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The annual nitrogen (N) budget was measured in a soybean-cultivated upland field during the first year after conversion from a paddy field on gray lowland soil, which is typically found on the Sea of Japan side of northern Japan. Forage rice was cultivated on lysimeter fields for 4 consecutive years with applications of chemical fertilizer, immature compost, or mature compost (the control, immature compost, and mature compost plots, respectively), and then the fields were converted to upland fields for soybean (Glycine max [L.] Merrill cultivar Ryuho) cultivation. Input (seed, bulk N deposition, and symbiotic dinitrogen [N 2 ] fixation) and output (harvested grain, leached N via drainage water, and nitrous oxide emission) N flows were measured, and the field N budget was estimated from the difference between the input and output. The soybean plants in the immature and mature compost plots grew well and had higher yields (498-511 g m ) in the mature compost plot (27.7) was higher than those in the control (18.1) and immature compost plots (19.9). Percentages of soybean N accumulation derived from N 2 fixation ranged from 53% to 74%. N derived from symbiotic N 2 fixation accounted for more than 90% of the total N input, whereas harvested grain accounted for approximately 85% of the total N output. N leaching mainly occurred during the fallow period, accounting for 13-15% of the total N output. The annual N budgets were negative ()10.0, )14.2, and )6.4 g N m )2 year )1for the control, immature compost, and mature compost plots, respectively). The N loss from the immature compost plot was higher than that of the control plot, because the N output in harvested grain was higher, and the N input by N 2 fixation was similar between plots. While the N loss from the mature compost plot was lower than that of the control plot because the N output in harvested grain was higher, as was the case in the immature compost plot, the N input by N 2 fixation was also higher. Preceding compost application--whether immature or mature compost--to paddy fields increased the subsequent soybean yield during the first year after conversion. This result suggests that N loss and the following decrease in soil N availability in the field could be mitigated by increased N 2 fixation resulting from mature compost application with an appropriate application practice.

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).