Effects of Water Table Control by Farm-Oriented Enhancing Aquatic System on Photosynthesis, Nodule Nitrogen Fixation, and Yield of Soybeans

We evaluated the effects of the water table controlled by a water table controlling system, farm-oriented enhancing aquatic system (FOEAS), on soybean productivity. FOEAS was constructed in a heavy soil paddy field; we made plots with water tables maintained at -32 (water table depth [WTD]32) and -20 cm (WTD20) from the soil surface and, as a control, an open-ditch drained plot (ODD). Soybean cultivar Tachinagaha and non-nodulating cultivar En1282 were cultivated in 2006 and 2007. The moisture of the topsoil in the water table-controlled plots showed less fluctuation owing to rainfall. The roots and nodules were distributed only in the upper soil layer in ODD, but more were distributed in deeper soil layers in WTD32 and WTD20. In Tachinagaha, the SPAD value and stomatal conductance were higher in WTD32 and WTD20 than in ODD, resulting in a higher apparent photosynthetic rate. The aboveground total dry weight and nitrogen accumulation of Tachinagaha were higher in the water table-controlled plots than in ODD; in En1282, this difference was insignificant. The relative ureide value which is an index of nodule activity, in ODD was depressed by both excess and deficit soil moisture; however, that in WTD32 remained relatively high during the growing stage. Tachinagaha showed higher yield in FOEAS plots, especially in WTD32 than in ODD in both years. The results indicate that control of water table by FOEAS increases nitrogen fixation, photosynthesis, and yield of soybeans in upland fields converted from paddy fields.