农田排水口高度对地表径流氮磷流失的影响

马瑛骏，万 辰，张克强，姜海斌，王 风，沈仕洲

农田排水口高度对地表径流氮磷流失的影响

马瑛骏1,2,3，万 辰1,3,4，张克强1,3，姜海斌1,3，王 风1,3，沈仕洲1,3※

（1. 农业农村部环境保护科研监测所，天津 300191； 2. 东北农业大学资源与环境学院，哈尔滨 150030；3. 国家农业环境大理观测实验站，大理 671004； 4. 云南农业大学资源与环境学院，昆明 650201）

洱海流域农田径流氮磷污染严重，大量氮磷污染物随雨水进入洱海，导致洱海水质雨季下降。为从源头控制氮磷污染物的输出，该研究采用人工模拟降雨的方法，探究5、10、15、20、25 cm 5种不同高度的排水口对农田径流氮磷流失的控制作用。结果表明，农田排水口较低会造成产流初期硝态氮和颗粒态氮浓度升高，将排水口高度提高到15 cm以上可有效降低径流中各形态氮磷浓度，并稳定在较低水平；排水口高度从5 cm提高至15～25 cm产流中总氮、颗粒态氮、铵态氮、硝态氮流失量分别降低了85.60%～93.13%、88.39%～95.77%、84.59%～91.72%、63.05%～65.15%，总磷、颗粒态磷流失量分别降低了86.75%～92.66%，61.64%～94.61%，且排水口设置在15 cm高度处氮、磷流失量削减效果突出，在15 cm基础上继续提高排水口不会对氮磷流失量产生明显影响。综上所述，将排水口提高到15～25 cm对农田径流污染控制效果优越。结合洱海流域多年降雨资料及建设成本，推荐将农田排水口设置于距土壤表面15 cm高度处，对控制农田养分流失，减少面源污染起到显著效果。

农田；径流；排水口高度；模拟降雨；氮磷流失

0 引 言

农田径流污染是指在雨水的冲刷作用下，大气沉降物及农田里各种污染物质随径流进入水体环境造成的污染[1]，是农业面源污染的主要来源[2-3]，也是引起水体富营养化的重要原因[4]。近年来，国内外学者针对农田径流污染展开了大量研究，2015年美国环保局[5]发布数据显示农田径流污染对水资源污染的贡献率接近50%，更是河流氮的主要来源（占70%）。根据中国2020年发布的《第二次全国污染源普查公报》[6]显示农田径流产生的总氮排放量达71.95万t，总磷排放量达7.6万t，占农业源总氮排放量的51.2%，总磷排放量的35.4%。洱海地处西南山区，降雨量大，且降雨比较集中，是典型的径流易发区，项颂等[7]研究表明2019年洱海流域农田径流总氮、总磷排放量为1 173.8、100.7 t，在总等标污染负荷中占比最高，为42.6%和38.8%。研究表明，在作物种植期间，少数几次大的流失事件往往决定了氮磷等养分的年流失总量，高超等[8]研究发现施肥后立即降雨会导致磷素大量流失，单次流失量达到当季总流失量的39.8%，三次强降雨中磷的流失量占整季磷流失量的72.0%。因此，控制由强降雨引发的径流氮磷流失极为关键，对农田面源污染防控具有重要意义。

