Hidehiro Inagaki

Increasing nitrate removal at low temperatures by incorporating organic matter into paddy fields

Abstract Denitrification of paddy fields is a key process for improving water quality in fields where nitrate concentrations are high. The objective of the present study was to understand the effects of incorporating organic carbon (C) into soil on the denitrification rate of paddy fields in winter. On 11 December 2007, separate paddy field plots were prepared by incorporating 5 Mg ha−1 of rice straw (RS), 11 Mg ha−1 of rice straw compost (RSC) or a control. A field with a high concentration of nitrate in the water (averaging 18 mg N L−1) was irrigated until 29 March. During the experiment, the daily average soil temperature at a depth of 0.05 m ranged between 3 and 15°C. The nitrate concentration in the surface water in the RS plot, where the residence time was 2 days, decreased more than the concentration in the control or RSC plots. The total estimated nitrate removal from each plot in relation to the other plots was RS > RSC = control. Measurements of the soil from each plot on 29 February 2008 showed that incorporation of RS significantly increased the denitrification potential, even at low temperatures (5–10°C). Furthermore, the RS plot contained more dissolved organic C than the control or RSC plots. This result indicates that supplying RS effectively increases denitrification under low-temperature conditions.

Sustainable Rice Agriculture by Maintaining the Functional Biodiversity on Ridges

The ridges between rice paddy fields, originally constructed for irrigation, also serve as habitats for many organisms, including the rice stink bug Oebalus pugnax (the most important insect pest of rice) and natural enemies of the insect pests of rice. Therefore, the ecological management of these ridges is important for pest control and for the conservation of the natural enemies of rice pests. We identified the following ecological management practices for the conservation of the natural enemies of rice pests and for the control of rice stink bugs: (1) mowing increases the population of wolf spiders (family Lycosidae), which prey on stink bugs; (2) mowing at a high level effectively controls Poaceae plants, which host rice stink bugs; (3) the use of cover plants on ridges increases or maintains the population of wolf spiders, as well as those of crickets (family Gryllidae), which feed on the seeds of harmful plants. The above ecological management practices should decrease the populations of rice pest insects and serve as laborsaving techniques for sustainable rice agriculture.