徐 敏,伍 鈞,張小洪,楊 剛

四川農(nóng)業(yè)大學環(huán)境學院, 成都 611130

隨著全球人口的增加及氣候變暖的加劇,糧食供應與人口的矛盾日益突出,預計到2050年全球人口將達96億[1- 2]。氣候變暖對糧食產(chǎn)量及可持續(xù)發(fā)展產(chǎn)生巨大的影響。研究表明,溫度升高會抑制作物光合作用、降低水分利用效率,從而降低作物產(chǎn)量,在一定范圍內(nèi),溫度與作物產(chǎn)量呈線性負相關(guān)關(guān)系[3]。土壤碳庫是生態(tài)系統(tǒng)中重要的碳庫和匯,其碳儲量高達27000億t,土壤碳庫的變動對全球溫室氣體有著舉足輕重的作用。氣候變暖及糧食危機對土壤固碳減排及多產(chǎn)的要求日趨增加,而氣候變暖[3- 6]及土壤退化是降低作物產(chǎn)量的主要原因[7- 10]。生物炭 (Biomass Charcoal, BC) 是指農(nóng)業(yè)廢棄物、林業(yè)廢棄物、水生生物、人類及動物排泄物、工業(yè)廢棄物等有機廢棄物[2],在250—700℃缺氧或低氧條件下,熱解炭化而成的高度芳香化的含碳物質(zhì)[7- 12]。生物炭原料來源廣泛,成本低廉,具有多孔、比表面積大、吸附能力強、碳穩(wěn)定性強等特點[13- 15],是一種經(jīng)濟、環(huán)境友好型的土壤改良劑,在土壤培肥[7,16- 19]、溫室氣體減排[7,20- 23]方面表現(xiàn)出巨大的潛力,具有較高的生態(tài)經(jīng)濟效益[13,20,24- 26]。以生物炭進行固碳減排,每年每公頃農(nóng)田能固定CO27.6t,預計到2100年,全球?qū)⒂?.5×1010t碳封存于土壤中[5],每年可抵消因人為引起的12%的C排放[6]。生物炭施用能增加土壤碳儲量,減少溫室氣體排放,緩解因高溫引起的糧食減產(chǎn)[3],同時,還能提高土壤質(zhì)量,進一步提高農(nóng)田產(chǎn)出,維持農(nóng)田可持續(xù)發(fā)展[1,4]。

本文總結(jié)前人研究,主要從生物炭減少溫室氣體排放及提高農(nóng)田產(chǎn)出兩方面進行總結(jié)歸納,闡明生物炭作用機理及影響因素,以期為生物炭應用于緩解溫室效應及提高農(nóng)田可持續(xù)發(fā)展提供理論依據(jù)。

1 固碳減排

1.1 固碳減排潛力

引起溫室效應的氣體主要是CO2、CH4、N2O[5,27- 28],其中,農(nóng)田生態(tài)系統(tǒng)是重要的排放源之一,占全球溫室氣體總排放量的10%—20%,且在不斷增加[29],因此,減少農(nóng)田溫室氣體排放具有重要意義。我國絕大部分土壤有機碳含量為100t/hm2,低于世界平均水平121t/hm2,耕層土壤碳密度低于全球平均值的70%,旱地耕層土壤碳密度僅為平均值的1/3,可見,我國土壤生態(tài)系統(tǒng)具有巨大的固碳減排潛力[4,30]。

