2024, Vol. 45
2. 河北省农田生态环境重点实验室, 保定 071001;
3. 河北大学化学与材料科学学院, 保定 071002;
4. 河北大学生态环境系, 保定 071002;
5. 河北省山区农业技术创新中心, 保定 071001;
6. 河北农业大学河北省山区研究所, 保定 071001
, WANG Wan-qing1,2 , RONG Sa-shuang3 , LI Yu-xin4 , WANG Xin-xin5,6 , LIANG Pan1,2 , LIU Wei3
2. Key Laboratory for Farmland Eco-Environment of Hebei, Baoding 071001, China;
3. College of Chemistry and Materials Science, Hebei University, Baoding 071002, China;
4. School of Eco-Environment, Hebei University, Baoding 071002, China;
5. Hebei Agricultural Technology Innovation Center, Baoding 071001, China;
6. Mountain Area Research Institute of Hebei Province, Hebei Agricultural University, Baoding 071001, China
在过去的几十年里, 塑料制品由于性质稳定和成本低廉已广泛应用于农业生产. 我国是农膜消费大国, 统计显示2012年我国农膜使用量约为238.3万t, 覆盖面积1 758.2万hm2；2017年农膜使用量达252.8万t[1]. 由于回收困难, 每年大约有18.6%的农膜残留在农田土壤[2]. 残留在土壤的农膜碎片在光照和微生物等多因素的综合作用下不断老化破碎形成微塑料（microplastics, MPs）. 在第二届联合国环境大会上, MPs污染被列入环境与生态科学领域第二大科学问题[3], 成为与全球气候变化和臭氧耗竭并列的重大全球环境问题. 聚乙烯是乙烯经加成聚合得到的一种热塑性树脂, 以聚乙烯树脂为基材, 添加少量抗氧化剂、爽滑剂和增塑剂等塑料助剂后造粒制成的塑料称为聚乙烯塑料[4]. 聚乙烯塑料是需求量最高的塑料类型, 约占总量30%, 主要用于生产农膜、包装袋和各类中空容器[5]. 聚乙烯塑料结晶区排列紧密, 对微生物和酶具有较大的空间位阻, 并且聚乙烯塑料C—C骨架中无亲水基团, 以上因素均使其在土壤中降解较为困难[6], 易积累. 根据北京[7]、山东[8]、韩国[9]和瑞士[10]等地的农田MPs污染情况可知, 聚乙烯是农田最常见的塑料类型并且数量较多.
土壤氮素是植物生长的必需营养元素之一, 是构成蛋白质的主要成分, 与作物产量和品质密切相关, 也是土壤肥力中最活跃的元素. 在有氧条件下, 铵态氮在氨氧化细菌或古菌作用下, 先氧化成亚硝态氮再经硝酸杆菌作用转化为硝态氮. 低氧条件下, 土壤中的硝态氮在反硝化细菌的作用下, 经硝酸还原、亚硝酸还原、一氧化氮还原和氧化亚氮还原最终产生氮气和含氮的中间产物. 可见, 土壤中多种氮素形态之间的转化依赖于微生物驱动, 参与氮转化的微生物种类、数量和活性以及功能基因编码的多种关键酶的酶促反应控制着氮素转化的过程和快慢, 影响氮转化的终产物[11]. 近期研究表明, 聚乙烯微塑料（polyethylene microplastics, PE-MPs）可通过物理、化学和生物等多种途径改变土壤物理化学性质影响微生物的生存环境[12, 13]. 同时, MPs亦可作为碳源[14, 15]、提供生态位[16, 17]和促进微生物的表面定殖, 直接影响土壤微生物的群落结构[18], 从而影响氮素转化过程. 如Huang等[19]研究发现低密度聚乙烯（low density polyethylene, LDPE）可能通过干扰土壤中的微生物群落组成和脲酶活性影响土壤氮循环过程. 还有研究指出PE-MPs增加nirS的基因丰度, 显著促进N2O的排放[20].
