常 远,李若琪,李 珺,詹亚斌,魏雨泉*,许 艇,李 季

好氧堆肥腐殖酸形成机制及促腐调控技术概述

常 远1,2,李若琪1,2,李 珺1,2,詹亚斌3,魏雨泉1,2*,许 艇1,2,李 季1,2

(1.中国农业大学资源与环境学院,北京 100193；2.中国农业大学有机循环研究院(苏州),江苏 苏州 215100；3.湖北省农业科学院,植保土肥研究所,湖北 武汉 430064)

基于堆肥腐殖酸形成机制,重点综述了工艺参数优化、外源添加剂等促进堆肥腐殖化进程的有效方法,系统地总结了各调控手段对堆肥腐殖酸形成过程的影响机理,旨在为堆肥快速腐熟调控技术发展提供理论依据.由于堆肥过程是不断波动变化的,多种调控手段在实际生产应用中并不能完全达到预期的效果.因此在堆肥实际生产应用中,应更多关注堆肥中与腐殖酸形成相关的因素或过程,进一步针对性地加强各种调控手段的研究并建立彼此之间的关联,优化其在堆肥过程中的影响,以实现提高最终产品质量的目标.

腐殖化；腐殖酸形成；快速腐熟；工艺参数；新型添加剂

2019年中国各类有机废弃物产生量干重总计22.69亿t[1].由于有机废弃物产量巨大,其处理工艺一直是研究重点.有机废弃物主要处理手段有:卫生填埋、焚烧、好氧堆肥等技术[2].作为传统处理工艺,填埋和焚烧不仅资源化程度低,而且对生态环境和人体健康造成了严重危害.相较于传统处理技术,好氧堆肥因无害化处理率高、环境友好性、成本低等优势,逐渐发展成处理有机废弃物实现肥料化的主要技术手段[3].好氧堆肥实质是微生物主导有机物质分解并通过降解和聚合生成稳定腐殖酸等高附加值产品的腐殖化过程[4].腐殖酸作为堆肥终端产物,在土壤修复、改善植物生长及土壤肥力方面发挥着重要作用[5].传统好氧堆肥依靠土著菌进行降解发酵,未添加任何生物菌剂或添加剂,堆肥温度较低,周期长,一般1～3个月,产品质量不稳定.随着有机废弃物产生量逐渐增大,有机废弃物堆肥处理效率要求愈来愈高,传统好氧堆肥已经不能满足日益增长的有机废弃物处理需求,因此快速堆肥应运而生.

快速堆肥是利用反应器、工艺优化、外源添加剂等调控手段人为强化堆肥关键可控因素,促进腐殖化进程,缩短堆肥周期[6],一般仅1～3个星期.目前仍存在腐熟不充分、腐殖化效率低和产品质量差等缺点[7].腐殖化过程通常被认为是含碳化合物积累和储存的一种形式,是堆肥固碳的关键环节.腐殖酸类物质是有机废弃物堆肥腐殖化过程的主要产物之一,腐殖酸数量和质量直接影响堆肥腐熟度和产品质量,促进堆肥腐殖酸稳定形成,可有效加快堆肥腐殖化效率,提升堆肥品质.因此,在有机废弃物堆肥无害化、减量化的基础上,调控好氧堆肥腐殖酸形成已成为新兴领域.然而,由于有机废弃物来源广泛、结构多样,各类腐殖酸前体可能相互作用、彼此联系,造成堆肥腐殖酸形成过程的复杂性与多样性[8].因此深入研究堆肥腐殖酸形成机制、调控堆肥腐殖化进程并建立腐殖酸稳定化调控方法极其必要,既符合农业绿色发展中有机肥相关产业需求,也是国内外研究的热点和难点问题.

广大学者对堆肥腐殖化调控的研究主要集中在改善堆肥环境、加强形成途径,以达到加快堆肥腐殖化的目的[4],主流快速腐熟调控技术有:优化工艺参数、外源添加剂.优化堆肥工艺参数是最常见的调控技术,温度、含水率、碳氮比(C/N)、pH值等都对堆肥腐殖化有直接影响[9].通过调节原始理化参数、协调原料配比、物化辅助加强等手段维持堆肥环境因子在最佳水平,有利于微生物生长代谢,促进堆肥腐殖化.添加外源添加剂是一种高效的促腐策略,包括常规强化添加剂和新型功能添加剂.微生物菌剂[10]、腐殖酸前体[11]、腐熟堆肥[12]等是堆肥中常用的外源强化添加剂,能直接加强腐殖酸形成途径,促进腐殖酸形成,提高堆肥腐熟度.新型功能添加剂如生物炭、矿物质等,不仅能调节堆肥环境如生物炭可增加堆肥C/N[13]、矿物质可作为调节剂增加pH值[14],还能吸附堆肥腐殖酸形成有机无机复合体起到保护作用[15],综合了前两种促腐调控技术的优势.同时由于这类物质在高温下具有较强的稳定性,与熟料混合还可提高堆肥产品的附加值,因此被称为新型功能添加剂.上述调控手段通过不同的促腐殖化机理,共同促进了快速堆肥技术的发展.

