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

Jorge Mazza Rodrigues, associate professor, Dept. of Land, Air, and Water Resources, UC Davis

The goal of this research is to determine whether nutrient use efficiency, yield, nitrogen acquisition, and translocation are linked to nitrification of common nitrogen fertilizers in California rice systems.

Three ongoing experiments were conducted in 2018. The first was a laboratory incubation to investigate whether mixed ammonium and nitrate nutrition would increase nitrification potential in rice paddy soils. The second experiment was a pot study in controlled environment growth chambers. The third experiment examined the importance of nitrification at four field sites.

In the first experiment, soils were collected from fields at the Rice Experiment Station (RES). Three soil cores were sampled from each of eight plots in an established nitrogen rate trial. All vegetation from the soil was removed and nitrification potential was tested in the laboratory.

Results suggest that the application of nitrate with ammonium suppresses soil nitrification potential but has no effect on nitrate consumption. More work is needed to determine whether the application of nitrate with ammonium promotes the soil nitrification process in paddy soils with rice planted, restrains the mobility of fertilizer nitrogen, and decreases nitrogen use efficiency and rice yield.

The second experiment sought to explore the importance of nitrification in California rice and its connection with the mobility of fertilizer nitrogen, nitrogen use efficiency, and rice yield. As in the previous year, the three rice cultivars used in the 2018 test were CM-101, M-206, and M-401. Nylon mesh bags with different pore sizes were used to isolate the influence of rice roots on soil processes. Fertilizer was applied at field-recommended rates to dried soils collected from RES. Rice was allowed to grow in a chamber to late tillering stage.

The results support the assumption that nitrification occurs at significant rates in California rice, and the mobility of fertilizer nitrogen increases under root influences. However, the magnitude of that influence varies across cultivars. This suggests that higher nitrification potential causes higher nitrogen mobility that results in higher, variable nitrogen uptake across cultivars.

Ongoing analyses of plant tissues and the soil microbial community will help elucidate the relationships among nitrification potential, nitrogen use efficiency, and rice yield. Results show a strong link between nitrate assimilation and increased rice yields through nitrification of nitrogen fertilizer in the root zone. Researchers have performed high throughput sequencing of the rhizosphere microbial community and are currently identifying DNA sequencing reads of microorganisms associated with the rice root zone.

Also, M-401 showed higher nitrate reductase (enzyme) activity in rice leaves than the other varieties, suggesting that it has the potential to use nitrate more efficiently. Further research on nitrogen cycling, especially on nitrogen loss during the process of nitrate assimilation, is needed.

In the third experiment, research focused on the importance of nitrification in connection with the mobility of ammonium fertilizer, nitrogen use efficiency, and rice yield. M-206 was grown in fields located near Williams, Meridian, and Nicolaus, as well as at RES, and treated with varying rates of nitrogen. The dynamics of soil ammonia and nitrate contents at different rice stages, and the relationships among nitrification potential, abundance of ammonia oxidizers, and rice yield were tested.

Results showed that ammonia content was low after rice elongation. Nitrate was the major nitrogen source after rice elongation and prior to panicle initiation. This work supports the assumption that nitrification is important in California rice and that the mobility of fertilizer nitrogen increases under root influences. However, the degree of these influences varies greatly among cultivars.

Ongoing plant tissue analyses, additional cultivar testing, and further studies of microbial communities will help increase understanding of nitrification potential, nitrogen use efficiency, and rice yield.

The results of research thus far indicate the important role that ammonia oxidizers in the root zone and nitrate enzyme activity in rice leaves have on nitrogen use efficiency and ultimately on rice yield.