华北平原潮土区粮田氮淋失阻控措施及效果分析

引用本文

孟凡乔, 王坤, 肖广敏, 王开永, 胡正江, 张海霞, 许秀春, 张薇, 杨轩. 华北平原潮土区粮田氮淋失阻控措施及效果分析[J]. 中国生态农业学报（中英文）, 2021, 29(1): 141-153. DOI: 10.13930/j.cnki.cjea.200523

MENG F Q, WANG K, XIAO G M, WANG K Y, HU Z J, ZHANG H X, XU X C, ZHANG W, YANG X. Nitrogen leaching mitigation in fluvo-aquic soil in the North China Plain[J]. Chinese Journal of Eco-Agriculture, 2021, 29(1): 141-153. DOI: 10.13930/j.cnki.cjea.200523

基金项目

国家重点研发计划项目（2016YFD0800104，2017YFD0800605）资助

作者简介

孟凡乔, 主要研究方向为农业生态系统碳氮循环。E-mail:mengfq@cau.edu.cn

文章历史

收稿日期：2020-06-30
接受日期：2020-10-23

Contents Abstract Full text Figures/Tables PDF

华北平原潮土区粮田氮淋失阻控措施及效果分析^*

孟凡乔¹, 王坤², 肖广敏¹, 王开永³, 胡正江³, 张海霞³, 许秀春¹, 张薇¹, 杨轩¹

1. 中国农业大学资源与环境学院/农田土壤污染防控与修复北京市重点实验室北京 100193;
2. 淄博市农业农村事业服务中心淄博 255033;
3. 桓台县农业农村局桓台 256400

收稿日期：2020-06-30；接受日期：2020-10-23

* 国家重点研发计划项目（2016YFD0800104，2017YFD0800605）资助

孟凡乔, 主要研究方向为农业生态系统碳氮循环。E-mail:mengfq@cau.edu.cn

摘要：华北平原潮土区是我国重要的粮食主产区，改革开放40多年来该区农业经历了以高水肥投入为主要特征的集约化进程，相应的氮淋失导致的面源污染自20世纪90年代以来不断加剧。本研究针对华北平原潮土为主要类型的粮田，对过去40多年间主要研究文献进行全面分析，梳理氮肥和水分投入与氮淋失之间的定量关系，比较主要农田管理措施对氮淋失的阻控效果及其机理，以期为我国农业面源污染提供决策支持。研究发现，氮肥和灌溉是影响华北平原潮土区粮田氮淋失的主要因素，其中氮淋失与氮盈余量之间呈指数关系，比与施氮量的指数关系更显著。基于机器学习的随机森林回归模型能够考虑包括施肥、灌溉、土壤条件和气象等多因素对氮淋失的影响，未来在定量预测中有较好前景。同等氮肥投入条件下，由于氮供应与作物吸收契合度高，有机无机配施能显著降低氮淋失。以缓控释肥、尿酶和硝化抑制剂为代表的肥料增效剂可以降低约1/3的氮淋失，值得重点推广应用。秸秆还田可以实现包括提高土壤有机物和微生物氮库、增加无机氮缓冲容量等综合效益，有利于降低氮淋失风险（降低比例达10%），但免耕的阻控效应较低且呈现较大不确定性。调整种植制度、休耕、间作套种和种植填闲作物等措施会影响粮食产量，推广过程中应慎重。氮淋失的阻控效果更多受到社会、经济和政策等因素的影响，今后应采取包括生态补偿等手段发挥农民主动性，从政策和法律法规层面创造实施氮淋失阻控措施的社会环境。

关键词：潮土区粮田氮淋失施氮肥灌溉肥料增效剂种植制度生态补偿

Nitrogen leaching mitigation in fluvo-aquic soil in the North China Plain^*

MENG Fanqiao¹, WANG Kun², XIAO Guangmin¹, WANG Kaiyong³, HU Zhengjiang³, ZHANG Haixia³, XU Xiuchun¹, ZHANG Wei¹, YANG Xuan¹

1. College of Resources and Environmental Sciences, China Agricultural University/Beijing Key Laboratory of Prevention, Control and Restoration of Farmland Soil Pollution, Beijing 100193, China;
2. Zibo Center of Rural and Agricultural Affairs Service, Zibo 255033, China;
3. Huantai Bureau of Rural Affairs and Agriculture, Huantai 256400, China

* This study was funded by the National Key Research and Development Program of China (2016YFD0800104, 2017YFD0800605)

