Ammonia-oxidizing bacteria dynamics affected by plantain under synthetic cattle urine patches

Plantain has been suggested as a nitrous oxide (N2O) and nitrate (NO3-) leaching mitigation option as it may induce biological nitrification inhibition (BNI) via plantain root exudation, which affects the activity of ammonia-oxidizing bacteria. This preliminary study compared the abundance of the ammonia monooxygenase gene (amoA) in soils containing either plantain and white clover, or ryegrass and white clover. Plants were sown in pots and grown in a greenhouse. Two months after sowing, synthetic cattle urine was applied to half the pots, and rhizosphere and bulk soil samples were collected 30 and 90 days after urine application. The abundance of the amoA gene was measured using real time quantitative PCR. The abundance of amoA genes in rhizosphere soil around ryegrass plants and in bulk soil under ryegrass/white clover were higher in pots treated with urine than the no-urine controls. AmoA gene abundance in bulk soil under plantain/white clover was higher in pots treated with urine (P<0.05) but not in rhizosphere soil around plantain plants (P>0.05) compared with the control. Furthermore, amoA gene copy numbers in the rhizosphere soil around plantain plants were lower than for ryegrass plants (P<0.05). However, there was no difference in the abundance of amoA genes in bulk soil of either combination of plant species evaluated (P>0.05). The results suggest that, in the time frame of our experiment, plantain could induce a BNI effect in the rhizosphere soil but not in the bulk soil.