Impact of depth of groundwater table on nitrogen dynamics in soil under sewage irrigation

Irrigation with industrial and domestic sewage has become increasingly common in most areas in China that are short of fresh water.Unlike fresh water,sewage is rich in nutrients,and irrigation with sewage will therefore give rise to a change in nitrogen dynamics in soils.In this paper we present an experimental study of the impact of groundwater table on the movement and transformation of nitrogen in soil under sewage irrigation.The experiment was carried out in a lysimeter,and the groundwater table was controlled at depth from 2 m to 4 m.The experimental crop was winter wheat and was irrigated with sewage at 900m3·hm-2 and 1200m3·hm-2 respectively.The results indicated that the nitrate concentration in both soil and groundwater increased substantially after the sewage irrigation.The increase of nitrate concentration in groundwater depends on the depth of groundwater table and the amount of irrigation.When irrigated at 900m3·hm-2,the nitrate concentration in the groundwater increased 34.67%,24.94% and 20.88% for groundwater depth at 2 m,3m and 4 m,respectively;whilst when the amount of irrigation increased to 1200m3·hm-2,the associated nitrate concentration in the groundwater increased 58.42%、38.98%、27.21%,respectively.The sewage used in our experiment provided more nutrients than what the crop needed.As a result,the risk of groundwater being polluted by nitrogen increases when groundwater becomes shallow.Since the crops adsorb the nutrients supplied by the sewage,for a given sewage,there should be a critical groundwater depth beyond which the risk of groundwater being polluted becomes negligible.