** Corresponding author, MENG Fanqiao, E-mail: mengfq@cau.edu.cn

Received Jun. 30, 2020; accepted: Oct. 23, 2020

Abstract: The North China Plain is a grain production region with fluvo-aquic soil and has seen rapid agricultural development over the past four decades. Excessive fertilization and frequent irrigation have increased nitrogen (N) leaching and nonpoint source pollution since the 1990s. This study screened published nitrogen leaching data on the North China Plain grain farmlands to identify the relationship between fertilization and irrigation with N leaching and to evaluate the primary N leaching mitigation measures. The results showed that regional groundwater during the 1970s was shallow and then deeper. During the 2010s, the regional cropping system changed from one to two crops per annum, and the annual N fertilizer rapidly increased to 600 kg(N)·hm^-2·a^-1 but then slowly decreased to 500–550 kg(N)·hm^-2·a^-1. Since the 1990s, irrigation increased from zero (rainfed during the 1980s) to 150–400 mm per annum, crop straw had gradually been incorporated into farmlands, and the fertilizer synergist technology had been accepted. The soil organic matter and total N improved by 38%–47%, pH decreased by 0.5 units, and available potassium decreased slightly. Fertilization and irrigation were the main influencing factors of N leaching, and the exponential relationship between N leaching and the N fertilizer balance (N fertilizer rate - crop above-ground N uptake) was better than the relationship between N leaching and N fertilizer rate. Random forest (RF) regression modeling based on machine learning was used to determine the relationship between N leaching and impacting factors such as irrigation, soil properties, and climate; the prediction results were satisfactory. At the same rate of N fertilization, organic fertilization combined with chemical fertilization significantly decreased N leaching because the N supply and crop demand were synchronized. Fertilizer synergists, such as control-release fertilizers, ureases, and nitrification inhibitors, mitigated N leaching by 1/3 and should be used in the North China Plain. Crop straw incorporation microbially improved N fertilizer in the short term and increased the long-term soil total N stock and inorganic N buffering capacity and reducing N leaching by 10%. The no-tillage mitigation effects were low and variable among farmlands. Fallow farmland and rotation/intercropping of deep root and shallow root crops, leguminous crops with cereal crops, and grains with vegetable crops were effective at reducing N leaching, but the crop yields also reduced. Therefore, these techniques required careful examination during technical dissemination. Governmental support, technical training, and proper planning should be implemented during the 14^th Five-Year Plan of China to prevent and mitigate N pollution. Ecological compensation and an agricultural sector water use charge could also be used to encourage farmer participation.

Keywords: Fluvo-aquic soil area Grain farmland Nitrogen leaching Nitrogen fertilization Irrigation Fertilizer synergist Cropping system Ecological compensation

改革开放40多年来, 华北平原等地区氮肥用量不断增加, 作物产量快速提高, 是地下水面源污染的重要原因^[1-4]。20世纪90年代初, 氮肥等引起的面源污染就开始进入人们视线, 周健等^[5]提出通过降低化肥数量、优化施肥技术等, 有效降低面源污染, 并建议2020年全国化肥投入总量控制到5.16×10⁷ t。张维理等^[6]建议华北地区二熟制粮食作物氮肥用量不超过400 kg(N)∙hm^-2∙a^-1。进入21世纪, 面源污染逐步加剧。刘光栋等^[7]对华北第一吨粮县——山东桓台调查发现, 由于连续多年大量施用氮肥, 1999年全县地下水硝态氮浓度 > 10 mg∙L^-1的污染区域占1/5。第1次和第2次中国污染源普查表明, 2007年和2017年全国种植业总氮排放量分别达1.59×10⁶ kg和7.2×10⁵ kg, 远超过工业与生活源。除了统计口径和计算方法, 肥料投入和氮淋失阻控措施的实施是两次污染物排放量出现差异的重要原因^[8-9]。

从20世纪60年代以来, 全球范围内, 美国、中国和荷兰等农业集约化国家对于氮淋失的研究开始快速增加, 对象主要是小麦(Triticum aestivum)、玉米(Zea mays)和草场, 又以在中国华北地区的研究更为充分^[10]。如在河北栾城的长期定位试验表明, 年施氮量为400 kg(N)∙hm^-2情景下, 硝态氮淋溶损失为47.0~65.5 kg(N)∙hm^-2, 占肥料氮的11.8%~16.4%^[1]。在夏玉米季, 全国范围内硝态氮淋失损失平均水平为27.6 kg(N)∙hm^-2(占氮肥投入的13.3%), 其中华北潮土区为10~35 kg(N)∙hm^-2[11]。从农田淋失的氮绝大部分进入地下水体, 造成华北地区地下水硝酸盐等逐年增加, 已经对该区水环境和人群健康造成重大潜在影响, 急需采取阻控措施, 降低粮食生产过程中的氮淋失。

华北平原是中国主要的粮食产区^[12], 目前在该区围绕氮淋失与氮肥和水分之间的数量关系、肥料增效剂等主要农田管理措施的阻控效果进行了大量研究, 但这些个案研究效果各异、甚至相反, 需要从区域尺度, 采用数据整合分析等方法, 对这些研究的整体性规律进行归纳总结, 并对不同结果的原因进行分析。本研究对过去40多年来的研究文献进行全面分析, 梳理氮肥和水分投入与氮淋失之间的定量关系, 比较主要农田管理措施对氮淋失的阻控效果及其机理, 提出适用于华北平原粮田的源头减量和过程阻断与拦截技术, 为该区面源污染防控和管理工作提供科技支撑。

