2. 中国科学院大学 北京 100049
2. University of Chinese Academy of Sciences, Beijing 100049, China
氮和磷是维持农作物生长发育所必需的养分, 为保障粮食安全和维持作物高产而大量投入的肥料通过淋溶损失到地下水, 威胁着饮用水安全, 也是造成沿海地区地表水富营养化的关键驱动力[1-2]。研究显示, 在欧美等发达国家, 每年因淋溶损失的硝酸盐(NO3-)和总磷分别为34.1 kg∙hm–2和1.13 kg∙hm–2[3-4], 即使在最佳施氮量下, 仍然有15%~65%的肥料氮素会随着NO3-的淋溶而损失[5], 一旦肥料用量超过作物的需要量, 淋失量就会急剧增加[6]。而在发展中国家, 如中国和印度, 化肥施用量远高于欧美, 淋溶损失程度尤为严重[7-8]。因此, 迫切需要降低氮磷淋溶损失以保证全球农业以环境友好型的方式可持续发展。
冻融交替是一种普遍存在于中高纬度、高海拔及部分温带地区的自然现象。据统计, 全球有6600万km2的土地会发生季节性土壤冻结[9], 北半球每年有850万km2的作物遭受冻结的影响[10], 60%的地区土壤冻结时间长达60多天。每年遭受冻害的农田多发生在美国、加拿大、中国和北欧的玉米(Zea mays)、小麦(Triticum aestivum)和大豆(Glycine max)生产密集的地区[11]。冻融交替不仅能改变土壤水热状态和土壤结构, 影响土壤的养分循环[12-15], 加之冻融期多为植物非生长季, 作物氮磷利用率低从而增加了淋溶损失风险[16]。受全球气候变化的影响, 土壤冻融格局势必发生变化, 如:气温升高导致积雪覆盖厚度变薄、持续时间变短, 土壤冻结深度变深、强度变大, 冬季冻结时间缩短, 春季融化期延长等。迄今为止, 对冻融交替如何影响氮磷淋溶仍然缺乏全面的认识, 这对建立有效的阻控措施、维持季节性冻土区农田生态系统的可持续发展至关重要。本文对国内外已有研究结果进行归纳和梳理, 总结了冻融交替对土壤氮磷淋溶损失的影响机制, 并分析了现有的阻控措施(图 1), 为减少肥料损失, 实现农田生态系统的可持续发展和未来相关研究工作的开展奠定基础。
冻融交替的模式受当地气候, 如气温、降雪量的直接影响。而不同的冻融模式会对土壤物理、化学和生物性质造成不同程度的影响, 进而改变氮磷养分淋溶损失的潜力和淋溶方式。气温对土壤冻融状态的影响可能是最直接的, 气温越低, 土壤冻结越深, 增加土壤团聚体的破碎、根系和微生物细胞的死亡, 从而导致土壤中养分含量增加[17]; 到融化期, 土壤溶液中的硝态氮和铵态氮含量也越高, 氮素淋溶损失越大[18]。气温还会影响土壤冻结的速度, 影响冰的大小和微生物对环境变化的适应性, 当土壤以高冷冻速率发生冻结时, 微生物量会减少, 而在较低冷冻速率下, 微生物可能不会受到影响[19]。另外, 土壤由多种矿物成分组成, 每种矿物成分在冻融交替下都有其自身的变形和应力行为, 而应力取决于温度变化的大小:温度变化越大, 应力就越大[20]。
积雪覆盖与否、覆盖厚度及持续时间会影响土壤温度和湿度, 进而会改变冻融交替模式[21]。积雪能够反射多达90%的太阳能, 积雪的减少会导致反射能量的减少, 太阳辐射的吸收增加, 使系统热量增加, 因此, 积雪的存在与否控制着能量的流动, 影响着土壤冻融的模式[22]。由于积雪的隔热性能, 有积雪覆盖的土壤与外界空气温度隔绝, 减弱冻融的作用力[23]; 相反, 没有积雪覆盖会增加土壤温度的日变化, 增加冻融交替的频率, 影响土壤冻结深度, 提前土壤冻结和融化日期[24-25], 从而改变土壤的理化过程和微生物活性, 促进碳和养分的淋失[26-28]。另一方面, 积雪融化形成大量的水, 为养分的淋溶提供了载体, 这可能导致在一段时间内土壤系统的养分大量流失[29-30]。