尽管当前已有成果对MPs积累下土壤碳氮磷元素循环动态进行了归纳和总结, 但对MPs影响元素循环的关键因子和机制研究的综述仍非常有限. 聚乙烯作为农膜的主要类型, 数量多, 残留时间长, 对土壤功能微生物种群结构和生物活性存在长期影响和关键基因改变作用, 尤其PE-MPs污染土壤的氮转化过程值得关注. 本文基于国内外研究结果, 总结了PE-MPs影响氮转化的关键因素（土壤理化性质、氮转化相关微生物和酶等途径（图 1）, 以期为PE-MPs污染土壤中氮素营养调控和生态风险评估提供参考.
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图 1 PE-MPs影响氮转化的主要方式 Fig. 1 PE-MPs influence pathways of nitrogen transformation |
土壤团聚体是微生物的重要栖息地, 不同粒径的土壤团聚体具有独特的理化性质, 形成了明显分化的微生物生态位, 决定氮转化相关微生物的分布与活性[21, 22]. 粒径0.25~2 mm的团聚体由于具有合适的微环境是氨氧化的热点区域；粒径在0.053~0.25 mm的微团聚体中反硝化细菌丰度最高；固氮螺菌属（Azospirillum）在 < 0.053 mm团聚体丰度最高[23, 24]. 土壤中MPs的存在可能会改变土壤颗粒的结合机制, 使得团聚体的数量和粒径重新分配[25~27], 影响氮转化相关微生物的丰度和分布. 例如, 0.1%的高密度聚乙烯微塑料（high density polyethylene microplastics, HDPE-MPs）明显降低土壤中粒径 > 2 mm和粒径 < 0.063 mm水稳性团聚体的比例, 显著增加粒径为0.063~0.25 mm水稳性团聚体数量, 其中土壤中粒径 > 2 mm水稳性团聚体数量对照比处理增加60%[28]. PE-MPs抑制小团聚体向大团聚体的聚集, 并且表现出一定的剂量效应. 陈荣桓等[29]研究表明粒径 > 2 mm的水稳性团聚体的比例随着LDPE-MPs含量的增大而减少, 而 < 0.25 mm的水稳性团聚体的比例随着LDPE-MPs含量的增大而增加. 有机碳是水稳性团聚体的重要粘合剂[30], PE-MPs对大团聚体产生的负面影响可能和土壤有机碳含量的降低有关[31]. 此外PE-MPs还可能通过在团聚体中引入断裂点, 影响土壤团聚体稳定性, 并表现出粒径效应和形状效应. 如膜状和泡沫状PE-MPs均降低土壤团聚体数量, 膜状LDPE-MPs随含量（0.1%、0.2%、0.3%和0.4%）的增加减少土壤团聚体数量, 而泡沫状LDPE对土壤团聚体数量的影响表现为随着土壤中泡沫状LDPE含量的增加, 土壤团聚体的数量也表现出增加的趋势[27]. 因此, PE-MPs可能直接嵌入土壤微结构降低团聚体的稳定性, 也可通过影响土壤有机碳的含量间接影响土壤团聚过程. 研究团聚体与氮转化相关的功能基因关系发现, 不同粒径团聚体的氨氧化基因（amoA、amoB和amoC）对PE-MPs响应不同, PE-MPs增加了大团聚体（> 0.25 mm）的氨氧化基因丰度, 降低了微团聚体（0.1~0.25 mm）和小团聚体（< 0.063 mm）的氨氧化基因丰度[32].
