Reassessing Soil Nitrogen
Availability and Fertilizer Recommenda-
tions under Alternative Rice Residue
Management Practices-2000


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Project Leader and Principal UC Investigators

William R. Horwath, associate professor, Dept. of Land, Air and Water Resources, UC Davis


Rice straw management has entered a new era since growers must implement alternatives to burning. This project is seeking to find how prolonged straw incorporation affects maximum yield potential and how it affects weed and disease pressure under different nitrogen fertilizer rates.


Past research has shown that 50 to 80 percent of the nitrogen taken up by rice plants comes from the soil, not directly from-fertilizer applications. Incorporating residues adds an average 75 pounds/acre (compared to about 25 pounds/acre under burning). This residual nitrogen accumulates and is gradually released over subsequent seasons. How well can this nitrogen be used to reduce fertilizer inputs? Winter flooding of straw incorporated or rolled fields has also shown significant increases in soil nitrogen availability 30.jpg (20668 bytes)The nutrient benefit of this straw-derived nitrogen is of considerable importance, since fertilizer use efficiency is generally poor. Emerging results in this study suggest that nitrogen applications can he reduced by 50 pounds/acre. It remains to be seen what the long-term effect straw incorporation will have on fertilizer use efficiency Another important question is how specific straw management practices such as straw removal through baling versus incorporation will impact long-terin soil quality and nutrient availability.

Grain yield

Alternative rice straw management practices - baling, rolling, incorporation in spring and incorporation in fall (with and without flooding) - had been studied for six years at Biggs and for seven years at another location near Maxwell. Grain yields have not been significantly affected, whether straw was incorporated or removed, at present fertilizer application rates at the Maxwell site. Similar results were observed at the Biggs site before it was discontinued due to lack of funding. The Maxwell site is now the only longterm site examining the transition from straw burning to alternative straw management practices.

Nitrogen fertilizer input has been constant since the Maxwell experiment started. Overall grain yield remained steady in 2000. The bale treatment continues to show a downward trend in grain yield compared to the other treatments regardless of winter flooding. This may indicate nutrient deficiencies other than nitrogen. Previous nitrogen rate trials show that burning and baling straw led to a decline in soil nitrogen fertility and grain yield in unfertilized plots.

When straw has been incorporated and winter flooded for seven years or more, no further yield increase was observed when the rate exceeded 100 pounds/acre. Therefore, fertilizer nitrogen applications can be reduced to this amount in fields managed this way for five or more years.

Weeds and diseases

31.jpg (28257 bytes)Changes in agronomic practices that impact crop yield often take up to 10 years or longer before they are realized. Furthermore, the direction of change - positive or negative - is difficult to know with any certainty during the transition to alternative practices such as from burning rice straw to incorporating it into the soil.

Changes in soil fertility often affect ot I her c r properties, including the ability to compete against weeds and other pathogens. This project has been tracking the severity of diseases under a wide range of nitrogen fertilizer application rates and long-term straw incorporation and burning. Weeds and diseases are the most likely nonnitrogen factors that limit plant production. Incidence of water grass was always higher in the incorporated and rolled plots.

No weeds were found on plots with no added nitrogen, suggesting that soil nitrogen availability primarily determines the incidence o( weeds. A decline in weed numbers in winter flooded plots suggests a decline from burial or foraging by waterfowl to reduce the weed seed bank. Additionally, there was a significant decline in total weed biomass at time of harvest when waterfowl had access to winter flooded rice fields. The data show convincing evidence of the value of winter flooding, as well as an increase in soil nitrogen availability.

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