2 冻融交替影响农田氮磷淋溶的机制 2.1 土壤物理性状的变化 2.1.1 影响土壤团聚体的稳定性低温作用下, 土壤中水形成冰晶, 导致体积膨胀, 土壤孔隙变大, 从而聚集更多的水分形成冰晶, 直至完全冻结, 如此产生的膨胀力会破坏土壤颗粒间的联结, 使粗质颗粒中大团聚体破碎为小团聚体, 黏土颗粒中小团聚体向中等大小团聚体聚集[20, 31-32]。且含水量越高的土壤产生的冰晶越多, 膨胀力越强, 对土壤团聚体的破坏作用也越强[20, 33-34], 当土壤处于最大持水能力时, 团聚体稳定性下降的幅度最大[35-36]。团聚体的破碎和聚集一方面导致固持在其中的碳、氮、磷等营养物质释放, 为随后的矿化提供底物。另一方面使有机-矿物复合体暴露出新的矿物表面, 影响养分的吸附和解吸[14]。但是, 冻融过程中土壤团聚体的稳定性还与土壤质地相关, 黏粒含量高土壤的颗粒之间有更多或更强的黏土桥, 面对相同的冻融条件, 团聚体的稳定性会随黏粒含量的增加而增加[37]。此外, 黏土对养分的吸附能力也高于砂土, 因此, 黏粒含量高的土壤, 养分的淋溶风险低于砂土[4, 38]。
2.1.2 促进大孔隙优先流的形成土壤冻结过程中, 因为充满水的小孔隙冻结后被冰堵塞, 而大孔隙在冻结时仍然充满空气, 进入冻土层的水必须经大孔隙流动从而形成优先流通道[39-40]。如果前期含水量非常小, 冻土可以迅速渗透大量的积水, 而土壤通常不能立即吸收所有的水分, 第一次解冻可能会导致较大的入渗量, 氮磷等养分会随融化的冰和雪水通过优先流途径迅速输送到地下[41]。而当土壤处于水饱和态时, 大孔隙含水量增多也会被冰阻塞, 气温升高时, 表层土壤先融化, 而深层土壤仍处于冻结状态, 从而形成滞水层或重新冻结成冰, 降低了水分渗透性, 但含水量增加, 冰晶的膨胀力更大, 当冰层融化时就会创造新的大孔隙促进流动。另外, 在土壤冻结过程中出现的干燥裂缝及根系死亡后留下的通道, 也会促进优先流通道的形成[42], 当雨季来临时, 冻融期形成的大孔隙优先流也为氮磷等养分随雨水的淋溶提供了通道。而且研究发现, 虽然黏粒含量高的土壤团聚体在冻融过程中表现出更大的稳定性, 但更容易形成大孔隙优先流, 且优先流形成的比例随黏粒含量的增加而增加, 与以基质流途径为主的砂土相比, 形成了垂直运输的捷径[43]。
2.1.3 影响土壤水热分布土壤冻融过程中, 土壤水分运动和热量传输是耦合的。因为土壤冰比土壤水具有更高的热导率和更低的热容, 在冻融过程中, 随着土壤水分的相变, 土壤的水、热性质也会发生变化, 冻结期土壤的感热通量和潜热通量均低于土壤冻结前和融化后[44]。土壤冻结过程中, 表层土壤温度低于深层, 表层土壤的液态水先发生相变, 冻成冰, 下层未冻结土层的水分因为水势的原因向上层冻结的土层中迁移, 导致冻融区上部(冻层)的含水量大于底部(未冻层)含水量; 解冻期, 表层土壤吸收太阳的辐射能优先融化, 积累的融水随土壤向深层融化过程向下迁移[45], 伴随着溶质的运输, 影响着微生物的活动, 土壤温度的波动还会影响微生物酶的活性, 进一步影响微生物的群落结构及其参与的养分循环。