PE-MPs通过改变土壤孔隙大小和通气透水性影响厌氧和好氧微生物的相对丰度及其种间作用关系, 改变氮素转化进程. 一般而言, 土壤孔隙大小和分布受PE-MPs粒径和含量的影响, 大粒径（1 000 μm）PE-MPs主要影响土壤大孔隙（> 9 μm）和有效孔隙（0.2~9 μm）；小粒径（25 μm）PE-MPs可作用于土壤微孔隙（< 0.2 μm）. ω（PE-MPs）为0.02%、0.05%和0.5%时土壤大孔隙（> 9 μm）、有效孔隙（0.2~9 μm）和微孔隙（< 0.2 μm）所占比例显著增加；而ω（PE-MPs）为5%时上述3种孔隙所占比例显著下降[33]. PE-MPs对土壤孔隙的影响一定程度上也与土壤自身结构的稳定性有关, 砂土等结构性较差的土壤对PE-MPs的响应更明显[34]. PE-MPs引起的土壤孔隙变化直接影响土壤含水量和持水性能. 例如, PE-MPs在土壤中积累可能堵塞孔隙, 造成水分积累[35, 36]. 王志超等[37]向土柱中添加质量分数为0.5%、1.0%和2.0%的PE-MPs, 所有深度的土壤含水率均表现出随PE-MPs含量的增加而增加. PE-MPs还可能通过增加水分运移通道促进水分蒸发降低土壤含水量, 并且大粒径的PE-MPs效应更明显[38]. 就持水性能而言, PE-MPs含量较低的处理（0~0.5%）能增强土壤的持水能力, 小粒径（9 μm）的PE-MPs提升效果更明显；PE-MPs含量较高的处理（0.5%~5%）显著降低土壤的持水能力, 大粒径（1 000 μm）的PE-MPs降低效果更明显[33]. 而Huang等[39]研究发现LDPE-MPs可在不改变土壤总孔隙体积情况下, 降低土壤水分容量, 促进土壤硝化作用. 总之, PE-MPs通过影响土壤孔隙的形成, 增加水分运移通道, 堵塞孔隙等途径影响土壤含水率及持水性能, 且PE-MPs的添加量及粒径是关键因子.
土壤pH值是影响土壤微生物群落结构和分布的重要化学因子[40], pH高低直接影响元素的生物可利用性及其引起的微生物毒性效应, 改变氮循环关键基因丰度, 影响氮素循环过程[41]. 如表 1所示, HDPE-MPs和LDPE-MPs在土壤中累积均可影响土壤pH值. 由于HDPE-MPs自身老化过程中释放酸性降解产物, HDPE-MPs进入土壤可降低土壤pH值[28, 42]. Bandow等[43]研究发现HDPE颗粒在光热条件下氧化后, 淋洗液的pH值明显降低. ω（HDPE-MPs）为0.1%时土壤pH值较对照下降0.61[28], 随着HDPE-MPs含量的增加土壤pH下降越明显[44]. 此外, 不同粒径土壤团聚体的pH值对HDPE-MPs添加亦有差异响应, ω（HDPE-MPs）为28%时, 粒径为0.25~2、0.053~0.25和 < 0.053 mm团聚体的pH值较对照分别降低9.7%、14.3%和30%[42]. 对于LDPE-MPs而言, Zhao等[45]和Gao等[46]的研究结果表明LDPE-MPs处理的土壤pH值较对照显著升高. 如ω（LDPE-MPs）为6%和18%的土壤pH值较对照分别增加0.18和0.28[46]. 这可能是由于部分聚乙烯塑料以CaCO3和Na2CO3等物质为填料, 随着聚乙烯塑料的环境老化, 释放出碳酸根离子在土壤溶液中水解, 使土壤pH值升高[47]. 此外LDPE-MPs诱导的土壤pH变化可以部分归因于对土壤生物区系的扰动, 如LDPE-MPs可以改变氨氧化细菌的丰度和硝化过程, 该过程释放H+, 进而影响土壤pH[48]. 此外, 土壤类型也会影响土壤pH值对PE-MPs的响应. Li等[49]研究表明0.2%的PE-MPs添加显著降低红壤pH, 升高水稻田pH, 对潮土pH无显著影响. 总之, PE-MPs对土壤pH的影响是多种因素共同作用的结果, 降解产物以及添加剂的释放可能是PE-MPs影响土壤pH的主要因素.
