ROOT EXUDATES FROM CANOLA EXHIBIT BIOLOGICAL NITRIFICATION INHIBITION AND ARE EFFECTIVE IN INHIBITING AMMONIA OXIDATION IN SOIL

A range of plant species produce root exudates that inhibit ammonia-oxidizing microorganisms.This biological nitrification inhibition(BNI)capacity can decrease N loss and increase N uptake from the rhizosphere.This study sought evidence for the existence and magnitude of BNI capacity in canola(Brassica napus).Seedlings of three canola cultivars,Brachiaria humidicola(BNI positive)and wheat(Triticum aestivum)were grown in a hydroponic system.Root exudates were collected and their inhibition of the ammonia oxidizing bacterium,Nitrosospira multiformis,was tested.Subsequent pot experiments were used to test the inhibition of native nitrifying communities in soil.Root exudates from canola significantly reduced nitrification rates of both N.multiformis cultures and native soil microbial communities.The level of nitrification inhibition across the three cultivars was similar to the well-studied high-BNI species B.humidicola.BNI capacity of canola may have implications for the N dynamics in farming systems and the N uptake efficiency of crops in rotational farming systems.By reducing nitrification rates canola crops may decrease N losses,increase plant N uptake and encourage microbial N immobilization and subsequently increase the pool of organic N that is available for mineralization during the following cereal crops.