环境科学  2024, Vol. 45 Issue (4): 2313-2320   PDF    
引用本文
黄艺华, 佘冬立, 史祯琦, 胡磊, 潘永春. 土壤盐分变化对N2O排放影响：基于Meta分析[J]. 环境科学, 2024, 45(4): 2313-2320.
HUANG Yi-hua, SHE Dong-li, SHI Zhen-qi, HU Lei, PAN Yong-chun. Effect of Different Soil Salinities on N2O Emission: A Meta-analysis[J]. Environmental Science, 2024, 45(4): 2313-2320.
土壤盐分变化对N2O排放影响：基于Meta分析
黄艺华1,2, 佘冬立1,2, 史祯琦1,2, 胡磊1,2, 潘永春1,2     
1. 河海大学农业科学与工程学院, 南京 210098;
2. 江苏省农业水土资源高效利用与固碳减排工程研究中心, 南京 210098
收稿日期: 2023-04-16; 修订日期: 2023-07-07
基金项目: 国家自然科学基金项目（42177393, U20A20113）
作者简介: 黄艺华（1998~）, 女, 硕士, 主要研究方向为土壤氮素循环与温室气体, E-mail：huangyihuaedu@163.com
通信作者: 佘冬立, E-mail：shedongli@hhu.edu.cn
摘要: N2O是导致臭氧层空洞和全球变暖的主要大气污染物, 农业生产活动是其主要来源, 而土壤盐分则是影响N2O排放的关键因素. 基于21篇同行评议文献中528对实验组及对照组形成的数据集, 运用R语言Metafor软件包进行Meta分析, 进而评估土壤盐分对土壤N2O排放的影响. 结果表明, 土壤盐分累积对N2O排放量有显著正效应, 中度和高度盐渍土N2O排放量比非盐渍土高75.57%和28.85%. 室内培养实验测定结果表明, 林地和农田的土壤盐渍化导致N2O排放量增加124.79%和131.64%, 而野外定位监测试验结果表明, 草地、裸地和农田中, 土壤盐分对N2O排放的影响均不显著. 盐分对N2O排放的影响趋势则因土壤（NH4+∶NO3-）、pH、土壤砂粒含量和粉粒含量的差异而发生改变, 影响程度依次是：（NH4+∶NO3-）＞pH＞砂粒含量＞粉粒含量. 通过揭示土壤盐分升高对不同土地利用类型下N2O排放的刺激作用, 明确了环境因子和盐分交互作用对N2O排放的影响, 对土壤盐渍化地区N2O排放预防阻控和环境改善具有重要科学意义.
关键词: N2O累积排放量      盐渍化      反硝化作用      土地利用方式      Metafor软件包     
Effect of Different Soil Salinities on N2O Emission: A Meta-analysis
HUANG Yi-hua1,2 , SHE Dong-li1,2 , SHI Zhen-qi1,2 , HU Lei1,2 , PAN Yong-chun1,2     
1. College of Agricultural Science and Engineering, Hohai University, Nanjing 210098, China;
2. Jiangsu Province Engineering Research Center for Agricultural Soil-Water Efficient Utilization, Carbon Sequestration and Emission Reduction, Nanjing 210098, China
Abstract: N2O is the main air pollutant that causes the ozonosphere hole and global warming. Agricultural production activities are the cause of soil N2O production, and soil salinity is the key factor affecting N2O emissions. In order to evaluate the effect of soil salinity on N2O emissions, we used the Metafor software package of R language to analyze the data sets formed by 528 pairs of experimental and control groups in 21 peer-reviewed literatures. The results showed that soil salt accumulation had a significant positive effect on N2O emissions. The N2O emissions of moderately and highly salinized soils were 75.57% and 28.85% higher than those of non-saline soils, respectively. The results of laboratory experiments showed that salt had no significant effect on N2O emissions in grassland and bare land, whereas for forest land and farmland soil, soil salinization led to an increase in N2O emissions of 124.79% and 131.64%, respectively. However, the results of field experiments showed that the influence of soil salinity on N2O emissions was not significant in grassland, bare land, and farmland. The influence trend of salinity on N2O emission was changed by the difference in soil (NH4+∶NO3-), pH, soil sand content, and silt content. The degree of influence of environmental factors on N2O emission was (NH4+∶NO3-) > pH > sand content > silt content. Our research revealed the stimulating effect of increasing soil salinity on N2O emissions from different land use types. We identified the interaction of environmental factors and salinity on N2O emissions. This article has important scientific significance for reducing N2O emissions in salinized land soil and improving the environment.
Key words: N2O cumulative emission      salinization      denitrification      land use pattern      Metafor software package     