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表 1 PE-MPs对土壤pH值的影响 Table 1 Effects of PE-MPs on soil pH value |
氮转化微生物中反硝化微生物为异养微生物, 需要通过氧化有机碳得到电子和碳源以保证反硝化作用的顺利进行, 因此反硝化强度常表现出与可溶性有机碳（dissolved organic carbon, DOC）含量呈正相关[50], 硝态氮与DOC的比例变化改变氮的转化过程[51]. 研究推测, 进入土壤的PE-MPs可能通过增加土壤DOC的含量改变微区碳循环, 间接影响氮转化相关微生物群落结构和功能. 聚乙烯塑料的碳含量约为90%, 其中0.11%~0.48%的碳以DOC的形式被释放[52]. 向土壤中添加LDPE-MPs可使DOC含量产生波动, ω（LDPE-MPs）为1%时, 土培15 d和22 d后, 土壤DOC含量分别较对照增加20.35%和23.56%[53]. Wang等[54]向土壤中添加0.1%、1%和10%的PE-MPs也得到了相似的结论, 与对照相比DOC含量显著增加了7.12%~27.1%. 而Ren等[55]研究表明当ω（PE-MPs）为5%时, 土壤DOC含量无明显影响, 但改变了DOC的组成, 更有利于芳香族官能团的形成. PE-MPs对土壤DOC含量的影响可部分归因于PE-MPs通过增加土壤的孔隙度增加土壤有机碳与空气、水分和微生物接触的机会, 促进土壤有机质矿化释放更多的DOC[56]. 此外PE-MPs本身较高的碳含量可能对土壤DOC含量有一定的贡献.
2 PE⁃MPs对土壤氮转化关键微生物的影响土壤中MPs作为外源疏水性基质, 是微生物特殊的栖息地, 可形成微生物生长繁殖独特的生态位[61], 改变氮循环过程[19]. 已有研究证明, PE-MPs进入土壤后细菌网络的复杂性和模块性发生明显变化, 微生物的周转时间加快, 参与土壤氮循环特定类群的相对丰度选择性改变[62~64]. 如LDPE-MPs污染土壤, 微生物群体表现出更高的氮元素和硫元素的循环代谢能力, 反硝化能力也有明显加强[65]. MPs具有粒径小和比表面积大的特点, 会吸附土壤介质中有机无机营养物质, 成为真菌、细菌和病毒等微生物的聚集地, 进一步形成区别于广域土壤的“塑料际”[66]. 由于“塑料际”极端的碳氮比, 微生物可能通过增加氮输入降低氮损失来缓解氮限制. Bryant等[67]研究发现“塑料际”具有更高的nifH基因丰度. Hu等[68]研究结果也表明PE-MPs表面显著增加了编码固氮基因nifA/S、编码氮水解ureC/GDH2和编码同化还原为铵的nasA/D和nirA/B的平均丰度, 降低了部分反硝化基因的平均丰度.
如表 2所示, PE-MPs对土壤氮转化相关微生物的影响具体表现为：LDPE-MPs污染土壤中显著富集亚硝化细菌门（Nitrospirae）、变形菌门（Proteobacteria）和酸杆菌门（Acidobacteria）的相关微生物[19]. 硝化螺菌门（Nitrospirae）是亚硝酸盐氧化菌, 可将亚硝酸盐氧化为硝酸盐, 在土壤硝化过程中起着主要作用[69]. 大多数氨氧化细菌均为变形菌门（Proteobacteria）的成员, 可以催化铵离子转化为亚硝酸离子. 酸杆菌门（Acidobacteria）众多成员具有催化有机氮和无机氮代谢的基因, 可以有效减少硝酸盐及亚硝酸盐积累[70]. LDPE-MPs明显增加了土壤中伯克氏菌科（Burkholderiaceae）[71, 72]和生丝微菌属（Hyphomicrobium）的相对丰度[65]. ω（LDPE-MPs）为7%时, 显著降低土壤固氮弓菌属（Azoarcus）的相对丰度[48]. 伯克氏菌科（Burkholderiaceae）、生丝微菌属（Hyphomicrobium）和固氮弓菌属（Azoarcus）微生物在土壤氮素硝化和固氮等生化过程中发挥重要作用[73, 74]. 土壤中LDPE-MPs的存在也会影响反硝化细菌的丰度. Rong等[48]和Sun等[65]研究发现LDPE-MPs的添加可富集反硝化细菌类固醇杆菌属（Steroidobacter）、芽孢杆菌科（Bacillaceae）和红螺菌科（Rhodospirillaceae）的微生物；降低慢生根瘤菌属（Bradyrhizobium）的相对丰度[75]. 反硝化微生物噬纤维菌科（Cytophagaceae）的丰度随LDPE-MPs含量的增加先增加后减少, ω（LDPE-MPs）为0.1%、0.5%和1%时, 其丰度明显高于对照[46]. HDPE-MPs促进放线菌门（Actinobacteria）微生物的富集, 并表现出一定的剂量效应, HDPE-MPs含量较高时微生物响应更明显[18]. 