土壤pH影响氧化亚氮(N2O)排放的研究进展

2017-08-24 04:20潘亚男王娅静曹文超
安徽农学通报 2017年15期
关键词:氧化亚氮硝化

潘亚男 王娅静 曹文超

摘 要:氧化亚氮(N2O)是第三大温室气体,也是21世纪内平流层臭氧(O3)的首要分解者。在过去约150年间,大气中N2O的浓度持续增加,其主要原因在于化肥和有机肥料刺激下土壤中N2O的大量排放。因此,理解土壤中N2O的排放机制与影响因素,已经成为估算N2O排放清单和制定N2O减排政策的关键科学问题。土壤pH是影响N2O排放的重要环境因子之一,但目前对其相对重要性和影响机制尚不明确。该文基于已有文献的梳理,总结了原位观测和室内培养研究中土壤pH与N2O排放之间的统计结果,发现多数研究中N2O排放与土壤pH呈显著负相关关系;并且从生物硝化、生物反硝化和非生物过程3个方面探讨了土壤pH影响N2O排放的微观机制。在此基础上,本文对今后的研究工作提出展望,以期为后续的研究提供参考和依据。

关键词:氧化亚氮;土壤pH;反硝化;硝化;非生物转化

中图分类号 S154.1 文献标识码 A 文章编号 1007-7731(2017)15-0019-7

Abstract:Nitrous oxide(N2O)is the third most important anthropogenic greenhouse gas,with a global warming potential 298 times that of CO2 over a 100-y time period. It is also the largest anthropogenic contributor to stratospheric ozone decomposition for the remainder of this century. Redox processes involving nitrogen(N)in soils contribute more than half of the global anthropogenic N2O emission,primarily fueled by reactive N from synthetic fertilizers and animal manure,added to agricultural lands. Understanding on the mechanisms of soil N2O emission have,therefore,become an important perquisite to construct sound N2O emission inventory and to make scientific N2O mitigation policy. Though soil pH has been found as the important environmental factor influencing N2O emission,its relative importance and operation mechanisms are not understood well. Based on the analysis of previous literatures,this review summarized the statistic relationship between N2O emission and soil pH in field and laboratory researches. Most studies showed that N2O emissions were negatively correlated with soil pH. In addition,this review probed the influencing mechanisms of soil pH and N2O emission,involving biological nitrification,biological denitrification and abiotic mechanism. Some prospects were also proposed and highlighted for the future related works. In future studies,more attentions should be paid to the influencing mechanisms of soil pH on N2O emission,considering both magnitude and gas product stoichiometry.

Key words:Nitrous oxide;Soil pH;Denitrification;Nitrification;Abiotic transformation

氧化亚氮(N2O)是大气中的痕量组分,其对地球环境具有重要的影响[1],是仅次于二氧化碳(CO2)和甲烷(CH4)的第三大温室气体,其增温潜势是CO2的298倍[2,3]。在氟里昂排放得到有效控制后,N2O已成为消耗平流层臭氧的最主要物质[4-7]。大气中N2O的来源包括自然源和人为源,其中自然源包括海洋、湖泊、草地、森林等,人为源包括农田土壤、工业排放等[8]。工业革命以后,大气中N2O的浓度已经从0.27μL/L上升至0.319μL/L[5,10-11]。其中,化学氮肥和有机肥的施用是土壤N2O排放的主要原因[8-9,12-14]。因此,在深入理解排放机制的基础上,实现土壤N2O科学减排,对缓解全球气候变化具有重要的现实和科学意义。

影响土壤中N2O排放的因素众多,概括起来可分为环境因子和人为因子2类,环境因子包括温度、降水量等气候条件和pH、有机质含量、土壤含水量、质地等土壤物理化学属性[5,15-16,18];人为因子包括氮肥输入量、氮肥类型、施肥方式、农作物类型、土地耕作及灌溉方式等[5,19-24]。相对于人为因子,环境因子具有明显的地带性规律,控制著土壤N2O排放的区域性差异。作为重要的环境因子,土壤pH在区域尺度和实验室规模下均对N2O的排放具有显著影响[25-28]。但目前相关研究较为零散,对土壤pH的影响机制和相对重要性尚缺乏系统性的认识。为此,本文基于对已有研究结果的梳理与归纳,总结了原位观测和室内培养研究中土壤pH与N2O排放之间的表观统计学关系,探讨了土壤pH影响N2O排放的可能微观机制,并提出对今后研究工作的展望。