我國每年產(chǎn)生的生物質(zhì)量巨大,據(jù)統(tǒng)計,每年可利用農(nóng)林生物質(zhì)總量為6.05×108t,其中,農(nóng)業(yè)生物質(zhì)占83%,林業(yè)生物質(zhì)占17%[30- 31]。農(nóng)業(yè)生物質(zhì)主要為水稻、小麥和玉米秸稈,分別約1.1×108、1.3×108、2.3×108t,其中,1/3-1/4的秸稈被直接焚燒[11,30- 32],還有一部分直接還田。李飛躍等[30]估算得出我國作物秸稈每年因焚燒排放的總碳量為4.77×107t。以焚燒產(chǎn)生的黑炭及秸稈直接還田兩種方式還田,不穩(wěn)定性碳比例均高于生物炭,不利于碳的固定[33- 34]。與原材料相比,熱解的生物炭具有更多穩(wěn)定性碳[33- 34],因此,將農(nóng)林廢棄物以生物炭的形式封存于土壤中,更利于碳的固定。若以生物炭形式,對廢棄生物質(zhì)進行封存,我國每年緩解溫室效應總潛力為5.45×108t,凈固碳潛力約5.32×108t,其中,焚燒部分以生物炭固存,每年平均固碳量可達0.96×108t[30- 31]。生物炭施入土壤對碳的封存效果取決于多方面的因素,如生物炭性質(zhì)、土壤性質(zhì)、氣候條件、管理措施等[4,6]。Sheng等[4]研究認為,pH較低的土壤能加速土壤碳的礦化,固碳效果差,而對于酸性耕地占總耕地面積27%的中國來說,高估了其土壤固碳潛力。

1.2 固碳減排機制

生物炭施用于土壤是一種有效的固碳減排手段[4,35],與不施生物炭相比,施用生物炭降低了土壤N、C的礦化作用,減少土壤生態(tài)系統(tǒng)CO2、N2O等溫室氣體排放[36- 37]。其固碳減排機制主要表現(xiàn)在(表1)：一是自身含碳較高,且穩(wěn)定性高[23,38- 39]。生物炭中易氧化態(tài)碳比例低,芳香化碳比例高,約占60%以上,且隨著溫度升高而增加[38],這部分不易被分解[40-42],是生物炭具有固碳減排效益的前提[41]。Wu等[21]研究表明,生物炭中的碳平均滯留時間為617—2829年,表現(xiàn)為對碳封存作用極佳,且具有長期效果[20]。二是對作物生長的促進作用,提高碳利用效率,具有固碳效應[1]。Liu等[43]認為生物炭可通過增加土壤有機質(zhì)及養(yǎng)分含量來,促進作物生長[19,37],提高土壤碳的利用效率。三是減少化肥投入,可降低生產(chǎn)化肥消耗的能源及肥料施用排放的溫室氣體[1,44]。Zhang等[44]采用平衡施肥法配以生物炭施用,能在提高玉米產(chǎn)量的同時減少N2O的排放,利于農(nóng)田的低碳可持續(xù)發(fā)展。四是生物炭抑制溫室氣體排放[1,44],主要表現(xiàn)在：1)通過調(diào)節(jié)土壤性質(zhì)及微生物活性,調(diào)節(jié)硝化及反硝化反應,抑制N2O產(chǎn)生,其一是增加了土壤的透氣性,抑制了反硝化作用[28,45]；其二是提高土壤pH值,增強了N2O還原酶活性,降低N2O含量[28,46],而Ameloot等[28]研究表明,生物炭抑制了N2O的產(chǎn)生,但并未促進N2O向N2的轉(zhuǎn)化,其原因還需進一步探究。2)生物炭C/N較高,N的循環(huán)被抑制[10,47]。3)降低酶活性,抑制N礦化,有助于提高土壤N含量[10]。4)影響土壤微生物群落結(jié)構(gòu)及功能,抑制了某些微生物的活性,從而抑制對有機質(zhì)的降解[48- 49],降低了C、N的排放[50]。5)生物炭吸附C、N及有機質(zhì),生成更穩(wěn)定的物質(zhì)[20,45- 46,51],Nguye和Lee[52]及Xu等[23]研究表明,生物炭巨大的比表面積及一定的礦物質(zhì)含量(如FeOH、CaCO3等)對CO2具有較強的物理、化學固定作用,物理及化學固定作用促使生物炭與土壤及有機質(zhì)形成有機-無機復合體,生成更緊密的團聚體。孟凡榮等[53]3年田間試驗表明,生物炭的施用提高了黑土中黑炭縮合度,提高了胡敏素HA芳香性,其原因主要是：a)生物炭自身的C=C增加黑炭中C=C比例；b)生物炭的施入提高了微生物活性,分解黑炭中一部分C-C,還有一部分形成更穩(wěn)定的C=C,穩(wěn)定性增加。生物炭主要以自由有機碳儲存在土壤粘粒中,同時,能促進土壤中的有機碳由大團聚體向小團聚體轉(zhuǎn)變[6]。6)降低酶活性,抑制土壤呼吸作用[43,54],侯亞紅等[55]研究表明,與秸稈還田相比,秸稈生物炭施用降低了脫氫酶、β-糖苷酶活性,顯著降低了土壤呼吸,認為生物炭施用是一種低碳的還田模式,利于碳長期封存[54]。而Plaza等[20]及Liu等[43]研究表明,生物炭施用對土壤呼吸無影響,但仍認為生物炭固具有固碳效應,主要是因為：一方面,生物炭施用后,未刺激土壤碳的礦化,即具有固碳效應；另一方面,生物炭施用促進作物生長,提高土壤碳利用效率,具有固碳效應。

