Effects of Freeze-thaw on Soil Nitrogen Conversion: Research Progress

Yu He, Xie Hongbao, Chen Yimin, Wang Yao, Sui Yueyu, Jiao Xiaoguang

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Chinese Agricultural Science Bulletin ›› 2021, Vol. 37 ›› Issue (26) : 88-92. DOI: 10.11924/j.issn.1000-6850.casb2021-0121

Effects of Freeze-thaw on Soil Nitrogen Conversion: Research Progress

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Abstract

Nitrogen is one of the important nutrients limiting plant growth, and its transformation degree in soil is affected by many factors. As an important driving force for soil N transformation in mid, high latitude or high altitude areas, the effects of freeze-thaw also has a great impact on the process of soil N transformation. Based on existing research results at home and abroad, this paper summarized the effects of soil freeze-thaw cycles, freeze duration and freeze intensity on the process of soil N transformation. We summed up the general rules of soil N transformation caused by the changes of freeze-thaw patterns: changes in freezing and thawing patterns were all conducive to the mineralization of soil N. The increase of freeze-thaw intensity could significantly increase the content of soil nitrate N. Changes in freeze-thaw patterns will also increase N2O emissions.

Key words

effect of freeze-thaw / freeze-thaw frequency / freeze-thaw duration / freeze-thaw strength / nitrogen mineralization

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Yu He , Xie Hongbao , Chen Yimin , Wang Yao , Sui Yueyu , Jiao Xiaoguang. Effects of Freeze-thaw on Soil Nitrogen Conversion: Research Progress. Chinese Agricultural Science Bulletin. 2021, 37(26): 88-92 https://doi.org/10.11924/j.issn.1000-6850.casb2021-0121

0 引言

土壤冻融作用是指在中、高纬度或高海拔及部分温带地区由于气温的变化[1],导致土壤热量也随之变化进而引起土壤水分发生相变,土壤出现反复“结冻—融化”过程,土体收缩或膨胀交替[2],引起土壤结构被破坏和性状发生改变的过程[3]。冻融过程在影响土壤水热条件变化的同时[4,5],还通过影响土壤理化性质、微生物活性发生变化[6],进而影响土壤养分循环与利用[7]。1960年后国际上已陆续展开对于冻土带和季节冻土区土壤冻融生态效应的研究[8]。然而,中国的相关研究相对较晚,对于土壤冻融格局改变对土壤生态效应影响的研究仍十分有限[1,9-10]
氮素是生物体构成的重要元素、是植物生长发育必需的营养元素之一,也是最易消耗和植物生长的限制元素之一[11]。土壤中氮素主要以有机态氮和无机态氮两大类存在,氮总量的95%以上为有机氮,植物根系可直接吸收利用小部分可溶性有机氮,但多数情况下,只有无机氮才能被植物有效吸收利用[12,13]。有机氮与无机氮在土壤中的相互转化即为土壤氮素的转化过程,其转化过程受诸多人为和自然因素干扰,有研究指出冻融作用对于土壤氮素循环起着调控因子作用[14],是土壤氮素循环的主要驱动力,因此冻融格局(冻融频数、结冻持续时间以及结冻强度)的改变对于氮转化均有较大影响。

1 冻融作用对土壤氮矿化的影响

氮矿化过程是氮转化的起始过程,是土壤氮素在微生物的作用下由有机态转化为无机态的过程,对土壤氮素迁移转化以及氮素的生物地球化学循环具有重要意义[13]。全球变暖现象日渐加剧,土壤温度随着大气温度的变化而有所改变。冻融频数、冻结持续时间、冻结强度等冻融格局的改变对中、高纬度或高海拔地区土壤氮素转化过程有何影响也已逐渐成为研究的焦点问题[11,15]

