氮、磷養(yǎng)分有效性對(duì)森林凋落物分解的影響研究進(jìn)展

林成芳, 彭建勤, 洪慧濱, 楊智杰,*, 楊玉盛

1 福建師范大學(xué)地理科學(xué)學(xué)院, 福州 350007 2 福建師范大學(xué)濕潤(rùn)亞熱帶山地生態(tài)國(guó)家重點(diǎn)實(shí)驗(yàn)室培育基地, 福州 350007

氮、磷養(yǎng)分有效性對(duì)森林凋落物分解的影響研究進(jìn)展

林成芳1,2, 彭建勤1,2, 洪慧濱1,2, 楊智杰1,2,*, 楊玉盛1,2

1 福建師范大學(xué)地理科學(xué)學(xué)院, 福州 350007 2 福建師范大學(xué)濕潤(rùn)亞熱帶山地生態(tài)國(guó)家重點(diǎn)實(shí)驗(yàn)室培育基地, 福州 350007

通過對(duì)相關(guān)研究文獻(xiàn)的綜述結(jié)果表明,氮(N)和磷(P)是構(gòu)成蛋白質(zhì)和遺傳物質(zhì)的兩種重要組成元素,限制森林生產(chǎn)力和其他生態(tài)系統(tǒng)過程,對(duì)凋落物分解產(chǎn)生深刻影響。大量的凋落物分解試驗(yàn)發(fā)現(xiàn)在土壤N有效性較低的溫帶和北方森林,凋落物分解速率常與底物初始N濃度、木質(zhì)素/N比等有很好的相關(guān)關(guān)系,也受外源N輸入的影響；而在土壤高度風(fēng)化的熱帶亞熱帶森林生態(tài)系統(tǒng)中,P可能是比N更為重要的分解限制因子。然而控制試驗(yàn)表明,N、P添加對(duì)凋落物分解速率的影響并不一致,既有促進(jìn)效應(yīng)也有抑制效應(yīng)。為了深入揭示N、P養(yǎng)分有效性對(duì)凋落物分解的調(diào)控機(jī)制,“底物的C、N化學(xué)計(jì)量學(xué)”假說、 “微生物的N開采”假說以及養(yǎng)分平衡的理論都常被用于解釋凋落物分解速率的變化。由于微生物分解者具有較為穩(wěn)定的C、N、P等養(yǎng)分需求比例,在不同的養(yǎng)分供應(yīng)的周圍環(huán)境中會(huì)體現(xiàn)出不同的活性,某種最缺乏的養(yǎng)分可能就是分解的最重要限制因子。未來的凋落物分解研究,應(yīng)延長(zhǎng)實(shí)驗(yàn)時(shí)間、加強(qiáng)室內(nèi)和野外不同條件下的N、P等養(yǎng)分添加控制試驗(yàn),探討驅(qū)動(dòng)分解進(jìn)程的微生物群落結(jié)構(gòu)和酶活性的變化。

凋落物分解；N；P；養(yǎng)分有效性；微生物；酶

植物凋落物的分解向土壤歸還養(yǎng)分,向大氣釋放CO2,是生態(tài)系統(tǒng)物質(zhì)循環(huán)和能量流動(dòng)的重要環(huán)節(jié)[1-2]。自從Bocock and Gilbert于1957首次使用凋落物分解袋技術(shù)以來,關(guān)于凋落物分解的文章逐年穩(wěn)定遞增,利用Web of Science進(jìn)行搜索,發(fā)現(xiàn)迄今為止文章總數(shù)已經(jīng)超過8000篇(圖1)。1990年后,人們密切關(guān)注全球升溫及與之相伴的大氣CO2濃度升高等環(huán)境問題,因此對(duì)分解過程調(diào)控機(jī)制的深入了解變得尤為迫切[3-5]。

圖1 近50年有關(guān)凋落物分解發(fā)表的SCI文章Fig.1 The papers on litter decomposition published included in SCI in the past 50 years