表1 生物炭對溫室氣體排放的影響及機制

1.3 負效應

也有學者研究認為生物炭施用對溫室氣體排放無影響或促進其排放(表1)[22,45]。Purakayastha等[22]研究表明,生物炭雖然增加了土壤pH、CEC,但對碳礦化作用無顯著影響,表現(xiàn)為碳中性?？扇苄杂袡C質(zhì)是土壤移動性最強、生物有效性最高的部分[56],在土壤碳循環(huán)過程中占重要地位[57],可溶性有機質(zhì)含量越高,越易礦化[45]。研究表明,生物炭的施用提高了土壤孔隙水中[58]及土壤[59]可溶性有機質(zhì)含量。同時,生物炭施用也提高了土壤孔隙度[18,60- 61]及土壤含水量[62- 63],而Sun等[64]研究認為,土壤水含量為70%時,土壤碳礦化作用最強,由此,生物炭施用提高了土壤水分及可溶性有機質(zhì)含量,也可能促進碳的礦化。此外,Wang等[57]研究顯示,生物炭還會促進本土有機質(zhì)轉(zhuǎn)化為可溶性有機碳,可能是因為生物炭：a)提高了土壤pH值,增加了可溶性有機質(zhì)的釋放；b)提高了腐殖膠體表面的電負性官能團；c)增加了鹽基離子,特別是K+含量,競爭粘土及有機質(zhì)表面的陽離子交換位點,從而增加可溶性有機質(zhì)的釋放。Subedi等[45]研究還表明,2%的糞便生物炭施用增加了土壤N2O的排放,主要是因為：a)生物炭的可溶解性有機物的礦化作用[57]；b)微生物數(shù)量及活性的增加,促進生物炭、土壤有機質(zhì)或動植物殘體的分解。

1.4 影響因素

不同的研究結(jié)果可能與生物炭的原料[10,54]、施用量[64]、土壤因素[4- 5,54]、人為因素[44]、試驗時間[4,22]等有關(guān)(表2)。影響因素在不同條件下,占有不同的地位,如Luo等[10]研究表明1%—3%生物炭單獨施用提高了土壤pH,對土壤N礦化影響不大,表明,pH的增加不是N礦化的主要影響因素,高C/N是主要限制因素；而與尿素配施后,N礦化與pH呈顯著負相關(guān)關(guān)系,施用1%生物炭相關(guān)系數(shù)為R=-0.709 (P<0.05),施用3%生物炭相關(guān)系數(shù)為R=-0.753 (P<0.05),此時,pH是主要影響因素。

表2 影響農(nóng)田溫室氣體排放因素

1.4.1 生物炭性質(zhì)