1.1 冻融频数对土壤氮矿化的影响

周幼吾等[16]根据冻融持续周期将土壤状态分为短时冻土、季节性冻土以及隔年冻土、多年冻土。蔡延江等[11]在对去除雪被越冬期原位采样试验中,研究发现,3个月的冻结持续时间对氮矿化速率无明显影响,而结冻持续4~5个月时氮矿化速率显著增加,说明长时间的冻结有利于土壤氮矿化。在较长时间尺度上冻融频次的变化对氮矿化有着强烈的影响[17]。贾国晶等[18]通过对长白山森林土壤的研究发现,在经过35次冻融循环后,相比一次冻融土壤无机氮质量分数在多次冻融后显著提高。土壤中无机氮质量分数随着冻融次数的增加而增加,高频的冻融循环促进了无机氮在土壤中的积累[19]。这与Sulkava等[20]研究结果相似。Amador等[21]研究发现在第一次冻融时,土壤净氮矿化速率比较快,但随着冻融频数的增加净矿化速率逐渐降低,表明较少的冻融循环次数有利于土壤氮的矿化。其原因主要为:(1)同干湿交替或氯仿熏蒸处理结果类似,矿质氮直接来自于土壤微生物细胞破裂的释放[22];(2)部分死亡微生物的残体为余下的微生物活动提供了充足的碳源,激发了微生物活性,有利于土壤有机氮的矿化过程[19];(3)冻融交替使土壤孔隙扩张与收缩交替,最终引起土壤晶格构造发生改变,释放出被固定的铵态氮[23,24]
Hentschel等[25]研究表明,土壤氮矿化速率随着冻融循环频数的增多而增强。而Herrmann[19]在40天内将农田土壤进行了20次冻融循环,发现随着冻融频次的增加,土壤氮矿化速率逐渐下降。胡霞等[26]在苔原生态系统、森林生态系统中也得到了类似的研究结果。净氮矿化速率随冻融频数的增加而减弱可能是由两方面原因导致的:一是较野外原位试验相比,室内的模拟试验涵盖因素尚不全面,缺少植物根系对氮素的吸收利用以及降水导致氮素的淋溶损失,氮矿化速率被土壤中大量积累的矿化氮所抑制[18,21]。此外,冻融作用下部分微生物死亡生物量减少,微生物活性下降,同样会影响氮矿化速率[20]。从而干扰了冻融作用对氮矿化影响效应的长期分析。

1.2 冻融强度对土壤氮矿化的影响

周旺明等[27]通过对湿地土壤氮和氮矿化的研究表明,-5~5℃冻融温差处理的土壤矿化氮累积量低于-25~5℃处理,这与Deluca的研究结果一致[28]。Groffman等[22]和Neilsen等[29]则发现相对较高的冻结温度对土壤氮矿化无影响,冰冻温度为-13℃时氮矿化增强;Larsen等[30]发现氮矿化量在冻结温度为-4~2℃时明显低于未冻结和永久冻结处理。所以重度冻结状态下的土壤产生的氮矿化累积量高于轻度冻结,氮矿化量随着冻融交替的温差波动幅度增加而升高,极端的冷冻温度会增加土壤氮的矿化作用[31]
冻融格局的改变可以对土壤氮的矿化作用产生积极的影响,近期研究表明,冻融频数的增加、冻融强度的增大以及持续时间的增强,均有利于土壤无机氮质量分数的增多和氮矿化速率的加快,但胡霞等[26]在研究中得出了不同结论。因此在研究冻融作用对于土壤氮矿化的影响时,不仅要从氮矿化作用的本身生物学过程考虑[11],还应综合土壤性质,植物类型以及植物根系吸收,土壤微生物状况等环境因素进行分析。

2 冻融作用对硝化作用与无机氮流失的影响

硝化作用是指在好氧环境下,铵离子在硝化微生物作用下转化为亚硝酸盐或硝酸盐的过程。降水淋溶过程是陆地生态系统氮循环过程中无机氮与可溶性有机氮迁移和损失的重要途径[32]

