Nitrogen uptake and ammonium-N content of flooded soil in direct-seeded rice cultivation in Hokkaido

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  • 北海道の水稲湛水直播栽培における土壌中アンモニア態窒素と窒素吸収
  • ホッカイドウ ノ スイトウタンスイ ジカマキ サイバイ ニ オケル ドジョウ チュウ アンモニアタイ チッソ ト チッソ キュウシュウ

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In central Hokkaido, it is common practice in the cultivation of direct-seeded rice (Oryza sativa L.) to drain the soil after seeding for 1 to 3 weeks and then reflood the soil after emergence to improve seedling establishment. The objectives of the present study were to investigate the influence of nitrogen (N) uptake at early spikelet differentiation (ESD) on grain yield and quality and to clarify the relationship between exchangeable soil ammonium and N uptake at ESD under flooded soil conditions in direct-seeded rice cultivation. In 2007-2011, 51 direct-seeded rice plants were collected from various fields in central Hokkaido. In 63% of samples, N uptake at ESD was below the recommended optimal uptake level of 40 kg ha^<-1>. These plants yielded fewer kernels per area and lower overall grain yields. Nitrogen uptake at both heading and maturity was positively correlated with protein content of milled rice and negatively correlated with percentage of ripened grains. These results indicate that increased N uptake during the initial growth stages is important to stabilize not only rice yield, but also rice quality. We monitored soil ammonium content at a depth of 0-0.1 m approximately every 2 weeks following reflooding through ESD. Soil samples were collected from 23 commercial or experimental fields and were extracted using 10% KCl solution. Nitrogen uptake at ESD was positively correlated with exchangeable soil ammonium at the three-leaf stage, irrespective of soil type and cropping history of the field. It was also positively correlated with soil ammonium at both the five-leaf stage and ESD, but these correlations were not as strong as that at the three-leaf stage. Our findings demonstrate that 40 mg kg^<-1> of soil ammonium in the plow layer at the three-leaf stage is sufficient to result in 40 kg ha^<-1> of N uptake at ESD.

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