中国北方农田氮磷淋溶损失污染与防控机制

Mitigation of nitrogen and phosphorus leaching from cropland in northern China

突破厚包气带农田根层氮磷淋溶与地下水污染复杂定量关系和阻控机理是国际研究难点。本文系统梳理了重点研发专项"农田氮磷淋溶损失污染与防控机制"项目取得的主要进展,项目包括以下4方面研究内容:1)北方主要农区农田根层氮磷淋溶时空规律;2)根层-深层包气带氮磷淋溶机制和主控因子;3)黑土、潮土和褐土氮磷淋溶阻控机制及其效果;4)典型农区氮磷淋溶风险与区域消减途径。主要科学发现包括:1)受土地利用类型、地下水埋深、包气带岩性、水文地质条件等综合因素的影响,黑土区、潮土区和褐土区根层氮磷淋溶规律与地下水硝酸盐超标率体现出空间不一致和较大差异性。黑土区虽然根层淋溶较小,然而受地形地貌影响,地下水水质对淋溶响应更强烈,应该进一步研究黑土区地下水水质对淋溶的响应机制。华北潮土区和褐土区厚包气带具有明显氮阻控能力,应该进一步加强厚包气带对氮磷淋溶减排机理与途径研究。2)基于长期施肥定位试验和12 m深观测井对包气带农田土壤氮盈余累积特征和淋失规律的研究发现,华北平原区的环境安全施氮量约为200kg(N)·hm-2·a-1,超过环境安全阈值的多投入氮肥中有51%淋失到1m根层以下,不合理灌溉、强降水、大孔隙和裂隙是造成土壤硝酸盐淋溶的主要因素,对包气带累积硝态氮的淋失作用可影响至6m以下土层。3)利用深层取样和生物学方法结合,对厚包气带0~10.5m原位土壤微生物的反硝化活性和微生物区系组成的研究结果表明,表层土壤是微生物进行反硝化的主要场所,深层土壤中反硝化作用显著减弱,"碳饥饿"是限制底层土壤反硝化微生物丰度与活性的关键因素;室内培养试验证实添加碳源可有效激活土壤微生物的反硝化活性,为"根层截氮包气带脱氮"的淋溶阻控机理找到了突破口。4)利用黑土、潮土和褐土区氮磷淋溶阻控试验、全国农业面源污染国控监测网、北方农区地下水硝酸盐监测网和NUFER (NUtrient flows in Food chains, Environment and Resourcesuse)模型,提出了养分损失脆弱区区划和区域氮磷污染削减草案,可为农业绿色发展和面源污染阻控提供科学依据。

Understanding on nitrogen(N)and phosphorus(P)leaching from root-vadose-groundwater system and mitigation options is research gaps in the world.We presented the main research progresses of a project‘Mitigation of Nitrogen and Phosphorus Leaching from Cropland in China’in National Key Research and Development Program in this paper.Four research contents are included:1)clarifying temporal and spatial variation of N and P leaching out of root zone;2)analyzing nitrogen and phosphorus leaching in the‘root-vadose-groundwater’system;3)exploring mitigation options of leaching in cereals and vegetables fields of three soil types,namely black soil,cinnamon soil and fluvo-aquic soil;and 4)exploring regional strategies for decreasing N and P leaching in the‘root-vadose-groundwater’system.Main scientific findings are as follows:1)the N leaching in root zone shows different trends with exceeding standard rates of nitrate in groundwater in three soil type areas,due to their differences in land use,groundwater depth,lithologies of vadose zones and hydrogeological conditions etc.In black soil areas,although the N leaching is not high in root zones,the interactions between groundwater quality and N leaching in root zone are more sensitive because of the topography.Therefore,more researches are needed to explore the interaction mechanism between groundwater quality and leaching N in black soil areas.In cinnamon and fluvo-aquic soil areas in the North China Plain,vadose zones are deep and can buffer N leaching from root zone to groundwater.Thus,it is also necessary to further explore the N leaching mitigation mechanism of vadose zones.2)Based on the long-term fertilization experiments and observation in 12 m deep borehole,we analyzed the accumulation characteristics of N surplus in vadose zones of farmland.The results show that the safe N application rate in the North China Plain is about 200 kg(N)·hm−2·a−1.If the threshold is exceeded,51%of the N would leach to out of the root areas(1 m).The unreasonable irrigation,heavy rainfall,macrovoids and crack are the main causes of soil nitrate leaching.It can lead accumulated nitrate in vadose zone to be leached to below 6 m.3)Combined deep sampling and biological method,we analyzed denitrification activity and floristic composition of soil microbe in 0−10.5 m of vadose zones.The results show that surface soil is main site for microbial denitrification,while in deep soil layer the denitrification weaken significantly,which indicates that“Carbon Starvation”is the key factor for limiting the abundance and activity of denitrifying microbes in the bottom soil.Furthermore,the indoor incubation experiment proved that adding carbon could effectively activate the soil denitrifying microbes,which explained the mechanism of“the nitrogen interception in root areas and denitrification in vadose zones”.4)Using the data from N and P leaching mitigation experiments,national agricultural non-point source pollution monitoring network,groundwater nitrate monitoring network in northern China and NUFER(NUtrient flows in Food chains,Environment and Resources use)model,we compartmented nutrient losses vulnerable zones.Based on this,we explored regional mitigation strategies of N and P leaching,which could provide scientific support for non-point sources pollution.