长江三角洲农田地下水反硝化对硝酸盐的去除作用

Groundwater Nitrate Removal through Denitrification under Farmland in Yangtze River Delta

长江三角洲(简称"长三角")农田氮素投入量高,但是否像其他高氮投入农田一样在土壤剖面累积了大量硝酸盐尚不清楚。通过连续两年的野外观测结合室内培养实验,发现长三角地区3种不同类型的高氮投入农田1～4 m地下水硝态氮(NO_3^--N)剖面分布特征存在明显差异,水稻田地下水NO_3^--N浓度始终很低(<1mg·L^(-1)),不同深度之间无差异。蔬菜地和葡萄园1 m处地下水NO_3^--N年平均浓度分别为5.6和17.5 mg·L^(-1),但是地下水NO_3^--N浓度随着深度增加急剧下降,至4m处,NO_3^--N浓度降至小于1 mg·L^(-1),与水稻田无差异。蔬菜地和葡萄园地下水高浓度NO_3^--N仅出现在施肥期间,非施肥期地下水NO_3^--N浓度较低,这表明长三角农田不存在明显的NO_3^--N累积。原状土柱培养实验结果表明,0～4 m土壤均存在较强的反硝化活性。通过对地下水中反硝化产物N2及N2O的直接定量测定,发现反硝化对地下水NO_3^--N的去除效率随着深度而增加,至4m处,反硝化对地下水NO_3^--N的去除效率分别为86%(水稻田)、93%(蔬菜地)和89%(葡萄园)。这表明反硝化能有效去除地下水NO_3^--N,是长三角地区农田土壤剖面未产生NO_3^--N累积的重要原因。反硝化产生的溶解性气态氮主要通过地下水流入临近水域,对于蔬菜地和葡萄园而言,溶解性气态氮流失量与NO_3^--N淋溶损失量相当,是一个重要的氮素去向,值得关注。

【Objective】The Yangtze River Delta is one of the areas,where the input of nitrogen(N)fertilizer is very high,in China,but it is not quite clear whether nitrate(NO3--N)has accumulated in the soil profile in the region as in the North China Plain.【Method】In the present study,two-year field observation combined with indoor incubation experiments were carried out to investigate distribution of NO3--N and denitri?cation capacity in subsoil and groundwater as affected by arable land systems.【Result】The groundwater NO3--N concentration in 1~4 m depth was varied significantly with type of the cultivation system.In the paddy field,groundwater NO3--N was always low(<1 mg·L-1),and did not vary much with depth.While in the vegetable field and vineyard,the average NO3--N concentration reached 5.6 and17.5 mg·L-1 in 1 m depth,but the NO3--N concentration dropped sharply with the depth and to a very low concentration(<1 mg·L-1)in 4 m depth which was comparable to that in the paddy field.High groundwater NO3--N were only observed in vegetable field and vineyard during the fertilization period and NO3--N was low in non-fertilization period.The findings indicated that no apparent NO3--N accumulation in the farmland soil profile occurs in the Yangtze River Delta.The indoor incubation experiment using undisturbed soil columns shows that denitri?cation activity was high through the 0~4 m soil profile.The removal efficiency(RE)of NO3--N by denitrification was estimated by direct determination of N2 and N2 O,products of denifrifiction,in groundwater.The RE increased with depth and removed 86%,93%and 89%of the groundwater NO3--N in 4 m depth in paddy field,vegetable field and vineyard,respectively.【Conclusion】All the findings in this experiment demonstrated that denitrification can effectively remove groundwater NO3--N,which explains why NO3--N does not accumulate in farmland soil profiles in Yangtze River Delta.Highly dissolved N2 produced by denitrification accumulate in groundwater,and flow into nearby water bodies mainly via groundwater.In the case of vegetable field and vineyard,soil N loss in the form of dissolved gaseous N is nearly equal to that of NO3--N and hence an important fate of soil N,which deserves close attention.