生物炭部分不穩(wěn)定性C會被生物分解,穩(wěn)定性C則會固存于土壤[65],穩(wěn)定性碳含量多少與生物炭的制備溫度及原料密切相關(guān)[57,66- 67]。一般而言,隨著熱解溫度增加,生物炭脂肪族碳含量降低,芳香族碳含量增加,C穩(wěn)定性更高[38,68- 70],固碳減排效益越好[22]。Ahmad等[71]認為高溫制備生物炭含有更多的穩(wěn)定性碳,可在土壤中存在至少1600年。研究表明,農(nóng)林廢棄物在500—700℃條件下制備的生物炭的H/C小于0.7[71],穩(wěn)定性C含量在98%以上[68],穩(wěn)定性高。此外,高溫制備生物炭可礦化態(tài)C及N含量低[45,70],對土壤微生物數(shù)量無影響,對C、N的礦化及釋放無促進作用[71]。不同原料生物炭固碳減排效益表現(xiàn)為：木質(zhì)生物炭高于秸稈生物炭[46],這可能與木質(zhì)生物炭：a)微孔數(shù)較高 (V微孔/V總數(shù)=61.5%),吸附性能較高[72]；b)C/P較低[73]；c)灰分含量及不穩(wěn)定性碳含量較低[45,65]密切相關(guān)。而幾種農(nóng)業(yè)秸稈廢棄物生物炭固碳減排效益表現(xiàn)為：玉米秸稈優(yōu)于稻殼及小麥秸稈[22]。糞便及污泥生物炭含灰分較多,減排效益較弱[45,70,74]。

1.4.2 施用量

生物炭施用量也影響溫室氣體排放效果,一般而言,隨著生物炭施用量的增加,對C、N的吸附作用增強,溫室氣體排放減少[29,43,54,75]；改善土壤理化性質(zhì),碳利用率更高[75]。Paneque等[75]研究認為,生物炭施用量為15t/hm2時,能提高作物生長,利于碳的固存[43]。李露等[29]研究表明施用生物炭20t/hm2對水田溫室氣體N2O和CH4的排放無顯著影響,施用40t/hm2生物炭能顯著降低CH4的排放。而程效義等[76]等研究則表明,施用20t/hm2降低NH3、N2O的排放量,施用40t/hm2效果則相反。Xu等[60]及Gascó等[38]研究表明,8%生物炭施用量能降低土壤呼吸,小于8%施用量則提高了土壤呼吸作用。不同的施用量標準還與土壤性質(zhì)及氣候等因素有關(guān)。

1.4.3 土壤因素

土壤因素,包括土壤pH、有機質(zhì)含量、含水量、溫度等。Sheng等[4]研究表明,與對照相比,酸性土壤施用生物炭促進了本土有機質(zhì)及生物炭的分解,提高了CO2的排放量,提高幅度約1.5—3.5倍,其原因是酸土壤：a)革蘭氏陽細菌比例增加 (增幅為25%—36%),促進生物炭及有機質(zhì)降解；2)生物炭提高了C/O比例,在底物有限的情況下,增加了CO2排放。Luo等[10]研究表明生物炭與尿素配施,解除C/N高的抑制作用后,N礦化與pH呈顯著負相關(guān)關(guān)系。Purakayastha等[22]研究表明,生物炭施用抑制了有機質(zhì)含量高的松軟土的土壤呼吸,促進了有機質(zhì)含量低的老成土土壤呼吸。土壤溫度、濕度也是影響碳礦化作用的影響因素之一[64]。Sun等[64]研究表明,生物炭施用條件下,田間持水量為70%時,碳礦化量最高。且隨著土壤溫度的升高,土壤可礦化態(tài)碳含量增加,碳循環(huán)周期及穩(wěn)定性碳含量均降低[64]。

