Nitrogen balance in a paddy field planted with whole crop rice (Oryza sativa cv. Kusahonami) during two rice-growing seasons

This paper focuses on N balance in a paddy field planted with whole crop rice (Oryza sativa cv. Kusahonami). The experiment was conducted with two treatments during two rice-growing seasons: one was fertilized with N (160 kg N ha−1; 16N plot) and the other unfertilized (0N plot); both plots were fertilized with P and K. The N input from precipitation was 15 and 12 kg N ha−1 in 2002 and 2003, respectively. The N input from irrigation water reached as much as 123 and 69 kg N ha−1 in 2002 and 2003, respectively. This was because irrigation water contained higher NO3− concentrations ranging from 4 to 8 mg N l−1. The N uptake by rice plants was the major output: 118 and 240 kg N ha−1 in the 0N and 16N plots in 2002 and 103 and 238 kg N ha−1 in 2003, respectively. N losses by leaching were 4.8–5.3 and 6.5–7.3 kg N ha−1 in 2002 and in 2003, respectively. Laboratory experiments were carried out to estimate the amounts of N2 fixation and denitrification. Amount of N2 fixation was 43 and 0 kg N ha−1 in the 0N and 16N plots, respectively. Denitrification potential was quite high in both the plots, and 90% of the N input through irrigation water was lost through denitrification. Collectively, the total N inputs were relatively large due to irrigation water contaminated with NO3−, but N outflow loading, expressed as leaching−(irrigation water + precipitation + fertilizer), showed large negative values, suggesting that the whole crop rice field might serve as a constructed wetland for decreasing N.