EFFECTS OF LANDFILL GAS ON GROWTH AND NITROGEN-FIXATION OF 2 LEGUMINOUS TREES (ACACIA-CONFUSA, LEUCAENA-LEUCOCEPHALA)

A study was made on the effects of landfill gas on ARA (acetylene redu cing activity) of nodules of two woody legumes (Acacia confusa and Leu caena leucocephala) widespread on landfill sites in Hong Kong. The eff ects of the three main components of landfill gas, O-2, CO2 and CH4, w ere first measured separately over a I-hr period. Maximum ARA was foun d at 20% O-2 (close to atmospheric partial pressure) and ARA decreased as the O-2 decreased in the range of 16-1%. Acacia confusa nodular AR A was significantly inhibited at 30-50% CO2, but not Leucaena leucocep hala nodular ARA. CH4 had no significant effect on ARA of either speci es. As the landfill gas concentrations in the landfill topsoil were mo stly > 10% O-2 and < 10% CO2, root nodules should fix N-2 effectively over these ranges of gases. A four-week test was conducted to assess t he long-term influence of landfill gas on seedlings of the two legumes . Landfill gas and elevated CO2 both suppressed their growth and their nodular ARA. Even under the influence of the gases, however, seedling s with nodules formed a higher biomass than seedlings lacking nodules. The growth of the two legumes under actual landfill conditions was in vestigated by transplanting non-inoculated and pre-inoculated seedling s to two landfill sites in Hong Kong: Junk Bay and Shuen Wan Landfill. After six months, most of the non-inoculated seedlings became infecte d: Acacia confusa 63 and 70%, Leucaena leucocephala 17 and 89%, respec tively, at the test sites. The results indicate that there were free r hizobia at these landfill sites to infect the legumes and they had for med effective nodules to fix N-2 under landfill conditions.