本文系统概述了堆肥腐殖酸形成途径、多种快速堆肥腐殖化进程调控技术及其机制,以寻求处理效率高、经济效益好且环境友好的快速堆肥技术,为建立堆肥腐殖酸稳定化机制、提高堆肥效率和产品质量提供重要的理论基础,促进有机废弃物资源化利用快速发展.

1 堆肥腐殖酸及其形成机制

腐殖酸作为堆肥的重要次生产物,其主要包括胡敏酸(HA,碱溶酸不溶)和富里酸(FA,酸溶碱不溶)两种组分.胡敏酸分子量大且结构复杂缜密,主要以芳香化合物为主,芳香化程度高而解离程度小;富里酸分子量小且结构简单松散,主要以烷烃类化合物为主,含有较多的羟基和羧基.完整的腐殖酸大分子主要通过这两种化合物结合而成,基本结构以芳环和脂环为主,环上连有羧基、酚羟基、醇羟基、羰基、醌和甲氧基等多种基团[16],同时结构表面还存在糖类、木质素单体、酚酸和脂肪酸以及大量的非木质素来源的芳香族化合物,丰富的结构表面赋予了腐殖酸生物活性[17].在堆肥过程中,微生物通过分解有机质或自身合成释放出含有上述基团的小分子有机化合物,这些多功能有机化合物通过随机氧化、疏水作用、电荷转移、氢键等方式聚合或缩合形成腐殖酸[16].

堆肥腐殖酸形成机制的研究起源于土壤腐殖质的研究,堆肥类似于土壤腐殖化的一个快速过程[18].堆肥中有机组分(蛋白质、多糖、脂质和木质素)转化与降解过程中形成的小分子化合物(多酚、羧基、脂肪酸、糖类以及氨基酸等化合物)被称为腐殖酸前体[19].不同前体分子对腐殖质形成具有不同的影响:芳香族结构的多酚和羧基化合物对HA的形成有明显的促进作用,而氨基酸、多糖和还原糖等烷烃类化合物则主要影响FA的形成[20].因此腐殖酸形成并不是前体生物分子单体间的简单加和,多种不同来源的前体物彼此间需通过一定机制才能最终聚合成腐殖酸大分子.

1.1 以非生物途径为主的形成机制

经过多年腐殖酸合成途径研究逐渐形成了多种腐殖酸形成机制假说(图1),其中以非生物途径为主的形成机制假说包括木质素-蛋白学说、多酚-蛋白假说、多酚自缩合以及美拉德反应[21-23].木质素-蛋白学说强调不完全分解的木质素转化形成具有酚羟基和羧基基团的分子,进而作为腐殖酸核心骨架与含氮化合物聚合形成腐殖酸.由于这类含氮化合物存在形式主要是蛋白质和氨基酸,故又有学者提出类似的多酚-蛋白质假说,认为酚、醌以及氨基酸等是构成腐殖酸骨架的核心物质[24].多酚-蛋白假说与木质素-蛋白学说高度相似,但多酚-蛋白假说涵盖范围更广、适用性更强[11].多酚除了和含氮化合物聚合形成腐殖酸外,其还可通过环断裂与其他酚类化合物缩合形成腐殖酸,即所谓的多酚自缩合[25].美拉德反应则与上述机制不同,主要强调糖和氨基的作用,还原糖和氨基酸之间的缩合被认为是堆肥初期腐殖酸形成的主要原因,且此过程没有酶和酚的参与[26].由此看出腐殖酸形成途径相互独立,但多种腐殖酸前体相互作用又使得各腐殖酸形成途径在堆肥环境中彼此联系,因此往往多种非生物途径共同作用于腐殖酸形成.

