Effects of Rhizosphere Dissolved Oxygen Content and Nitrogen Form on Root Traits and Nitrogen Accumulation in Rice

Dissolved oxygen and nitrogen form have important effects on rice root growth and nitrogen availability. An indica hybrid rice, Guodao 1, and a conventional japonica rice, Xiushui 09, were cultured in hypoxic nutrient solution with NH4NO3 or (NH4)2SO4 as the nitrogen source for six weeks in pools. A portion of the Guodao 1 seedlings after treatment in the pools for four weeks were transferred to a split-root system at different dissolved oxygen contents and cultured for an additional two weeks. Biomass, root morphological traits and nitrogen accumulation were recorded. Under the low rhizosphere dissolved oxygen content (0–1.0 mg/L), plant biomass was significantly increased under NH4NO3-N supply by about 69% in Guodao 1 and 41% in Xiushui 09 compared with those under NH4+-N alone. Similar results were observed for root number, maximum root length, root dry weight and root activity. Nitrogen accumulations in roots and shoots were increased by 60% and 52% for Guodao 1, and by 41% and 33% for Xiushui 09, respectively, in the NH4NO3-N treatment. In the split-root system, the high rhizosphere dissolved oxygen content (8.0–9.0 mg/L) promoted root growth and development. Root biomass was increased by 21.6%, root number by 27%, maximum root length by 14%, and root volume by 10%. Moreover, nitrogen accumulation in roots was increased by 11% under high rhizosphere oxygen conditions. In conclusion, enhanced dissolved oxygen content and combined ammonium-nitrate nitrogen source have positive effects on root growth and nitrogen accumulation of rice plants.