Nitrification inhibition activity, a novel trait in root exudates of rice

BACKGROUND AND AIMS Nitrification is an important process in soil--plant systems for providing plant-available nitrate (NO(3) (-)). However, NO(3) (-) is less stable in soils compared with ammonium (NH(4) (+)) and is more easily lost through leaching, runoff or denitrification. This study tested whether biological nitrification inhibition (BNI) activity is present in the root exudates of rice (Oryza sativa) and also the extent of variation between different genotypes. METHODOLOGY The BNI activity of root exudates was estimated by a bioluminescence assay using a recombinant Nitrosomonas europaea strain. Afterwards, the effect of a single application of concentrated root exudates and that of exudates deposited in the rhizosphere soil was tested on BNI using soil incubation. Soil was added with (NH(4))(2)SO(4) and water to reach 60 % of the water-holding capacity and incubated at 30 °C for different periods. Amounts of NH(4) (+) and NO(3) (-) were determined using a continuous-flow auto-analyser. PRINCIPAL RESULTS In an initial screening experiment, BNI activity in the exudates of 36 different rice genotypes was evaluated using a bioassay based on a recombinant Nitrosomonas strain. Significant genotypic variation was detected with the upland cultivar IAC25 demonstrating consistently high BNI activity, while modern lowland varieties like Nipponbare or IR64 exhibited lower activity. Subsequent experiments ruled out the possibility that BNI activity is simply due to non-specific (solute) leakage from roots. Soil incubation studies with concentrated root exudates of IAC25 showed significant reductions in NO(3) (-) formation. This effect was confirmed by detecting lower NO(3) (-) levels in incubation experiments using rhizosphere soil obtained from IAC25. CONCLUSIONS Our results provide first evidence that root exudates of rice can reduce nitrification rates in soil. Having shown this for a model crop, rice, offers possibilities for further exploitation of this phenomenon through molecular and genetic tools.