1.2 以生物途径为主的形成机制

堆肥腐殖化是一个复杂的微生物主导的生物生化过程,有机质分解和腐殖酸形成是微生物活动的结果[27].堆肥中微生物活动主要存在分解(有机物矿化)和合成(腐殖酸形成)两种代谢路径[28].两种路径共同作用,相辅相成,确保腐殖化过程的顺利进行.堆肥中微生物合成腐殖酸强调以生物途径为主(图1),主要涉及微生物合成假说、微生物多酚学说、细胞自溶学说及活性氧(ROS)理论.微生物合成假说认为微生物利用植物物质作碳源和能源在细胞内合成高分子腐殖质物质,微生物死亡后再释放到土壤中,在细胞外再降解为HA和FA[25].微生物多酚学说强调微生物合成的多酚在微生物分泌的酚氧化酶作用下氧化成醌,最终与含氮化合物缩合形成腐殖酸.细胞自溶假说则认为腐殖物质是植物和微生物死亡之后的自溶产物,原先的细胞成分,如糖、氨基酸、酚和其他芳香族化合物通过自由基进行缩合和聚合而成.此外,最近的研究表明,ROS在堆肥腐殖酸形成过程中起到关键作用,微生物通过在交替有氧和厌氧条件下介导铁或腐殖质氧化还原循环的耦合过程,产生的羟基自由基(·OH)、超氧阴离子自由基(·O2−)、过氧化氢(H2O2)等活性氧物质可以将木质纤维素分解成小分子前体有机物,进而聚合缩合形成腐殖酸[29].

图1 堆肥腐殖酸形成机制

有机废弃物来源广泛、结构组成复杂,不同物料因其结构组成上的差异使得其在堆肥过程中被不同的功能微生物类群分解转化,进而形成多种腐殖酸前体物质,使得堆肥过程中腐殖酸形成的主要途径存在区别.目前对于秸秆、园林枯枝落叶等植物源有机固体废弃物堆肥腐殖化过程研究较多,一般认为富含木质素的有机废弃物堆肥腐殖化过程主要依赖于以木质素-蛋白质复合体为核心的木质素-蛋白学说,对于富含结构相对简单的纤维素类有机固体废弃物如杂草、尾菜等,其堆肥腐殖化过程中,多酚学说和微生物多酚学说更为主导,同时美拉德反应也发挥一定作用.然而动物源等高蛋白类有机废弃物由于有机碳组分在堆肥中直接合成腐殖酸的比例相对较小,无法像植物源有机废弃物一样提供充足的典型腐殖酸核心骨架,易降解有机组分相对较高,微生物代谢更为活跃,有机物在堆肥过程中易以气体形式排放实现稳定化,不利于腐殖酸前体物的积累和聚合,但在堆肥过程中仍可发生明显的腐殖化过程[30].因此,微生物介导的动物源高蛋白类堆肥腐殖化过程更为复杂,其堆肥腐殖酸形成机制可能更多是以生物途径为主.

综上所述,非生物途径和生物途径组成了堆肥腐殖酸形成的基本机理.但无论是非生物途径还是生物途径,都离不开腐殖酸前体物质.在堆肥进程中,微生物主导的生物途径会与非生物途径竞争底物,大量腐殖酸前体被微生物作为生长活动所需营养物质利用消耗,致使堆肥腐殖化进程缓慢[25].因此,为了加速堆肥腐殖化进程,优化工艺参数、添加功能添加剂等多种快速腐熟技术普遍应用于促进堆肥腐殖酸形成,缩短堆肥周期,提升堆肥产品质量,使得快速腐熟技术在堆肥领域越来越受到关注.

2 堆肥快速腐熟调控技术

堆肥存在堆肥周期长、氮素损失、腐熟度低等缺点.腐殖酸是堆肥腐熟的重要指标,未腐熟的堆肥可能缺乏腐殖酸或腐殖酸不稳定[4].腐殖酸形成受堆肥基本参数、原料特性、微生物活性、外源添加剂等因素影响[31].

2.1 堆肥参数优化

2.1.1 堆肥过程基本工艺参数调节 温度是影响堆肥微生物代谢及种群动态的重要环境变量[32].根据温度,堆肥过程分为升温期、高温期、降温期(腐熟期)三个时期,每个时期微生物群落影响堆肥腐殖化的机理不同.升温和高温期中嗜热微生物占主导地位,优势嗜热微生物对HA前体产生起决定性作用[33],且对多糖、木质素、蛋白质、脂肪的降解与高温有关[34].温度过高不利于HA前体形成[32].此外,堆肥进入高温期越早,有机质降解越彻底,越有利于缩短堆肥腐殖化周期[35].堆肥降温腐熟阶段是HA形成的关键时期,此时嗜温微生物发挥主要作用,促进前体物缩合聚合形成HA[36].