N2O是导致臭氧层破坏和全球变暖的主要大气污染物, 虽然与CO2相比其在大气中的含量很低, 但增温潜势却是CO2的298倍[1]. 农业生产活动导致土壤成为N2O排放的主要来源, 而土壤盐分是影响N2O排放的关键因素. 全球约有8.3亿hm2土地受到土壤盐渍化影响[2], 仅我国存在土壤盐渍化问题的土地面积就高达1亿hm2[3], 主要分布于东北、华北、西北及沿海地区. 盐分通过直接影响微生物功能基因丰度[4~8]和土壤阴阳离子组成[9, 10], 从而影响硝化过程和反硝化过程[11], 间接使得生态系统氮循环发生改变. 针对N2O排放, 盐分分别在N2O的生成和逸散过程中体现其影响, 即调节相关酶的活性[12]和N2O在土壤溶液中的溶解度[13].

有研究表明, 低盐渍化水平下, 盐分与N2O排放呈负相关, 且N2O排放量对盐分变化的响应较强烈, 而高盐渍化情况下, 盐分与N2O排放呈正相关, 但N2O排放量变化幅度较小[13]. 而其他研究表明, N2O排放量随着盐分的增加先增加后降低[14]. 此外, 无机氮含量、土壤pH、土壤质地和温度等环境因子是影响N2O排放的直接因素[15], 而在空间变化上, 土地利用类型是影响N2O对盐分变化响应的间接因素[13, 16]. 不同土地利用类型中土壤生物因素与环境因素交互影响, 导致土壤N2O排放过程对盐分变化的响应机制十分复杂. 但目前, 由于各项研究对土壤盐渍化水平的划分不统一, 对土壤盐分梯度变化如何影响N2O排放, 以及环境因子作用下盐分对N2O排放量变化趋势的影响如何等问题还没有达成共识. 因此, 整合同一研究主题的实证分析案例以进行定量总结, 得出N2O排放对盐分变化的总体响应趋势十分必要.

本文通过收集不同土地利用类型和实验条件下盐渍土N2O排放规律的相关研究, 将盐分按照美国盐土实验室标准划分为非盐渍土、轻度盐渍土、中度盐渍土和重度盐渍土, 结合土壤的理化参数进行分析, 确定生物因素和环境因素交互作用下, 土壤盐分变化对N2O排放的作用及其影响机制, 以期为盐渍化地区土壤N2O排放调控提供科学依据.

1 材料与方法 1.1 数据收集

通过Web of Science核心合集和中国知网, 以“盐度（salinity）” “电导率（EC）” “氮（nitrogen）” “N2O” “N（氮）”为关键词, 检索到1989~2023年期间有关盐分对氮素循环影响的文章10 079篇. 所选文献纳入标准如下.

（1）可直接从文本、表格或图片中计算或提取平均值、样本量和标准偏差或标准误差.

（2）为减少误差, 提取不同盐分处理下实验周期大于一个季度的野外定位监测试验和实验周期大于一个月的室内培养实验测定的N2O累积排放量数据或平均值, 并提取实验结束时土壤的NH4+和NO3-含量.

（3）包含盐渍化处理和非盐渍化对照的室内培养实验和野外定位监测试验的N2O排放量数据, 且至少有3个重复, 盐渍化处理包括：①人为向土壤添加梯度盐分水溶液；②长期使用盐水灌溉；③盐分含量高的内陆土壤和沿海湿地土壤. 非盐渍化处理包括：低盐分原状土壤和淡水脱盐土壤.