洱海是云南省第二大高原淡水湖，是国家重点保护水域，是大理人民的饮用水源，受自然条件和人类活动加剧的影响，近年来洱海水质呈现旱季较好、雨季超标，水体富营养化程度逐步加重等特点[9-11]。氮、磷是引起水体富营养化的主要因子。洱海流域为典型性的农业流域，水稻种植面积大，占该流域总面积的10%左右，且水稻种植季正处该流域年内降雨高峰月份，暴雨事件多发，由此引发的农田营养盐流失是影响洱海水质雨季下降的关键环境因子[12-14]。目前，针对洱海流域农田氮磷流失的问题已展开大量研究，研究主要侧重于施肥管理和轮作模式[15-18]，而利用农田排水口进行防控的研究鲜有报道。农田排水口是农田水利设施的组成部分之一，具有防洪排涝、灌溉排水等功能，同时也是陆源污染物进入江河、湖泊等水体的通道[19]。因此，通过提高排水口高度减少降雨产生的农田排水，可为削减氮磷等营养物质的输出提供有效方法。基于以上背景，该研究采用人工模拟降雨的方法，根据该流域农田排水口高度设计5种不同高度的排水口，在流域最大降雨强度70 mm/h条件下探究洱海流域农田径流氮磷流失特征，筛选有效控制农田径流污染的排水口高度，为洱海流域农田排水口高度的设定提供科学依据。

1 材料与方法

1.1 试验材料与装置

供试土壤取自国家农业环境大理观测实验站（北纬25°53′34″，东经100°10′27″），0～20 cm供试土壤pH值为7.2、容重为1.4 g/cm3、有机质含量为36.2 g/kg、全氮为3.1 g/kg、全磷为0.9 g/kg、有效磷为35.3 g/kg。根据洱海流域农业施肥习惯，选用YNFHFL2021-00345复合肥，N+P2O5+K2O≥25%，配比为13:5:7，施肥水平N 195 kg/hm2、P2O575 kg/hm2、K2O 105 kg/hm2。

模拟降雨试验在国家农业环境大理观测实验站降雨厅内进行，采用南京南林电子科技有限公司NLJY-10人工模拟降雨系统，共设4组降雨喷头，每组由3种不同大小的喷头组成，降雨区域为6 m×4 m，雨强在15～240 mm/h范围内连续可调，降雨均匀度系数大于88%。大理州气象局发布的气候公报[20-21]显示该流域降雨高峰期（7－8月）暴雨事件多发，最大降雨强度达70 mm/h，结合试验目的，确定70 mm/h为本试验模拟降雨强度。试验土槽箱结构如图1所示，规格为0.70 m×0.50 m× 0.55 m（长×宽×高）。根据洱海流域农田排水口高度实测结果（距土壤表面5～10 cm）和降雨高峰期水稻植株高度（50～100 cm），设置5种不同高度的排水口，排水口底端距土壤表面5、10、15、20、25 cm记为H1、H2、H3、H4、H5。共计5个处理，每个处理设3次重复。

1.2 试验过程及指标测定

将采集的0～20 cm土壤充分混匀，填充于15个土槽箱中，测定土壤容重为1.4 g/cm3。土壤填充后，向土槽箱内注入自来水，直至田面水深度达到5 cm，并对自来水进行各形态氮磷浓度检测，自来水总氮浓度为0.66 mg/L，颗粒态氮浓度为0.14 mg/L，铵态氮浓度为0.21 mg/L，硝态氮浓度为0.28 mg/L，总磷浓度为0.10 mg/L，颗粒态磷浓度为0.03 mg/L。将复合肥均匀撒入土槽箱内，静置一周后对箱内田面水进行氮磷浓度测定，田面水总氮浓度为63.59 mg/L，颗粒态氮浓度为14.32 mg/L，铵态氮浓度为38.65 mg/L，硝态氮浓度为1.34 mg/L，总磷浓度为7.71 mg/L，颗粒态磷浓度为0.73 mg/L。降雨开始后，记录每个土槽箱的初始产流时间，产流后进行60min径流取样，前30min内每5 min采集一次径流，后30min每隔10 min采集一次，同时记录流量。降雨结束后，记录停止产流时间。将收集的径流液带回实验室进行分析测定，径流液总氮（Total Nitrogen，TN）含量采用碱性过硫酸钾紫外分光光度法，总磷（Total Phosphorus，TP）采用钼酸铵分光光度法测定；将水样经0.45m滤膜过滤后用紫外分光光度法、纳氏试剂分光光度法、碱性过硫酸钾紫外分光光度法、钼酸铵分光光度法测定滤液中硝态氮（Nitrate Nitrogen，NO-3-N）、铵态氮（Ammonium Nitrogen，NH+4-N）、溶解态总氮（Total Dissolved Nitrogen，TDN）和溶解态总磷（Total Dissolved Phosphorus，TDP）含量；径流液中颗粒态氮（Particulate Nitrogen，PN）和颗粒态磷（Particulate Phosphorus，PP）的含量利用差减法计算得出。