微生物在堆肥过程中活动的最佳pH值范围为5.5～8.0[37],可通过调节pH值来提高堆肥微生物活性及其丰度,以促进堆肥腐殖化进程[34].低pH(3.0～7.5)会使得超分子腐殖酸失稳,并与小分子富集[38];而pH小于2.0会使腐殖物质聚合形成大分子[39].此外, pH对于控制氨挥发造成的氮损失至关重要[9].

碳氮比是评价堆肥启动、影响堆肥产品生产工艺和质量的重要指标[9].最佳碳氮比的确定决定了堆肥的稳定性和腐熟度,初始C/N较低时,碳源少,氮源相对过量,过量的氮转化成氨气挥发导致氮损失;C/N过高时,微生物的活性会降低,有机物分解速度会变慢[40].过高或过低的C/N均不利于堆肥腐熟,被认为最适合堆肥的C/N为25～30[20].

合适的含水率是堆肥成功的关键条件,水分对堆肥物理结构、生物活性以及腐熟程度等均产生影响[41].堆体含水率过高或过低都不利于堆肥腐殖化进程,含水量过高,会形成厌氧发酵,影响堆体升温;含水量过低,不利于微生物生长繁殖,延缓堆肥腐熟效率[42].堆肥含水率可以通过添加膨胀剂调节, 40%～60%被广泛认为是堆肥的最佳含水率范围[43],不同的堆肥反应系统及堆肥原料最佳含水率可能有所差异.随着堆肥的进行,水分会通过蒸发、渗滤流失,需对含水率进行合理调控,以营造适宜微生物生长繁殖的环境.

多种参数共同调控了影响堆肥中腐殖酸快速形成的直接或间接因子(表1),各个参数之间并不是独立作用,而是彼此之间相互联系、共同协调促进腐殖化进程.因此在实际调控堆肥参数时,往往不能只关注某个参数,应联系多参数之间的连锁效应展开进一步研究.

2.1.2 原料配比 堆肥原料的类型是影响腐殖化过程中的一个重要参数.单一物料堆肥时会受到自身理化性质及组成的限制,而多种物料按照合适比例进行堆肥时可以加速腐熟的进程[45].秸秆、园林废弃物等这类含水率低、碳氮比、孔隙度和木质化程度高的堆肥原料,虽然其木质素分解产生的酚类物质是腐殖酸形成的重要前体,但单一秸秆等物料的堆肥腐殖化程度低[3].而畜禽粪便含水率高、碳氮比和孔隙度低,并含有大量的微生物,作为调节剂与秸秆等合理配比混合堆肥,可以促进腐殖物质形成,加快腐殖化进程[46].因此提出了原料组分配比堆肥:不同类型的农业废弃物配比堆肥具有更高的效率,并且成分丰富能生产高质量的堆肥产品[47].配比堆肥是基于原材料的物理和化学性质,按照一定比例组合将含水率和碳氮比调整到能够有利于微生物生长繁殖的最佳条件,以加快升温速度、缩短堆肥腐熟进程[48].在配比堆肥过程中,不同材料的组合可能会加速或减缓堆肥速度,选择合适的原料对腐殖质的形成至关重要,各种原料腐殖质化的途径机制值得进一步研究.

表1 堆肥基本工艺参数对腐殖酸形成的影响

2.1.3 新兴物化辅助策略 近年来为克服传统堆肥的缺点,开发了多种物化辅助策略(图2).超高温预处理堆肥(HPC)是基于物化辅助策略优化堆肥工艺从而衍生出的一种新型堆肥工艺,利用超高温反应器对堆肥物料进行预处理,再进行传统堆肥(TC)[49]. Yamada等[50]所研发的超高温反应器保持在100℃,持续2个小时.已有研究表明,HPC可加剧腐殖酸形成,使有机质演化指数提高30%～50%[50];并且HPC 可有效减少总氮损失,氮保留较TC增加49%[51]; HPC还可通过调控前体产生来促进HA的形成, Huang等[52]表明,HPC中腐殖酸形成前体的浓度增加了44%～92%.此外,细菌群落的变化被认为是HPC过程中腐殖化率和HA产生增加的主要原因,从而导致成熟期缩短[53].Cao等[49]通过13C NMR波谱发现HPC中提取的腐殖质物质中的芳烃百分比更高,芳结构富集更早.因此,HPC通过加速堆肥腐殖化和缩短成熟期,在堆肥质量和效率上均具有优越性.