（4）研究内容不包括施肥和淹水等其他处理. 从文献中获得的数据包括作者、实验地点、实验方法、土壤容重、土壤盐度、土壤水分、实验温度、土壤pH、土壤质地（砂粒、粉粒和黏粒含量）和土壤的NH4+和NO3-含量（对于盐分含量高的原状土壤, NH4+∶NO3-为原始土壤铵态氮和硝态氮含量的比值, 对于人为添加盐分导致土壤盐分含量高的土壤, NH4+∶NO3-为实验培养周期结束后土壤铵态氮和硝态氮含量的比值）、N2O累积排放量和处理组与对照组的样本量（n）. 由于每篇文献的盐度测定方法不同, 盐度表示方法有电导率（EC1∶1、EC1∶1.25、EC1∶1.5）和含盐量（%）等, 根据美国盐土实验室的标准[17]将电导率统一换算成ECe, 将土壤盐化程度划分为非盐渍化、轻度盐渍化、中度盐渍化和重度盐渍化这4个水平（表 1）, 并将列入研究案例的室内培养实验测定的N2O累积排放量（以N计）统一为mg·kg-1, 将野外定位监测试验测定的N2O累积排放量（以N计）统一为kg·hm-2. 在土地利用方式的划分中, 裸地指无植被覆盖的地块, 草地指草本植物群落覆盖的地块, 农田指长期种植农作物的地块, 林地指木本植物群落覆盖的地块（表 2）. 通过Web Plot Digitizer 4.6软件对文献中的图表信息进行数字化, 最后累计纳入文献共计21篇（表 2）, 包括528对处理组和对照组数据.

表 1 土壤盐化分级标准 Table 1 Soil salinization grading standards

表 2 研究案例主要信息 Table 2 Overview of the studies used for analysis

1.2 数据分析

以非盐渍化水平下的N2O累积排放量为对照, 其他3个盐分水平为处理, 使用R 4.2.0软件Metafor包[34]的escalc命令计算盐分对N2O累积排放量影响的效应值（yi）和案例内方差（vi）. 用最大似然法估算案例间方差（τ2）, 通过案例内和案例间方差对每个研究案例赋予权重（wi）, 通过随机效应模型计算累积效应值（y）. 公式如下：

(1)
(2)

式中, ye、se和ne为处理组的均值、标准差和样本量, yc、sc和nc为对照组的均值、标准差和样本量.

(3)
(4)
(5)

式中, wi为单个研究的权重, y为累积效应值, 95% Cl为yi的95%置信区间[如果95%的置信区间不与零重叠, 则认为具有显著性差异（P＜0.05）, 反之不显著].

使用Higgins I2统计和Q检验评估数据的异质性程度, 当Qt和I2越大, 则yi越离散, 数据的整体异质性越高, 表明有其他因素造成了这种偏离, 需要引入解释变量, 用混合效应模型计算解释变量对效应值的影响, 当数据均质时, Qt服从K-1的卡方分布, 则不需要引入解释变量. 用Qm评估解释变量对效应值的影响程度, Qm在P＜0.05时解释变量对效应值有显著影响. 计算公式如下：

(6)
(7)
(8)
(9)

式中, Qt为数据的整体异质性, Qm为由某一已知因素引起的异质性, Qe为未知因素的残留异质性, yj为解释变量的水平为j时的累积效应值, wi为对应权重.

采用SPSS进行偏相关分析以确定盐分和非生物因素之间的相关度. 在3个变量中, 任意两个变量之间的偏相关系数是在控制剩下的一个变量的作用后计算得到的, 称为一阶偏相关系数, 公式如下：

(10)

式中, rxy（z）为控制变量z时, 变量x与y的偏相关系数, rxy、rxz和ryz分别为变量x与y、x与z和y与z之间的简单相关系数.

在4个变量中, 任意两个变量之间的偏相关系数是在控制其余两个变量的影响后计算得到的, 称为二阶偏相关系数, 二阶偏相关系数是由一阶偏相关系数求得, 公式如下：

(11)

式中, rxy(z1z2)为控制变量z1和z2时, 变量x与y的偏相关系数, rxy(z1)为控制变量z1时, 变量x与y的一阶偏相关系数, rx(z2z1)为控制变量z1时, 变量x与z2的偏相关系数, ry(z2z1)为控制变量z1时, 变量y与z2的偏相关系数.

存在3个或更多控制变量时, 通过类推计算x和y的偏相关系数.