芽单胞菌属（Gemmatimonas）含有与反硝化相关的功能基因（如nirS和nosZ）, 可能是一种反硝化细菌[76]. 有研究表明, HDPE-MPs降低芽单胞菌属（Gemmatimonas）、慢生根瘤菌属（Bradyrhizobium）和固醇杆菌属（Steroidobacter）等可以催化反硝化作用菌群的相对丰度[58]. 综上可知, PE-MPs进入土壤, 相对丰度发生明显变化的主要是与土壤固氮和反硝化作用相关的微生物. PE-MPs污染土壤较高的碳氮比刺激固氮作用以缓解氮限制可能是土壤固氮微生物相对丰度发生明显变化的原因之一. PE-MPs对反硝化细菌相对丰度的影响可能部分归因于MPs可以为反硝化细菌提供生态位和PE-MPs污染土壤较高的碳含量更有利于反硝化细菌的富集. 事实上, 微生物之间的相互作用关系极其复杂, 未来的研究可以通过构建土壤氮转化微生物的功能分子生态网络, 揭示MPs污染对土壤氮转化微生物网络结构的影响.
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表 2 PE-MPs对土壤氮转化相关微生物的影响 Table 2 Effects of PE-MPs on nitrogen function microorganism |
3 PE⁃MPs对土壤氮转化功能基因及关键酶活性的影响
土壤微生物携带nifH、amoA、amoB、nirK和nirS等参与土壤氮转化过程相关的功能基因, 分别在土壤的固氮、硝化和反硝化过程中扮演着重要角色. 其中nifH主要用来编码钼铁固氮酶的电子转移蛋白, 是检测环境中具有固氮作用微生物的标记基因[77]. amoA和amoB编码的氨单加氧酶, 是硝化作用的主要限速酶[78]. nirK和nirS编码的亚硝酸还原酶, 主要控制亚硝酸盐向一氧化氮转化, 是反硝化过程主要的限速酶[79].
PE-MPs的存在改变了与氮转化相关的功能基因丰度. Rong等[48]研究发现LDPE-MPs进入土壤初期显著增加nifH的基因丰度, amoA的基因丰度表现出先降低后增加的趋势, nirS的基因丰度在培养的7 d和15 d显著高于对照, nirK的基因丰度在培养的90 d显著高于对照. 不同地区土壤由于pH和微生物群落结构等差异, 对PE-MPs的响应也有所不同. 如Yu等[20]探究了不同地区水稻土的氮转化过程对PE-MPs的响应, 发现PE-MPs显著增加了江西和江苏地区水稻土amoA的基因丰度以及湖南、江西和江苏这3个地区水稻土nirS的基因丰度. 此外, 也有研究表明PE-MPs会抑制nirS基因丰度. Gao等[46]将HDPE-MPs添加到酸性土壤中发现：amoA和nirS的基因丰度显著降低, ω（HDPE-MPs）为18%时, amoA和nirS基因丰度分别降低34.13%和38.46%.
细菌群落是土壤中酶的主要生产者, 而酶是土壤中生化反应的关键催化剂[80, 81]. 土壤酶活性变化一方面可反馈功能微生物的活性, 另一方面决定土壤生态系统中酶促反应的进行速度[82]. PE-MPs在改变土壤群落结构的同时也在影响土壤酶活性, 最终影响氮循环的速度和方向. 荧光素二乙酸酯酶属氧化还原酶类, 在一定程度上荧光素二乙酸酯酶可以间接反映氮转化相关微生物活性. PE-MPs存在会抑制土壤中荧光素二乙酸酯酶的活性[71, 83]. 如Yi等[83]研究发现, ω（PE-MPs）为2%时, 在培养的14 d和29 d荧光素二乙酸酯酶的抑制率分别为26%和21%. 而Wang等[58]研究结果表明HDPE-MPs显著增加土壤荧光素二乙酸酯酶活性, 添加量10%的处理较1%的处理效应更明显. 土壤脲酶是催化有机氮转化为铵的关键酶, 其活性高低可以反映土壤中有机态氮素的周转过程[84]. PE-MPs可通过提高土壤脲酶活性, 促进氮素的矿质化. Huang等[19]研究表明LDPE-MPs可提高土壤中脲酶的活性, 在培养15 d和90 d后, 脲酶活性分别较对照提高234%和175%. Yang等[57]研究表明HDPE-MPs粒径越小添加量越高, 土壤脲酶活性提高越明显. Shi等[85]研究发现PE-MPs除了可以显著提高脲酶活性之外, 作用于反硝化过程第一步的硝酸还原酶活性也显著增加. 脲酶和硝酸还原酶等与氮转化相关的关键酶均会对PE-MPs的添加做出响应, 响应程度可能与PE-MPs的粒径和添加量有关. 总之, PE-MPs的存在影响荧光素二乙酸酯酶活性, 提高土壤脲酶和硝酸还原酶的活性.