超高温堆肥(HTC)同样被提出作为另一种新型堆肥工艺,通过接种超嗜热微生物提高好氧发酵的温度,无需外源加热[54].HTC工艺的最高温度可超过90甚至100°C,明显高于TC的温度,从而提高了有机质生物转化效率[55].HTC通过发展超嗜热微生物群落,形成超高温阶段,加速有机质降解[56].在HTC中,HA前体的氧化水平是决定聚合度和腐殖化程度的关键因素.有研究表明HTC中类蛋白物质在超高温条件下通过强烈生物氧化反应产生高浓度含氮前体物,在酶的作用下形成稳定的腐殖质物质[57].由此可见,HTC的快速腐殖化过程归因于超嗜热菌驱动有机质快速降解和转化形成前体,前体深度氧化进而加速HA形成.与传统堆肥相比,超高温堆肥依靠其独特的嗜热微生物群落在提高堆肥效率和减少气体排放上具有潜力性[58].HTC系统的关键问题是缺乏合适的超嗜热菌持续降解有机质,评估其他超嗜热菌在HTC系统中的应用可行性值得研究.

电场辅助好氧堆肥(EAC)是一种新颖且有效的促腐工艺.好氧堆肥在本质上是微生物驱动下的氧化还原过程,该过程产生的电子可被电活性细菌转移到细胞外电子受体,EAC可以增加电活性细菌的相对丰度,从而加速有机物的生物降解,提高堆肥腐熟度[59].Cao等[60]研究证明施加直流电场丰富了堆肥中细菌丰度及其代谢,促进腐殖酸形成.直流电场可将TC的温度提高到70～75℃,但直流电场作用下的梯度水分分布影响了微生物代谢热,限制了堆肥温度的升高[61].与直流电场不同,交流电场(AEF)可促进堆肥堆体中水分的均匀分布,进一步将温度提高到90℃,促进有机质的生物降解和腐殖化过程[62].此外,AEF还可富集嗜热菌,其代表了一种新颖且适用于HTC快速腐殖化调控的可行策略.但堆肥材料导电性差、电子传递效率低等影响了EAC的效率和适用性[63].因此,提高堆肥系统的电导率对于确保EAC系统快速腐殖化调控的有效性非常重要.

2.2 常规堆肥添加剂强化

添加剂被认为是促进堆肥腐殖化的一种高效且易于掌握的策略.添加剂可以促进有机质的分解,保留堆体营养物质,从而获得腐殖物质和营养物质丰富的堆肥产品[65].

2.2.1 微生物菌剂 微生物作为堆肥腐殖化过程中物质循环和能量流动的主要推动者,有机物在其作用下聚合或缩合形成腐殖酸[66].堆肥中,土著微生物菌群主导有机物的分解转化;土著微生物菌群多样性不足或受到某些环境参数和原料特性的不利影响,则会导致分解能力差、堆肥周期长、产品质量低[67].因此,接种微生物菌剂是一种有效的促腐策略,通过改变微生物群落及其代谢功能,可加速简单化合物的降解和复杂化合物的形成,促进腐殖化程度,提升堆肥产品质量[68].

堆肥中主要微生物种类是细菌(包括放线菌)和真菌,在不同时期中都有其独特的优势菌群,共同促进物料的分解和腐熟[69].在堆肥过程中细菌群落多样性越高,对有机物的降解越有利[70].有研究表明细菌接种可增加细菌群落多样性,产生更多功能性细菌,促进堆肥腐殖化[71].细菌作为主导升温期的菌群,对发酵升温起主要作用.Li等[72]证明接种微生物菌剂可加速温度上升,缩短堆肥周期.事实上,堆肥产生的热量主要来自微生物分泌的降解有机物的酶[70]. Duan等[73]报道,堆肥中接种枯草芽孢杆菌可增强纤维素酶、蛋白酶和淀粉酶的分泌,促进纤维素和蛋白质的生物降解,从而形成稳定的腐殖质.真菌接种加强堆肥进程也尤其显著,堆肥中重要且广泛使用的真菌之一是白腐真菌.它产生由锰过氧化物酶、木质素过氧化物酶和漆酶组成的细胞外酶系统,可用于降解木质纤维素[74].研究发现,接种白腐真菌是增强最终堆肥产品特性的有用策略,有机物降解和堆肥成熟的有效性取决于真菌类型[75].Zhang等[76]研究证明接种黄孢原毛平革菌(Phanerochaete chrysosporium对木质素等稳定性有机质的分解有积极作用.此外,放线菌在堆肥微生物群落中起着重要的作用,不仅可生产木质纤维素水解酶,还可在高温下形成孢子以抵抗堆肥过程中的恶劣环境[77]. Zhao等[69]发现,多阶段接种从堆肥样品中筛选出的纤维素降解嗜热放线菌可提高纤维素酶活性,在加速纤维素降解、提高腐殖酸含量的同时降低了CO2排放量.