1 土壤pH影响N2O排放的统计学分析

1.1 原位观测研究 基于众多的原位观测结果,研究者可以从空间差异的角度分析各种因素对土壤N2O的影响。类似研究表明,土壤pH是影响N2O排放的最重要的因素之一,土壤N2O的排放量随pH增加而显著降低[21,25,29-32]。在流域尺度上,Weslien等[30]发现瑞典森林土壤中N2O排放通量与土壤pH之间的关系最为显著。在德国南部的森林地区,Borken和Brumme[29]发现长期施用石灰的土壤所排放的N2O比对照土壤低9%~62%。韩琳等[25]分析了全球38个森林野外原位观测结果,发现土壤pH是影响土壤N2O排放的最主要因子,其重要性大于氮沉降强度和气候因子。Bouwman等[31]整理分析了全球846个农田土壤的N2O排放量數据,结果表明土壤pH是显著影响N2O排放的主要因素之一,但该研究未能比较各因素之间的相对重要性。类似,Stehfest等[21]整合了全球1215个农田和自然土壤N2O排放数据,也发现土壤中N2O排放量随土壤pH显著下降。

1.2 室内培养研究 在实验室条件下,通过比较各处理间N2O排放的差异同样可以发现土壤pH的显著影响。多数研究结果表明,向酸性/酸化土壤中添加石灰、白云石或生物质炭等物质在提升pH的同时能够显著降低N2O排放[33-36]。Shaaban等[34]发现,向酸性土壤中添加白云石可以显著降低硝化过程中N2O的排放。Obia等[35]的研究发现,向酸性土壤中添加生物质炭可以有效抑制反硝化过程中N2O的净产量,主要原因在于生物质炭提高了土壤pH。Quin等[36]发现桉树生物质炭的添加可以增加土壤pH,进而抑制了反硝化过程中N2O的产生。但另外一些研究发现,土壤pH的增加也可能会促进N2O的产生。Feng等[33]研究发现,向酸性矿质土壤中加入石灰后反硝化过程中的N2O排放量随土壤pH升高而显著增加。因此,土壤pH对N2O排放的影响可能因土壤属性、培养条件等因素而有所不同。在实验室条件下还可以进一步探究气体产物比N2O/(N2O+N2)与土壤pH之间的关系,从而有助于理解土壤pH影响N2O的微观机制。Raut等[37]发现,集约化种植模式导致的土壤酸化增加了反硝化产物中N2O/(N2O+N2)比值。Sun等[38]研究结果表明,在中国东北部地区的草地和森林土壤中,土壤pH是影响反硝化速率和产物N2/N2O比值最主要的控制因素。Qu等[39]研究发现,土壤酸化能够提高反硝化的气体产物比,从而促进土壤N2O排放。

2 土壤中N2O产生的主要途径

如图1所示,土壤中N2O可以通过多种途径产生。一般认为,硝化反应第一步中(表1,式1)好氧自养细菌利用铵(NH4+)作为电子供体、氧气(O2)作为电子受体,产生的中间产物羟胺(NH2OH)通过自身分解或与亚硝酸盐(NO2-)等其他化合物反应生成N2O[40](式2~3);反应第二步由一类特定的自养细菌将NO2-氧化成硝酸盐(NO3-)(式4)。体系中的NO3-随后通过异养细菌和真菌的反硝化作用,依次被还原为一氧化氮(NO)、N2O和氮气(N2),该过程含氮化合物作为电子受体、有机碳作为电子供体,在氧气含量很低或厌氧条件下进行[41]。硝化反应第一步中的氨氧化细菌可以把电子转移给硝化作用或其它过程生成的NO2-,并生成N2O(式5),这一过程分别被称为硝化细菌的反硝化作用(Nitrifier denitrificiation)[41]和厌氧氨氧化过程(Anammox)[42]。此外,土壤中铁锰等金属的离子和氧化物可以在氮素转化过程中作为电子供体或受体,并产生N2O。在特定微生物的参与下,Fe3+或氧化铁在厌氧条件下可以将NH4+氧化成N2O和N2,该过程称为FEAMMOX[43](式6~7)。某些微生物还可以通过催化Fe2+与NO3-/NO2-之间的电子转移获取能量,同时伴有NO、N2O、N2等气体产生,这个过程通常被称为硝酸盐依赖型铁氧化(NDFO)[44](式8~11)。