凋落物分解的調(diào)控機(jī)制極其復(fù)雜,涉及物理、化學(xué)和生物學(xué)過程,但主要可以歸納為兩個(gè)方面,其一是凋落物本身質(zhì)量,即其物理化學(xué)性質(zhì)和養(yǎng)分的含量,其二是分解的外在環(huán)境,特別是氣候因子(溫度、水分)以及土壤或枯枝落葉層的養(yǎng)分含量和有效性[1,6-7]。過去的研究表明在全球和區(qū)域的尺度上,氣候是調(diào)控凋落物分解的首要因素,但在局域尺度上,表征凋落物質(zhì)量的養(yǎng)分含量指標(biāo)以及分解環(huán)境中的養(yǎng)分有效性則起至關(guān)重要的作用[8-13]。N、P是陸地生態(tài)系統(tǒng)植物生長(zhǎng)最重要的兩種限制性養(yǎng)分[14-16],在全球變化背景下,自然或人為驅(qū)動(dòng)的這兩種元素輸入變化,必將對(duì)凋落物分解產(chǎn)生重要影響,進(jìn)而影響全球的碳平衡或碳預(yù)算。然而,N、P養(yǎng)分有效性的改變?nèi)绾斡绊懙蚵湮锏姆纸?以及分解過程中的微生物學(xué)機(jī)制等尚待闡明,直接影響到碳循環(huán)模型的構(gòu)建和對(duì)生態(tài)系統(tǒng)碳吸存潛力的預(yù)測(cè)。

1 不同來源的N對(duì)凋落物分解的影響

化石燃料的燃燒、化肥的使用以及固N(yùn)作物的栽培等人類活動(dòng),把N從惰性的形態(tài)(N2)轉(zhuǎn)化為活性的形態(tài),如NH3(Haber Bosch 制氮法)、NOx。而活性N經(jīng)脫N作用形成N2的速度小于活性N形成的速度,導(dǎo)致這些活性N在大氣、土壤和水體中儲(chǔ)藏[17],并在未來相當(dāng)長(zhǎng)的時(shí)間內(nèi)持續(xù)增加[18]?；钚訬的增加引發(fā)了平流層臭氧耗竭、酸沉降、海岸帶富營(yíng)養(yǎng)化以及淡水的生產(chǎn)等環(huán)境問題,同時(shí)也提高了生態(tài)系統(tǒng)的凈初級(jí)生產(chǎn)力[19-20],增加了生態(tài)系統(tǒng)C、N儲(chǔ)量。為了進(jìn)一步了解N輸入持續(xù)增加情況下,生態(tài)系統(tǒng)碳輸出的變化,需要明確N對(duì)凋落物分解的效應(yīng)。

大量的凋落物分解研究表明,分解速率與N相聯(lián)系的各種指標(biāo)有很好的相關(guān)關(guān)系。這體現(xiàn)在凋落物自身的N含量、C/N、木質(zhì)素/N,土壤環(huán)境中的C/N、N含量以及外源添加的N量對(duì)分解產(chǎn)生的不同效應(yīng)。

1.1 凋落物底物的N含量對(duì)分解的影響

“底物的C、N化學(xué)計(jì)量學(xué)”假說認(rèn)為,微生物分解者與分解底物之間存在C、N化學(xué)計(jì)量學(xué)的差異,這種對(duì)分解者生理學(xué)上的基本約束驅(qū)動(dòng)凋落物分解和養(yǎng)分的釋放[21-23]。一般來說,新鮮凋落物的C/N比高于微生物分解者,微生物需要從周圍環(huán)境(土壤或降水)獲取養(yǎng)分來維持其自身的生長(zhǎng)需求[24-25]。此外,大量的研究發(fā)現(xiàn)在分解早期,凋落物初始N含量以及與N相關(guān)的底物質(zhì)量指標(biāo)和凋落物分解速率有很好的相關(guān)關(guān)系[20-21],證明N對(duì)凋落物分解的限制作用。盡管以上證據(jù)都是間接的,但C/N比和木質(zhì)素/N的比率等指標(biāo)常被用于預(yù)測(cè)凋落物的分解速率[20,23,26-27],同時(shí)也被一些碳循環(huán)模型采用。

