Abstract:In northern Japan, declines in soil nitrogen fertility have occurred in paddy-upland rotation systems with soybean cultivation. A six-year lysimeter experiment was conducted to evaluate the nitrogen budget in paddy-upland rotation (three-year for upland soybean, then three-year for flooded paddy rice) and to clarify the effect of preceding compost application (immature or mature compost over four consecutive years of forage rice cultivation) on the nitrogen budget and soil nitrogen fertility. Available soil nitrogen throughout the experimental period and soybean and rice yields in both compost application plots tended to be higher than those in the control plot. The nitrogen budgets during both soybean and rice cultivation were negative, and the amount of nitrogen loss in both compost application plots tended to be higher than that in the control plot. The nitrogen loss during rice cultivation (−2.3 to −4.3 g N m −2 year −1 ) was less than that during soybean cultivation (−9.6 to −14.6 g N m −2 year −1 ). Nitrogen loss estimated based on the nitrogen budget agreed well with that estimated based on changes in soil nitrogen storage during soybean cultivation but not during rice cultivation, suggesting underestimation of nitrogen loss from the rice paddy.

To reduce the over-production of rice, the paddy-upland rotation system, which alternates every few years between paddy rice cultivation and upland crop cultivation in drained (converted) paddy fields, is now commonly practiced in Japan. Recently, depletion of available soil nitrogen (N) and a subsequent decline in soybean yield in converted upland fields with repeated rotation have been reported in northern Japan. To evaluate the N budget in the paddy-upland rotation field with soybean and rice, a 6-year lysimeter experiment was conducted. In the rotation system, a considerable loss of N occurred in both the upland soybean and paddy rice cultivation periods (−11.9 and − 2.3 g N m−2 y−1, respectively). To mitigate the N loss in the rotation system, N supply from organic matter application is required. The effects of applying different types of organic matter (leguminous green manure, hairy vetch, and livestock manure compost) on the N budget in soybean cultivated fields were investigated. Compared to the N loss in the control plot without organic matter application, the N loss was mitigated in the hairy vetch plot, and N accumulation occurred in the livestock manure compost plot (−13.7, −3.5, and +11.8 g N m−2 y−1, respectively).

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