Soil N Availability & Fertilizer Recommendations under Alternative
Rice Residue Management Practices-97

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Project Leader and Principal UC Investigators

William R. Horwath - Assistant Professor, Department of Land Air and Water Resources, UC Davis

Chris van Kessel - Professor, Department of Agronomy and Range Science, UC Davis


As California rice growers implement alternative methods of disposing of rice straw, of how these new practices affect the land's capacity to cycle nutrients is becoming increasingly important. This project examined soil nitrogen availability and fertilizer recommendations at an ongoing rice straw residue management trial near Maxwell and at another trial near the Rice Experiment Station.

Under different straw management practices, researchers studied the fate of fertilizer nitrogen using a stable isotope that traced nitrogen through the rice plant, microbial biomass and other soil pools. Results from the winter-flooded burned and winter-flooded incorporated plots showed:

  • Only about one third of the fertilizer nitrogen was taken up by the rice crop.
  • Nitrogen uptake was completed by the seventh week following planting.
  • The majority of nitrogen uptake by the rice i (60 percent) was from the soil, not from fertilizer.
  • A significant amount of fertilizer (30 percent) was apparently lost through volatilization and denitrification.
  • A significant portion of the fertilizer nitrogen was found in the microbial biomass late in the growing season.

The low fertilizer use efficiency underscores the importance of soil nitrogen in supplying nitrogen to the rice crop, researchers believe. This experiment also shows that the microbial biomass not only competes strongly with rice for fertilizer nitrogen but also controls the rate at which soil nitrogen becomes available to the rice crop.

Results from the winter non-flooded treatments were not available but researchers anticipate results will follow a similar trend.

The disposition of fertilizer under alternative straw management practices was also studied near the Rice Experiment Station. In general, straw incorporation had a positive effect on the nitrogen status of the soil. Straw incorporation significantly increased available soil nitrogen prior to the growing season but decreased in availability after harvest. Researchers believe that the increased nitrogen availability may have been from the decomposition of prior years' rice residue, which shows the importance of straw in maintaining a sustainable nitrogen cycle. In the fall, the straw incorporated plots showed a tendency to immobilize nitrogen, which would help prevent the loss of soil nitrogen through denitrification during the wet winter months. These observations contrast with the Maxwell site, where winter flooding increased available straw nitrogen regardless of treatment.

This information has obvious implications for fertilizer nitrogen recommendations under different flood and residue treatments. As nitrogen becomes more available under straw incorporation management, it may be possible to reduce fertilizer nitrogen application rates in the future. However, the effect of straw on soil fertility may take up to five to 10 years to be realized. As soil quality in rice systems is enhanced from straw incorporation, it will become increasingly necessary to reassess fertilizer nitrogen recommendations as nitrogen availability can affect the incidence of weeds and diseases.

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