2 结果与讨论 2.1 盐分对N2O排放的影响

通过随机效应模型计算得到, 盐分对N2O排放量影响的累计效应值为0.28[P = 0.001 2, Cl：（0.11, 0.45）], Qt = 16 572.8, τ2 = 0.76, I2 = 99.46%[图 1（a）], 表明盐分对N2O排放有显著正影响且每个案例之间呈现显著异质性, 需要引入解释变量分析其他环境因子对效应值的影响. 通过亚组分析表明, 与非盐渍化相比, 轻度盐渍化对N2O排放量影响不显著, 中度和重度盐渍化使得N2O排放量分别提高75.57%和28.85%[计算公式：(ey-1)×100%[35], 图 1（b）].

（a）中的灰色虚线为0效应值, 红线表示盐渍土N2O累积平均效应值 图 1 不同盐分水平对N2O排放的影响 Fig. 1 Effects of different salinity levels on N2O emission

在本研究中, 随机效应模型中Qt的P值显示盐分和N2O排放量呈现极显著相关[图 1（a）]. 盐分变化引起土壤N2O排放过程的差异一方面归结于盐离子化学作用改变了土壤中NH4+吸附和释放过程, 调节了NH4+可利用性, 以及改变了N2O在土壤中的溶解度. 同时, 更多学者将盐分对N2O排放过程的影响归因于硝化-反硝化微生物对盐分胁迫的耐受性差异. Li等[36]研究发现, 盐渍土和非盐渍土细菌群落组成和丰度存在显著差异, 在盐渍土壤中, 亚硝酸盐还原微生物群落的丰度增加导致N2O排放量增加[亚硝酸盐还原过程由nirS基因编码的亚硝酸盐还原酶（cd1⁃nir）和含Cu的nirK基因编码的亚硝酸还原酶（Cu⁃nir）催化[37]]. Han等[38]研究发现, 土壤含盐量从＜0.5%增加到0.7%时, 土壤N2O还原酶的应激细胞亲和力显著降低, N2O排放量增加.

2.2 不同土地利用方式下盐分对N2O排放的影响

室内培养实验测定结果表明, 草地和裸地土壤中盐分变化对N2O排放影响不显著, 而林地和农田的土壤盐渍化导致N2O排放量增加124.79%和131.64%[计算公式：(ey-1)×100%, 图 2（a）]. 有研究表明, 林地土壤中盐分含量增高会促进硝化作用[39]；Chauhan等[40]研究也表明, 林地土壤中盐度为23.53 dS·m-1时, 亚硝酸盐氧化细菌（nitrite-oxidizing bacteria, NOB）和氨氧化细菌（ammonia-oxidizing bacteria, AOB）的丰度分别为0.24×104 g-1和0.12×104 g-1, 而盐度为13.09 dS·m-1时, NOB和AOB的丰度分别为0.12×104 g-1和0.04×104 g-1, 土壤硝化细菌数量随土壤盐分含量增加显著增加. 因此, 林地土壤中盐分提高导致N2O累积排放量增加的主要原因是硝化作用的增强. Ghosh等[12]研究发现, 盐渍化农田土壤N2O排放量比非盐渍化农田提高42.2%, 且盐渍化农田土壤N2O排放量主要归因于硝化-反硝化共同作用[41]. Li等[14]研究发现, 盐渍土NO2-含量高于非盐渍土, 盐渍土中以NO2-为底物的硝化细菌反硝化过程受到促进, 导致N2O排放量增加.

括号中的数值为样本量（n） 图 2 不同土地利用方式下盐分对N2O排放的影响 Fig. 2 Effects of salinity on N2O emissions under different land use patterns

野外定位监测试验结果表明, 草地、裸地和农田中, 盐分对N2O排放的影响均不显著[图 2（b）]. 有研究表明, 室内培养实验对单一变量进行控制且控制较为精准, 而野外定位监测试验由于样地占地面积大等因素不能准确控制单一变量[42], 导致土壤有机氮、有机碳、pH和土壤机械组成等不一致, 而这些因素共同影响N2O产生过程. 此外, Helton等[43]研究表明, 在室内培养实验中, 随盐分升高, NH4+在土壤孔隙水中累积, 进而导致N2O排放量增加, 而在野外定位监测试验中, 土壤NH4+含量随盐分升高而降低, 导致N2O排放随之降低.

2.3 影响盐渍化土壤N2O排放的环境因素分析

通过SPSS进行偏相关分析并利用混合效应模型对盐分和环境因子交互作用进行分析, 发现盐分对N2O排放的影响受土壤的NH4+∶NO3-、pH、土壤砂粒含量和粉粒含量影响, 影响程度依次是：（NH4+∶NO3-）＞pH＞砂粒含量＞粉粒含量（表 3、表 4和图 3）.