1 潮土区土壤特点与农田管理措施

潮土(Alluvial soils, Fluvo-aquic soils)是河流沉积物受地下水运动和耕作活动影响而形成的土壤, 因有夜潮现象而得名。潮土为发育于富含碳酸盐或不含碳酸盐的河流冲积物, 受地下潜水作用, 经过耕作熟化而形成的半水成土壤^[13], 相当于美国《土壤系统分类》的淡色始成土纲(Ochrepts), 以及世界土壤分类中的饱和始成土(Eutric Cambisols)和石灰性始成土(Calcaric Cambisols)^[14]。我国潮土主要分布于华北平原(黄淮海地区)和长江中下游平原北部, 其中河南省4.16×10⁶ hm^2[15]、山东省4.70×10⁶ hm^2[16]。华北地区潮土粉粒含量较高(65%~80%), 多为粉壤土, 而源于潮白河的沉积物的潮土粉粒少。潮土发育时间短, 土壤有机质和养分含量(钾除外)较低, 土壤pH为7.0~8.5, 除长江及支流沉积物发育潮土外, 其他地区碳酸钙含量较高^[16]。历史上潮土区地下水位较浅(1.5~3.0 m), 近年来, 随着全球气候变化以及用水需求的快速增加, 潮土区地下水位不断下降, 相应的化肥等面源污染源对地下水体的影响也逐步加深。

改革开放以来, 华北平原农业经历了快速集约化进程, 其主要特征包括: 1)种植制度从一年1熟[冬小麦/夏玉米/棉花(Gossypium spp.)/豆类/薯类]逐步向一年两熟(冬小麦和夏玉米)转变, 作物由粮食作物向蔬菜和林果作物转变^[17-18], 并在部分地区试点推广“一季休耕、一季雨养”和退耕冬小麦等措施^[19-20]。2)氮肥投入经历了快速增加到顶点、然后逐步下降的过程, 从20世纪80年代的每年200~300 kg(N)∙hm^-2增加到2010年代每年的400~600 kg(N)∙hm^-2。2010年代以后, 随着缓控释肥、复合肥以及增效肥等新型肥料和技术的引进, 施肥机械、秸秆还田和施肥技术的推广, 目前该区冬小麦和夏玉米单季的肥料氮投入量为200~ 250 kg(N)∙hm^-2, 比2010年代最高氮肥量下降15%~33%^[21]。3)灌溉量快速增加。华北平原正常气候年份的降水基本可以满足夏玉米季生长需求。冬小麦季的降水则无法满足作物生长, 从20世纪80年代的雨养演替为灌溉2~5次, 灌溉量150~ 350 mm^[22]。随着水肥一体化技术和节水滴灌技术推广应用, 冬小麦季灌溉量有进一步降低的空间^[23]。4)作物秸秆逐步还田。随着人民生活水平的提高和机械改进, 两季作物秸秆逐步全部还田^[21], 或通过种植食用菌、饲料化以及生物质处理等再还田^[24]。5)肥料增效剂包括尿酶和硝化抑制剂、缓控包膜等得到推广应用, 对提高氮素利用率和作物产量、降低氮肥损失具有显著效果^[22-26]。

华北平原潮土区的农业集约化, 很大程度上改变了该区的潮土性状和生产力^[27]。对51个潮土肥力点的长期监测表明, 潮土有机质、全氮等含量整体呈上升趋势, pH呈下降趋势^[28]。潮土有机质含量虽然低于棕壤和褐土, 但改革开放后25年间的增长速率是3类土壤中最快的^{[16, 29]}。从土体构型上, 随着农业集约措施的应用, 潮土区普遍存在着土壤有效耕层变浅、犁底层加厚、耕层土壤质量减少等问题^[30]。

2 潮土区氮淋失与主要农田阻控措施的关系

氮淋失是指土壤中氮素在水分投入(灌溉、降水)之后随土壤水分向下运移直至进入到作物根区无法到达的区域, 最终不能被作物吸收所导致的损失^[31]。对于氮淋失与氮肥施用等诸多农田管理措施之间关系的深入分析, 有助于制定和实施氮淋失的阻控措施。本文以“面源污染” “氮淋失” “氮淋溶”以及“华北”等以及相应英文为关键词, 针对“中国知网”和“ISI-Web of Science”等数据库1980—2019年的文献进行搜索, 选取代表性文献进行分析。

2.1 化学氮肥施用

氮肥过量施用, 多余氮素在土壤中积累, 遇到降雨或灌溉事件时, 就有可能发生淋洗。合理施氮条件下, 土体氮素收支平衡, 氮淋失数量很小。针对冬小麦和夏玉米生产中, 氮淋失与氮肥之间定量关系的整合分析表明(表 1): 1)指数关系比线性方程更适合对氮淋失的定量估测。华北地区冬小麦施氮量在90~170 kg(N)∙hm^-2范围内, 硝酸盐淋溶损失极低, 而施氮量达到240 kg(N)∙hm^-2后, 硝酸盐淋失量大多数情况下呈指数关系增加^[32-33]; 高施氮量下, 线性方程严重低估氮淋失^[34-35]。研究对象比较专一, 比如冬小麦和夏玉米分开分析, 一般获得指数关系而不是线性关系模型。2)随着研究的增加和深入, 近年来氮淋失的估测数量比早期估测数量低, 主要表现在指数方程的各参数数值减小^[36-37]。3)氮肥施用后, 作物吸收一部分氮、且数量相对稳定, 因此对氮淋失进行预测时, 应用氮盈余量(即施氮量减去地上部作物吸收量)比施氮量更能准确预测氮淋失, 但在实际工作中增加了监测指标数据量, 较难实施。基于以上分析, 可以认为配方施肥、水肥一体化、叶片硝酸盐测定等精准施肥可以大幅度降低氮肥投入数量, 因此能有效降低农田氮淋失。