1.4.4 管理措施

1.4.5 其他影響因素

實驗長短也是影響生物炭固定效益的重要因素。生物炭施用初期C、N礦化增大,隨后則降低,主要表現(xiàn)在試驗初期,外加有機質(zhì)提高微生物活性,礦化作用增強[58]。另外,生物炭穩(wěn)定性高[22],且具有較高的C/N[10],長期來看,C、N礦化作用較弱。生物炭的施用對土壤有機質(zhì)的提升及穩(wěn)固作用也是一個長期的過程,大部分室內(nèi)培養(yǎng)及田間試驗均較短,對土壤C、N礦化及土壤碳庫的提升作用不明顯[6],Fungo等[6]2年田間試驗表明,生物炭單獨施用對土壤團聚體大小無顯著影響,主要是因為：a)團聚體形成初期,耕作、除草等人為因素的破壞；b)時間較短。

氣候引起土壤環(huán)境的變化,如干濕交替、干旱等環(huán)境,會加速碳的礦化,促進CO2的排放[77]。Wang等[57]研究則表明,與恒濕相比,干旱及干濕交替更能促進本土有機質(zhì)分解轉(zhuǎn)化為可溶性有機質(zhì),主要原因為二者：a)增加了微生物作用有機質(zhì)的幾率；b)提高了微生物活性,促進了土壤有機質(zhì)分解。

2 農(nóng)田效應

生物炭在固碳減排的同時[3,61],提高農(nóng)田土壤產(chǎn)出[10,37],達到一舉多贏的目的[35,65,78- 79]。大量研究表明,生物炭的施用能改善土壤物理性質(zhì)[37,80],提高土壤養(yǎng)分含量[79,81],促進作物生長,提高作物產(chǎn)量[44,82]。已有研究表明,生物炭施用能提高小麥[9]、水稻[19,82]、玉米[44]、生菜[83]、向日葵[75]、草莓[83]等作物生物量及產(chǎn)量,增產(chǎn)幅度為13%—112%[3]。以生物炭的方式還田,與同施用量的秸稈還田及單施化肥相比,增產(chǎn)效果更好,環(huán)境及經(jīng)濟效益更佳[19,37]。

2.1 增產(chǎn)機制

生物炭提高作物產(chǎn)量表現(xiàn)在：一是生物炭促進作物生長[19]；二是調(diào)節(jié)土壤環(huán)境[84],且二者與作物產(chǎn)量均呈顯著正相關(guān)關(guān)系[37]。

2.1.1 提高作物生長

生物炭促進作物生長表現(xiàn)在(表3),生物炭的施用可提高：1)作物葉片胡蘿卜素、葉綠素含量[37]及葉片天冬氨酸、谷氨酸含量,為促進作物生長,提高作物生物量及產(chǎn)量奠定了基礎(chǔ)[84]；2)提高根系生物量、長度,提高根系傷流速度及氧化力[85]；3)提高作物莖葉及籽粒氮、磷、鉀含量[37,65,80,86]；4)提高水分利用率[75],Paneque等[75]研究表明,地中海氣候下,生物炭施用促進了葉片氣孔關(guān)閉,從而提高葉片含水量。6)生物炭降低病蟲害發(fā)生幾率[83],主要是因為：a)提高了土壤養(yǎng)分含量,特別是K,促進作物生長,利于抗??；b)促進根際土有益微生物生長,抑制病菌入侵；7)降低了重金屬在作物體內(nèi)的轉(zhuǎn)移系數(shù),降低作物體內(nèi)重金屬含量[32,82,87],特別是可食用部分重金屬含量[15,32,84,87- 89],主要是因為生物炭：a)能通過提高土壤pH、吸附、共沉淀、官能團螯合等作用固定重金屬[87- 88]；b)提高可食用部分生物量[87],“稀釋”了作物體內(nèi)重金屬含量[82,84],且隨施用量增加,降低幅度增加[89]。Mohamed等[90]研究認為1.5%施用量是較合適的,能降低作物重金屬積累量,保證人類健康。8)降低有機污染物毒性,促進種子萌發(fā)及作物生長[84,89,91]。生物炭施用降低土壤污染物的毒性,減少了植物螯合肽的合成與分泌[84,89]。

