Suppression of nitrification and nitrous oxide emission by the tropical grass Brachiaria humidicola

Nitrification by soil nitrifiers may result in substantial losses of applied nitrogen through NO3- leaching and N2O emission. The biological inhibition of nitrification by crop plants or pasture species is not well known. This study was conducted to evaluate the ability of three pasture species, Brachiaria humidicola, B. decumbens and Melinis minutiflora to inhibit nitrification. Plants were grown in a growth chamber for sixty days, fertilized with (NH4)(2)SO4. After harvesting, the soil was incubated with (NH4)(2)SO4 for 24 days. Ammonium oxidizing bacteria (AOB), NH4-N levels, and N2O emission were monitored at 4 d intervals. Among the species studied, B. humidicola inhibited nitrification and maintained NH4-N in soil to a much greater extent than the other two species. This nitrification inhibition lasted for 12 days after initiation of soil incubation study (i.e. from 60 DAS when the plants were harvested). The AOB populations and N2O emission from the soil were significantly lower in the soils where B. humidicola has been grown compared to the other two species. Root exudates and soil extracts of B. humidicola suppressed AOB populations, whereas those of B. decumbens and M. minutiflora did not. The results are in consistence with the hypothesis that B. humidicola suppressed nitrification and N2O emissions through an inhibitory effect on the AOB population.