表 1 华北平原潮土区冬小麦和夏玉米生产中氮淋失估算模型 Table 1 Estimation models for nitrogen leaching from winter wheat and summer maize production in Fluvo-aquic soil areas of the North China Plain

作物 Crop	模型Model	参数Parameter	文献来源 Reference
冬小麦Winter wheat	Y=4.93e^0.0057X (R²=0.50^**)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[36]
	Y=13.59e^0.009X (R²=0.28^**)	Y为淋失的总氮; X为氮盈余量, 即施氮量减去地上部作物吸收量。 Y is the amount of total nitrogen leached; X is the surplus N, i.e., fertilizer N minus crop aboveground uptake N.	[37]
	Y=17.3e^0.0063X (R²=0.31^**)	Y为淋失的硝态氮; X为氮盈余量, 即施氮量减去地上部作物吸收量。 Y is the amount of nitrate leached; X is the surplus of N, i.e., fertilizer N minus crop aboveground uptake N.	[21]
	Y=8.42+0.13X (R²=0.22^***)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[41]
	Y=16.86e^0.003X (R²=0.17^***)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[41]
	Y=3.63e^0.008X (R²=0.27^**)	Y为淋失的总氮, X为施氮量。 Y is the amount of total nitrogen leached, X is the fertilizer N rate.	[42]
夏玉米 Summer maize	Y=2.38e^0.0041X (R²=0.63^**)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[36]
	Y=25.31e^0.0095X (R²=0.55^**)	Y为淋失的总氮; X为氮盈余量, 即施氮量减去地上部作物吸收量。 Y is the amount of total nitrogen leached; X is the surplus N, i.e., fertilizer N minus crop aboveground uptake N.	[37]
	Y=12.14e^0.0061X (R²=0.35^**)	Y为淋失的总氮; X为氮盈余量, 即施氮量减去地上部作物吸收量。 Y is the amount of total nitrogen leached; X is the surplus N, i.e., fertilizer N minus crop aboveground uptake N.	[35]
	Y=25.87e^0.0072X (R²=0.39^**)	Y为淋失的总氮, X为施氮量。 Y is the amount of total nitrogen leached, X is the fertilizer N rate.	[35]
	Y=10.7e^0.006X (R²=0.30^**)	Y为淋失的总氮, X为施氮量。 Y is the amount of total nitrogen leached, X is the fertilizer N rate.	[42]
	Y=42.0e^0.0072X (R²=0.32^**)	Y为淋失的硝态氮; X为氮盈余量, 即施氮量减去地上部作物吸收量。 Y is the amount of nitrate leached; X is the surplus N, i.e., fertilizer N minus crop aboveground uptake N.	[21]
	Y=3.83+0.25X (R²=0.51^***)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[41]
旱地作物 Upland crops	Y=0.0192X+4.1463 (R²=0.0479)	Y为淋失的总氮, X为降雨量。 Y is the amount of total nitrogen leached, X is the precipitation.	[34]
	Y=0.046X+4.2936 (R²=0.0938^*)	Y为淋失的硝态氮, X为灌溉量。 Y is the amount of nitrate leached, X is the irrigation amount.	[34]
	Y=0.0491X+7.6368 (R²=0.1883^**)	Y为淋失的总氮, X为施氮量。 Y is the amount of total nitrogen leached, X is the fertilizer N rate.	[34]
	Y=0.0594X+5.8362 (R²=0.4802)	Y为淋失的总氮, X为施氮量。 Y is the amount of total nitrogen leached, X is the fertilizer N rate.	[43]
	Y=0.0459X-6.5932 (R²=0.3652)	Y为淋失的总氮, X为降雨量。 Y is the amount of total nitrogen leached, X is the precipitation.	[43]
	Y=-12.16+0.041X₁+0.035X₂ (R²=0.522)	Y为淋失的总氮, X₁为施氮量, X₂为降雨量。 Y is the amount of total nitrogen leached, X₁ is the fertilizer N rate, X₂ is the precipitation.	[43]
	Y=0.01X(2.84+0.030X) (R²=0.22^***)	Y为淋失的总氮, X为施氮量。 Y is the amount of total nitrogen leached, X is the fertilizer N rate.	[44]
	Y=0.1+1.4e^0.027X (R²=0.84^***)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[45]
	秸秆还田 Straw returning: Y=1.3453e^0.0074X(R²=0.94) 秸秆不还田 No straw returning: Y=1.2469e^0.008X(R²=0.95)	Y为淋失的硝态氮, X为施氮量。 Y is the amount of nitrate leached, X is the fertilizer N rate.	[46]
Y和X的单位均为kg(N)×hm^-2(X为降雨量或灌溉量时单位为mm); R²为方程的决定系数。、和*分别表示方程显著性水平为P < 0.05、P < 0.01和P < 0.001。The unit of both Y and X (X is precipitation or irrigation amount, its’ unit is mm) is kg(N)×hm^-2. R² is the coefficient of determination. , and * indicate the significance of regression at P < 0.05, P < 0.01 and P < 0.001 levels, respectively.