表3 生物炭對農(nóng)田產(chǎn)出的影響及機制

2.1.2 提高土壤肥力

生物炭的施用能提高土壤質(zhì)量[19,46,50,84],促進作物生長,特別是在養(yǎng)分、水分缺乏的條件下[83]。生物炭提高農(nóng)田效益主要歸因于生物炭：1)自身含有較高的養(yǎng)分,促進作物生長[44,83]；2)施用能降低水分、養(yǎng)分的淋失[42],且溫度越高,固持效果越好[92]。一般而言,生物炭施用后土壤田間持水量提高15.1%,土壤飽和導水率增加25.2%[81]。3)改變根際微生物群落組成[83],提高有益細菌的生物量[87]；4)利于土壤團聚體的形成,提高其穩(wěn)定性[6,20],促進土壤保肥保水[20],主要是因為生物炭：a)促進根系及微生物(特別是真菌)的生長,從而促進團聚體形成[58]；b)提高可交換性陽離子如Ca的交換能力,抑制粘粒的分散作用[6]；c)通過比表面積或含氧官能團吸附粘粒,提高微團聚體形成[6]；d)提高微生物量[6]。5)一定程度上提高了土壤溫度,利于微生物活動,Usowicz等[62]研究表明,施用20—30t/hm2生物炭,能降低土壤導熱率及熱擴散率；還會增加土壤顏色,降低反射率[62]。6)改善土壤物理性質(zhì),如降低土壤容重[62],提高含水量及透氣性[80]等。生物炭能顯著降低土壤容重[62,93],且隨著生物炭的施用量增加而降低幅度增加[63,81]。G?b等[63]研究表明,土壤容重y與生物炭施用量x相關(guān)關(guān)系可表示為：y= -0.0856x+1.6559 (R2=0.903)。此外,生物炭還能提高鹽堿土孔隙度,改善透氣性,提高土壤團聚體,促進作物生長[80]。7)提高土壤動物數(shù)量,特別是無脊椎動物,其原因是生物炭的施用：a)提高了微生物數(shù)量,為無脊椎動物提供了更多的食物[85]；b)降低土壤污染物的有效性[85]。

2.2 負效應

也有一些研究結(jié)果表明生物炭的施用對作物產(chǎn)量無影響,甚至出現(xiàn)降低的趨勢[83,94]。生物炭抑制作物生長的主要原因是(表3)：1)生物炭含有含有一定量重金屬、多環(huán)芳烴等有毒物質(zhì)[5,17],抵消了生物炭因改善土壤的生物學效應,表現(xiàn)為對產(chǎn)量無影響或?qū)е庐a(chǎn)量下降[17,93]。生物炭的有毒物質(zhì)[38],是阻礙生物炭應用的重要原因[95]。因此,施用前應對生物炭進行毒性檢測[11,17,96]。根據(jù)歐盟生物炭聯(lián)盟的建議生物炭中重金屬As、Cd、Pb及PAHs的含量應低于12—100mg/kg、1.4—39mg/kg、70—500mg/kg、4—12μg/g[24,94]。2)降低了有效水含量,Carvalho等[97]研究表明,施用32t/hm2的木質(zhì)生物炭對水稻產(chǎn)量無影響,可能是因為生物炭降低了土壤植物有效水含量,這部分作用抵消了生物炭對土壤有機質(zhì)的提升作用；3)較高的C/N比,降低有效N的含量[32]；4)提高了土壤的pH,抑制了根系活性[17]；5)養(yǎng)分過高,特別是K[5]。Buss等[98]分析了19種生物質(zhì)制備而成的生物炭的元素組成,表明,19種生物炭中K元素富集系數(shù)及含量最高,較多的K元素提高了土壤滲透壓及pH,抑制了種子發(fā)芽。6)降低微量元素(如Fe、Cu)有效性,抑制葉綠素產(chǎn)生,降低光合[58]；7)速效養(yǎng)分含量降低,且隨著溫度升高及施用量增加,降低幅度增加。隨著熱解溫度升高,生物炭比表面積增大,吸附的速效養(yǎng)分增加[99]；Seneviratne等[89]及Xu等[80]證實了,隨著生物炭施用量增加,土壤有效N、P含量出現(xiàn)降低的趨勢,二者呈現(xiàn)負相關(guān)關(guān)系。