2.2 土壤微生物的变化 2.2.1 土壤微生物量微生物对初次冻融非常敏感, 大约50%的微生物细胞会在第一次冻融时死亡, 土壤冻结产生的冰晶一方面对微生物细胞造成机械损伤, 一方面导致细胞内外渗透压失去平衡造成细胞死亡[46-47]; 此外, 低温会破坏细胞膜的完整性, 从具有功能状态的液相变成凝胶相, 细胞失去转运蛋白的能力导致死亡[48]。但即使在-20 ℃的土壤环境条件下, 仍然有微生物可以存活, 例如进入低代谢活性的休眠状态或脱水保护细胞膜免受破坏等[48-49]。在土壤融化后, 团聚体破坏、根系残体裂解以及死亡微生物会释放出大量营养物质[50-51], 有效水和通气性的增加, 为这些存活的微生物修复受损细胞及大量生长提供了必要条件[51-52], 使得短期内微生物生物量和活性快速增加, 但这种增加并不会持续, 一方面养分会随融水向下层土壤淋溶导致有效基质下降, 另一方面如果脱水的微生物细胞不能快速适应渗透势的突然上升, 也可能导致微生物细胞膜的破裂[53]。这些不同原因的相对影响在不同地区和年份可能有所不同, 而且由于不同研究的冻融条件、测量手段等试验方法上的差异, 土壤微生物生物量对冻融的响应并不一致[54]。有些研究显示冻融交替对微生物量没有影响[55-56], 而有研究报道冬季土壤冻结会导致微生物量和活性下降, 且土壤增温后也不能恢复[57], 还有研究报道土壤冻结不会改变细菌生物量却会降低真菌生物量[58], 而Sjursen等[59]观察到延长冷冻时间并不会影响真菌的生物量, 但会导致细菌数量的下降。因土壤微生物量氮能反映土壤中氮的生物固持作用, 土壤微生物量氮的形成能减少土壤中可提取氮含量, 从而减少硝态氮淋溶[60]; 但大部分研究结果显示冻融后土壤微生物量氮的含量是降低的[61-62]。此外, 土壤微生物生物量又是土壤和地表水中磷的重要来源[63]。综上, 在土壤冻融期间, 存活的微生物活性很低, 死亡的微生物又会释放出大量的氮磷化合物, 因此, 冻融对微生物量的影响有增加氮磷淋溶的风险。
2.2.2 土壤微生物群落不同种类的微生物对土壤冻融的强度、持续时间和频率有不同的反应。Monteux等[64]研究发现, 在永冻层仅嗜热丝菌门(Caldiserica)和厚壁菌门(Firmicutes)的相对丰度就占总细菌丰度的50%以上; 在斯瓦尔巴特群岛的阿森达伦山谷(78°11′N, 15°55′E), 活动层和永冻层的细菌群落组成显著不同, 永冻层的细菌群落以放线菌(Actinobacteria)为主, 平均相对丰度可达70%, 而活动层以变形菌门(Proteobacteria, 24%)、疣微菌门(Verrucomicrobia, 16%)、酸杆菌门(Acidobacteria, 14%)和放线菌门(Actinobacteria, 9%)为主[65]。但是经历多次冻融交替后, 优势门的特征就会减弱, 此时, 起主要作用的门类包括放线菌门(Actinobacteria)、厚壁菌门(Firmicutes)、酸杆菌门(Acidobacteria)、疣微菌门(Verrucomicrobia)、变形菌门(Proteobacteria, Alpha-、Beta-和Gamma-亚门)、拟杆菌门(Bacteroidetes)、浮霉菌门(Planctomycetes)和绿弯菌门(Chloroflexi)[66-67]。真菌因与细菌具有不同的形态、生长策略和生态位, 对冻融交替的响应也不同[68]。一般认为, 细菌在寒冷的温度下比真菌更脆弱, 对于冻结比较严重且时间较长的土壤, 真菌可能在群落中占主导地位[69]。而对于冻融交替频次较高的土壤, 群落结构可能以微生物碳氮比较低的细菌为主, 其原因是土壤解冻时, 土壤温度较高、水分充足、有机质质量高且数量充足, 细菌会优于真菌做出响应, 从而成为优势微生物群落[70]。