1.3 数据分析

采用Excel 2019进行数据处理和图表制作，SAS 9.0软件对数据进行方差分析（显著性差异水平设置为 0.05）和相关性分析。

2 结果与分析

2.1 农田排水口高度对产流时间的影响

图2显示了不同的排水口初始产流时间，H5初始产流所需时间最长，平均达到186.44 min，显著高于其他4组。H1与田面水高度接近，初始产流所需时间最短，在降雨后2～3 min立即开始产流。初始产流时间与排水口高度之间呈对数关系。

2.2 农田排水口高度对径流氮素浓度变化的影响

图3展示了5种不同高度的排水口所产径流中总氮、铵态氮、硝态氮、颗粒态氮的浓度变化趋势。由图3a可知H1在产流过程中总氮浓度从63.90 mg/L下降至23.34 mg/L，且产流60 min内各时段总氮浓度均显著高于其他处理；H2产流过程中总氮浓度表现为平缓下降趋势，浓度从36.01 mg/L降低至18.72 mg/L；H3、H4、H5在产流过程中总氮浓度无显著性变化，浓度均稳定在2.22～6.91 mg/L内。图3b展示了铵态氮浓度的变化趋势，H1、H2径流中铵态氮浓度呈现平缓下降趋势；H3、H4、H5在产流过程中铵态氮浓度无显著变化，稳定在1.53～3.85 mg/L范围内，且各时段浓度均显著低于H1、H2。图3c、3d显示了硝态氮、颗粒态氮浓度变化趋势，分析发现H3、H4、H5在产流60 min内硝态氮、颗粒态氮浓度均保持在0.30～0.55、0.15～6.28 mg/L范围内，呈稳定状态；而H1在产流过程中硝态氮、颗粒态氮浓度则随着产流时间的增加呈现先上升后下降的趋势，经分析，H1中硝态氮浓度和颗粒态氮浓度之间相关性系数达到0.949，相关性强。

2.3 农田排水口高度对径流磷素浓度变化的影响

图4a可知，H1产流中总磷浓度不断下降，从5.98 mg/L下降至2.17 mg/L；H2中总磷浓度呈现平缓下降趋势，且H1、H2在产流60 min内各时段总磷浓度显著高于其他3组；H3、H4、H5径流中总磷的浓度随产流时间的增加未发生明显变化，浓度稳定在0.20～0.48 mg/L。图4b展示了颗粒态磷浓度变化趋势，H1产流过程中颗粒态磷浓度不断降低，从0.93 mg/L降至0.13 mg/L；而H2、H3、H4、H5产流中颗粒态磷浓度相接近，且各时段无明显变化。

2.4 农田排水口高度对径流氮、磷流失量的影响

由图5可知，提高农田排水口高度能够显著降低氮磷流失量。图5a、5b中H5总氮、颗粒态氮、铵态氮、硝态氮流失量最低分别为1.77、0.52、1.06、0.21 kg/hm2；H1总氮、颗粒态氮、铵态氮、硝态氮流失量最高分别为25.80、12.29、12.84、0.62 kg/hm2。对于磷素，图5c，H4、H5总磷、颗粒态磷的流失量最低分别为0.15～0.16、0.01～0.02 kg/hm2；H1总磷、颗粒态磷流失量最高分别为1.98和0.22 kg/hm2。

