Can Allelopathic Rice (Oryza sativa) Effectively Inhibit Nitrification?.

Allelopathy is a beneficial plant trait that can be used to enhance the competitiveness of crops against weeds. Further, some allelochemicals have been shown to suppress ammonium oxidation, the first step in nitrification. We studied the effect of different concentrations (0.0, 0.05, 0.50 and 1.00 mg ml^-1) of water-soluble root exudates (RE) and root extracts (RT) from rice (Oryza sativa) on (i) lettuce seedling root growth and (ii) ammonium oxidation (AO) by Nitrosomonas europeae. Two allelopathic rice varieties (PI312777, PI338046), one non-allelopathic rice variety (IR55423) and a new rice breeding line (IR80508-B-57-3-B) with unknown allelopathic potential were tested. Rice plants were grown in a capillary mat system with and without water stress. Water soluble RE and RT were collected from 14 day-old rice seedlings and used to inoculate lettuce root growth and AO bioassays.  In AO bioassay, AO potential of N. europeaea grown in a NH₄⁺ enriched growth media was measured for 6 h after inoculation. Uninoculated controls were included in both bioassays.

At 0.05 mg ml^-1 RT and all concentrations of RE, AO rates increased 50-100% compared to controls; however, lettuce root growth and AO rates decreased significantly with increasing concentrations of RT. At 1.00 mg ml^-1 RT from  PI312777 and PI338046, AO inhibition was equivalent to that achieved by 5 ppm nitrapyrin. Significant effects  (p=0.05) of rice variety and water stress on AO rates and lettuce root growth were evident at RT concentrations of 0.50 and 1.00 mg ml^-1. Lettuce root growth was inhibited by 0.50 mg ml^-1 RE (0-30%) and all three concentrations of RT from PI312777, PI338046 and IR80508-B-57-3-B (1-65%). RT from all  rice varieties tested inhibited AO and lettuce root growth more than RE.  Allelopathic rice varieties may prove useful for combating weeds and inhibiting nitrification and needs to be investigated further under field conditions.