成的生物質(zhì)炭40 t?hm-27個(gè)月后, 土壤團(tuán)聚體的平均重量直徑(MWD)增加了28%, 但是施用20 t?hm-2則沒(méi)有效果。Sun等[47]發(fā)現(xiàn)施用6%草本植物生物質(zhì)炭180 d后, 黏土的團(tuán)聚體平均重量直徑提高了21%, 然而施用木本植物生物質(zhì)炭則沒(méi)有效果。Herath等[24]比較了550 ℃和350 ℃下用玉米芯制備的生物質(zhì)炭對(duì)粉壤土團(tuán)聚體形成的影響, 發(fā)現(xiàn)在有機(jī)質(zhì)含量低的土壤上, 前者的效果大于后者, 但在有機(jī)質(zhì)含量高的土壤上則相反。也有相反的報(bào)道, 生物質(zhì)炭對(duì)土壤團(tuán)聚作用沒(méi)有顯著影響。Peng等[48]在紅壤中施用2 t?hm-2?a-1稻草生物質(zhì)炭, 沒(méi)有檢測(cè)到土壤團(tuán)聚體穩(wěn)定性發(fā)生變化。可見(jiàn), 生物質(zhì)炭對(duì)土壤礦物質(zhì)顆粒團(tuán)聚作用的影響, 可能與生物質(zhì)炭種類(lèi)、施用量及土壤性質(zhì)有關(guān), 目前尚沒(méi)有形成統(tǒng)一的理論, 還需要廣泛的研究[49]。

生物質(zhì)炭促進(jìn)土壤礦物質(zhì)顆粒團(tuán)聚作用機(jī)理可能包括直接作用和間接作用兩個(gè)方面。直接作用包括: 1)土壤有機(jī)質(zhì)在土壤團(tuán)聚體形成過(guò)程發(fā)揮著重要作用[50], Zornoza等[51]發(fā)現(xiàn)土壤團(tuán)聚體穩(wěn)定性與土壤惰性有機(jī)碳含量呈正相關(guān), 在施用生物質(zhì)炭的土壤中也是如此。2)由于生物質(zhì)炭具有巨大的比表面積和大量負(fù)電荷, 生物質(zhì)炭可以提高土壤陽(yáng)離子交換量, 進(jìn)而促進(jìn)土壤團(tuán)聚體的形成[52]。3)生物質(zhì)炭表面有羥基和羧基等多種官能團(tuán), 帶有大量的負(fù)電荷, 也帶有一定量的正電荷, 可以通過(guò)靜電引力直接與礦物質(zhì)顆粒表面的金屬離子結(jié)合, 亦或通過(guò)多價(jià)離子的鍵橋作用, 將礦物質(zhì)土粒團(tuán)聚在一起, 形成具有水穩(wěn)定性的團(tuán)聚體[53-54]。4)生物質(zhì)炭表面陽(yáng)離子(如Ca2+)還可以通過(guò)置換土壤黏粒表面的Na+等陽(yáng)離子, 從而阻止膠體分散, 促使礦物質(zhì)膠體絮凝, 形成膠團(tuán)[55]。

生物質(zhì)炭促進(jìn)土壤礦物質(zhì)顆粒團(tuán)聚的間接作用與土壤微生物和植物有關(guān)。盡管生物質(zhì)炭富含惰性芳香碳, 但仍然含有一定量的易分解的有機(jī)物質(zhì), 可作為微生物基質(zhì), 施入土壤后可提高土壤微生物量[15], 微生物細(xì)胞本身可作為膠結(jié)劑, 將礦物質(zhì)土粒團(tuán)聚在一起。Luo等[56-57]報(bào)道生物質(zhì)炭對(duì)土壤微生物群落結(jié)構(gòu)也有顯著的影響, 不同微生物與礦物質(zhì)顆粒相互作用差異很大, 對(duì)土壤團(tuán)聚體形成的促進(jìn)作用應(yīng)也有區(qū)別。生物質(zhì)炭由于C/N比較高, 可能有利于真菌生長(zhǎng), 從而促進(jìn)團(tuán)聚體形成[58]。此外, 生物質(zhì)炭通過(guò)影響微生物的代謝產(chǎn)物, 以及對(duì)植物根系生長(zhǎng)與發(fā)育的影響, 從而間接地影響土壤團(tuán)聚體的形成。生物質(zhì)炭可以促進(jìn)作物生長(zhǎng), 間接地提高土壤有機(jī)物質(zhì)的投入, 如根系及其分泌物, 進(jìn)而間接地提高土壤團(tuán)聚體穩(wěn)定性[50,59]。