综上所述,接种微生物菌剂是提高堆肥腐殖化和效率的有效措施之一,其机理主要包括:1)丰富微生物群落及其功能多样性,共同参与腐殖质合成.2)增加核心菌落丰度,合成大量酶促进有机物降解.3)改善矿化,保留碳氮,增强有机物转化为腐殖质.尽管如此,微生物接种的有效性仍受微生物种类、接种时间、堆肥具体操作条件、原料特性等因素影响[78],调控堆肥快速腐殖化的最适微生物接种技术是什么的问题仍然存在.在未来,需要确定最佳接种微生物浓度;需要考虑堆肥过程中微生物的接种剂种类、功能、生理、适应性和稳定性;需要针对堆肥过程中的微生物作用机制进行研究,以期找到堆肥过程中最适宜添加的微生物菌剂.

2.2.2 外源腐殖酸前体物 前体在腐殖酸形成过程中起着关键作用.堆肥腐殖酸前体可通过有机物分解和微生物合成两种途径形成[22].在生物和非生物腐殖化途径下通过氧化和亲核反应聚合形成腐殖酸[79].多酚作为腐殖酸的芳香族骨架,促进腐殖酸的芳香性以确保其结构稳定性[80].羧基对增加腐殖酸的脂肪族化合物和不饱和度起着重要作用[24].氨基酸是含氮有机物的水解产物,被还原可作微生物的氮源,为HA的形成提供氮素[81].还原糖和多糖作为微生物的主要能量和碳源,可促进FA转化成HA[23].因此,前体作为促进腐殖酸形成的重要调节因子,通过添加外源前体物是促进堆肥快速腐殖化的有效方法.

外源前体物促进堆肥腐殖化主要是通过以下几方面(图3):1)直接参与腐殖酸的形成[82];2)提高FA向HA的转化速率,增加HA的不饱和度[83];3)促进木质纤维素降解[4],并改变细菌群落功能以调节前体数量[82];4)作为微生物的能源物质和养分来源,提高微生物代谢能力以产生腐殖酸前体[84];5)减弱微生物对前体的利用,使得更多前体形成腐殖酸[11].外源前体物促进HA形成的多种途径并不是单独存在的,而是多途径相互联系、共同作用[85],因此未来研究应探究各前体在堆肥不同时期下多途径之间的相互关系,精准定位不同外源前体物的添加时期,提高促腐效率.

图3 外源腐殖酸前体促进堆肥腐殖化机制

2.2.3 腐熟堆肥回流 腐熟堆肥(MC)作为堆肥稳定发酵产品,具有低含水率、低碳氮比、高孔隙及富含微生物的特点[86].MC属于常用的堆肥添加剂,并兼具调理剂、膨胀剂与接种剂的功能[12].添加MC可以有效促进堆肥快速腐殖化,缩短堆肥周期.

MC通过调节堆肥物理结构和微生物群落结构两个方面来影响堆肥的矿化和腐殖化进程.一方面,MC用作调理剂,可提高堆肥孔隙度并降低堆肥含水率[87].另一方面,MC用作膨胀剂和接种剂,可为堆体接种内源微生物,加速微生物演替,缩短堆肥周期[88].因此,MC理论上是一种促进快速腐殖化的堆肥辅料调节剂和微生物接种剂综合体.腐熟堆肥回流有利于腐殖酸前体的形成与积累,有效降低腐殖质损失率,显著提高腐殖质含量[12].在经济效益上,腐熟堆肥可代替商业微生物接种物,降低堆肥成本[89].此外,MC可回收利用,价格低廉且易于获得.但利用腐熟堆肥作为堆肥添加剂需要很高的添加比例,但过高的添加量会抑制堆肥过程中腐殖质聚合度和芳构化程度的增长,因此在实际生产中要注意控制腐熟堆肥的添加用量,以确保堆肥效果.

2.3 新型功能添加剂

在研究外源添加剂提高堆肥快速腐熟的同时,还应尽可能考虑产品的多功能性.为提升堆肥产品附加值,新型功能添加剂如生物炭、矿物质等被发掘应用.新型添加剂施加在堆肥中不仅可稳定形成腐殖酸,还可增强堆肥产品的农艺功能[5].

2.3.1 生物炭 生物炭是一类碳含量极高、活性官能团丰富、离子交换能力强、芳构化程度高、性质稳定的碳基物质[90].大量研究表明,生物炭在促进堆肥腐殖化中发挥积极作用.