1.2 土壤N有效性對(duì)凋落物分解的影響

在熱帶山地生態(tài)系統(tǒng)的土壤年齡序列中,凋落物在養(yǎng)分有效性高的土壤中分解最快,而在養(yǎng)分有效性低且限制了地上部分生產(chǎn)力的土壤中分解最慢[28-29]。Hobbie[30]在美國(guó)明尼蘇達(dá)州的研究發(fā)現(xiàn),分解速率與土壤N有效性有顯著的相關(guān)關(guān)系。土壤養(yǎng)分有效性對(duì)分解的限制作用還表現(xiàn)在分解者和植物之間的正反饋環(huán),在低養(yǎng)分的土壤中,凋落物分解受到限制而減緩養(yǎng)分循環(huán),進(jìn)一步降低植物的養(yǎng)分有效性,限制植物的生產(chǎn)力；而較低生產(chǎn)力的植物,凋落物歸還的數(shù)量和質(zhì)量都較低,抑制了分解[31]。在溫帶森林生態(tài)系統(tǒng)研究表明,高土壤N有效性提高細(xì)根底物的質(zhì)量,進(jìn)而影響了細(xì)根的分解速率[32]。

1.3 外源添加的N對(duì)凋落物分解的影響

分解養(yǎng)分限制的直接證據(jù)只能通過施肥實(shí)驗(yàn),在增加外源提供的養(yǎng)分情況下,考察凋落物分解速率是否提高。然而凋落物分解對(duì)外源N輸入的響應(yīng)并不一致,既有促進(jìn)作用[33-34]、抑制作用[35-36],還有研究發(fā)現(xiàn)沒有明顯的作用[33,37]。

N添加對(duì)凋落物分解的促進(jìn)或抑制作用以及影響的程度大小與很多因素有關(guān),除凋落物本身質(zhì)量外,還有諸多其他因素,如氣候、大氣N沉降水平以及施N肥的類型、網(wǎng)袋的孔徑大小等。Knorr 等[38]的綜合分析表明,N施肥改變了凋落物的分解速率,但分解響應(yīng)的方向和程度受到施肥的量、大氣氮沉降水平以及凋落物質(zhì)量交互作用的調(diào)節(jié)。當(dāng)施肥量為氮沉降水平的2—20倍時(shí),或者氮沉降水平達(dá)5—10 kg N hm-2a-2,再或者凋落物質(zhì)量差(高木質(zhì)素含量)時(shí),凋落物分解受到抑制；在高N施肥的情況下(大氣氮沉降的20倍),或氮沉降水平在5kg N hm-2a-2以下,或高質(zhì)量的凋落物(低木質(zhì)素含量)時(shí),凋落物分解被促進(jìn)。因此,特定生態(tài)系統(tǒng)中,分解過程對(duì)N施肥的響應(yīng)受到N沉降水平的影響,長(zhǎng)期高N沉降下的生態(tài)系統(tǒng)可能獲得了適應(yīng)性,在人為施N肥的情況下凋落物分解反應(yīng)程度較小。

從有機(jī)質(zhì)中獲得N的過程非常復(fù)雜,N分布在多種等級(jí)的混合物以及腐殖大分子中,因此微生物的N獲取策略與特定種群的C底物偏好有密切聯(lián)系[39-40]。在Moorhead和Sinsabaugh[41]提出的以分解者集團(tuán)為基礎(chǔ)的凋落物分解模型中,把微生物分成三大類：分解活性蛋白質(zhì)的機(jī)會(huì)主義者(opportunists)、需要外源N輸入分解木質(zhì)纖維素的專性分解者(decomposers)和利用氧化酶打開腐殖質(zhì)獲得所需的C和N 的“淘金分解者”(miners)。

