Reassessing Soil N Avail-
ability and Fertilizer Recommenda-
tions under Alternative Rice Residue Management Practices - 98

Project Leader and Principal UC Investigators

William R. Horwath, assistant professor, Department of Land, Air and Water Resources, UC Davis

At Maxwell and RES four treatments were examined: incorporated and winter flooded; residue burned, winter flooded; residue incorporated, non-winter flooded; and residue burned, non-winter flooded. These treatments are representative of the most diverse residue management options currently in use. Enriched urea fertilizer was used at all three sites, although different methods of application were utilized. Soil samples were taken every two months throughout the year, while samples for plant nitrogen were taken about every three weeks.

The experiments indicate that alternative rice straw management practices are affecting the availability of fertilizer nitrogen but have not shown an effect on grain yield. Soil nitrogen availability was increased by winter flooding at both the Maxwell and RES sites, probably the result of decomposition of prior yearsÆ rice straw residue combined with winter flooding. Winter flooding has consistently been found to increase available inorganic nitrogen at the Maxwell and RES sites. However, the available soil nitrogen has not been available for plant uptake in the spring, most likely because it was lost through denitrification following flooding and seeding.

Overall, straw incorporation tended to have positive effects on the nitrogen status of the soil. At the Maxwell site, the fall incorporated plots showed a tendency to immobilize added fertilizer. Researchers believe immobolization increases sustainable nitrogen cycling by preventing loss through denitrification during the winter months.

The fate of fertilizer nitrogen was again traced with a stable isotope through rice plants, microbial biomass and other soil pools. This research revealed that only about one third of the fertilizer nitrogen was taken up by the rice crop at Maxwell in 1997 and only 15 percent at the Yuba site in 1998. This large difference in fertilizer use efficiency, as well as the significant amounts of nitrogen lost from the plant-soil system, can be attributed to differences in soil and climatic factors between the two sites and years. Furthermore, the generally low fertilizer nitrogen use efficiency found at both sites underscores the importance of soil nitrogen for rice crop uptake.

At Maxwell a significant portion of the fertilizer nitrogen was found in the microbial biomass and soil organic matter late in fall and the following spring. This shows the importance of maintaining these soil nitrogen sources with fertilization. Microbial biomass controls the rate at which soil nitrogen becomes available for rice crop uptake. The majority of the nitrogen taken up by the rice was soil nitrogen. This shows the dependency of the rice on the mineralization activity of the microbial biomass.

Results also show that a significant amount of fertilizer was apparently lost through volatilization, denitrification activity and leaching. Incorporating rice straw appears to make more fertilizer nitrogen available, which could have a significant cumulative impact. A larger potentially available soil nitrogen pool is of even greater importance in rice systems because of rice's heavy reliance on nitrogen derived from the soil.

Seasonal, above-ground biomass accumulation and total nitrogen was also traced with the labeled fertilizer nitrogen. As expected, the highest residual fertilizer effect was found for the incorporated/winter flooded treatment; the lowest for the burned treatments. Incorporating large amounts of residue leads to immobilization and protects inorganic nitrogen loss through leaching, volatilization and denitrification. This study clearly shows that the below ground contribution of residual fertilizer nitrogen to subsequent rice crops is much more important than the amount of fertilizer nitrogen from above-ground sources. Winter-flooded treatments tended to have higher amounts of nitrogen during the growing season.

After five years there appears to be no adverse impact from alternative rice straw management practices on grain yields at either the Maxwell and RES sites. Incorporating rice straw has lead to an increase in weed and pest pressure, but has been manageable thus far.

In order to develop a better understanding of the ability of the soil to supply nitrogen to rice, a nitrogen rate addition experiment was conducted to determine nitrogen availability in the different straw management treatments. At the Maxwell site, rice straw incorporation showed a strong positive response to grain yield, clearly suggesting that straw incorporating leads to a higher nitrogen supplying power. At the Biggs site, however, a different picture emerged. Straw incorporation did not lead to higher yield when no fertilizer was applied. Movement of the soil by planing may have influenced these results.