土壤微生物群落组成或优势菌群的变化会影响土壤氮磷养分循环。对于参与土壤氮转化的细菌群落, 研究发现, 硝化和反硝化作用相关的群落随着季节性土壤冻融的时间会发生巨大变化, 冻结时多样性最低, 融化时迅速增加, 而且反硝化细菌对冻结温度的承受能力大于硝化细菌[71], 一旦土壤开始解冻, 反硝化功能就会迅速恢复[72], 这可能导致NO3-被消耗, 从而掩盖了冻融过程对硝化作用的积极影响[73-74]。对于硝化微生物种群, 实时定量PCR的研究发现整个冻融期间氨氧化古菌(AOA)和氨氧化细菌(AOB)均维持较高的数量, 且AOA的丰度始终高于AOB[75]。而AOA被认为是农业生态系统中硝酸盐淋失的驱动因素[76-77], AOA和AOB在农业生态系统中占据不同的生态位, AOA在酸性土壤中主导硝化过程, 而AOB在中性、碱性和氮富足的土壤中占据主导地位[78]。土壤微生物群落中优势种的变化还会影响生物量磷释放的数量和形式[79]。例如, 不同种类的菌根真菌对养分淋溶的影响是不同的, 与没有菌根真菌的对照土壤相比, Glomus claroideum使黑麦草(Lolium perenne)微区渗滤液中难反应磷降低13%, 而Glomus mosseae使PO43-浸出增加了46%[80]; 另外, 形成包囊或孢子的细胞很可能释放细胞壁和细胞质中的含磷化合物, 经磷酸酶水解后可以变成可溶性磷, 随积雪融化产生的下渗水流向深层土壤或地表水迁移[63]。
2.3 土壤化学性质的变化 2.3.1 土壤氮循环早在1992年, DeLuca等[36]发现, 冻融会显著提高耕地土壤的氮矿化速率, 且春季解冻后的净氮矿化可能提供了土壤剖面中总NO3-的很大一部分。随后Herrmann等[81]发现经过冻结处理后农田土壤净氮矿化增加了2~3倍。作为有机质矿化的产物, NH4+通常在冬天土壤冻结时积累[71], 再经土壤的硝化作用转化为NO3-。作为冻融期间土壤氮素转化的主要过程, 硝化作用是冻融期间NO3-积累的主要原因[71, 82-83]。根据Campbell等[29]利用氧同位素研究结果, 淋溶液中来自土壤硝化作用产生的NO3-比例可达80%。也有研究显示土壤的硝化作用似乎不受冻融的影响, 但可能是由于反硝化作用的增强对该过程造成掩盖的结果[84]。据估计, 农田土壤因冻融导致的氧化亚氮(N2O)排放占农业总排放的17%~28%[10], 而且大部分N2O是在春季土壤解冻时以脉冲方式释放[85]。2017年, Song等[74]和Gao等[86]分别开展了冻融对土壤氮素循环的meta分析, 结果均显示, 冻融后土壤NH4+和NO3-含量显著增加, 土壤总氮和土壤微生物量氮含量显著下降, 可溶性无机氮和可溶性有机氮含量及N2O排放显著提高, 土壤NO3-的淋溶损失更是增加了66.9%。2019年, 隽英华等[87]对3种农田土壤进行相同冻融频次的室内模拟试验结果也显示, 随着冻融频次的增加, 土壤可溶性无机氮、可溶性有机氮和可溶性全氮含量均显著增加。
2.3.2 土壤磷循环土壤对磷的吸附能力在磷的淋溶损失中发挥着至关重要的作用, 如果土壤对磷的吸附作用比较强, 即使磷含量很高淋溶风险也很低[4]。但是冻融对团聚体的破坏作用一方面造成比表面积增大, 吸附位点增多; 另一方面有其他离子与磷酸盐共同竞争吸附位点, 而且磷的吸附和解吸附同时受土壤黏土含量、水分含量以及铁铝化合物等的共同影响, 因此, 土壤冻融对磷的吸附和解吸的影响与土壤本身的性质有关[88-89]。磷的同位素试验研究发现, 土壤冻融过程中不发生磷的固定作用[90], 但是冻融对植物残体、微生物生物量以及土壤团聚体的物理破坏会刺激土壤磷的矿化作用[91], 导致土壤淋溶液中总磷浓度和不同形态磷比例发生显著变化[92]。有研究表明, 冻融会显著降低土壤微生物量磷含量[90], 提高土壤溶液中总溶解磷(TDP)含量[90-91, 93], 并且溶解性有机磷(DOP)对TDP贡献率达50%以上[91]。