甘蔗集约化种植区施肥显著增加入河硝态氮污染:基于氮氧同位素的流域示踪

Fertilization in intensive sugarcane planting areas significantly increased riverine nitrate pollution:Evidenced by nitrogen and oxygen isotopes in a watershed

【目的】过量施用肥料会导致土壤养分过剩,在降雨径流冲刷下极易流失进入河湖水体,引起地表水体污染。本研究旨在阐明农区施肥与入河硝态氮污染的关系,为有效防控农业面源污染提供科学依据。【方法】选择广西集约化蔗区客兰水库水源区的那辣流域,应用双稳定同位素(δ^(15)N-NO_(3)-、δ^(18)O-NO_(3)-)示踪技术,确定流域内3个子流域(S1、S2和S3)入河硝态氮的来源比例,结合地表水主要离子测定,探讨了丰水期和枯水期硝态氮主要来源贡献的时空变化机理及其对施肥的响应。【结果】那辣流域水体NO_(3)^(-)-N浓度范围是1.24~27.90 mg/L,超过国家地表水Ⅲ类水质标准。丰水期施肥量较多,化肥在降雨径流的冲刷下大量流失进入水体,致使NO_(3)^(-)-N浓度在丰水期(均值为22.01 mg/L)显著高于枯水期(均值为5.64 mg/L),丰水期和枯水期NO_(3)^(-)-N浓度从上游向下游呈逐渐下降的趋势,可能是下游区域做植物河道梯级拦截试验所致。δ^(15)N-NO_(3)^(-)、δ^(18)O-NO^(3)-的组成表明流域施肥(化肥和有机肥)与土壤氮素是入河水中硝态氮污染的主要来源,占入河所有硝态氮污染源贡献的90%以上。在丰水期,流域入河硝态氮来源贡献的大小顺序为化肥(42.9%)>土壤氮(28.6%)>有机肥(19.2%)>大气沉降(9.3%);枯水期入河硝态氮来源的贡献大小顺序为土壤氮(39.7%)>化肥(37.6%)>有机肥(15.3%)>大气沉降(7.4%)。【结论】流域施肥与入河硝态氮污染呈正相关,不合理的施肥方式,如肥料撒施,是集约化蔗区入河硝态氮污染加剧的主要原因。因此,避免肥料撒施、强化肥料深施,是提高肥料利用率、减少氮肥流失入河污染,从而保障人畜饮水安全和农业绿色发展的重要途径。

【Objectives】Excessive application of fertilizer will lead to excess soil nutrient accumulation,which will be washed into rivers and lakes by rainfall runoff,resulting in surface water pollution.This study aims to clarify the relationship between fertilization in agricultural land and nitrate(NO_(3)–-N)pollution downstream for effectively controlling agricultural non-point source pollution.【Methods】In the intensive sugarcane growing area of Guangxi,we selected Nala Watershed flowing into Kelan Reservoir as case study watershed.This watershed includes three sub-watersheds,S1,S2 and S3.We identified the source proportion of NO_(3)^(–)-N from three sub-watersheds into stream by using double stable isotope tracer(δ^(15)N-NO_(3)^(–),δ^(18)O-NO_(3)^(–))technique,and determination of main ions in combined surface water.Integrating with watershed monitoring,we analyzed the spatial-temporal variations in NO_(3)–-N sources contributing to the stream and their response to fertilization in wet and dry seasons.【Results】The concentration of NO_(3)--N in the stream of Nala Watershed ranged from 1.24 mg/L to 27.90 mg/L,exceeding the national standard for Type III surface water quality.A larger application of fertilizer was in wet season than in dry season resulted in significantly higher NO_(3)--N concentration of stream during wet season(mean 22.01 mg/L)than that in dry season(mean 5.64 mg/L).NO_(3)--N concentration gradually decreased from upstream to downstream in both wet and dry seasons,which might be caused by the cascade interception experiment applying vegetated ditches in the downstream area.The composition ofδ^(15)N-NO_(3)-andδ^(18)O-NO_(3)-indicated that fertilization(chemical fertilizer and organic fertilizer)and soil nitrogen were the main sources of riverine nitrate pollution,accounting for more than 90%of all nitrate pollution contributions in the river.The contribution of NO_(3)--N sources to the stream in Nala Watershed decreased in the following order:fertilizer(42.9%)>soil nitrogen(28.6%)>organic fertilizer(19.2%)>atmospheric deposition(9.3%)in wet season,and soil nitrogen(39.7%)>chemical fertilizer(37.6%)>organic fertilizer(15.3%)>atmospheric deposition(7.4%)in dry season.【Conclusions】Riverine nitrate pollution is positively correlated with fertilization in the intensive sugarcane growing watershed.Irrational application such as fertilizer spreading was the main cause for NO_(3)--N export.Thus,fertilizer deep application is strongly recommended for improving fertilizer utilization and reducing nitrogen inflow pollution.This may be an important practice to ensure the water safety of both people and livestock and agriculture green development.