除了施肥类型和数量, 其他因素如土壤性质和气象因素, 也会对潮土区氮淋失产生重要影响。单纯利用氮肥和水分对氮淋失进行预测, 往往效果较差, 因此, 计算能力强、考虑多因素影响和复杂关系的机器学习方法凸显优势。Ying等^[11]首次引进基于机器学习的随机森林模型, 利用气温、氮肥投入、表观水平衡(灌溉量+降水量-蒸散量)、土壤pH、土壤有机质、土壤质地等多因素对玉米季农田氮淋失进行预测, 取得了很好的效果。另外, 研究表明磷肥和钾肥配合施用, 可以增加作物氮吸收, 从而减少氮肥在土壤中的残留和淋失风险^[38]。一次施用大量氮肥不利于小麦生产, 在小麦生长关键时期分次合理施用, 比如适当增加追肥比例, 更能降低氮淋失^[39-40]。

2.2 有机无机配施

在同等施氮情况下, 与化肥单施相比, 有机无机配施一般可以显著降低氮淋失^[47-51], 主要机理在于:第一, 有机肥中的氮绝大部分以有机态形式存在, 其分解和释放速率远低于化肥氮; 第二, 有机肥与化肥氮配合, 逐步释放被植物吸收, 与植物需求的时间规律契合较好, 从而实现阻控氮淋失效果; 第三, 有机肥中的有机碳还可以提高土壤对氮素的固定, 降低氮淋失。单施有机肥的土壤淋溶液中硝酸盐浓度(31.8 mg∙L^-1)高于施用化肥农田(15.5 mg∙L^-1), 但是两种肥料硝酸盐总淋溶量无显著差异^[52]; 也有研究发现, 单施有机肥的氮淋失小于单施化肥^[53-54], 这与有机肥类型、土壤性状和作物类型都有密切关系。需要指出的是, 这些研究肥料氮投入大多处于较低水平, 远低于当前华北平原每年两季作物的氮肥量。

2.3 降水和灌溉

氮淋失的发生必须有足够水分。土壤水分含量高于田间持水量的情况下, 重力水才会携带氮下移到深层土壤或地下水, 因此水分投入是影响潮土区农田氮淋失的另一个主要因素。与氮肥类似, 在分析氮淋失量与水分投入之间关系时, 采用水量平衡(灌溉量+降水量-蒸散量)比应用单纯的灌溉或降水量更准确、更符合逻辑关系^[11]。

华北地区每年的降水75%发生在夏玉米季的7—9月份(350~450 mm)^[20], 因而由于降水引起的氮淋失量夏玉米季远高于冬小麦季。冬小麦生长季降水较少, 但由于多在播种和春季返青拔节两个施肥时期进行灌溉, 因而氮淋失风险也很高, 这从表 1中的估测模型可以看出。在氮肥投入水平一致的情形下, 灌溉量越高, 氮淋失量越大^{[38, 49, 55-56]}。当前华北潮土区的灌溉方式仍以大水漫灌为主, 灌溉量远超过作物需水量, 灌溉后土壤含水量极易超过田间持水量, 因此降低灌溉量、优化灌溉方式是本区水资源管理和氮淋失控制的有效措施^{[38, 57]}。

华北潮土区冬小麦-夏玉米的根系主要集中在0~180 cm土体中, 其中90%以上的根系集中在0~90 cm土层, 因此控制0~90 cm土层土壤无机氮的残留, 是阻控氮淋失的重要原则^[58-60]。多次、小水量灌溉比少次、大水量灌溉会显著减少氮淋失^[58-60], 减缓水分与氮素向2 m土层以下渗漏, 但在砂质构型土壤中这类灌溉方式不适用。

2.4 肥料增效剂

近年来以缓控释肥、尿酶和硝化抑制剂为代表的肥料增效剂在华北潮土区逐渐推广应用。缓控释肥按照设定的释放率和释放期控制养分释放, 肥效长、与作物养分的需求强度和容量配合程度高, 可以减少养分淋洗^[61-62]。硝化抑制剂可以阻断硝化反应发生, 减少硝态氮的积累从而减少硝态氮损失^[63]。施加3, 4-二甲基吡唑磷酸盐(DMPP)和双氰胺(DCD)可以减少氮淋溶45%~64%^[64-65]。脲酶抑制剂维持土壤氮主要以铵态氮形态存在, 减少淋洗的发生, 与硝化抑制剂类似, 可以降低氮淋失30%以上^[66-68]。同时添加硝化抑制剂和脲酶抑制剂可以有效降低氨挥发和土壤硝酸盐积累, 比单施可进一步减少氮淋溶^[68-69]; 也有研究^[70-71]认为, 同时添加硝化抑制剂和脲酶抑制剂会刺激硝化抑制剂的分解, 增加氮淋溶风险, 主要原因在于大量灌溉将硝态氮快速淋溶到深层土壤, 增加了深层氮淋失, 因此增效剂等还应注意与优化灌溉等技术配合。最新整合研究^[11]表明, 与常规氮肥相比, 增效肥料(缓控肥和硝化抑制剂)可以降低35%的氮淋失, 其中华北和华南地区分别为28%和39%, 在碱性和黏壤土上降低氮淋失效果优于酸性和砂壤土, 还能促进作物增产。此外, 羧甲基纤维素钠、聚丙烯酰胺、聚丙烯酸钠以及保水剂、腐植酸和沸石等也可通过相对固持施入土壤中的肥料氮, 控制其挥发和转化成硝酸盐的速度, 减少氮淋失、提高氮利用效率^[72-73]。