2.3 影響因素

生物炭對農(nóng)作物生長的促進作用隨生物炭類型及熱解溫度、土壤類型、施用量、氣候的不同而不同(表4)[75,78,86]。不同影響因素,在不同條件下所占地位不一致。Hagner等[35]研究認為,樺木生物炭的施用效果與作物類型及施用量關(guān)系密切,而熱解溫度對土壤性質(zhì)及作物生物量的影響作用較小。而對于較干旱的地中海氣候,生物炭對水分的固持作用越好,對作物促進作用越好,此時,原料占主導[75]。

表4 生物炭影響作物生長因素

2.3.1 生物炭性質(zhì)

不同原料生物炭對作物的增產(chǎn)效果不一致[85]?？v觀前人研究,可以得出,不同原料生物炭對土壤改良及對作物促進作用大致表現(xiàn)為：豬糞生物炭>秸稈生物炭>竹炭生物炭、葡萄藤生物炭>木質(zhì)生物炭[19,61,85- 86,100]。糞便及污泥生物炭重金屬及鹽分含量較高[67],對作物生長抑制作用較強。另外,Paneque等[75]研究表明,松樹、紙漿、污泥生物炭持水量較低[97],對地中海氣候下向日葵萌發(fā)及生長有抑制作用,而葡萄藤生物炭具有促進作用,還與地中海較干燥的氣候條件有關(guān)。

施用效果與生物炭的制備溫度密切相關(guān)[70]。一般而言,低溫(300—400℃)制備生物炭：1)有效N、P含量較高,對作物生長促進作用較好[80]；2)比表面積小,對養(yǎng)分固定作用小,利于根系生長[6]；3)含不穩(wěn)定可溶性有機碳較多[70],促進微生物生長,Ahmad等[71]研究表明,300℃制備生物炭提高了土壤細菌、放線菌、真菌、叢枝菌根的豐度,而700℃制備生物炭施用,微生物數(shù)量表現(xiàn)出恒定的狀態(tài)；4)影響酶活性,Wang等[99]研究表明,生物炭熱解溫度從450℃增加到600℃,脲酶活性逐漸降低,不同酶種類表現(xiàn)出不同的趨勢。Gascó等[38]施用8%的豬糞生物炭,表明300℃制備生物炭提高土壤脫氫酶、磷酸單酯酶、磷酸二酯酶活性,而500℃制備的生物炭對土壤酶活性無影響。

2.3.2 施用量

施用量也是影響生物炭施用效果的重要因素[81]。如Hagner等[35]及Paneque等[75]研究表明,低劑量生物炭施用對土壤pH、EC、含水量及微生物活性無影響,因此,對作物生長無促進作用,提高施用量能提高作物產(chǎn)量,認為施用量應在1.5t/hm2以上。但施用量過高,超過5%則出現(xiàn)抑制[89,93,98]。施用量過高,土壤養(yǎng)分(特別是K)[98]、pH[98]、有毒元素[5,17]含量較高,抑制作物生長,同時會降低水分及養(yǎng)分有效性[32,97]。因此,選擇合適的生物炭施用量是作物增產(chǎn)的前提[65,75,93- 94]。Seneviratne等[89]及Buss等[98]研究認為2.5%施用量對作物生長及養(yǎng)分的提高效果最佳[89]。