此外, 不同土壤类型磷素组分对冻融交替的响应不同, 相同冻融条件下, 矿质土壤中醋酸提取磷的含量没有发生显著变化, 而有机土壤中醋酸提取磷的含量几乎增加1倍; 相似地, 冻融后矿质土壤中DOP的增幅比例最大, 而在有机土壤中, DOP、钼酸活性磷(MRP)以及除DOP和MRP之外的溶解磷含量都大幅度增加[93]。胡钰等[94]对我国东北地区的黑土、暗棕壤和水稻土的研究发现, 多次冻融提高了黑土和水稻土的有效磷含量, 但降低了暗棕壤的有效磷含量。与此相似, 张迪龙等[95]对我国东北地区的旱田和水田的研究也发现, 冻融交替会增加不同土层深度土壤TDP、MRP和DOP释放量。由此可见, 冻融交替对土壤磷循环的影响会增加磷素的淋溶损失风险。
2.3.3 土壤有机质在土壤冻融过程中, 土壤有机质质量和数量的变化从多个方面影响土壤氮磷淋溶[68, 96]。土壤有机质与矿质颗粒的结合作用, 对土壤团聚体的形成和稳定起着重要作用。研究发现有机质含量高的土壤, 团聚体对冻融交替表现出更高的稳定性[37]; 高土壤有机质含量意味着较强的阳离子交换能力, 对水和养分的持有能力也更强[97]; 另外, 土壤有机质还是土壤食物网的主要能源物质, 影响着微生物群落结构, 初始有机质含量高的土壤, 微生物的丰富度和多样性更高, 在冻融交替胁迫下会表现出更高的抗性或恢复力[62], 与低有机质含量的土壤相比, 高有机质含量的土壤冻融后真菌与细菌生物量之比下降更明显[68]; 土壤有机质和氮的变化还会影响土壤的碳氮比, 当土壤碳氮比小于微生物碳氮比时, 微生物的矿化作用会增强。其他土壤化学性质, 如pH不仅会影响土壤的离子交换能力, 还会通过影响矿物表面电荷及吸附位点影响矿物对有机质的吸附, 在氮磷养分淋溶中也发挥重要的作用。
3 冻融交替背景下土壤氮磷淋溶的阻控措施 3.1 合理施肥肥料施用过量和利用率低是农田土壤氮磷淋失的主要因素。研究显示, 当氮肥施用量超过225 kg∙hm-2时, 大量的肥料氮素就会进入地表水, 目前很多地区的施氮量远高于225 kg∙hm-2, 甚至达600 kg∙hm-2[98]。实际上, 许多作物并不需要如此多的肥料就能保证粮食产量, 例如, 在我国, 谷物的施氮量在150~ 180 kg∙hm-2时就能保障产量在4.9~5.9 t∙hm-2[98]。而且不同作物的需氮量不同, 因此, 应根据种植作物需求提出合理的施肥意见, 制定标准, 并鼓励农民按此标准施肥[99-100]。另外, 氮磷养分在土壤中的积累和运移也受施肥方式、施用时间、灌溉等因素的影响, 例如, 与传统施肥相比, 堆肥会降低硝酸盐的淋溶损失[101], 通过灌溉系统定期施用液态氮肥也可降低硝酸盐的淋溶损失[102]。因此, 在人口不断增长、对粮食需求不断增加的背景下, 从“源头”着手, 合理施肥是减少氮磷养分淋失的关键。
3.2 增施生物炭生物炭是一种土壤改良剂, 是生物质在部分或完全无氧气的情况下, 于300~600 ℃下热分解的产物。无论是田间原位试验还是室内培养试验均发现, 不同类型的生物炭可以不同程度地降低土壤中氮磷养分的淋溶损失[103]。李美璇等[104]研究发现, 施加生物炭后, 东北黑土因冻融作用引起的氮素淋失降低50%以上; 周丽丽等[105]对冻融期棕壤磷有效性的研究发现, 增施生物炭降低了冻融期间土壤磷素的淋溶损失。但是师澜峰等[106]的研究表明, 虽然不同比例的秸秆生物炭施入量均降低了冻融期黑土表层无机磷淋溶量, 但高生物炭施入量处理的无机磷淋溶量高于低生物炭施入量处理。生物炭影响冻融期养分淋溶的潜在机制可能包括: 1)生物炭的多孔结构和较大的比表面积会影响土壤的物理性质, 包括团聚体的稳定性和孔隙分布特征, 影响养分的持有能力[107-108]。