Possibility of Nitrification Suppression by a Tropical Grass : The Effect on Multiplication of Ammonium-Oxidizing Bacteria and Nitrous Oxide Efflux

Abstract

We examined the effect of three tropical grasses (Brachiaria decumbense, B. humidicola and Melinzis minzutiflora) on nitrification in soils. These three grasses were grown in pots filled with loamy Andisol for 2 weeks and then ammonium-N was applied to the soil as ammonium sulfate. After ammonium-N applications, the number of ammonium-oxidizing bacteria in soils increased more than ten times for two grasses compared from B. humidicola. But in the case of B. humidicola, the number of ammonium-oxidizing bacteria did not increase and kept the same value for 27 d. Three grasses were grown in 1/5000 a Wagner pots for 4 weeks and then ammonium-N was applied to the soils for the determination of nitrous oxide emissions. From one day after the applications of ammonium-N, emission of nitrous oxide from the soils in which B. decumbense and M. minutiflora were grown was observed and continued more than 4 d. However, there was little emission of nitrous oxide from the soil in which B. humidicola was grown. Moreover, after determination of nitrous oxide emissions, plant roots were removed from the soil and ammonium-N was reapplied to the soils. Only in the soil in which B. humidicola was grown, was nitrification delayed. From these data, it seems likely that the tropical grass, B. humidicola, inhibits nitrification in soil; in particular, it suppresses the multiplication of ammonium-oxidizing bacteria in the soil as well as the emission of nitrous oxide to the atmosphere.