在多樣的C和N獲取策略情況下,外源N添加試驗(yàn)中發(fā)現(xiàn)土壤胞外酶活性的響應(yīng)差異便不難理解了。因此,近年來很多研究者傾向于從酶活性變化的角度來解釋N添加對(duì)分解的影響[42]。用酶活性的變化來解釋N添加對(duì)分解的影響得到了多個(gè)研究的支持[42-46]。如Fog[47]發(fā)現(xiàn)在高N的情況下,白腐擔(dān)子菌被分解纖維素的子囊菌所競(jìng)爭(zhēng)性排除,這種對(duì)木質(zhì)素分解的抑制作用導(dǎo)致添加N對(duì)分解的中性效應(yīng)或負(fù)效應(yīng)。Craine 等[48]研究也認(rèn)為,是N的添加降低負(fù)責(zé)分解惰性C的微生物活性C有效性,抑制了負(fù)責(zé)惰性有機(jī)C中“N開采”的酶(如酚氧化酶、過氧化物酶)的生產(chǎn)[41,49],從而限制了凋落物的分解。添加N不僅改變了酚氧化酶,還有其他微生物獲取C、N、P酶的活性,這些酶活性的變化能較好的解釋凋落物分解速率的變化[49-51]。酶活性對(duì)外源N的不同響應(yīng),與樣地土壤養(yǎng)分有效性[52-53]、凋落物木質(zhì)素含量[53-54]、凋落物和土壤的C/N比[46,55]以及微生物生物量[56]有關(guān)。測(cè)定酶活性使人們能夠直接跟蹤微生物群落對(duì)凋落物性質(zhì)和環(huán)境變量的功能響應(yīng)[44],但由于植物-凋落物-微生物交互作用具有潛在的復(fù)雜性,在地表植被、凋落物化學(xué)性質(zhì)、土壤C/N比和N有效性不同的多種林地上,凋落物分解速率和土壤的酶活性同時(shí)會(huì)對(duì)外源N添加做出何種響應(yīng),目前尚不清楚。目前,DGGE、PLFA以及高通量測(cè)序技術(shù)的成熟和應(yīng)用,必將推進(jìn)對(duì)N沉降背景下不同生態(tài)系統(tǒng)凋落物分解過程中微生物群落和酶活性的動(dòng)態(tài)變化的準(zhǔn)確刻畫。

2 P對(duì)凋落物分解的影響

長(zhǎng)期復(fù)雜的成土過程中形成了土壤在地理學(xué)上的分異,中髙緯度地區(qū)的土壤較為年輕, N含量較少,而低緯度地區(qū)土壤古老, P含量有限[57]。P和N對(duì)初級(jí)生產(chǎn)力以及其他生態(tài)系統(tǒng)過程的限制作用已經(jīng)在多種多樣的陸地生態(tài)系統(tǒng)中被證明[16]。與N不同,土壤P來源于巖石風(fēng)化,每個(gè)生態(tài)系統(tǒng)發(fā)生時(shí)都有一個(gè)固定P儲(chǔ)量,隨生態(tài)系統(tǒng)發(fā)育,P不斷流失且不能輕易得到補(bǔ)充。這導(dǎo)致老的土壤中P的總量和生物有效性都較低,從而對(duì)生態(tài)系統(tǒng)NPP、凋落物分解等其他生態(tài)學(xué)過程產(chǎn)生了深刻的影響[28,15]。

