Nitrate Contamination of Groundwater and Soil Management

The Japanese Government set the environmental quality standard for nitrate (NO3) in groundwater at 10 mg N L1 in 1998, based on a level considered acceptable for avoiding infant methemoglobinemia. In 1998, 6.3% of groundwater in Japan contained NO3 exceeding 10 mg L¡¦, with agriculture regarded to be a primary source of the NO3 (Environmental Agency, Japan, 1999). This paper aims to define the mechanisms of NO3 contamination of groundwater associated with soil management in arable land. The author gives an overview of the relation between nitrogen (N) fertilization and groundwater contamination. First of all, the utilization efficiency of N fertilizers for outdoor cultivation of vegetables is usually 50% or less (Nishio, 2001; Vance, 2001). Although N fertilizer is essential for crop production, excessive N could leach out of arable soils and eventually cause NO3 contamination of groundwater. However, conversely, excessive N is necessary as insurance in some cases, such as when there is heavy rainfall immediately after fertilization. It should be also noted that some vegetables physiologically require a high content of N in soil even at harvest. Nitrate leaching from different fertilizers was monitored for 7 years and the data were evaluated using an N and water balance equation (Maeda et al., 2003). Excessive N from chemical fertilizers caused substantial NO3 leaching, while compost application was promising to achieve high yields and low N leaching during a few years but led to the same level of NO3 leaching as that in the plots subjected to chemical fertilizer application over longer periods of time. Thus, it is of importance to predict the N mineralization rates both for manure and for soil under natural conditions. Experimental results of this kind can provide full information on N dynamics in fields for policy decisions or regulations to reduce NO3 leaching while maintaining crop yields. Likewise, we must consider other influencing factors such as soil types, climatic conditions, and cropping systems for this purpose