2.5 秸秆还田和耕作

秸秆还田是当前华北平原秸秆资源化利用的主要方式。秸秆对于氮淋失的影响, 一方面短期内通过微生物作用固定铵态氮和硝态氮, 减少土壤无机氮含量, 降低氮淋失^{[38, 74-75]}; 另一方面长期秸秆还田下, 通过形成土壤有机质而实现固氮, 减少氮损失。秸秆还田对氮淋溶的影响取决于还田秸秆本身的碳氮比, 低碳氮比的秸秆还田增加氮淋溶风险^[76-77], 而高碳氮比秸秆还田则相反^[78]。秸秆还田后, 深层土壤的氮固持或反硝化作用也会得到加强, 进而降低氮淋失^[79]。施氮量 < 140 kg(N)∙hm^-2时, 秸秆还田的氮淋失稍高, 而 > 140 kg(N)∙hm^-2时, 秸秆不还田稍高, 但都低于10%^[46]。

秸秆还田可以通过防止水分蒸发、提高土壤保水能力、减少水分渗漏而降低土壤氮淋溶^[80], 但碳氮比较低且秸秆数量较大时, 有可能增加氮淋溶^[81-82]。秸秆还田可以固定肥料氮, 如在玉米季固定的肥料氮达100~280 kg(N)∙hm^-2[83], 这些被固定的氮发生淋溶风险较低, 是作物需求和养分供应之间重要的缓冲库^[84-85]。因此, 通过秸秆还田、维持土壤氮库稳定并将环境污染风险降到尽可能低的程度, 是当前华北潮土区的有效氮淋失阻控措施, 值得重点推广^[86]。

耕作通过影响土壤水分和温度而影响氮转化和迁移, 但因自然条件和作物类型, 研究结果不一致。小麦根系主要分布在土壤较浅层次, 免耕条件下土壤水分蒸发较少, 小麦对深层养分吸收较少, 氮淋失较高; 玉米收获后, 在0~180 cm土体中免耕处理的土壤硝酸盐累积量最高, 氮淋失增加^[38]; 此外也有研究发现免耕可能降低氮淋失^[87]。因此, 耕作对于氮淋失的影响, 还需要更多长期定位试验分析和验证。

2.6 土壤性状

土壤性状如质地等对氮淋失有重要影响^[11]。在质地粗糙、通透性好的土壤中容易发生氮淋失, 沙壤土中氮素淋溶损失量占施氮量的16%~32%, 而在黏壤土中仅为5%左右^[88-89]。也有研究发现, 与粗质地土壤相比, 由于大孔隙会产生优先流(preferential flow), 细质地土壤的硝态氮淋溶损失更多^[90-91]。同延安等^[92]的研究发现, 黏粒含量较低的土壤质地疏松, 硝化作用强容易造成硝态氮积累增加淋溶风险。在pH≥7的时候, 氮淋失量显著高于pH < 7时; 类似的, 在土壤有机碳≥10 g×kg^-1、土壤全氮≥1.0 g×kg^-1以及年降水量 > 700 mm时, 显著高于低于这些条件的土壤中的氮淋失^[44]。土壤性状对于氮淋失的综合影响较大, 因此在最新的定量模拟研究中被列入重要影响因素^[11]。

2.7 种植制度

近年来, 休耕政策的实施逐步改变了华北地区种植制度, 有些地区减少了耗水量高的冬小麦面积, 有些地区则增加了蔬菜、树木或者覆盖作物^[51], 或在冬季或作物茬口之间种植填闲作物^[93], 这些种植制度有些减少了农田氮淋失^[94], 而如果施肥量增加较多(如设施蔬菜), 氮淋失数量反而会增加。与大豆(Glycine max)、花生(Arachis hypogaea)和马铃薯(Solanum tuberosum)等浅根系作物相比, 较深根系作物(冬小麦和夏玉米)更能有效吸收利用土壤氮素, 减少氮淋失^[95]。小麦和玉米间作可以更好地吸收利用硝态氮, 与单作相比, 降低淋失量达30.75%^[96]。种植黑麦草(Lolium perenne)和高丹草(Sorghum sudanese)等填闲作物也是目前华北平原较好的提取深层土壤累积硝态氮和降低氮淋失的措施, 其次是种植甜高粱(Sorghum saccharatum)^[97]。

3 潮土区农田氮淋失阻控措施效果评估

华北平原潮土区的自然条件和农田管理措施相似度较高, 在全面分析当前国内外研究和实践基础上, 对适合本区的主要氮淋失阻控措施的效果、技术可行性、成本以及对作物产量影响等方面进行了综合评估(表 2)。

表 2 华北平原潮土区农田氮淋失阻控措施评价 Table 2 Evaluation of mitigation measures on nitrogen leaching from farmland of Fluvo-aquic soil areas in the North China Plain