在熱帶亞熱帶的酸性土壤里, P易于被其中的鐵鋁氧化物固定,發(fā)生強(qiáng)的吸附作用, N相對(duì)過剩而由礦石來源的P等必要養(yǎng)分處于耗竭狀態(tài),全球氣候變化下的N/S沉降更加劇了生態(tài)系統(tǒng)潛在的P限制[31,58-60]。研究已證實(shí)P有效性限制了熱帶、亞熱帶生態(tài)系統(tǒng)的NPP(凈初級(jí)生產(chǎn)力)。凋落物分解的初級(jí)階段經(jīng)常需要富集P,說明新鮮凋落物中的P養(yǎng)分不足以維持分解者的生長(zhǎng),而凋落物分解速率常與P及其相聯(lián)系的凋落物質(zhì)量指標(biāo)顯著相關(guān)[1,61-65],表明凋落物分解可能受P限制。此外熱帶生態(tài)系統(tǒng)土壤高度風(fēng)化,N過剩而P有效性低,在此土壤上生長(zhǎng)的凋落物也具有高N/P比的特征[58,66]。因此相比于N,微生物在分解中更難以從凋落物本身及周圍環(huán)境獲得P,根據(jù)李比希最小值定律(Liebig′s law of the minimum),P可能是該區(qū)域凋落物分解更為重要的限制性因子。然而,現(xiàn)有的生態(tài)系統(tǒng)C循環(huán)機(jī)理模型很少考慮P的循環(huán)[40,67]。

不同于N的循環(huán),P循環(huán)可分為生物和地球化學(xué)循環(huán)。生物循環(huán)是指植物和微生物吸收的P在死亡后變成土壤有機(jī)P,這部分的P能夠被重新礦化和吸收；地球化學(xué)循環(huán)則是由于母巖風(fēng)化或外源添加提供給生態(tài)系統(tǒng)的P,與土壤礦物發(fā)生化學(xué)反應(yīng)被固定在土壤中[68-70]。但有證據(jù)表明,微生物能迅速利用外源添加的P,在生物吸收和土壤吸附有效P的競(jìng)爭(zhēng)中起支配作用[71-73]。已有研究通過測(cè)定呼吸速率,探討P添加后高度風(fēng)化土壤凋落物分解過程中微生物活性變化,來解釋P有效性對(duì)分解的限制作用[73-75]。而直接測(cè)定P添加對(duì)分解過程中微生物動(dòng)態(tài)效應(yīng)的研究很少,這是由于當(dāng)前的生態(tài)系統(tǒng)模型都把土壤微生物當(dāng)作一個(gè)黑箱[76-78]。

與N相比,人們對(duì)P與微生物結(jié)構(gòu)和功能的關(guān)系了解較少[61]。在外源P添加的情況下,P的有效性提高,微生物運(yùn)用更少的能量獲得P[43,79],那么微生物是否重新分配它的資源,轉(zhuǎn)向提高獲得C、N的胞外酶活性呢？ 這些酶的活性能用于解釋P添加情況下凋落物分解速率的變化么？土壤化學(xué)性質(zhì)：如 pH,N、P等養(yǎng)分有效性和凋落物質(zhì)量的變化不僅影響酶的合成,同時(shí)影響細(xì)菌和真菌的比例及其群落結(jié)構(gòu)的變化[45,78,80]。如富含養(yǎng)分的活性有機(jī)質(zhì)由快速生長(zhǎng)的細(xì)菌侵染,因其快速的細(xì)胞分裂需要大量富P的RNA,而養(yǎng)分貧乏難分解的有機(jī)質(zhì)易于生長(zhǎng)真菌[81-83]。因而直接測(cè)定酶活性和主要微生物類群結(jié)構(gòu)對(duì)凋落物性質(zhì)和環(huán)境變量的響應(yīng),能獲得對(duì)凋落物分解機(jī)理更為清晰的了解。

3 其他元素對(duì)凋落物分解的影響

N和P是限制生態(tài)系統(tǒng)過程的兩種關(guān)鍵元素,被大部分分解試驗(yàn)關(guān)注。目前的研究表明,北方森林和溫帶森林,富N比貧N的凋落物分解快[84]；而低地?zé)釒в炅諲相對(duì)豐富, P含量隨時(shí)間不斷耗竭[85],從而使P在該區(qū)域凋落物分解中起主導(dǎo)作用[33,46,59,67]。除這兩種元素外,尚有少量研究表明其它元素也在分解中有重要作用。