最近的研究发现, 施加生物炭能显著改善土壤团聚体稳定性[109-110], 提高冻融期间土壤的保水能力[111]。2)通过改变土壤的化学性质影响土壤对养分的持有能力。一方面, 生物炭的分解很慢, 在氧化过程中表面产生带负电荷的官能团[112]会增加与养分循环有关的阳离子交换能力, 而这是生物炭养分保留的最基本的表面化学特性之一[113-114]; 另一方面, 生物炭一般具有较高的pH, 可以通过改变土壤pH间接改变土壤养分的溶解度[103]。3)通过影响土壤微生物活动和群落结构改变土壤对养分的保留[103]。多孔结构为土壤微生物提供了适宜的栖息地[115], 从生物炭表面解吸的营养物质和有机质为微生物生长提供了条件, 导致养分循环的改变[116]; 此外, 生物炭具有很高的碳氮比, 也在一定程度上诱导土壤氮素固定。生物炭对水和养分固持能力的影响为植物生长创作了条件, 从而还会通过提高生长季作物对肥料的利用效率[117-118], 减少冻融期可供淋溶的养分含量。
3.3 覆盖作物覆盖作物是在只有1个生长季的地区, 为了覆盖土壤, 在主经济作物收获后种植的淡季作物[119]。覆盖作物在改善土壤质量、减少农业投入和提高环境可持续性方面发挥着越来越重要的作用。Abdalla等[120]对涵盖不同国家和地区的106项研究分析显示, 覆盖作物能显著降低土壤氮素的淋溶损失。这可能是由于: 1)非豆科类的覆盖作物可以吸收土壤中过量的硝酸盐用于自身生物量的生长[121]; 2)覆盖作物能增加土壤有机碳的储量, 改善土壤结构, 增强土壤的持水能力, 影响微生物种群大小和活性[122]; 3)春季土壤融化时, 覆盖作物对水分的吸收也可以减缓水分向下运移的速度。但是, 覆盖作物对磷淋溶的影响是不稳定的, Aronsson等[123]对斯堪的纳维亚(Scandinavia)南部和芬兰地区农田土壤的研究发现, 覆盖作物对总磷淋溶的影响在增加86%和减少43%之间变化。这可能因为土壤冻融对植物细胞的破坏会增加土壤中溶解磷的含量, 且随冻融交替频次的增加释放更多的磷素到土壤中[14, 124]。Liu等[14]对寒冷气候下覆盖作物对土壤可溶性磷和总磷损失的研究发现, 覆盖作物可降低易受侵蚀地区土壤颗粒磷的损失, 但倾向于增加非侵蚀性土壤中可溶性磷的损失。对总磷损失的影响在各种研究中不一致, 受到土壤、气候和管理因素的复杂影响, 还与覆盖作物的品系显著相关[123-125]。此外, 覆盖作物生长过程中吸收了大量水分, 竞争了部分可以被主作物吸收的养分, 可能导致干旱和半干旱地区水分含量下降, 主作物产量降低[126]。
3.4 免耕秸秆覆盖还田免耕秸秆覆盖还田作为一种保护性耕作措施, 在农业可持续发展方面发挥着重要作用。在对养分淋溶的影响上: 1)可以通过提高肥料利用率, 降低可供淋溶养分含量。最新研究发现, 免耕秸秆覆盖可以通过提高土壤水分, 激活植物有效态氮和Olsen-P的释放, 从而提高根系活力、根系表面积、干物质积累速率, 进而提高水肥利用效率, 与免耕无秸秆覆盖相比, 秸秆覆盖处理的玉米平均氮肥利用率提高36.9%[127], 冬季油菜(Brassica campestris)氮素利用效率提高51.6%[128]。Zhang等[129]和Truong等[130]利用15N同位素标记的肥料研究发现, 秸秆覆盖能有效地将肥料氮固持在土壤中, 显著减少肥料氮素损失。Dong等[131]也发现秸秆覆盖能在一定程度上减缓土壤硝态氮在深层的积累。2)免耕秸秆覆盖起到很好的保温作用, 影响冻融期土壤剖面的水热分布及土壤冻融模式。