2.1 冻融频数对硝化作用与无机氮流失的影响

宋长春等[33]在对沼泽湿地季节性冻融期CO2、CH4和N2O排放研究时发现,在较高循环次数下可溶性氮组分中仅NH4+-N含量显著增加,而NO3--N含量明显降低。出现这种现象的可能原因是:高频次的土壤冻融循环使土壤水分由液相变为固相,被固定在土壤孔隙中,使土壤中氧气含量减少,土壤处于嫌气状态,好氧微生物活性被抑制,厌氧微生物活性增强,硝化过程被削弱而反硝化过程被促进[34],所以,NO3--N的含量减少了。而且反硝化细菌对低温的耐受程度强于硝化细菌[35,36],只要温度升高土壤开始处于解冻融化状态,反硝化功能就会迅速恢复。因此,在高频数的冻融循环条件下,NO3--N的消耗量增加,冻融循环对硝化过程的促进作用就会被反硝化过程掩盖。
殷睿等[37]研究表明,浅雪被森林土壤在频繁的冻融循环作用下,土壤氮淋溶损失以NO3--N为主,其次为可溶性有机氮、再次为NH4+-N。NO3--N淋溶量高于NH4+-N淋溶量的原因在于带正电荷的NH4+-N易附着于带负电荷的土壤胶体上,而带负电荷的NO3--N更易淋失,故NO3--N淋溶量较高[38,39],因此冬季土壤较强的硝化作用所导致的NO3--N含量的增加也是淋溶液中NO3--N含量高的重要原因。

2.2 冻融持续时间对硝化作用与无机氮流失的影响

徐俊俊等[40]研究发现随着冻融时间的增加,可溶性有机氮、NH4+-N 和NO3--N含量呈先增加后下降的趋势。李源等[23]研究发现4天的短期冻融循环使硝化速率和矿化速率降低,氨化速率提高,NH4+-N含量有所提高。而138天的长期自然冻融提高了矿化速率和氨化速率,在土壤水含量较高时,长时间的反硝化和淋溶作用抑制了硝化作用发生,使NO3--N含量显著降低,NH4+-N含量明显提高。土壤处于长期冻结情况下,土壤中存留的NH4+-N、NO3--N含量均会升高,而剧烈的冻融条件会使这部分氮素的流失加剧[41]

2.3 冻融强度对硝化作用与无机氮流失的影响

冻融强度的波动必将影响到土壤中氮素的矿化和硝化作用[23,39],有研究表明-10℃的弱冻有利于NO3--N含量增加,使NH4+-N含量降低,-20℃强冻使NO3--N增加量显著大于NH4+-N减少量。当温度在10℃以下时,土壤中的氨化速率通常高于硝化速率[42]。强冻可能不是通过促进NO3--N的生成量来增加土壤的矿化作用,而是通过减少NH4+-N的转移量。在内蒙古草原地区土壤冻融过程中氮素矿化的研究发现,与冻结前相比,土壤融化后NH4+-N总量降低了56%,NO3--N总量增加了84%[43]。低温结冻处理后,云杉林和枫榉树林土壤NH4+-N含量均显著提高[44]
不同冻融格局因素的改变对土壤硝化作用影响都不尽相同,随着冻融频数与冻融持续时间的增加,土壤NO3--N含量趋于降低更易淋失;而随着冻融强度的加剧,土壤NO3--N的含量趋于增加。当然土壤中NO3--N含量的改变量不一定能真实的反映出土壤硝化作用的变化,因此,如何明确在冻融过程NO3--N中含量的变化与土壤硝化作用的关系应需进一步研究讨论。

3 冻融作用对反硝化作用与N2O排放的影响

反硝化作用则是指在嫌气环境下,硝酸盐或亚硝酸盐在反硝化微生物作用下被还原为气态NO、N2O和N2的过程[11]。N2O是一种存留时间长、可进入平流层,并引起臭氧的损耗的温室气体。目前多数研究认为冻融循环中反硝化作用是N2O产生的主要途径[1],土壤理化性质、有机质含量、微生物活性以及冻融格局等影响土壤反硝化过程的因素都会影响N2O排放[45]