3 生物質(zhì)炭對(duì)土壤水分的影響及其機(jī)理

土壤水分運(yùn)移和儲(chǔ)存在養(yǎng)分運(yùn)輸和植物生長(zhǎng)過(guò)程中發(fā)揮著重要作用, 土壤水分特征主要包括土壤持水能力(飽和含水量、田間持水量、萎蔫點(diǎn)持水量等)、飽和導(dǎo)水率、非飽和導(dǎo)水率和土壤水勢(shì)等等。生物質(zhì)炭對(duì)土壤水分特征的影響已經(jīng)成為近期研究的熱點(diǎn)之一[60-62]。目前已有關(guān)于生物質(zhì)炭影響土壤水分特征的研究多集中在生物質(zhì)炭對(duì)土壤田間持水量、有效水含量和飽和導(dǎo)水率方面, 而生物質(zhì)炭對(duì)非飽和導(dǎo)水率和入滲率影響的研究較少[21]。生物質(zhì)炭對(duì)土壤水分特征的影響包括直接影響和間接影響, 直接影響主要是生物質(zhì)炭本身的多孔性對(duì)田間持水量、有效水含量及飽和導(dǎo)水率影響等方面, 而間接影響體現(xiàn)在生物質(zhì)炭通過(guò)改善土壤團(tuán)粒結(jié)構(gòu), 提高土壤有機(jī)碳含量, 促進(jìn)作物根系生長(zhǎng)等從而間接地影響土壤水分特征[63-65]。

3.1 生物質(zhì)炭對(duì)田間持水量和有效水含量的影響

生物質(zhì)炭具有多孔性并有較大的比表面積, 因此可以通過(guò)增加生物質(zhì)炭顆粒和水分之間的吸附力提高田間持水量[66-67]。Laghari等[4]發(fā)現(xiàn)向兩種砂土中添加0 t?hm-2、15 t?hm-2、22 t?hm-2和45 t?hm-2生物質(zhì)炭, 培養(yǎng)8周后發(fā)現(xiàn)施用45 t?hm-2生物質(zhì)炭分別顯著提高了兩種砂土田間持水量的33%和26%, 而施用22 t?hm-2及以下的生物質(zhì)炭處理并沒(méi)有顯著影響田間持水量。Karhu等[67]報(bào)道添加9 t?hm-2生物質(zhì)炭, 壤土的田間持水量提高了11%。Lu等[68]向黏土中添加2%、4%和6%生物質(zhì)炭, 培養(yǎng)180 d后, 土壤田間持水量分別提高了12%、20%和31%, 說(shuō)明生物質(zhì)炭施用量與田間持水量呈正相關(guān)。但是也有個(gè)別相反的報(bào)道, 如Major等[69]發(fā)現(xiàn)施用20 t?hm-2生物質(zhì)炭對(duì)黏土田間持水量沒(méi)有顯著的影響?？梢?jiàn)生物質(zhì)炭對(duì)田間持水量的影響與土壤質(zhì)地、施用量有關(guān)。生物質(zhì)炭提高砂土田間持水量的原因一方面是生物質(zhì)炭本身持水能力強(qiáng); 另一方面是較小的生物質(zhì)炭顆粒可以填充砂質(zhì)土壤的大孔隙, 增加毛細(xì)孔, 從而增加田間持水量[70-71]。生物質(zhì)炭對(duì)黏質(zhì)土壤田間持水量的影響相對(duì)復(fù)雜。Lu等[68]研究表明生物質(zhì)炭(施用量為2%、4%和6%)提高黏土田間持水量的原因可能與生物質(zhì)炭增加了黏土孔隙度有關(guān)。但是向黏質(zhì)土壤中添加少量(<1%)生物質(zhì)炭也可能使生物質(zhì)炭的孔隙被黏粒堵塞, 從而導(dǎo)致生物質(zhì)炭提高土壤田間持水量的能力降低或保持不變。因此Blanco-Canqui[21]建議應(yīng)向黏土中施用大量生物質(zhì)炭以提高田間持水量。

