Effects of landfill gas on growth and nitrogen fixation of two leguminous trees (Acacia confusa, Leucaena leucocephala)

A study was made on the effects of landfill gas on ARA (acetylene reducing activity) of nodules of two woody legumes (Acacia confusa and Leucaena leucocephala) widespread on landfill sites in Hong Kong. The effects of the three main components of landfill gas, O-2, CO2 and CH4, were first measured separately over a I-hr period. Maximum ARA was found 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 ARA was significantly inhibited at 30-50% CO2, but not Leucaena leucocephala nodular ARA. CH4 had no significant effect on ARA of either species. As the landfill gas concentrations in the landfill topsoil were mostly > 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 the 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, seedlings with nodules formed a higher biomass than seedlings lacking nodules. The growth of the two legumes under actual landfill conditions was investigated by transplanting non-inoculated and pre-inoculated seedlings to two landfill sites in Hong Kong: Junk Bay and Shuen Wan Landfill. After six months, most of the non-inoculated seedlings became infected: Acacia confusa 63 and 70%, Leucaena leucocephala 17 and 89%, respectively, at the test sites. The results indicate that there were free rhizobia at these landfill sites to infect the legumes and they had formed effective nodules to fix N-2 under landfill conditions.