整体分析发现，5个处理中颗粒态氮、磷流失量明显低于溶解态总氮、磷流失量，经计算，溶解态总氮、磷的流失量可达到总氮、磷流失量的52.37%～83.64%和67.83%～92.29%；铵态氮流失量明显高于硝态氮流失量，铵态氮流失量占总氮的47.85%～80.80%，硝态氮流失量仅占2.15%～11.80%。

排水口高度变化对氮磷流失削减率的影响如表1所示。首先，对总氮、总磷流失量削减率进行分析，排水口高度从距离土壤表面5 cm提高到15 cm高度处时总氮、总磷流失削减效果突出，削减率达到85%以上，与排水口提高到10 cm高度处相比总氮、总磷流失削减率提高了43.43% 和47.41%；排水口高度从5 cm提高到20、25 cm高度处时总氮、总磷流失量削减率达到91%以上，削减效果最好，但与排水口提高到15 cm高度处相比削减率仅提高了5.82～7.53个百分点，削减率提升幅度很小。再对不同形态氮磷流失量削减率进行分析，排水口高度从5 cm提高到25 cm高度处时铵态氮、硝态氮流失量削减率最高，削减率分别为91.72%和65.15%，同样在排水口提高到15 cm高度处时削减率基本稳定，分别达到84.59%和64.49%。颗粒态氮流失量削减率在排水口提高到10 cm高度处时到达80.14%，削减效果突出，继续将排水口提高到15～25 cm高度处颗粒态氮流失量削减率仅有8.25～15.62个百分点的提升空间。颗粒态磷流失量削减率在排水口提高到20 cm高度处时基本达到稳定值90.98%，此后继续提高排水口高度对颗粒态磷流失量削减率的影响较低。

表1 不同高度农田排水口产流中各形态氮、磷流失量的削减率

注: “5→10”表示排水口从5 cm提高至10 cm，其余类似。

Note: ‘5→10’ indicates that the drainage outlet increases from 5 cm to 10 cm, other similar.

3　讨 论

田面水对地表有保护作用，可使土壤表层免受雨水直接冲击，5 cm的排水口高度较低，田面水少，受雨水冲击后浅层土壤被剥离，释放出细颗粒态氮；硝态氮在土壤中形态较为稳定，与土壤颗粒之间的作用力较弱，在雨水的冲刷下极易流失[22-24]。因此，本研究中排水口高度为5 cm的处理组在降雨初期产流中硝态氮和颗粒态氮浓度先上升，后随着降雨时间增加，雨水积累，田面水上升，土壤紧实度增加，不易被剥离，硝态氮和颗粒态氮浓度开始下降。

试验中氮磷主要的流失形式是溶解态，溶解态总氮流失量占总氮流失量的52.37%～83.64%，溶解态总磷占总磷流失量的67.83%～92.29%；铵态氮流失总量显著大于硝态氮，其中铵态氮流失量占总氮流失量的47.85%～80.80%，硝态氮流失量仅占2.15%～11.80%，说明总氮流失的主要形式是铵态氮。这一结果与前人研究结论一致[25-27]，原因是化肥中的氮磷主要以无机态形式存在，遇水极易转化为溶解态氮磷，直接撒施固体复合肥可使田面水中溶解态氮磷浓度迅速升高，极易随径流损失；另一方面，随着雨水的不断积累，田面水上升，表层土免受雨水直接冲击，因此颗粒态氮、磷流失程度较低。

