Reassessing Soil Nitrogen
Availability and Fertilizer Recommenda-
tions under Alternative

Rice Residue Management Practices-99

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Project Leader and Principal UC Investigators

William R. Horwath, Assistant professor, Dept. of Land, Air & Water Resources, UC Davis

Chris van Kessel, professor, Dept. of Agronomy and Range Science, UC Davis

How will changes in the way growers dispose of rice straw after harvest affect nitrogen availability?   Researchers have been examining that subject at an ongoing straw management trial near Maxwell and at the Rice Experiment Station.


Past research has shown that 50 to 80 percent of the nitrogen taken up by rice plants comes from native soil nitrogen.  On average incorporating rice residue will add about 75 pounds an acre — compared to about 25 pounds by burning residue.   Initially, residual straw nitrogen is tied up in the soil in an organic form but becomes available to the crop following decomposition.  How well can this nitrogen — and other nutrients — be used to reduce fertilizer inputs?   How will alternative straw disposal practices affect weeds and pests?  What would be the long-term impact of straw removal on potassium and phosphorous availability — something that should not be overlooked, especially on the east side of the Sacramento Valley?  Does winter flooding help or hinder decomposition of rice straw?   Answers to these questions have begun to emerge and should provide a measure of reassurance in how changing cultural practices are affecting soil fertility.

Comparable Grain Yields

Researchers have studied alternative practices —  baling, rolling, incorporation in spring and incorporation in fall (with and without flooding) — for six years at the Maxwell and Biggs locations.  At current fertilizer application rates, kept relatively constant over the years, none of these practices led to a grain yield increase or decrease.

soil 35.jpg (19986 bytes)Neither has the amount of straw produced been affected by any of the alternative practices.  However, further research into the method of fertilizer application and other management practices that could affect straw biomass is recommended.

Nitrogen Use Effeciency

Researchers calculated nitrogen uptake from applied fertilizer and native soil nitrogen at the Maxwell site in 1997 and at Biggs and another location in Yuba County in 1998.   Fertilizer use efficiency at Maxwell was 37 percent, 28 percent at Biggs and 15 percent at Yuba.  Native soil nitrogen supplied more N to the rice crop in incorporated plots than burned, showing the influence of straw incorporation on the availability of soil organic N.  Five successive seasons of straw incorporation have provided incorporated plots with greater N supplying power than burned plots.  Loss of applied fertilizer N below the root zone and to the atmosphere was similar at both long-term sites (36 percent) and among straw management treatments.  At the Yuba site, applied fertilizer loss was very high at 71 percent because of coarser soil texture and losses of fertilizer through leaching.  Nitrogen utilization efficiency, which can vary significantly from location to location, should be studied further.

Disease Pressure

The concern that grain yield would decline because of an increase in disease pressure has not been realized.  Under current fertilizer practices, it remains to be seen whether disease, pest or weed pressure will increase due to changes in weed seed bank and disease inoculum potential.  One problem worth watching is blast, which is sensitive to available soil nitrogen.

Fertilizer Recommendations

One of the main objectives of this project is to reevaluate nitrogen fertilizer recommendations and optimize nitrogen use efficiency in winter flooded and non-flooded rice cropping systems.  

Researchers recommend that when straw is incorporated for five or more years that fertilizer nitrogen applications do not exceed 100 pounds per acre.

Rice was grown without any nitrogen fertilization to determine differences in soil nitrogen supplying power affected by residue management.  At Maxwell, the highest grain yield occurred in incorporated or rolled plots, followed by winter flooding.   The lowest yield was observed in unflooded plots that were  baled, burned or rolled. 

At Biggs, grain yield without any nitrogen fertilization was much higher than at Maxwell, which suggests that the background fertility level at Biggs is higher.   However, it was more difficult to decipher a pattern at Biggs as the response to winter flooding and straw removal on grain yield remains variable at this site.  In addition, the high background nitrogen fertility at Biggs may create a residual nitrogen effect, masking the outcome of any treatment.

In unfertilized rice, incorporation of straw led to an increase in overall soil nitrogen availability and doubled grain yield compared to the burning of straw.   About 30 to 50 pounds fertilizer nitrogen per acre would be needed in burned plots to achieve an equivalent grain yield of non-fertilized rice in straw incorporated plots.   Clearly, when straw is incorporated, the plant available soil N pool is increased significantly.

  It is also clear that in straw-incorporated plots, nitrogen rates higher than 100 pounds per acre did not lead to any increase in grain yield.  Therefore, researchers recommend that when straw is incorporated for five or more years that fertilizer nitrogen applications do not exceed 100 pounds per acre.  At Maxwell, the rate of N application can be reduced at least 25 pounds from the currently recommended 140 pounds per acre.  Further study is recommended to ensure the accuracy of this recommendation for other rice growing areas.

Where straw is still burned, the nitrogen supplying power of the soil has declined to about half the grain yield where straw is incorporated.  Higher yields can be achieved with sufficient nitrogen.  In a rice system that incorporates straw for an extended period, factors other than available N, such as weeds and insect pests, would control maximum yield at higher fertilizer nitrogen rates.

Winter flooding showed a consistent increase in grain yield under different fertilizer nitrogen levels.  At present, researchers do not have a clear indication of what caused the increase.  Reduction in insect pests or weeds may be a factor.  For example, stem rot and rice water weevil may decline in winter flooded fields.

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