研究发现,在没有微生物参与的情况下N2O也可以通过一系列化学过程直接产生。化学硝化是指土壤中的氨或铵被三价铁等氧化生成高价氮的过程,其产物可以包括N2、N2O、NO和NO2[45-46](如,式6~7)。化学反硝化是指NO3-或NO2-被低价态金属离子(如,Fe2+、Mn2+)还原的过程,其气体产物中同样包含N2O等多种气体[45-46](式8~13)。作为土壤氮循环过程的活性中间产物,NH2OH和NO2?可以通过一系列化学过程向N2O转化。例如,NH2OH可以通过快速化学分解生成N2O和N2[32],也可以被铁(Fe)或锰(Mn)等金属氧化产生N2O[48-49](式15~16)。在酸性环境中(pH<5.5),NO2?容易通过自分解过程产生NO、N2O和HONO等含氮气体[40,47](式14)。此外,土壤中的腐植酸等有机物也可以通过化学过程将NO2-进一步还原成NO、N2O等气态化合物[40,47]。

3 土壤pH影响N2O排放的可能机制

由于N2O的产生途径众多,土壤pH的影响机制也十分复杂。目前,相关研究主要集中在生物反硝化、生物硝化和非生物过程3个方面。

3.1 反硝化作用 反硝化作用过程是土壤产生N2O的主要途径之一,而pH是其主要影响因素之一。大多数反硝化微生物生长的最适pH范围在6~8的中性环境中,在pH较低时(≤5)反硝化作用会进行的较为缓慢[50]但N2O的排放增加。研究表明,土壤pH可以通过改变反硝化微生物的群落多样性和丰度影响N2O产生量[50-54]。Fierer等[51]研究发现,反硝化细菌在中性土壤中多样性最高,酸性土壤中较低。Philippot等[52]发现土壤pH是影响反硝化细菌群落组成的重要因素。相比反硝化细菌群落本身的大小,总细菌群落内反硝化细菌所占的比例对反硝化速率的影响更重要[53]。同时,Chen等[55]研究发现,酸性土壤中产N2O的真菌群落更丰富,也具有较大的产N2O潜力。

从整个反硝化作用过程产生的酶来看,其反应需要硝酸盐还原酶(Nar & Nap)、亚硝酸盐还原酶(Nir)、一氧化氮还原酶(Nor)以及一氧化二氮还原酶(Nos)的催化,相应编码基因分别为narG和napA、nirK/S、norB以及nosZ[56]。多数研究认为,土壤pH是通过影响反硝化酶活性,尤其是N2O还原酶的活性来调控N2O的排放[13,37,57-59]。?imek等[59]研究发现,在等于或接近自然土壤的pH时反硝化酶活性最高,如果pH被人为改变,反硝化酶活性会下降。朱永官等[13]总结发现,当pH>7时N2O还原酶活性增强,而在pH<7时其活性逐渐减小[57]。Shaaban等[58]研究发现,通过添加白云石提高土壤pH后,可以增加土壤N2O还原酶的活性。从基因角度来说,相应编码基因的表达更易受到土壤pH的影响[60]。低pH会直接影响生物体产生功能性N2O还原酶的能力[39]。Liu等[60]研究表明,功能基因nirS、nirK和nosZ的丰度与pH呈显著正相关关系,但nosZ基因及其转录本的相对数量并未受土壤pH的直接影响。Bergaust等[61]以脱氮副球菌(Paracoccus denitrificans)为研究对象,发现pH为6时对N2O还原速率有很大的影响,但通过定量基因转录产物并不能解释这种现象。研究结果显示,nosZ基因转录产物的相对数量未受到影响(nosZ/norB比值),在低pH下(对比pH为6和7)甚至有些增加。也有研究发现,低pH对N2O还原酶的抑制作用是通过干扰蛋白质的合成或装配,进而影响了酶的活性[60-61]。此外,低pH也会降低土壤矿质氮和有机碳的可利用性[62]进而影响反硝化作用过程的进行。

3.2 硝化作用 氨氧化细菌(AOB)或氨氧化古菌(AOA)在氨单加氧酶和羟胺氧化还原酶的催化下将NH3氧化成NO2-,是硝化作用的关键步骤[63-64]。一般认为,由于AOB和AOA在土壤环境中占据的生态位不同[65-68],其对不同土壤pH的响应也不一致。如AOB在中性、碱性和高氮素投入的条件下是驱动硝化过程进行的主体,而AOA在酸性的自然生态系统中更能发挥作用[69]。即土壤pH会通过调控AOA和AOB的群落结构进而影响生物硝化作用中N2O产生量[63,72]。Yao等[72]研究發现强酸性土壤中AOA的丰度与潜在的硝化率直接相关,AOA是酸性土壤主要氨氧化菌,而AOB在低pH环境无法进行正常生长代谢。毛新伟等[63]研究发现微酸性土壤不利于AOA发挥作用,土壤pH下降至酸性时AOA的作用得到发挥。多数研究表明,土壤pH也会影响氨氧化菌基因和转录体的丰度从而影响硝化作用N2O的排放 [70-71]。Nicol等[70]研究发现古菌amoA基因和转录体的丰度随土壤pH升高而下降,而细菌amoA基因的丰度普遍较低,转录体随pH升高而增加。Gubry-Rangin等[71]研究发现古菌amoA基因的丰度和多样性随土壤pH升高而增加。此外,低pH条件不利于硝化作用底物NH3在土壤中的存留,因此土壤硝化潜势与pH呈显著正相关[70,73]。