土壤有效水含量通常指基質(zhì)勢(shì)為-33~-1 500 kPa土壤含水量之差, 一般說(shuō)來(lái), 土壤萎蔫點(diǎn)降低, 或土壤持水量提高, 土壤有效水含量也增加。生物質(zhì)炭對(duì)土壤有效水含量的影響, 不僅與生物質(zhì)炭種類(lèi)及用量有關(guān), 而且還可能與土壤條件和試驗(yàn)時(shí)間有關(guān)。如Peake等[72]向8種不同土壤添加0.1%、0.5%和2.5%(/)的生物質(zhì)炭, 培養(yǎng)48 h后發(fā)現(xiàn), 生物質(zhì)炭對(duì)土壤有效水含量的影響分別為-4.0%~13.4%、-9.8%~33.7%和0.3%~48.4%。G??b等[41]發(fā)現(xiàn)施用10 t?hm-2生物質(zhì)炭不會(huì)顯著影響土壤水分有效性, 但施用高量生物質(zhì)炭可提高土壤水分有效性。Burrell等[23]報(bào)道秸稈生物質(zhì)炭增加了土壤水分有效性, 然而木條生物質(zhì)炭則沒(méi)有影響土壤水分有效性。Herath等[24]報(bào)道添加11.3 t?hm-2和10.0 t?hm-2的350 ℃和550 ℃下制備的生物質(zhì)炭, 土壤有效水含量迅速增加13%和10%, 但培養(yǎng)295 d后, 土壤有效水含量與對(duì)照沒(méi)有差異。生物質(zhì)炭能夠增加土壤有效水含量的原因與生物質(zhì)炭提高土壤田間持水量的原因類(lèi)似, 即生物質(zhì)炭具有較大的親水表面積和孔隙度, 水分吸附在生物質(zhì)炭表面, 儲(chǔ)存在生物質(zhì)炭孔隙內(nèi)[21]。

3.2 生物質(zhì)炭對(duì)土壤飽和導(dǎo)水率的影響

水分入滲主要取決于土壤水勢(shì)和飽和導(dǎo)水率, 而非飽和導(dǎo)水率是土壤水分再分布的重要參數(shù)[73-74]。施用生物質(zhì)炭能夠降低土壤容重, 顯然也能夠改變土壤孔隙狀況, 無(wú)論是對(duì)土壤水分入滲, 還是水分再分布, 理論上應(yīng)有顯著的影響, 但是目前關(guān)于生物質(zhì)炭對(duì)土壤水分入滲和非飽和導(dǎo)水率影響的研究很少, 而關(guān)于生物質(zhì)炭對(duì)土壤飽和導(dǎo)水率的影響研究較多, 因?yàn)楝F(xiàn)有生物質(zhì)炭試驗(yàn)多在室內(nèi)培養(yǎng)條件下進(jìn)行, 飽和導(dǎo)水率比非飽和導(dǎo)水率容易測(cè)定[21]。生物質(zhì)炭對(duì)不同質(zhì)地土壤的飽和導(dǎo)水率影響不一致, 通常生物質(zhì)炭可以降低砂質(zhì)土壤的飽和導(dǎo)水率[69,75], 而增加黏質(zhì)土壤的飽和導(dǎo)水率[65-76]。生物質(zhì)炭降低砂質(zhì)土壤飽和導(dǎo)水率的原因是生物質(zhì)炭較小的顆?？梢远氯寥揽紫痘蚺c土壤無(wú)機(jī)礦物結(jié)合減少土壤孔隙。Zhang等[75]發(fā)現(xiàn)施用粉末生物質(zhì)炭比顆粒生物質(zhì)炭更能降低土壤的飽和導(dǎo)水率。而與砂質(zhì)土壤不同, 黏質(zhì)土壤孔隙度低, 施用生物質(zhì)炭通?？梢栽黾羽ね量紫抖群惋柡蛯?dǎo)水率[35]。生物質(zhì)炭的施用量和原料與土壤導(dǎo)水率也有關(guān)。Omondi等[35]對(duì)76組數(shù)據(jù)進(jìn)行了大數(shù)據(jù)分析, 發(fā)現(xiàn)施用高量生物質(zhì)炭(80 t?hm-2)可以顯著提高土壤導(dǎo)水率, 但低量生物質(zhì)炭對(duì)土壤導(dǎo)水率沒(méi)有顯著的影響。施用木質(zhì)和動(dòng)物糞便制備的生物質(zhì)炭, 土壤導(dǎo)水率分別顯著提高了35.7%和6.6%, 而草本植物生物質(zhì)炭則對(duì)土壤導(dǎo)水率沒(méi)有顯著影響[38]。

總而言之, 生物質(zhì)炭對(duì)土壤水分特征的影響與生物質(zhì)炭特性及土壤性質(zhì)有關(guān), 但是目前對(duì)其機(jī)理還沒(méi)有完整的認(rèn)識(shí), 可能包括以下幾個(gè)方面:

1)生物質(zhì)炭表面吸持水分: 生物質(zhì)炭表面具有親水和疏水雙重特性, 二者的比例及親水表面特性, 在很大程度上決定了生物質(zhì)炭施入土壤后, 土壤水分特性的變化。大多數(shù)生物質(zhì)炭表面有大量的親水性官能團(tuán)[76], 具有比較強(qiáng)的持水能力, 持水量是本身重量的10倍之多[77-78]。顯然, 添加此類(lèi)生物質(zhì)炭, 能夠顯著提高土壤持水性。但是如果生物質(zhì)炭疏水表面比較大, 則會(huì)增強(qiáng)土壤的斥水性, 降低土壤持水性, 容易產(chǎn)生徑流, 加劇土壤侵蝕。

2)生物質(zhì)炭孔隙吸持水分作用: 生物質(zhì)炭為多孔材料[36], 含有大量的微孔(<2 nm)、中孔(2~50 nm)和大孔(>50 nm), 根據(jù)拉普拉斯定律公式:

式中:為孔隙半徑,為不同孔徑下的毛管力,為表面張力(0.073 N×m-1),為接觸角度(<90°)。半徑<10 nm孔隙所吸持的水分植物很難利用。不同生物質(zhì)炭的孔性差異很大, 吸持水分量及所吸持水分的有效性也不同, 施入土壤后, 對(duì)土壤吸持水分性能的影響也應(yīng)不同。

3)生物質(zhì)炭顆粒堵塞效應(yīng): 生物質(zhì)炭是剛性顆粒狀材料, 大顆粒生物質(zhì)炭可以增加土壤孔隙度, 但小顆粒生物質(zhì)炭可能會(huì)堵塞土壤孔隙, 從而減少土壤孔隙。盡管其填充堵塞土壤的過(guò)程尚不清楚, 但是堵塞效應(yīng)仍被認(rèn)為是生物質(zhì)炭降低土壤導(dǎo)水率的主要原因[42]。

4 生物質(zhì)炭對(duì)土壤熱性質(zhì)的影響與機(jī)理

土壤熱性質(zhì)在能量平衡方面扮演著重要的角色, 是土壤溫度的基礎(chǔ), 主要受土壤容重、孔隙、含水量和有機(jī)質(zhì)等因素影響[79]。生物質(zhì)炭直接或間接地影響到土壤這些性質(zhì), 因此施用生物質(zhì)炭的土壤, 其熱性質(zhì)也發(fā)生變化。Zhang等[80]報(bào)道施用4.5 t?hm-2和9 t?hm-2玉米秸稈生物質(zhì)炭5年后, 潮土熱導(dǎo)率顯著降低3.48%和7.49%, 可能是生物質(zhì)炭降低土壤容重所致。此外, 生物質(zhì)炭增加了近紫外光和藍(lán)光(350~513 nm)的反射率, 但是降低了近紅外光(520~2 350 nm)的反射率, 從而降低了土壤溫度。劉志鵬等[81]也發(fā)現(xiàn)生物質(zhì)炭降低了土壤導(dǎo)熱率、熱容量和熱擴(kuò)散系數(shù), 因?yàn)樯镔|(zhì)炭降低了土壤容重, 增加了土壤孔隙度。但是, Usowicz等[82]發(fā)現(xiàn)木屑生物質(zhì)炭對(duì)草地和荒地土壤的熱性質(zhì)影響不一致, 生物質(zhì)炭顯著降低了荒地土壤容重, 增加土壤孔隙度, 從而降低了土壤熱導(dǎo)率, 而對(duì)草地土壤并無(wú)顯著影響。這說(shuō)明生物質(zhì)炭對(duì)土壤熱性質(zhì)的影響, 不僅與生物質(zhì)炭本身的特性有關(guān), 而且還與土壤質(zhì)地有關(guān), 目前相關(guān)研究十分欠缺, 對(duì)不同生物質(zhì)炭及不同施用量對(duì)不同土壤熱性質(zhì)的影響及其機(jī)理的了解很少。

5 研究不足與亟待解決的問(wèn)題

5.1 研究不足

生物質(zhì)炭作為土壤調(diào)理劑的研究方興未艾, 也已取得顯著的進(jìn)展, 但有關(guān)生物質(zhì)炭對(duì)土壤物理性質(zhì)的影響及其機(jī)理的研究尚不充分, 而且在研究?jī)?nèi)容和研究方法上存在嚴(yán)重缺陷, 主要體現(xiàn)在4個(gè)方面:

1)定性描述性的研究為主, 而定量研究很少?，F(xiàn)有的研究大多是選擇一種或幾種生物質(zhì)炭, 一個(gè)或幾個(gè)用量, 施用到某一個(gè)或幾個(gè)不同類(lèi)型的土壤, 再定期檢測(cè)土壤相關(guān)性質(zhì)的變化, 從而說(shuō)明生物質(zhì)炭對(duì)土壤某些物理性質(zhì)的影響, 幾乎沒(méi)有關(guān)注生物質(zhì)炭特性及其變化與土壤相關(guān)物理參數(shù)之間的定量關(guān)系, 致使重復(fù)性研究比較多, 也就無(wú)法從個(gè)案研究總結(jié)出普遍的規(guī)律性。

2)單一學(xué)科、單一因子研究較多, 而交叉學(xué)科的綜合研究沒(méi)有。現(xiàn)有的研究主要涉及到土壤物理學(xué)某些參數(shù), 很少將土壤物理參數(shù)與化學(xué)及生物指標(biāo)結(jié)合在一起, 所用的研究方法大多是傳統(tǒng)的土壤物理研究方法, 研究的深度和廣度十分有限, 解決的問(wèn)題大多也比較膚淺。

3)短期研究為主, 缺乏長(zhǎng)期定位試驗(yàn)研究。生物質(zhì)炭十分穩(wěn)定, 在土壤可保存數(shù)百甚至上千年, 在此期間, 生物質(zhì)炭必然發(fā)生很多變化, 與土壤礦物質(zhì)、有機(jī)質(zhì)的相互作用也可能改變; 況且, 與土壤化學(xué)穩(wěn)定性不同, 土壤物理性質(zhì)一般變化比較緩慢。顯然短期試驗(yàn)很難研究生物質(zhì)炭對(duì)土壤物理性質(zhì)的影響, 也很難了解其真實(shí)的過(guò)程與機(jī)理。

4)以室內(nèi)培養(yǎng)試驗(yàn)為主, 田間試驗(yàn)結(jié)果缺乏?，F(xiàn)有的研究大多為室內(nèi)模擬試驗(yàn), 與田間實(shí)際情況差異非常大, 根本無(wú)法與農(nóng)業(yè)生產(chǎn)活動(dòng)聯(lián)系在一起, 所獲得的結(jié)果不可能解釋真實(shí)田間條件下生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的原因。

5.2 亟待解決的問(wèn)題

1)明確生物質(zhì)炭特性與土壤物理參數(shù)的定量關(guān)系。生物質(zhì)炭對(duì)土壤物理性質(zhì)的影響在很大程度上主要取決于生物質(zhì)炭本身的特性, 其次與施用量及土壤某些性質(zhì)有關(guān)。應(yīng)在現(xiàn)有定性研究的基礎(chǔ)上, 利用現(xiàn)代儀器分析手段, 結(jié)合計(jì)算機(jī)及信息技術(shù), 定量研究生物質(zhì)炭特性與土壤孔性、土壤水分及熱性質(zhì)之間的定量關(guān)系, 重點(diǎn)研究生物質(zhì)炭與土壤水分及其運(yùn)動(dòng)之間的定量關(guān)系, 建立數(shù)學(xué)模擬模型, 從而為設(shè)計(jì)制造個(gè)性化的生物質(zhì)炭提供參考, 以滿(mǎn)足不同土壤及不同作物的需求。

2)揭示生物質(zhì)炭對(duì)土壤主要物理過(guò)程影響的機(jī)理。根據(jù)農(nóng)業(yè)生產(chǎn)、生態(tài)環(huán)境建設(shè)與保護(hù)的需要, 重點(diǎn)研究生物質(zhì)炭促進(jìn)土壤礦物質(zhì)顆粒直接及間接作用的過(guò)程與機(jī)理; 在土壤侵蝕嚴(yán)重的地區(qū), 應(yīng)大力開(kāi)展生物質(zhì)炭對(duì)土壤水分入滲過(guò)程影響的研究, 為減少地表徑流提供科學(xué)依據(jù); 在干旱地區(qū)應(yīng)著力開(kāi)展生物質(zhì)炭對(duì)土壤水分保持及水分再分布過(guò)程影響的研究, 提高土壤水分利用效率; 而在低溫地區(qū)應(yīng)開(kāi)展生物質(zhì)炭調(diào)節(jié)控制土壤熱性質(zhì)過(guò)程與機(jī)理研究。