由于洱海流域水稻种植季正处该流域年内降雨高峰月份（5－10月），降雨量占全年的75.26%～91.40%（1989－2019年），多年最大月降雨量达到356.0 mm（1999年8月），雨季月均降雨量为147.9 mm，24 h降雨量超过50 mm的强降雨情况时有发生[12]，结合不同高度农田排水口对氮磷流失的削减效果，将排水口从5 cm提高至15 cm削减效果突出，总氮流失量降低85.60%，总磷流失量降低86.75%；在15 cm基础上继续提高排水口高度对氮磷流失削减效果影响很小。因此，在距离土壤表面15 cm高度处设置农田排水口最合适。段四喜等[28]通过生态拦截系统对洱海流域农田尾水进行净化，研究结果显示生态沟渠+表流库塘系统对污染物总氮、总磷削减效果最好，削减率为59.70%、55.90%。姜海斌等[29]通过探究洱海流域水稻种植的合理施肥模式减少农田径流氮磷流失，结果显示有机无机配施总氮流失量降低31.60%～40.40%，但会引起籽粒产量下降11.80%～42.90%。杨世琦等[30]通过模拟试验探究了植物篱埂垄向区田技术对农田径流氮磷流失的控制作用，结果表明植物篱埂处理农田氮流失平均降低了19.70%。姚金玲等[31]则是通过探究合理的轮作与施肥方式对农田土壤径流氮磷损失进行控制，结果表明施用相应作物专用缓释掺混肥能够有效降低土壤径流氮磷损失，削减率达到10.70%～28.80%和17.10%～47.90%。对比以上氮磷减排措施，本研究中通过提高农田排水高度，氮磷流失削减率可达到85%以上，是一种较为有效的氮磷减排措施，对农业面源污染防治有一定价值。

4 结 论

1）农田排水口较低会造成产流初期硝态氮和颗粒态氮浓度升高，将排水口高度提高到15 cm以上可有效降低径流中各形态氮磷浓度，并稳定在较低水平。

2）提高农田排水口高度，径流中各形态氮磷流失量明显下降。将农田排水口高度从5 cm提高至15～25 cm产流中总氮、总磷流失量分别降低了85.60%～93.13%、86.75%～92.66%。且农田排水口设置在15 cm高度处氮磷减排效果突出，在15 cm基础上继续提高排水口则不会对氮磷流失量产生明显影响。

3）水田养分流失的主要形态是溶解态，溶解态总氮、磷流失量占总氮、磷流失量的52.37%～83.64%和67.83%～92.29%，氮素流失以铵态氮为主，占总氮流失量47.85%～80.80%。

从以上结论能够看出，农田排水口设置在距离土面15～25 cm高度处对农田径流氮磷流失控制效果优越，结合洱海流域多年降雨资料及建设成本，推荐将农田排水口设置于距土壤表面15 cm高度处，对控制农田养分流失，减少面源污染起到显著效果。由于模拟试验存局限性，自然环境下影响农田径流氮磷流失的因素更为复杂，将排水口提高到15 cm高度是否对作物产量造成影响仍需探讨，后续会在田间自然环境下进一步试验，为洱海流域农田排水口高度的确定提供科学依据。

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Effects of the heights of farmland drainage outlets on nitrogen and phosphorus loss from surface runoff

Ma Yingjun1,2,3, Wan Chen1,3,4, Zhang Keqiang1,3, Jiang Haibin1,3, Wang Feng1,3, Shen Shizhou1,3※

(1.,,300191,; 2.,,150030,; 3.,671004,; 4.,,650201,)