玉米秸秆覆盖下耕层土壤温度比裸地高1.0~2.8 ℃, 整个冻融过程缓慢且滞后于裸地[132], 表层土壤融化的时间延缓3~6 d, 融化速度每天降低0.4~0.8 cm[133]; 陈军锋等[134]对比了裸地和5种玉米秸秆覆盖厚度地块的水热分布情况, 裸地时0~40 cm土层的水热变化剧烈, 随着秸秆覆盖厚度增加水热变化的活跃层不断减小, 当秸秆覆盖为15 cm时可抑制土壤剖面水热的变化。3)秸秆具有较高的碳氮比, 添加后会促进土壤氮的固定作用, 降低土壤溶液中硝态氮浓度[135]。另外, 免耕秸秆覆盖还具有增加有机碳的固持、改善土壤结构、影响微生物群落组成、结构、功能等特点[136-138], 对养分淋溶的影响可能更为复杂。而且作物残体本身也是重要的养分来源[139], 还有研究认为秸秆覆盖有利于大孔隙优先流的形成[140], 从而促进养分的淋溶。因此, 还需要更多关于免耕秸秆覆盖对养分淋溶影响的研究。
此外, 施用硝化抑制剂、不同作物轮作在氮素淋溶方面也取得了不同程度的效果。硝化抑制剂的开发在控制氨氧化速率方面发挥了重要作用, 例如, 双氰胺(DCD)是一种非挥发性的商业可用化合物, 通过共价结合到活性位点使AMO氨单加氧酶失活, 抑制土壤的硝化作用, 达到减少硝态氮损失和N2O排放的目的[141]。
4 总结与展望土壤冻融是一种非生物应力, 土壤冻结和融化的过程实际上是水随温度的波动在土壤中的相变过程, 而土壤氮磷养分的淋溶损失必须同时具有两个条件:一是土壤中有氮磷养分积累可供淋失; 二是有下渗水流, 使得氮磷养分可随之迁移进而发生淋溶损失。因此, 概括来讲, 冻融交替对养分淋溶的影响机制主要是通过土壤物理、化学和生物性质的变化影响可淋溶养分的含量、水分和淋溶通道, 具体包括: 1)冻融对土壤团聚体、微生物细胞和植物根系残体的破坏作用会释放出有机质和不同形态的氮磷化合物, 养分可利用性的增加会刺激存活微生物的活性, 导致土壤矿化作用增强, 从而增加了土壤中可溶性矿质态养分含量; 2)冻融过程中水热分布特征, 以及孔隙结构的变化易形成大孔隙优先流, 促进养分向深层的运移, 同时影响着易发生淋溶的时间; 3)冻融循环对土壤微生物群落的影响会改变土壤物质转化的方向, 进而可改变可供淋溶的养分形态和含量; 4)冻融期间土壤化学性质的改变影响着淋溶液中养分的形态和浓度, 土壤有机质、pH和离子交换能力等性质的变化影响着土壤对养分和水分的持有能力、土壤结构的稳定性、微生物的活动和群落特征。
根据冻融对养分淋溶的作用机制, 可以通过以下4种措施降低冻融期间养分淋溶损失。1)合理施肥, 从“源头”降低可淋溶的养分含量; 2)增施生物炭, 通过影响土壤理化性质和微生物群落提高土壤对养分和水分的固持能力; 3)种植覆盖作物, 作物生长过程中对养分的吸收利用可减少可淋溶的养分含量和养分向下运移的速度; 4)实施免耕秸秆覆盖的耕作制度, 可提高生长季作物的氮素利用率, 从而降低可淋溶养分含量, 秸秆地表覆盖可以减缓冻融的作用效果, 通过对土壤物理、化学、生物多方面的变化影响养分的淋溶。
当前的许多研究受试验设备和条件的限制, 存在试验周期短、因素单一的缺点, 研究结果多基于室内模拟, 容易偏离实际情况; 冻融交替对养分淋溶损失的影响程度(正、负和无影响)又受冻融模式和土壤性质本身的影响; 淋溶过程涉及养分从表层向深层迁移的动态过程, 即使同一地区, 同一土层, 不同装置收集的淋溶液含量也存在较大差距。冻融对土壤氮磷养分淋溶的过程仍存在分歧, 很难评价冻融期间养分淋溶量及在全年养分淋溶损失中所占的比例, 现有阻控措施对养分淋溶的影响机理和作用效果还需要更全面的研究, 以建立完善的评估体系, 结合特定的土壤、管理和区域气候条件, 在保证土壤可持续发展的前提下, 有针对性地选择或建立更有效的阻控措施。
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