阻控措施 Mitigation measures		阻控效果¹⁾ Performances¹⁾	优点 Advantages	缺点 Disadvantages
施肥 Fertilization	1)优化氮肥数量:冬小麦和夏玉米每季施肥150~250 kg (N)×hm^-2, 采用土壤测试与计算、氮肥效应函数、氮素输入输出平衡、淋溶液硝态氮浓度控制等确定^[95] Optimization of fertilizer N rate: seasonal input of 150-250 kg (N)×hm^-2 for winter wheat and summer maize	+++++	节约肥料, 效果最好 Saving fertilizers with better performances	氮肥量确定方法中, 除氮肥效应函数法, 其他方法工作量大、技术难度较高, 较难实施, 适合于大规模种植 Except fertilizer nitrogen effect equation, other methods need high labor input and are technically difficult to be implemented.Applicable for large scale crop production
	2)有机肥和化肥配施 Integration of organic with chemical fertilizers	+++	充分发挥有机肥和化肥优点, 易实施, 提高土壤肥力 Achieving advantages both of organic and chemical fertilizers and easy to implement, improving soil fertility	需要养殖业配套 Requiring the coordination of livestock production
	3)适当提高追肥比例 Increase of the ratio of top-dressing fertilizer	++	实施成本低 Low implementation cost	基肥/追肥比例的确定技术要求较高 High technical competence for the determination of ratio of basal to topdressing fertilizers
	4)磷钾肥配施 Combined use of phosphorous and potassium fertilizers	++	容易实施 Easy to implement
	5)地下水高、砂壤等高风险地区减少氮肥 Reduction of N fertilization in regions of high groundwater and sandy soils	++++	节约肥料和灌溉水 Saving fertilizer and water	需提前进行土壤勘查, 影响作物产量 Soil needs to be investigated beforehand; crop yield may be reduced.
	6)施用缓控释肥和肥料增效剂, 如硝化抑制剂和脲酶抑制剂等 Use of control-released fertilizers and efficiency enhancement products, such as nitrification inhibitor and urease inhibitor	++++	减少肥料综合损失, 提高养分利用效率 Reducing fertilizer N loss, and increasing nutrient efficiency	增加成本 Increasing cost
灌溉 Irrigation	1)节水灌溉(喷灌、滴灌等) Saving water irrigation (sprinkle and drip irrigation)	+++++	节约水资源和成本 Saving water resources and cost	一次性设施投资较高, 设备质量要求较高; 运营期持续维护 High basic investment, high equipment quality, and continuous maintenance during implementation
	2)适当水分胁迫和增加根系深度的灌溉 Irrigation under water-stressed condition and increase of crop root depth	+++	节约水资源和成本 Saving water resources and cost	技术要求高, 田间水分监测和管理复杂 High technical competence, difficult to monitor field water and manage
	3)水肥一体化 Integrated fertilization and irrigation	++++	节约水肥等双重效益, 降低成本 Saving water and fertilizer, and decreasing cost	一次性投资较高, 设备质量要求较高; 运营期持续维护 High basic investment, high equipment quality, and continuous maintenance during implementation
秸秆管理和耕作 Straw management and tillage	1)秸秆粉碎还田 Pulverized straw incorporation	+++	提高土壤氮库和无机氮缓冲容量, 固碳减排 Improving soil N stock and buffering capacity for inorganic nitrogen with carbon sequestration and greenhouse gases reduction	增加机械投入等成本, 可能对出苗率有影响, 可能增加病虫害发生程度 Increasing mechanic input and cost, maybe influencing crop emerging rate and increasing pest and disease
秸秆管理和耕作 Straw management and tillage	2)秸秆深松还田 Deep straw incorporation	+++	有利于创造蒙金土土体构型, 提高土壤肥力和固碳减排 Building up soil structure with loam texture at top and clay texture at bottom of profile; improving soil fertility, carbon sequestration and greenhouse gases reduction	增加机械投入等成本, 可能对出苗率有影响 Increasing mechanic input and cost, maybe influencing crop emerging rate
秸秆管理和耕作 Straw management and tillage	3)改良土壤质地等土壤质量 Improvement of soil quality, such as texture	++	技术难度低 Low technical competence	成本较高, 较难实施 Higher cost and difficult to implement
种植制度和技术 Cropping structure and technologies	1)休耕 Fallow	++	技术难度低 Low technical competence	可能减产, 需要经济补偿 Maybe decreasing yield, and needing eco-compensation
	2)深根和浅根作物轮作、间作 Rotation/intercropping of deep root with shallow root crops	+++	提高肥料利用率, 降低病虫害发生程度 Increasing fertilizer efficiencies and controlling pests and diseases	需调整种植结构, 增加生产、储存和销售复杂程度 Requiring cropping structure adjustment, increasing the complexities of production, storage and marketing
	3)豆科与禾本科作物轮作、间作 Rotation/intercropping of leguminous with cereal crops	+++	固氮, 提高肥料利用率, 降低病虫害发生程度 N fixation, increasing fertilizer efficiencies and controlling pests and diseases	需调整种植结构, 增加生产、储存和销售复杂程度 Requiring cropping structure adjustment, increasing the complexities of production, storage and marketing.
	4)蔬菜与禾本科粮食作物轮作、间作 Rotation/intercropping of vegetables and cereal crops	+++	提高肥料利用率, 降低病虫害发生程度 Increasing fertilizer efficiencies and controlling pests and diseases	需调整种植结构, 增加生产、储存和销售复杂程度 Requiring cropping structure adjustment, increasing the complexities of production, storage and marketing.
	5)引进高产抗性品种 Introduction of high yield and high resistance crop varieties	++	容易操作, 降低农药等投入 Easy to implement, reducing pesticide input	寻找适合的高产抗性作物品种较困难 Difficult to find crop varieties with high productivity and resistance
	6)适当密植 Increase of sowing intensity	++	容易操作 Easy to implement	易出现倒伏 Prone to lodge
	7)优化播种数量和时期 Optimization of sowing timing and quantity	++	成本较低 Lower cost	需结合土壤墒情和天气状况监测, 操作难度增加 Requiring soil moisture and weather monitor, increasing implementation difficulty
政策 Policies^[96]	1)对氮淋失阻控措施进行生态补偿 Eco-compensation for practices of nitrogen leaching mitigation	++	节约水肥资源, 发挥市场手段, 促进农民主动参与 Saving water and fertilizers, using market tools to promote farmer's participation	需要政府配套措施, 价值估算技术难度高 Requiring government measures, and difficult to quantify the economic value of nitrogen leaching
政策 Policies^[96]	2)进行农业用水收费, 促进节水农业 Charging for water use in agriculture and promotion of water saving farming	++	节约水资源, 发挥市场手段, 促进农民主动参与 Saving water and fertilizers, using market tools to promote farmer's participation	可能增加农民成本 Maybe increasing farmers'cost
1)“+”越多, 说明阻控效果越好。1) More “+” indicates better mitigation effects for nitrogen leaching.