2.3.3 土壤類型

生物炭施用于不同土壤類型,作用效果亦不同[54,61,86]。整體來說：生物炭對粗砂土壤的改善效果優(yōu)于砂土[61]；對酸性土壤改良效果優(yōu)于鹽堿土[86,88,90]；生物炭對粗骨土的改善作用優(yōu)于壤土[61,81]；對干旱及半干旱區(qū)土壤的養(yǎng)分[44]及物理性質(zhì)[81]的改善作用更好[18]。生物炭可通過提高土壤pH、有機質(zhì)含量,改善土壤的性質(zhì)[101],簡言之,對酸[86]、瘦[83]、粗[61]、旱[102]、冷[19]類型土壤改良作用更佳[19,35,65]。Tender等[83]研究表明,3%的生物炭施用量對大田生菜生長及抗立枯絲核菌感染的能力無影響,而顯著提高了白炭泥土壤上草莓的鮮重及干重,提高了抗病能力,這與白炭泥土較低的pH及養(yǎng)分有關(guān)。此外,生物炭也能改善pH較高的鹽堿土物理性質(zhì),如孔隙度、通透性、團聚體,從而提高作物產(chǎn)量[80]。

2.3.4 管理措施

與單施生物炭相比,生物炭與化肥[3,80]、作物殘余物[40]、綠肥[6]配施,對作物光合作用、根系活力、土壤性質(zhì)(如微生物活性、團聚體含量)的提升效果更明顯,對作物生長的促進作用更好[5,37]。Xu等[80]研究認為,生物炭與P肥配施于鹽堿土,對作物的促進作用高于二者單獨施用,主要是因為生物炭對P素的吸附作用,抑制了P的有效性,而與P配施減少了生物炭對P的固定。

3 總結(jié)及研究展望

綜上,生物炭固碳及農(nóng)田效益受多方面因素影響,如生物炭性質(zhì)、土壤性質(zhì)、氣候等因素影響,因此,生物炭施用需因地制宜,選擇合適的生物炭[75]、合適的用量[94],是發(fā)揮固碳及增產(chǎn)效益的前提?；诒疚奶峒暗难芯拷Y(jié)論,綜合固碳及培肥兩個方面,筆者認為選擇300—700℃制備的農(nóng)林廢棄物生物炭,且施用量不超過5%為宜[45,67- 68,70,75, 89- 90,98]。

生物炭作為在減少溫室氣體排放及提高農(nóng)田產(chǎn)出方面表現(xiàn)出巨大的潛力,可通過大規(guī)模的施用,緩解溫室效應,提高農(nóng)田產(chǎn)出,維持農(nóng)田可持續(xù)發(fā)展。而大規(guī)模應用僅在發(fā)達國家較多,對于發(fā)展中國家應用還較少[1],因此,還有許多不完備之處,尚需開展以下等方面的研究：

(1)經(jīng)濟效益作為農(nóng)田生態(tài)系統(tǒng)重要屬性之一,生物炭施用的經(jīng)濟可行性是生物炭大規(guī)模應用的前提,特別是對于發(fā)展中國家,因此,還需對生物炭從生產(chǎn)到施用整個體系進行經(jīng)濟可行性分析,為大規(guī)模應用提供基礎(chǔ)；

(2)生物炭固碳潛力及農(nóng)田效益突出,如何量化生物炭作用效益,還需開展更多的工作；

(3)生物炭的施用對土壤有機質(zhì)的提升及穩(wěn)固作用是一個長期的過程,對土壤改良及固碳是否具有持續(xù)效果,還需要長期的定點觀測；

(4)土壤粘粒含量也是影響生物炭施用效果的因素之一,具體的影響效果及機制還需進行定性及定量探究；

(5)生物炭調(diào)節(jié)硝化及反硝化反應的作用機理還不清楚；

(6)生物炭對土壤酶活性的影響及其影響因素尚不清楚。

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