凋落物分解是個(gè)多種底物(如蠟類、酚類、木質(zhì)素和纖維素等)連續(xù)降解的過程,需要微生物合成多種(metallomic enzymes)金屬酶的參與[86],這個(gè)過程中會(huì)出現(xiàn)包括N、P等其它多種養(yǎng)分供應(yīng)的不足。Kaspari[86]在熱帶低地森林的分解試驗(yàn)發(fā)現(xiàn),P的添加提高凋落物分解速率33%而微量元素(如 B, Ca,Cu, Fe, Mg, Mn, Mo, S, Zn)的添加則提高了81%。這說明除P之外,另有其它元素可能參與并促進(jìn)凋落物的分解。據(jù)統(tǒng)計(jì),大約需要25種化學(xué)元素才能驅(qū)動(dòng)整個(gè)生態(tài)系統(tǒng)碳循環(huán)過程中的樹木生長(zhǎng)、分解中的微生物繁殖[87]。如果要對(duì)限制凋落物分解的養(yǎng)分重要性進(jìn)行排序,需設(shè)計(jì)析因?qū)嶒?yàn),然而這樣的試驗(yàn)?zāi)壳叭院苌賉88]。

4 N/P比率對(duì)凋落物分解的影響

Alfred Redfield在1958年觀測(cè)到,海洋浮游生物生物量中C、 N 、P原子比為 106∶16∶1,這與海水中C、 N 、P比例相似。這種化學(xué)計(jì)量學(xué)關(guān)系簡(jiǎn)潔的反映了有機(jī)體和環(huán)境之間的交互作用,被稱為“Alfred比率”,其有助于深入了解海洋生態(tài)系統(tǒng)NPP和C儲(chǔ)量受養(yǎng)分限制的性質(zhì)和程度,以及海洋中N、P生物地球化學(xué)循環(huán)過程。“Alfred比率”在海洋生態(tài)系統(tǒng)的預(yù)測(cè)能力,推動(dòng)生態(tài)學(xué)家在陸地生態(tài)系統(tǒng)尋找相似的養(yǎng)分格局和關(guān)系,因而誕生了一門新的學(xué)科——生態(tài)化學(xué)計(jì)量學(xué),這門學(xué)科著眼于了解生態(tài)學(xué)交互作用過程中的多種化學(xué)元素的平衡[89-90]。然而,我們當(dāng)前對(duì)化學(xué)計(jì)量關(guān)系以及其在陸地生態(tài)系統(tǒng)中的重要意義所知還很有限[91]。

微生物生物量C與土壤C有效性有很強(qiáng)的相關(guān)關(guān)系,同時(shí)也與土壤微生物中N和P的含量密切相關(guān),這種養(yǎng)分之間的線性相關(guān)關(guān)系表明土壤微生物生物量C的增加依賴于充足的土壤N和P養(yǎng)分供應(yīng)來維持微生物生長(zhǎng)所需要的養(yǎng)分元素化學(xué)計(jì)量比[90]。植物葉片養(yǎng)分比率反映了土壤N和P的相對(duì)豐度從低緯度到高緯度的增長(zhǎng)[58,92-93],而土壤微生物量中的N/P比率并不隨著緯度的變化而變化,也與土壤中N∶P供應(yīng)比率沒有相關(guān)關(guān)系,而是維持在類似“Alfred比率”的比例[94]。土壤微生物相對(duì)嚴(yán)格的養(yǎng)分需求以及低緯度地區(qū)土壤低P有效性解釋了P對(duì)這些生態(tài)系統(tǒng)微生物生物量和活性的限制[59,72,95-97],進(jìn)而限制凋落物的分解。由于N、P有效性在不同生態(tài)系統(tǒng)間的差異,因此有些研究者認(rèn)為N和P對(duì)生態(tài)系統(tǒng)過程(包括凋落物分解)限制的相對(duì)重要性也不同,當(dāng)N/P比低時(shí)受N限制,當(dāng)N/P比高時(shí)受P限制(通常認(rèn)為N/P比<14時(shí)受N限制,N/P比>16時(shí)受P限制,當(dāng)N/P比在14和16之間受到N、P的共同限制)[83]。人類活動(dòng)可以通過添加P而轉(zhuǎn)變P限制的生態(tài)系統(tǒng)為其它養(yǎng)分限制系統(tǒng),也可以通過有意和無意的影響其它養(yǎng)分(主要是N)的供給,導(dǎo)致P限制生態(tài)系統(tǒng)的出現(xiàn)。例如,歐洲西北地區(qū)由于極端高水平的大氣N沉降克服了許多生態(tài)系統(tǒng)的N限制而轉(zhuǎn)變成首先受P限制的生態(tài)系統(tǒng)[46,96],在北美地區(qū)進(jìn)行的控制試驗(yàn)也獲得類似的結(jié)果[97-99]。