3.3 非生物机制 土壤pH同样是影响N2O非生物产生过程的重要因子。在化学反硝化中(如,式8-13)质子(H+)是产物,因此高pH有利于化学反硝化过程的进行[76]。Van Cleemput等[75]推断,化学反硝化是碱性土壤(尤其是下层土壤)中N2O产生的重要途径。刘晶[46]对北京东灵山土壤进行灭菌后培养发现,N2O产生量随土壤pH升高而增加。然而基于热力学计算,徐香云[76]发现化学反硝化过程的气体产物比(如,NO/N2O、N2O/N2)随体系pH升高而下降。卢晋晶[74]、刘晶[46]的实验发现,不同土壤中,氮素化学转化的气体产物NO/N2O摩尔比值都随体系pH升高而下降。已有实验研究表明,化学反硝化的气体产物在酸性条件下以NO为主[40,77],中性条件下以N2O为主[78-79],碱性条件下则以N2或NH4+为主[80-81]。这意味着,尽管高pH有利于NO3-/NO2-的化学还原,但对N2O产生的贡献还不确定。理论上,化学硝化作用同样也可以产生N2O(如,式7、8),但目前对其研究很少。李良谟等[82]的实验发现,化学硝化过程中的气体产物可包括NO、N2O和N2等[45]。徐香云[76]通过热力学计算提出,化学硝化中NH4+的氧化速率及N2O/N2摩尔比值可能随环境pH的降低而增加。

作为氮素转化的活性中间产物,NO2-和NH2OH向N2O的化学转化同样受到土壤pH的影响[39,48,83-86]。在酸性条件下(pH<5.5),NO2-不稳定易发生自分解生成NO(式17),而后者可以通过化学过程被还原成N2O[40]。此外,NO2-还可以作为电子受体与土壤中还原性物质发生化学反应[47],土壤pH在其中的影响与前文中的化学反硝化类似。研究表明,硝化作用生成的NH2OH被高价态金属或NO2-(式15、16、3)氧化分解生成N2O[48]。在这些反应中H+作为产物,因此从热力学意义上降低pH更有利于反应发生。但是,Heil等[85]研究发现NH2OH非生物过程产生的N2O在pH较高时较多。类似地,马兰等[86]研究表明NH2OH非生物过程对N2O排放的贡献与土壤pH呈正相关。究其原因,可能是低pH加速了NH2OH的质子化从而减缓了氧化分解速率[48,83]。

4 研究展望

理解土壤N2O排放的驱动机制与影响因子是编制N2O排放清单和制定N2O减排策略的科学基础。土壤pH作为主要的环境因子之一,其对N2O排放的影响已经引起广泛的重视。由于土壤氮素转化过程的复杂性,对其影响的机制和程度一直没有系统的认识。在今后的工作中,如下方面可能需要关注。

(1)土壤中生成N2O的过程众多,而且不同条件下具有不同的相对重要性。目前的相关研究主要集中在生物学反硝化过程,对土壤pH在其他过程中的影响(如厌氧氨氧化、NDFO等)还知之甚少,需要在今后的研究中予以关注。此外,人们目前更多地关注N2O产生的生物学机制,对土壤pH在氮素非生物转化过程中的影响尚有待澄清。

(2)作为氮素转化过程的中间产物,N2O与其他氮素形态之间存在相互转化的关系。目前的研究往往仅关注N2O一种气体,忽略了不同pH下底物转化和气体产物间转化的差异性。在今后的研究中,应将N2O置于土壤氮素转化的整体框架下,同时关注土壤pH对底物转化速率和气体产物比的影响。

(3)目前,我国的农田、森林和草原土壤均出现区域性的酸化趋势[89-91]。在理解影响机制的基础上评价土壤酸化背景下N2O的排放规律对于建立科学的N2O排放清单、因地制宜制定N2O减排政策具有重要的科学与现实意义。

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(責编:张宏民)

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