3.1 冻融频数对反硝化作用与N2O排放的影响

冻融会使N2O气体大量逸出,冻融作用会破坏土壤团粒结构并释放大量营养物质,随着冻融频数增加,微生物虽逐渐适应频繁的冻融交替环境,但其可利用的有机底物和营养物质含量逐渐下降,微生物活性会呈先增强后减弱的规律,所以N2O排放量一般呈先增加后减少的趋势[46,47]。Papen等[48]和Wu等[49]在野外观测研究中发现,当气温缓慢动态上升至0℃以上时,N2O的排放仅小幅增加,而当冻融循环次数增加或温度波动上升明显时,N2O的排放会出现显著的峰值。由此可见,在野外自然条件下,在第一次冻融循环过程中一般不会形成土壤N2O的排放高峰,而是在冻融循环频数增加或温度显著升高时产生。在第一次冻融循环过程中,由于土壤水分的体积增大使土壤孔隙扩张,土壤晶格被破坏其固定的营养元素和一些活性有机物质的释放供微生物利用,从而可以促进矿化和反硝化作用的发生[50]。但多次冻融循环之后N2O的产生与排放量随着土壤本底碳、氮含量及有效性的逐渐降低而减少。

3.2 冻融持续时间对反硝化作用与N2O排放的影响

有研究发现,冻结11天的土壤N2O排放量比冻结3天的土壤N2O排放量要多22%[51],很可能是冰层的存在阻碍了下层土壤N2O的排放并造成累积,所以融化后N2O释放量急剧升高。Luo等[52]通过对德国森林土壤N2O排放动态监测发现,在15年中仅在其中年均土壤温度较低以及冻结期持续约3~4周较长的5年发现全年的N2O排放显著受融循环影响。这与Teepe和Ludwig的模拟试验结果相似,冻结持续时间越长,N2O的排放量越大[53]

3.3 冻融强度对反硝化作用与N2O排放的影响

徐星凯等[54]研究发现-18℃和-80℃冻结处理后冻土融化N2O排放脉冲峰值显著高于-8℃冻结处理,严重冻结后土壤NH4+-N释放量的增加可以刺激硝化作用,使得随后的反硝化作用也随之增强进而引起N2O排放量的增加;Koponen的研究结果表明,冻融交替期间土壤N2O气体的排放量在冻结温度为-15℃时远高于-1.5℃处理下土壤的N2O排放量[55]
冻融格局的改变均促进了土壤反硝化过程提高了N2O排放量,因此我们在有效地确定影响冻融作用对N2O排放机制的主导因素方面仍有很大的挑战。

4 结语与展望

作为普遍存在于中、高纬度或高海拔地区的非生物应力,冻融作用通过改变土壤的水热条件进而影响土壤的理化性质,影响土壤元素的养分循环[54]。氮素作为植物生长发育必需的限制元素之一,土壤冻融作用明显影响其转化过程。本研究从冻融格局改变的角度总结了目前关于冻融作用对于氮转化研究的一般性规律:随着冻融格局的改变,均有利于土壤氮的矿化;冻融强度的增加可显著提高土壤硝态氮的含量;冻融格局的改变也会提高N2O的排放。
目前为止关于冻融作用对于氮转化过程机理研究并不全面。由于15N同位素示踪法、乙炔抑制法等方法的局限性,仅通过测定土壤无机氮的改变量并不能真实反映冻融作用下土壤氮转化的动态特征[11]。冻融作用对于氮素转化的研究室内模拟实验多于野外原位观察,但这其中存在许多局限性。例如:(1)实验土壤脱离原土体,温度变化脱离实际情况,室内实验冻融循环温差大且变化迅速;然而,自然状态下土壤冻融极端温度不会太低且变化缓慢;(2)室内模拟实验一般忽略了在土壤融化时期氮素的径流损失等因素;(3)研究采样地点多集中于中、高纬度地区,而高海拔地区研究相对较少[56,57]
当前,冻融作用对于氮转化作用机理研究尚有不足,得出结论并不全面,对于室内模拟实验与野外原位观察所得出的有关土壤氮转化特征的异同尚不明确,相关研究亟待加强。

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