The Erhai Lake is the second second-largest freshwater resource in the Yunnan Plateau (southwest China). The lake is also a national protected area and drinking water source for Dali residents. The water quality of the Erhai Lake is has gradually aggravated eutrophication, even above an acceptable level in the rainy season, due mainly to global natural conditions and intensified human activities in recent years. Specifically, nitrogen (N) and phosphorus (P) are the main factors causing eutrophication. Furthermore, the rice planting area accounts for about 10% of the total area of Erhai Lake Basin, a typical agricultural basin. The rice planting season is in the peak month of rainfall in the basin, where rainstorm events occur frequently. The loss of nutrients in farmland subjected to rainstorms has been a key environmental factor to determine the water quality of Erhai Lake in the rainy season. Much effort has been dedicated to the loss of nitrogen and phosphorus in the farmland of the Erhai Lake Basin, particularly on fertilization management and rotation mode. Farmland drainage outlets can serve as the channels for terrestrial pollutants to enter rivers, lakes, and other water bodies. However, only a few studies focused on the farmland drainage outlets for pollution prevention and flood control. Taking the height of the drainage outlet in the Erhai Lake Basin as a research object, this study aims to find an effective way to reduce the nitrogen and phosphorus loss from surface runoff in farmland. An artificial rainfall simulation was also adopted. Five drainage outlets were set with different heights, according to the height of the farmland drainage outlet (5-10 cm from the soil surface), and the height of the rice plant in the peak period of rainfall in the study area. The bottom distances of the drainage outlet were 5, 10, 15, 20, and 25 cm from the soil surface. An investigation was finally made to evaluate the control of drainage outlets at different heights on nitrogen and phosphorus losses in farmland runoff. The results showed that: 1) The low drainage outlet of farmland resulted in the increase of nitrate nitrogen and particulate nitrogen concentrations at the early stage of runoff generation. The drainage outlet height of more than 15 cm effectively reduced the concentrations of nitrogen and phosphorus in various forms, all of which be stabilized at a low level. 2) The losses of nitrogen and phosphorus were significantly reduced, with the increase in the height of farmland drainage outlets. A better control was achieved at the height of the farmland drainage outlet increasing from 5 to 15-25 cm. Specifically, the losses of total nitrogen, particulate nitrogen, ammonium nitrogen, and nitrate nitrogen reduced by 85.60%-93.13%, 88.39%-95.77%, 84.59%-91.72%, 63.05%-65.15%, respectively. The losses of total phosphorus and particulate phosphorus decreased by 86.75%-92.66%, 61.64%-94.61%, respectively. Moreover, there was an extremely high reduction of nitrogen and phosphorus losses, when the farmland drainage outlet was set at a height of 15 cm. But there was no significant change over 15 cm. 3) The main form of nutrient loss was the dissolved state in the paddy field. The dissolved nitrogen and phosphorus losses accounted for 52.37%-83.64% and 67.83%-92.29% of the total nitrogen and phosphorus losses, respectively. The inorganic nitrogen loss in runoff was mostly ammonium nitrogen, accounting for 47.85%-80.80% of the total nitrogen loss. Consequently, the drainage outlet at the height of 15-25 cm can be expected to achieve a superior performance for the runoff pollution control in farmland. Anyway, it is strongly recommended to be 15 cm high for the farmland drainage outlet in the Erhai Lake Basin. This finding can provide a significant support to control nutrient loss in farmland, thereby to improve improving the ecological environment.

agriculture; runoff; heights of drainage outlets; simulated rainfall; loss of nitrogen and phosphorus

马瑛骏，万辰，张克强，等. 农田排水口高度对地表径流氮磷流失的影响[J]. 农业工程学报，2021，37(15)：114-120.doi：10.11975/j.issn.1002-6819.2021.15.014 http://www.tcsae.org

Ma Yingjun, Wan Cen, Zhang Keqiang, et al. Effects of the heights of farmland drainage outlets on nitrogen and phosphorus loss from surface runoff[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(15): 114-120. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2021.15.014 http://www.tcsae.org

2021-05-31

2021-07-01

国家重点研发计划项目（2017YFD0800103）；云南省科技创新开放基金（2017HC015）；云南省基础研究青年基金（2019FD120）；中央级公益性科研院所基本科研业务费专项（Y2021PT01）

马瑛骏，研究方向为农业面源污染防治。Email：ma_yingjun@126.com

沈仕洲，博士，助理研究员，研究方向为农业面源污染治。Email：shenshizhou@126.com

10.11975/j.issn.1002-6819.2021.15.014

S282

A

1002-6819(2021)-15-0114-07