Incorporation of winter grasses suppresses summer weed germination and affects inorganic nitrogen in flooded paddy soil

Laboratory experiments were conducted to investigate how incorporation of the winter grasses foxtail (Alopecurus aequalis) and milk vetch (Astragalus sinicus) into flooded paddy soil affects the germination of summer weeds and soil inorganic nitrogen (N) levels. Polyethylene terephthalate (PET) vessels (10.0 × 8.0 × 15.0 cm) were filled with 0.7 kg air-dried soil that had been collected from organic paddy fields located in Tochigi, Japan. Foxtail and milk vetch were then independently incorporated into the soil at rates of 0% (untreated control), 0.25%, 0.5%, 1%, and 1.5%, with four repetitions per treatment. The PET vessels were placed in a growth chamber (Biotron) set to day/night temperatures and humidities of 27/18°C and 70%/65%, respectively, and with an illuminance of 40,000 lx (12 h) for 28 days, and the total number of weeds and changes in the electrical conductivity (EC) of the surface water were investigated throughout this period, while changes in soil inorganic N were determined at the ends of the experiments. The incorporation of both foxtail and milk vetch significantly suppressed the germination of summer weeds by 56% and 50%, respectively, at incorporation rates of 1% and by 78% and 77%, respectively, at incorporation rates of 1.5% compared with their respective controls. There was a high and significant positive correlation between both the rate of winter grass incorporation and the degree of growth suppression and EC. Incorporation of foxtail at rates of 0.25% and 0.5% significantly increased the inorganic N contents of the sub-layer by an average of 68% compared with the control but had no significant effect in the top layer, whereas the incorporation of milk vetch at a rate of 1% significantly increased the inorganic N contents of both the top layer and sub-layer by 200% and 316%, respectively, compared with the control. Furthermore, the effect of milk vetch on inorganic N significantly increased with an increase in the rate of incorporation. These findings demonstrate that incorporation of the winter grasses foxtail and milk vetch into flooded organic paddy soil can suppress weed growth and act as a source of inorganic N.