4 邻苯二甲酸酯类物质对土壤氮转化的影响邻苯二甲酸酯类（phthalates, PAEs）是塑料生产过程中用以增加塑料制品的柔韧性、可塑性和弹性的塑料添加剂[86]. 其中邻苯二甲酸二（2-乙基己基）酯[bis（2-ethylhexyl）phthalate, DEHP]和邻苯二甲酸二丁酯（dibutyl phthalate, DBP）是塑料生产中常用的增塑剂, 具有较高的检出率[87, 88]. Xu等[89]对聚乙烯塑料包装袋中的PAEs的调查结果表明, ω（PAEs）范围为11.16~309.70 mg·kg-1. 由于PAEs与石油基之间仅通过范德华力和氢键连接, PAEs极易向环境中释放. Wang等[90]研究发现, 聚乙烯农膜残留30 d后土壤中ω（PAEs）仍能达到5.72 mg·kg-1. 值得注意的是, 随着塑料制品在环境中的不断老化, 塑料表面的扩散路径更短, MPs更容易释放PAEs[91]. MPs进入土壤后, PAEs会随着聚乙烯塑料的老化破碎逐渐释放到环境中, 并影响氮转化相关微生物及土壤氮循环关键酶活性[92]. 已有研究表明PAEs可以改变土壤氮转化过程的关键微生物、关键酶活性和关键基因丰度. 例如Wang等[93]研究发现当ω（DEHP） > 100 mg·kg-1时, 具有较强硝化反硝化功能的假单胞菌科（Pseudomonadaceae）明显增多, 群落结构显著改变. ω（DEHP）为200 mg·kg-1的酸性土壤明显富集了慢生根瘤菌属（Bradyrhizobium）和芽孢杆菌科（Bacillaceae）以及与固氮作用相关的固氮弓菌属（Azoarcus）的微生物[94]. 碱性土壤中DEHP和DBP的残留对伯克氏菌科（Burkholderiaceae）也有明显的富集[95]. DEHP添加下调了硝化基因（amoA）和反硝化基因（norB、nirK和nosZ）的丰度, 代谢通路分析高水平PAEs可以降低固氮、硝酸盐异化还原和反硝化过程中的基因丰度[96]. 事实上MPs向土壤中释放的PAEs可能是MPs影响土壤氮转化过程的重要因子. 首先, 相较于石油基, PAEs是微生物更容易利用的碳源. PAEs在土壤中降解, amoA的基因丰度降低, 硝化细菌的活性减弱, 对硝化作用产生竞争性负面影响[97]. 此外Zhu等[98]通过添加PAEs的聚氯乙烯微塑料（polyethylene microplastics, PVC-MPs）和不含PAEs的PVC-MPs的对照试验证明了PVC-MPs释放的PAEs是PVC-MPs影响氮转化的主要驱动因素. 由于PAEs也大量应用于聚乙烯塑料的生产, 因而PE-MPs对土壤氮转化的影响可能和PAEs的释放密切相关. 除此之外, 由于MPs自身具有比表面积大、疏水性强和表面结构复杂等特点, MPs表面容易吸附部分疏水性的有机物污染物协同影响氮转化功能微生物的丰度[99, 100]. 比如Bakir等[101]研究发现MPs对DDT（dichloro diphenyl trichloroethanes, DDT）有较高的吸附能力, 有利于DDT在环境中积累, 而DDT可使硝化螺菌属（Nitrospira）等与氮转化相关的微生物丰度发生变化[102].
