Contamination of groundwater with nitrate (NO3) derived from agricultural activity is serious problem in many countries worldwide. We investigated the annual (growing and non-growing seasons) behavior of NO3–N in the soil pore water of cropped Andosols and Gray lowland soils under eight crop groups (Type A: paddy rice, Type B: winter crops, Type C: vegetables 1, Type D: vegetables 2, Type E: vegetables and forages, Type F: legume crops, Type G: orchard, and Type H: grass). In the vegetable group (Type C) and the orchard group (Type G), which required large amounts of fertilizer and frequent top-dressing, NO3–N concentrations in the soil pore water were extremely high. In these agricultural lands, it was clear that the inorganic nitrogen produced by nitrification in surface soil was dominantly discharged from a depth of 90 cm in July to September. The descending order of the amount of discharge of NO3–N (N-discharge) was Type C > G > D > E > F > B > H > A for the Andosol, and Type G > C > F > D > E > B > H > A for the Gray lowland soil. If fertilization of the vegetables and orchard was performed based on the standard application amount, the annual average NO3–N concentration at a depth of 90 cm exceeds 10 mg L−1. To reduce the risk of groundwater contamination by NO3–N, we calculated the annual cumulative water flux density and annual cumulative NO3–N flux density. We examined the calculated fertilizer amount and proposed reduced fertilizer application amounts so that the annual average concentration of NO3–N in soil pore water would not exceed 10 mg L−1. The standard application amount of nitrogen fertilizer for vegetables should be reduced by 65.8 and 30.8 kg ha−1 in the Andosol and the Gray lowland soil, respectively. We also proposed that the standard application amount of nitrogen fertilizer be reduced by 59.9 and 40.7 kg ha−1 in Andosol orchards and Gray lowland soil orchards, respectively.
N-modeling N-discharge N-fertilizer recommendations Isopleth of NO3–N concentration