生物炭直接促进堆肥腐殖化通过以下机制实现:1)生物炭自身部分被氧化降解释放可溶性有机化合物和芳香化合物掺入腐殖质类物质,促进HA的形成及其稳定[91].有研究发现木材生物炭的水萃取部分含有更高水平的类富里酸和类胡敏酸物质,可直接用于形成类腐殖质样物质[13].2)堆肥形成的HA及其前体等物质通过配体交换和疏水作用在生物炭活性表面吸附保留[39].生物炭表面丰富的官能团(如羧基、羟基、羰基、酰基等)可作为吸附位点[92],将腐殖质吸附到其表面并保护它们免受微生物分解来促进腐殖化过程[91].

生物炭间接促进腐殖化则通过影响堆肥微生物群落实现[93].高比表面积可为微生物生长繁殖提供适宜的栖息环境.丰富的孔隙结构可加速氧气快速流通和能量传输,促进微生物的好氧代谢[94].生物炭表面能吸附堆肥中产生的水溶性碳和酚类等物质,作为底物为微生物生长代谢提供营养[95].生物炭可通过中和氢离子或促进有机酸的降解来调节堆肥体系的酸碱环境,增强微生物活性[13].此外,生物炭还可提高产品附加值.可通过静电吸引作用减少气体挥发和养分损失;依赖表面高阳离子交换能力及含氧官能团降低堆肥中重金属生物有效性;通过表面的电子供体受体吸附削减有机污染物的毒性[96].

综上所述,生物炭在促进堆肥腐殖化、改善堆肥产品质量上展现出巨大的优势.过往研究讨论了不同生物炭添加量对腐殖化的影响,建议添加约10%的生物炭以最大限度促进堆肥腐殖化[2].尽管如此,超过20%的添加量会抑制微生物的活性并干扰有机物的降解,同时生物炭较高的成本限制了其大剂量的使用.另外不能忽略的一个问题便是内源污染问题,如何有效避免因内源污染产生环境风险在未来值得进一步研究.

2.3.2 矿物质 堆肥作为一个快速、易变化的体系,堆肥所形成的类腐殖质物质不稳定易被降解,外源矿物质的添加可以缓解堆肥过程中类腐殖质物质的降解,提高堆肥腐殖酸形成稳定性[65].

矿物质通过以下四种主要机制促进腐殖化(图4):首先,直接催化美拉德反应[97].其次,通过改变微生物群落结构和多样性并产生协同作用.一方面,多孔特性和高比表面积为微生物活动或表面官能团催化腐殖化提供位点和空间[98].另一方面,通过调节堆肥理化性质来为微生物降解创造适宜的环境.其独特的晶体结构和内部纳米孔道的吸附性,保证了良好的保水保肥性和酸碱缓冲性能[99].同时,矿物不同形式的电子能量通过胞外电子传递方式影响微生物生长代谢,丰富微生物的能量获取途径,并且作为直接参与生长代谢的电子供体/受体,提高电子能量的传递效率,促进堆肥腐殖化[100].再次,可加速前体的形成和积累,从而促进堆肥腐殖化.最后,通过各种吸附机制与腐殖质相互作用形成有机无机复合体,保障腐殖酸稳定形成[101-102].在矿物质基底表面,腐殖酸通过疏水相互作用和阳离子桥键吸附形成有机无机复合体[103].在矿物质边缘表面,腐殖酸通过配体交换和静电吸引吸附形成有机无机复合体[101].

近年来,很多具有促腐殖化能力的矿物质添加剂被广泛研究, Pan等[104]发现蒙脱石和伊利石的添加主要促进了HA的非生物途径的形成.蒙脱石和沸石可用作路易斯酸,以非生物方式催化氨基酸和还原糖缩合以产生腐殖酸[19,105].蒙脱石基底表面的Si-O、Si-O-Al基团也参与了矿物边缘对HA的吸附,促进了HA的形成[36].膨润土可为微生物提供栖息地,还可吸附酶并增强酶活性和稳定性,从而促进微生物降解有机物,提高堆肥的稳定性和成熟度[106].碱性石灰可促进木质纤维素水解产生前体,还可调节堆肥的初始酸性pH值[14].黑电气石可延长高温期,改善腐殖化并减少氮损失[107].麦饭石可加速木质纤维素降解,促进HA形成[108].海泡石可增加堆体中高芳香组分占比,提高堆肥稳定性从而促进堆肥腐熟[109].凹凸棒土可加速有机物降解并促进动物粪便堆肥腐殖化[110].