表 3 效应值（yi）影响因素的偏相关系数 Table 3 Partial correlation coefficient of influencing factors of effect size (yi)

表 4 效应值（yi）与环境因子之间的关系 Table 4 Relationships between effect size (yi) and environmental factors

圆点大小为单组数据的权重（wi）大小, 灰色阴影部分为95%置信区间带, 黑线为数据点的线性拟合 图 3 效应值与环境因子之间的关系 Fig. 3 Relationship between effect size and environmental factors

盐分对N2O排放量影响的效应值随NH4+∶NO3-增大而显著增大, 即随NH4+∶NO3-增大, 盐分对N2O排放促进作用增大（表 4和图 3）. 有研究发现, 土壤NH4+含量升高为高盐土壤N2O产生过程提供底物[44], 且添加NH4+比添加NO3-对N2O产生的促进作用更强[45], 从而导致NH4+∶NO3-与N2O排放量呈线性正相关关系[26]. Xie等[46]研究表明, 盐分升高加速NH4+从土壤中释放, 同时导致参与异化硝酸盐还原为铵, 亚硝酸还原和反硝化过程相关细菌的基因丰度增加, 促进过程1和过程2, 从而导致NH4+增加, NO3-降低和N2O累积排放量增加.

盐分对N2O排放量影响的效应值随土壤pH增大而增大（表 4和图 3）. 这是由于pH升高时, 受盐碱胁迫影响, N2O还原酶活性受到抑制[47], 导致N2O还原酶竞争电子的能力变弱, 当电子供体不足时, N2O无法被还原, N2O排放量增加[48]. Duan等[49]研究表明, 自养硝化过程产生的N2O与pH呈现指数正相关关系, pH升高时土壤NH4+生成速率增大, 而较高的NH4+含量为自养硝化作用提供了底物. 此外, pH升高导致土壤AOB amoA基因丰度增高, 从而导致N2O排放量增加[50].

盐分对N2O排放影响效应值随土壤砂粒含量增多而增大, 随粉粒含量增多而减小（表 4和图 3）. 表明随着砂粒含量增多, 盐分对N2O排放促进作用增强, 而随着粉粒含量增多, 盐分对N2O排放表现为抑制作用. 土壤质地通过影响土壤氧气有效性、阳离子交换量和与N2O产生相关的微生物活性等, 进而影响N2O排放. Zhou等[51]在高砂粒含量土壤中研究表明, 土壤盐渍化显著促进氮矿化速率, 导致土壤NH4+增加11.7%, 为N2O产生提供底物, 从而增加高砂粒含量土壤的N2O排放. 此外, 也有研究表明真菌反硝化作用是引起受盐影响的高砂粒含量土壤产生N2O的主要过程, 由于真菌反硝化系统缺乏nosZ基因, 不具备将N2O还原为N2的能力, 使其能产生更多N2O. 粉粒含量高的土壤气体扩散率和氧气有效性均较低, 提高了N2O还原为N2的速率, 同时抑制自养硝化作用对N2O产生的贡献, 使N2O排放量较低[32, 52]. 此外, 粉粒含量高的土壤有较低的NO3-还原率和较高的阳离子交换量, 土壤能吸附较多的NH4+, 从而降低通过硝化过程产生N2O的底物来源[53].

3 结论

（1）通过Meta分析发现盐分促进N2O排放, 不同盐分水平对N2O排放的影响程度为：中度盐渍化＞重度盐渍化＞轻度盐渍化.

（2）室内培养实验结果表明, 林地和农田的土壤盐渍化导致N2O排放量增加124.79%和131.64%, 而野外定位监测试验结果表明, 草地、裸地和农田中, 盐分对N2O排放的影响均不显著.

（3）（NH4+∶NO3-）、pH、砂粒和粉粒含量与盐分交互作用共同影响N2O排放, 这些因素对效应值的影响程度依次是：（NH4+∶NO3-）＞pH＞砂粒含量＞粉粒含量.（NH4+∶NO3-）、pH、砂粒含量增加时, 盐分升高促进N2O排放. 粉粒含量增加时, N2O排放受到抑制.

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