The Influence Of Agricultural Nitrogen Pollutants Input In Vegetable-Soil-Groundwater System

In recent years, with the improvement of living standard, many people have put forward higher requirements of the quality of vegetables. Meanwhile, there has been increasing demand in vegetables. But unreasonable fertilization has caused huge nitrogen accumulation in vegetables, which has far beyond the edible standard. At the same time, the nitrogen which has not been uptake remaining in the soil, which also increased the leaching, and further polluted the groundwater.The influence of agricultural nitrogen pollutants input in Vegetable-Soil-Groundwater system has been expounded in detail. The experiment consequently clarify the origin of NO3- and biogeochemical processes (NH4+-N, NO3- -N, NO2--N) in an agricultural region in Shuanglin town in Huzhou city.The conclusions as followed:(1) 6.9% soil samples in organic matter reached the standard of II, 31% soil samples reached the standard of Ⅲ,62.1% soil samples reached the standard of IV. The whole contents of organic matter in vegetable system have a lower level.36.7% soil samples in total nitrogen reached the standard of Ⅲ,30% reached the standard of IV. Total nitrogen in soil has a high level. The total nitrogen in soil which has reached the standard of Ⅱ was located in the 30cm-60cm soil layer. This means the nitrate has been leaching downward in open-air vegetable system. Total potassium and total phosphorus were all gradually decreased with increasing depth. Total potassium in the depth distribution was uneven and the total phosphorus content in 60-90cm depth was reduced by 62.5% compared with the total phosphorus content in 0-30cm depth in soil. The average concentrations of NH4+ was 32.25mg/Kg in vegetable fields and The average concentrations of NO3- was 3.05mg/Kg. Nitrate in soil mainly concentrated in the surface. The concentrates of NO3- in soil was gradually increased with increasing concentrates of NH4+ in soil. NH4+ and NO3- were increased with the increase of total nitrogen in soil There was a significant correlation between the soil organic matter and total nitrogen. The correlation coefficient reached 0.866.This means the soil total nitrogen was increased with the increase of soil organic matter. With the increase of depth, the correlation is more obvious.(2) The relationship between NO3- content in different organs of the same vegetables is vegetable tissue stem> root> stem> vegetables cabbage leaves. The content of NO3- in vegetable leaves accounted for 46.6% of the total accumulation, NO3- content in root vegetables accounted for 33.4% of the total cumulative. The average concentrate of NO3- has reached 15.58g/Kg, which has serious exceed the IV level of vegetable nitrate pollution degree. The correlation coefficient of NO3- concentration in vegetables and NO3-concentration in soil was 0.43, significantly correlated (p<0.05). This means the the NO3- in vegetables increased with the increasing of the NO3- in soil. Traditional methods of fertilization (300 kg/hm2·a-500kg/hm2·a) has caused a large number of nitrate accumulation in vegetables. Excessive fertilizer was the main factor in causing substantial accumulation in vegetables.(3) The shallow groundwater had been contaminated. The main pollution factors was NH4+ and SO42-. The average concentration of NH4+ was 0.94mg/Kg.20.6% groundwater samples has reached the standard of V. The average concentration of SO42- was 144.14mg/Kg.19% groundwater samples have reached the standard of III. Groundwater pollution mainly came from the Nitrate of soil and fertilizer. This means groundwater pollution was closely related to the agricultural pollution input. The concentration of NO3-in Groundwater increased with the increasing of soil NO3- concentration. The concentration of NH4+ increased with the reducing of soil NH4+ concentration. The changes of NO3- and NH4+ in temporal and spatial variation of groundwater were different. The NH4+ maximum of groundwater appeared in early period after fertilize, and the maximum NO3- of groundwater appeared in the middle after fertilize. But they also had the same rules. The maximum values of NH4+ and NO3- appeared in the shallow underground water.NO3- and NH4+ in shallow groundwater decreased gradually as the time goes on. Opposite, the concentration of NO3- and NH4+ in deeper groundwater increased gradually. The average concentration of NO3- in groundwater was 1.61 mg/L, ranged from 0.27mg/L～9.81 mg/L。 The δ80 in groundwater was -2.0‰～18.8%o, the δ15N in groundwater was-0.8‰ ～20.8‰。 The Soil N and chemical fertilizer was the main source of Nitrate contamination in groundwater, rarely came from the organic manure and chemical fertilizer NO3-, no nitrate pollution came from atmospheric precipitation.