5 PE⁃MPs对土壤无机氮含量及氧化亚氮排放的影响目前关于MPs对土壤无机氮含量变化及氧化亚氮排放特征影响尚无明确且一致性结论. Ng等[72]研究表明, ω（LDPE-MPs）为0.2%和3%时, 土壤硝态氮和铵态氮含量及N2O的排放与对照相比无显著差异. Wang等[58]结果表明ω（HDPE-MPs）为1%和10%时, 土壤硝态氮含量显著降低；ω（HDPE-MPs）为10%时, 土壤硝态氮含量下降48%. Feng等[18]研究结果表明, ω（HDPE-MPs）为2%时土壤硝态氮和铵态氮的含量分别降低27.85%和81.2%. 与之不同的是Shi等[85]研究表明土壤中PE-MPs显著增加了土壤铵态氮的含量, 降低硝态氮和无机氮含量. Gao等[46]研究结果表明旱地土壤中HDPE-MPs对土壤N2O的排放无显著影响. 而Ren等[55]研究发现, PE-MPs显著抑制了施肥土壤N2O的排放. 这说明N2O的排放对PE-MPs的响应可能与无机氮含量有关, 无机氮含量较高的土壤, N2O的响应更明显. 此外, 在好氧条件下, MPs可通过增加微环境中氧含量抑制N2O的产生. 相较于旱地土壤, 稻田土壤长期处于淹水的状态下, 其土壤环境和微生物组成都与旱地土壤有较大区别, 土壤中的氮素转化过程对PE-MPs的响应亦有差异. Yu等[20]研究发现, 向水稻土添加PE-MPs后, 土壤N2O的排放显著增加3.7倍. PE-MPs对土壤无机氮含量及氧化亚氮排放的影响可以概括为：土壤中PE-MPs含量是影响土壤无机氮含量主要因素, 土壤无机氮含量和土壤类型是影响PE-MPs污染土壤氧化亚氮排放的重要因素.
6 展望当前虽然关于MPs对氮转化的研究进行了一定尝试, 但在复杂土壤体系中MPs与氮素的相互关系及作用过程还有待深入探究, 主要有以下4个建议：
（1）土壤是一个复杂的多因素体系, 目前多数研究均是聚焦在MPs对单一元素循环的影响, 未来可开展MPs对多种元素循环协同影响研究, 综合分析MPs对土壤营养元素释放、转化及植物供应的影响.
（2）化学添加剂可能是MPs影响土壤氮转化过程的重要因素, 未来研究应考虑MPs的化学添加剂与微生物和营养循环的相互作用.
（3）农业生态系统中的MPs不仅能影响矿质营养元素的循环过程, 还会间接影响种子发芽及幼苗的生长, 亟须综合分析MPs和纳米塑料对植物毒性和农产品健康风险的影响.
（4）当前研究使用的MPs添加量高、暴露时间短且多为MPs. 而土壤中的MPs经过自然老化其表面特征与初级MPs有明显区别. 未来应在环境浓度下开展长期试验, 明晰自然老化MPs的环境行为.
7 结论进入土壤的PE-MPs与土壤颗粒之间相互作用首先可以通过改变土壤团聚体组成和土壤孔隙度大小等物理性质, 改变土壤微生物生存的微环境, 改变好氧和厌氧微生物种群分布, 影响土壤元素循环. 其次PE-MPs可在土壤中因释放添加剂, 吸附污染物, 直接影响土壤氮转化过程；或者通过影响土壤pH和DOC含量对土壤氮转化产生间接影响. 最后PE-MPs的选择性富集、增塑剂的释放和为微生物提供碳源等原因, 促进部分特征微生物在其表面定殖, 使得微生物群落结构发生变化, 造成固氮和硝化反硝化相关功能基因的基因丰度也发生波动. PE-MPs对土壤氮转化的影响是多因素综合作用的结果, PAEs可能是短期内PE-MPs影响土壤氮转化过程的主要因子.
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