5 展望

凋落物分解是一個(gè)貫穿著淋溶、凍融粉碎等物理作用以及土壤生物參與的生物化學(xué)作用交織在一起的復(fù)雜過程,并最終通過微生物的作用釋放CO2到大氣,釋放養(yǎng)分回歸土壤中。對(duì)同一種底物而言,控制微生物活性的溫度、水分條件以及微生物所需的養(yǎng)分有效性,決定了凋落物最終的分解速率。因此,當(dāng)前凋落物分解研究存在以下主要問題：(1)影響凋落物分解的N、P養(yǎng)分因素與其他因素同時(shí)存在或發(fā)生交互作用,導(dǎo)致難以確定影響分解的主導(dǎo)因素。且確定影響分解速率的因素時(shí),常采用相關(guān)分析的方法。相關(guān)分析可以為確定主導(dǎo)因素提供很好的線索,然而需要控制試驗(yàn)加以驗(yàn)證；(2)較少考慮P有效性對(duì)凋落物分解的影響。早期的分解研究主要集中在北方森林和溫帶森林,研究結(jié)果認(rèn)為N及木質(zhì)素/N比是調(diào)控凋落物分解的主要因素,現(xiàn)有的生態(tài)系統(tǒng)過程模型也只考慮N的作用而忽略了P,然而近年來熱帶、亞熱帶發(fā)育在高度風(fēng)化土壤上生態(tài)系統(tǒng)的多項(xiàng)研究表明P對(duì)生態(tài)系統(tǒng)過程的限制作用。即使在熱帶和亞熱帶,不同生態(tài)系統(tǒng)或同一生態(tài)系統(tǒng)不同的發(fā)育階段受N、P養(yǎng)分限制情況也不一致；(3)分解底物的異質(zhì)性。即便同種植物,其不同位置或不同時(shí)間產(chǎn)生的凋落物N、P養(yǎng)分含量具有較大的差異。(4)較少考慮影響分解過程各種酶的活性和微生物群落結(jié)構(gòu)的變化。N、P無疑是生態(tài)系統(tǒng)過程中兩種最重要的養(yǎng)分,但它們對(duì)凋落物分解的影響及作用機(jī)制尚未明確的闡述,影響了對(duì)全球環(huán)境變化下生態(tài)系統(tǒng)過程的預(yù)測(cè)。今后應(yīng)著重通過室內(nèi)結(jié)合野外的控制試驗(yàn)獲取直接的證據(jù),主要考慮以下幾個(gè)方面的工作：

(1)養(yǎng)分添加試驗(yàn)是確定養(yǎng)分限制屬性的良好手段,然而需要認(rèn)真的考慮養(yǎng)分添加量以及觀測(cè)的時(shí)間。養(yǎng)分添加量會(huì)改變生態(tài)系統(tǒng)養(yǎng)分受限性質(zhì),有效體現(xiàn)養(yǎng)分添加效應(yīng)需要延長(zhǎng)觀測(cè)的時(shí)間。例如,現(xiàn)有的凋落物分解試驗(yàn)大部分少于2a,有的凋落物在此期間尚處于分解的初期階段。