表 2 华北平原潮土区农田氮淋失阻控措施评价 Table 2 Evaluation of mitigation measures on nitrogen leaching from farmland of Fluvo-aquic soil areas in the North China Plain

华北平原潮土区氮淋失的主要阻控措施是在不降低产量的前提下, 降低氮肥和灌溉水投入量以及优化施肥方式(表 1和表 2), 但该方法需要充分数据(土壤和作物)支持, 技术难度高, 因而小规模农户应用有一定困难。值得欣喜的是, 近年来由于种粮大户对生产资料水平和经济投入敏感, 土地流转和经营规模增加促进了氮肥和灌溉水减量。笔者在山东桓台县的长期跟踪发现, 近年来该县冬小麦和夏玉米每季氮肥量已经从20世纪90年代中期的300 kg(N)∙hm^-2降到230 kg(N)∙hm^-2以下, 其中种粮大户做出了重要贡献。水肥一体化、节水灌溉等优化灌溉技术面临的主要问题是一次性设施投资和后期运营维护成本较高。

肥料增效剂包括缓控释肥、尿酶和硝化抑制剂, 对于降低氮淋失效果显著、容易实施, 近年来随着施用量增加, 其成本也在逐渐降低。2017年以来笔者等对河北、山东和河南等华北粮区的调研发现, 含有肥料增效剂的肥料已经占据肥料市场的半壁江山, 在降低面源污染中发挥了重要作用。有机无机配施、改善土壤质量等具有包括降低氮淋失等综合效益的措施, 由于收益低, 难以大面积推广应用。在中国政府奖惩政策的持续推动和机械技术的支撑下, 秸秆还田目前已经在华北平原绝大部分粮区推广, 显著提高了土壤氮库和无机氮缓冲容量, 对降低氮淋失有较好效果, 但今后需要在完善机械秸秆还田质量、提高出苗和降低病虫害发生等方面继续进行研发。调整种植制度、休耕、间作套种和种植填闲作物等措施, 可以有效降低氮肥和水投入、减少氮淋失, 但该类措施会影响粮食产量, 因此各地在应用过程中需谨慎考虑。

对各类技术措施的分析还表明(表 2), 除了技术自身外, 氮淋失的阻控效果更多受到社会、经济和政策等因素的影响。首先, 氮淋失阻控措施推广后, 改善土壤质量, 保护水体质量和生态环境, 这些大多属于公共物品, 缺乏相应的补偿机制或奖励措施, 农民何谈主动降低面源污染？此外, 当前我国灌溉用水不收费, 对节水技术的推广也造成了一定障碍。因此实施生态补偿等市场手段进而发挥农民的主动性, 是今后我国农业发展和生态文明建设进程中, 从政策和法律法规层面需要着重考虑的途径。

4 结论

氮肥用量特别是氮盈余量与氮淋失之间总体上呈指数关系, 即氮肥超过一定阈值后氮淋失快速增加, 因此降低氮肥和水分投入是氮阻控的源头控制措施。通过施用缓控施肥和脲酶抑制剂、硝化抑制剂等肥料增效剂较单施化肥能减少33%左右的氮淋失, 应在华北地区大力推广。秸秆还田能够提高土壤有机氮库、促进微生物对无机氮的固定作用、增加土壤中无机氮的缓冲容量, 从而降低氮淋失风险(降低比例达10%)。免耕对氮淋失的阻控效应较施肥和灌溉措施低, 且具有较大不确定性。调整种植制度、休耕、间作套种和种植填闲作物等措施会影响粮食产量, 推广过程中应慎重。氮淋失的阻控效果更多受到社会、经济和政策等因素的影响, 今后应采取包括生态补偿等市场化手段发挥农民主动性, 从政策和法律法规层面创造实现氮淋失阻控措施的社会环境。

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