We estimated nitrogen flow in food and feed supply in 1992, 1997, 2002, and 2007 in Japan, and compared it with previously reported nitrogen flow in 1982 and 1987. Total nitrogen in domestic and imported food and feed in Japan increased until 1992, after which it decreased. The domestic ratio (domestic−exported)/(domestic−exported+imported) of the food and feed supply in Japan decreased from 42％ in 1982 to 30％ in 1997, and it stabilized at 28％ 2002 onwards. The domestic ratio decreased from 54％ in 1982 to 37％ in 2007 of diet food supply, 32％ in 1982 to 21％ in 2007 of feed supply for livestock farming, and rapidly from 33％ in 1987 to 15％ in 2002 of food and feed supply for food processing. The nitrogen load to the environment increased to its peak until to 1992, after which it decreased; this occurred due to a decrease in chemical fertilizer application. However, food and feed wastage in the system did not decrease, resulting in only a slight decrease in nitrogen load to the environment. The ratio of nitrogen load absorbed into domestic crop production to total nitrogen amount in domestic−exported+imported food and feed+crop residues decreased from 35％ in 1982 to 28％ in 1997, and was stable until 2007. The ratio of nitrogen load in waste recycled to farmland to total nitrogen amount in recycled waste+chemical fertilizer input to farmland increased from 26％ in 1982 to 39％ in 1997–2002, and decreased to 35％ in 2007. The ratio of nitrogen load from farmland to the hydrosphere per total nitrogen load to the hydrosphere decreased from 30％ in 1982 to 25％ in 2007.

Arbuscular mycorrhizal fungi (AMF) are known to colonize plant roots and improve the phosphorus (P) uptake by their host plants, including maize (Zea mays L.). However, few studies have addressed the extent to which the application of P fertilizer can be reduced in the presence of AMF. Thus, the present study aimed to determine which P fertilizer application rates could be used without reducing the yield of continuously cultivated maize in Konsen, Japan.

The results of our field experiments demonstrated that maize yield was not significantly affected by reducing the application rate of P fertilizer, except when no P fertilizer was applied. Relative early growth rate was a useful measure for evaluating the effect of reduced P fertilizer application on maize yield. P fertilizer application rates that did not result in reduced yield were different depending on factors such as soil type, available phosphorus content, and AMF colonization. Based on relative early growth rate, we tentatively determined that P fertilizer application rates could be reduced by 20％ of the present standard in fields where maize is continuously cultivated.

The application of P fertilizer to maize could be further reduced if the effects of AMF on the P uptake of host plants are quantified more precisely under various cropping conditions.

Subsurface drip fertigation (SDF) is a method of injecting liquid fertilizers through an irrigation system, which comprises a buried irrigation tube with earth piled on top. Here we examined the effects of SDF on Welsh onions by analyzing their growth and nutrient uptake. Because >45％ of the area under cultivation in Japan used to grow Welsh onions contains soil with an excessively high amount of phosphate, we examined the effects of subsurface drip irrigation (SDI) and SDF with a low-cost fertilizer application method using coated urea on such soil. 1) SDI enabled efficient and effective transition of soil moisture, enhanced the nitrogen utilization rate in the initial growth stage, and increased yield by 12％. 2) The use of liquid fertilizer in SDF resulted in significantly greater (by 13％) phosphate absorption in the hypothermic phase than did SDI. 3) Along with enhancing the nitrogen utilization rate, SDI using coated urea significantly increased the amount of phosphate absorption during all phases of growth and increased yield by 32–41％. 4) There was no significant difference in yield and nutrient uptake between SDF using coated urea effluent and SDI. These results indicate that the continuous supply of nutrients and water through SDF promotes utilization of phosphate accumulated in the soil, thereby contributing to enhanced growth and higher yields of Welsh onions.