Soil greenhouse gas fluxes from a humid tropical forest and differently managed urban parkland in Singapore

Abstract

Studies of soil greenhouse gas (GHG) fluxes (CO2, CH4 and N2O) from tropical soils are still scarce and the effects of urban green space management on soil GHG fluxes are poorly addressed. In order to establish reliable global GHG budgets, field measurements of soil GHG fluxes from different land uses and management practices in tropical regions are urgently needed. In this study, soil fluxes of CO2, CH4 and N2O were measured over a period of one year, from a forest and an urban parkland in Singapore. At the parkland site, two additional management practices were investigated, namely compost application to urban lawn and the replacement of lawn by legumes. The average CO2 efflux from the forest soil was 0.17 ± 0.01 g C m−2 h−1 and was 30% lower than that of the parkland soil. The forest soil was a CH4 sink (−36.03 ± 8.80 μg C m−2 h−1), while the parkland soil was a source of CH4 (43.79 ± 9.66 μg C m−2 h−1). Both soils were net emitters of N2O, with the forest soil releasing four times more N2O (23.23 ± 5.10 μg N m−2 h−1) than the parkland soil (6.47 ± 2.52 μg N m−2 h−1). During the entire observation period, compost application increased CO2eq by 30% compared with the untreated parkland soil. Shortly after compost addition, CO2 fluxes significantly increased and remained elevated until the end of the experiment. Soil CH4 emissions also increased, but the effect was small compared to CO2 (<1% CO2eq). Compost amendment caused an immediate short-term peak of N2O emissions. However, after the first day, which was characterized by a heavy rain event, the compost effect on N2O production was negligible. Compared to grass cultivation, legume planting resulted in a 20% reduction of CO2eq. This study shows that conserving mature secondary forests and the cultivation of herbaceous legumes on tropical urban soils, where open lawn spaces are not required, can reduce soil GHG emissions.