矿物质作为一种廉价易得、操作性强、时效长和经济成本低的材料,矿物质改性能够进一步提升快速堆肥腐殖化效果,如Pan等[104]发现热改性矿物较普通矿物促进堆肥腐熟效果更好.未来可进一步研究开发应用于堆肥促腐的矿物质改性方法,同时还需结合土壤矿物学理论和微生物学进一步剖析有机无机复合体在堆肥体系中稳定形成腐殖酸的机制.最后,从堆肥实际生产应用角度出发,应该进行更多的大规模试验,验证从小试或中试规模研究中获得的结果.

3 结语

快速堆肥是为了在短时间内获得完全腐熟的终端产物,腐殖酸含量是堆肥产品的重要评判指标,快速腐熟调控技术(图5)都是以促进腐殖酸稳定形成为目的.优化工艺参数是调控各种堆肥理化性质,为堆肥各阶段关键微生物提供最适宜的栖息环境,提高微生物多样性及其代谢活性,通过微生物间接促进腐殖质稳定生成.物化辅助策略是从原料预处理、接种超嗜热微生物、加速电子传递的角度来提高酶活性、增殖超嗜热微生物、加强微生物代谢活性,以实现快速腐殖化.常规强化添加剂是通过强化腐殖酸形成途径中的某一环节,如生物腐殖化途径、前体形成和积累等,以提高腐殖化效率,缩短堆肥周期.新型功能添加剂则是依靠其丰富的孔隙空间和表面官能团通过多种吸附作用吸附氨基酸、糖和酚等低分子量化合物,促进腐殖酸前体形成和积累,还可形成有机无机复合体以保护腐殖酸并促进腐殖酸稳定化,同时由于其稳定的性质还能保留在堆肥产品中,从碳固存、降低重金属有效性、削弱有机污染物毒性等多个方面提高堆肥附加价值.此外,本文针对各调控手段分别给予了一定研究展望.总的来说,由于堆肥环境的波动变化和微生物群落的演化,不论是调控参数还是外源添加剂对腐殖化的影响相对来说都是难以控制,而且大多数促腐殖化研究都是在实验室条件下展开,并未投入到实际生产应用中,对于这些手段促腐效果并没有一个综合的评价.因此,针对不同有机固废原料性质的差异性,选择合适的添加剂种类、添加量对于精细化控制堆肥进程是有意义的,未来还需深入研究.

图5 堆肥促腐调控技术

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Mechanism and regulation method of humic acid formation in composting-a review.

CHANG Yuan1,2, LI Ruo-qi1,2, LI Jun1,2, ZHAN Ya-bin3, WEI Yu-quan1,2*, XU Ting1,2, LI Ji1,2

(1.College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China；2.Organic RecyclingResearch Institute (Suzhou) of China Agricultural University, Suzhou 215100, China；3.Institute of Plant Protection and Soil Fertilizer, Hubei Academy of Agricultural Sciences, Wuhan 430064, China).2023,43(10)：5291～5302

The aim of this study was to review the mechanism of humic acid formation and the regulation method for improving the humification degree by process parameter optimization, the exogenous additives, and so on, in composting. The mechanisms of various regulation methods for accelerating humic acid formation process were also discussed, which provided theoretical basis for the development of fast composting technologies. Various regulatory methods may be interacted in applications due to the complex dynamic physiochemical environment factors in composting. Therefore, it is necessary to establish an integrated relationship among more factors related to humic acid formation based on practical compost production process. The future regulating methods will help to improve the quality of composting products.

humification；humic acid formation；rapid maturation；process parameters；new additives

X705

A

1000-6923(2023)10-5291-12

2023-03-05

国家自然科学基金资助项目(32071552);国家环境保护食品链污染防治重点实验室开放课题基金(FC2022YB01);苏州市科技计划项目(SS20200);安徽省科技重大专项(202003a06020003)

* 责任作者, 副教授, weiyq2019@cau.edu.cn

常 远(1999-),男,安徽马鞍山人,中国农业大学硕士研究生,主要从事生态工程与有机废弃物资源化处理研究.发表论文3篇. chy491@cau.edu.cn.

常 远,李若琪,李 珺,等.好氧堆肥腐殖酸形成机制及促腐调控技术概述 [J]. 中国环境科学, 2023,43(10):5291-5302.

Chang Y,LI R Q, LI J, et al. Mechanism and regulation method of humic acid formation in composting-a review [J]. China Environmental Science, 2023,43(10):5291-5302.