This project is evaluating the impact of grower field management practices on nutrient cycling. Its chief goal is to improve fertility management guidelines for rice growers. The focus of this work is now on how different early season water-management practices are affecting nitrogen use efficiency.

Many rice growers are beginning to change early season water management to facilitate the use of new herbicides and cultural weed control practices. One new approach requires draining the initial flood irrigation for the application of herbicides like Clincher®. With this approach, growers may dry down their fields for up to three weeks before reflooding. Similarly, some organic growers use deep flooding and lengthy dry-downs in the early season to shift weed pressures back and forth from water to dryland weeds to give rice the competitive edge. Other methods being tested involve drill seeding and the use of a “stale” seedbed. Both of these methods require flushing the soil to induce rice or weed seed germination.

The wetting and drying of rice fields, a practice common to all of the above weed control strategies, changes the redox potential of soils. This can cause soil nitrogen to transform from one form to another. If not managed correctly, these transformations can lead to nitrogen loss and will ultimately affect rice growth and yields.

In the first year of this study, the goal was to better understand nitrogen transformations in soil as a result of wetting, drying and rewetting cycles. This research also sought to determine the amount of ammonium converted to nitrate during dry-down periods and the amount of nitrate lost as gas through denitrification following reflooding. Experiments were set up in two grower fields with an early season dry-down for a Clincher® application. A second set of experiments at the Rice Experiment Station examined different rice establishment systems.

The on-farm experiment was conducted in two fields managed by the same grower using similar cultural practices, except straw was incorporated in one field and burned in the other. M-205 rice was grown in 30-inch diameter metal rings – drained and undrained. Soils were continuously monitored for a number of factors, such as moisture, temperature and redox potential, and several soil samples were taken to measure mineral nitrogen content.

During the first few weeks of the season, total nitrogen in the burned field was about half that of the incorporated field. No nitrate accumulated in the undrained plots, but it did in the drained plots –

35 pounds/acre in the straw incorporated field and 28 pounds/acre in the burned field. After the drained plots were reflooded, the nitrate disappeared within eight days, from denitrification losses or plant uptake. Measurements suggest that at least half and maybe more was lost. Also, yield and nitrogen uptake were lower in the drained treatment by 340 pounds/acre and 15 pounds/acre, respectively.

An analysis of different rice establishment methods indicates that the yield potential of these systems is similar. This result is consistent with the two previous years of data. However, the amount of nitrogen required to achieve the yield potential varies between systems. As with the Clincher® experiments, early season water management affects soil nitrogen availability early in the season. Flash-flooding to recruit weeds (in stale seedbed systems) or to germinate rice seeds (drill seeded system) can result in denitrification losses.

These studies indicate that nitrogen management practices need to be revised in these systems in order to achieve optimal yields, and efficient cost-effective fertilizer use. Additional research is needed to refine recommendations for growers using early season drainage or alternative crop establishment practices to control weeds. Preliminary results from the first year of these studies have been shared with rice growers and pest control advisers at a number of meetings.