(2)要充分考慮分解環(huán)境變化對(duì)N、P添加效應(yīng)的影響。例如熱帶雨林凋落物分解過程中淋溶起主導(dǎo)作用,因此養(yǎng)分添加對(duì)凋落物的干質(zhì)量損失沒有顯著影響,但卻促進(jìn)了凋落物淋溶部分物質(zhì)的礦化分解。

(3)養(yǎng)分平衡對(duì)凋落物分解的作用。微生物分解者的生長(zhǎng)有嚴(yán)格的化學(xué)計(jì)量學(xué)要求,是否某種養(yǎng)分有效性的缺乏限制了凋落物分解。

(4)通過對(duì)分解過程中微生物和酶活性變化的觀測(cè),揭示凋落物分解的機(jī)理,闡明養(yǎng)分添加對(duì)凋落物分解的不同效應(yīng),最終弄清楚不同生態(tài)系統(tǒng)凋落物分解的主要限制性因子。

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Effect of nitrogen and phosphorus availability on forest litter decomposition

LIN Chengfang1,2,， PENG Jianqin1,2, HONG Huibin1,2, YANG Zhijie1,2,*,YANG Yusheng1,2

1SchoolofGeographicalScience,FujianNormalUniversity,Fuzhou350007,China2StateKeyLaboratoryofSubtropicalMountainEcology(FoundedbyMinistryofScienceandTechnologyandFujianProvince),FujianNormalUniversity,Fuzhou350007,China

Nitrogen (N) and phosphorus (P) are the two most important elements for building plant proteins and genetic material, limiting forest production, and other ecosystem processes, and they profoundly affect litter decomposition. Litter-quality parameters, particularly initial lignin and N contents, ratios of C∶N, and lignin:N often correlate strongly with rates of litter mass loss in temperate and boreal forests. Furthermore, soil N availability and N addition also affect litter decomposition. In tropical and subtropical forests, where highly weathered soil is frequently observed, P could be more important than N in inhibiting litter decomposition. However, P has not been considered in most current ecosystem carbon cycling models, but fertilization experiments show different responses of litter decomposition rates to nutrition addition. Synergistic, antagonistic, and neutral effects can be observed. Both the “basic stoichiometric decomposition theory” and “microbial nitrogen mining” hypothesis have been used to explain litter decomposition rate variations with nutrition addition. Regarding rigid C∶N∶P ratios in microbial decomposers, different nutrition sources could result in altered microbial activity, and limited nutrient supplies could result in restricted litter decomposition. To clearly understand the effects of N and P regulation on decomposition, we need longer decomposition experiment durations, more intensive field and laboratory fertilization experiments, and simultaneously, microbe and enzyme dynamics in the decomposition process should be further investigated.

litter decomposition; N; P; nutrient availability; microbes; enzymes

國(guó)家自然科學(xué)基金面上資助項(xiàng)目(31270584)；國(guó)家自然科學(xué)基金重點(diǎn)資助項(xiàng)目(31130013)

2016- 08- 09;

2016- 11- 17

10.5846/stxb201608091636

*通訊作者Corresponding author.E-mail: zhijieyang@fjnu.edu.cn

林成芳，彭建勤, 洪慧濱, 楊智杰,楊玉盛.氮、磷養(yǎng)分有效性對(duì)森林凋落物分解的影響研究進(jìn)展.生態(tài)學(xué)報(bào),2017,37(1):54- 62.

Lin C F, Peng J Q, Hong H B, Yang Z J,Yang Y S.Effect of nitrogen and phosphorus availability on forest litter decomposition.Acta Ecologica Sinica,2017,37(1):54- 62.