3)探究生物質(zhì)炭對(duì)土壤物理過(guò)程與土壤化學(xué)及生物過(guò)程相互作用的機(jī)理。土壤物理性質(zhì)是土壤肥力的基礎(chǔ), 土壤物理過(guò)程在很大程度上決定土壤化學(xué)及生物過(guò)程, 應(yīng)大力開(kāi)展研究, 了解生物質(zhì)炭是否能夠“干涉”甚至調(diào)節(jié)控制主要土壤物理過(guò)程與主要土壤化學(xué)及生物過(guò)程的交互作用, 以期定向地放大或減弱這些相互作用, 從而發(fā)揮生物質(zhì)炭作為土壤調(diào)理劑的效能。

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Biochar effect on soil physical properties: A review*

DONG Xinliang1,2, LIN Qimei2**

(1. Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences / Key Laboratory of Agricultural Water Resources, Chinese Academy of Sciences / Hebei Key Laboratory of Water-Saving Agriculture, Shijiazhuang 050022, China; 2. College of Resources and Environment, China Agricultural University, Beijing 100193, China)

Biochar is charred biomass produced under limited or no oxygen (O2) supply and at high temperature. Rich in aromatic carbon (C), biochar can persist in soil for hundreds or thousands years. Biochar application in agricultural soils has attracted an intensive global research with extensive reviews of biochar characteristics, effects on soil chemical, biological and physical properties and on crop growth. However, the work on the effect of biochar on soil physical properties is not as detailed as those on soil chemical and biological properties. In this study, relevant publications on the effect of biochar on soil physical properties in recent decades were searched and summarized. The results indicated that biochar application reduced soil bulk density, improved soil aggregate stability, field water holding capacity, soil available water content, and decreased saturated hydraulic conductivity through direct and indirect ways. Biochar has huge specific surface area and porosity, which directly influences soil physical properties such as reducing soil bulk density and increasing water holding capacity and soil available water content. Furthermore, biochar particles combine with soil minerals to change soil structure and create the proper environment for soil microbial and plant root growth, which ultimately affect soil physical properties. Indirectly, the effect of biochar on soil physical properties includes improved soil aggregate stability and saturated hydraulic conductivity. The effect of biochar on soil physical properties is related to biochar application rate, feedstock type, pyrolysis temperature, particle size, soil texture and duration in the soil. Generally, the higher the amount of biochar applied, the more significant are the induced changes in soil physical properties (bulk density, water holding capacity and saturated hydraulic conductivity). Woody biochar significantly increases saturated hydraulic conductivity, while grass biochar has no effect on saturated hydraulic conductivity. In addition, as the duration after biochar application increases, its effect on soil physical properties decreases. However, very limited studies have investigated this relationship under long-term filed conditions, and field experiments have been even relatively scarce. Consequently, the further studies should focus on the long-term effects of biochar application on soil physical properties in filed condition, and investigate the mechanisms behind the biochar effect on soil physical properties, which is essential to provide scientific evidence for the real agricultural production and ecological improvement. Furthermore, the relationships between biochar characteristics and soil physical properties are not fully known. Thus, there are needs to further investigate the effect of biochar application on the interaction of soil physical and chemical, biological properties and the mechanisms.

Biochar; Soil bulk density; Soil water; Soil aggregation; Soil thermal property

, E-mail: linqm@cau.edu.cn

Mar. 19, 2018;

Jul. 20, 2018

S152

A

1671-3990(2018)12-1846-09

10.13930/j.cnki.cjea.180277

* 國(guó)家自然科學(xué)基金項(xiàng)目(41371243)資助

林啟美, 主要從事土壤生物與生物過(guò)程研究。E-mail: linqm@cau.edu.cn

董心亮, 主要從事土壤生物與生態(tài)系統(tǒng)健康研究。E-mail: xldong@sjziam.ac.cn

2018-03-19

2018-07-20

* This study was supported by the National Natural Science Foundation of China (41371243).

董心亮, 林啟美. 生物質(zhì)炭對(duì)土壤物理性質(zhì)影響的研究進(jìn)展[J]. 中國(guó)生態(tài)農(nóng)業(yè)學(xué)報(bào), 2018, 26(12): 1846-1854

DONG X L, LIN Q M. Biochar effect on soil physical properties: A review[J]. Chinese Journal of Eco-Agriculture, 2018, 26(12): 1846-1854