Thinking beyond the canopy
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Forest soil as sink or source of greenhouse gases

A case study of species effects on nitrous oxide and methane fluxes in Karura forest


Understanding the processes responsible for the fluxes of non-CO2 GHGs from soil will help in better predictions of future concentrations of these gases in the atmosphere. The study objectives were to (1) assess the effect of forest type on CH4 and N2O fluxes, (2) evaluate the effect of soil water content on CH4 and N2O fluxes and (3) determine the relationship between N availability and terrestrial N2O and CH4 fluxes. The study was carried out in Karura forest, in central Kenya. Gas fluxes were measured from a natural forest stand and three different exotic plantations (Eucalyptus saligna, Cuppressus torulosa and Araucaria cunningham species) using static chambers. The forest was a net consumer of CH4 with high values in the natural forest of -4.77±0.14 and lowest in the A. cunningham plantation at -3.23±0.11 kg CH4 ha-1yr-1. There was a net emission of N2O from all sites,with highest emission in the natural forest at 0.37±0.03 and lowest in the C. torulosa stand at 0.18±0.01 KgN ha-1yr-1. Both fluxes were significantly different between the seasons and between stands. Overall, the NO3 pool increased during the wet months while the NH4 pool decreased. The N stocks were lowest in the A. cunningham stand and highest in the natural forest. The study showed that the forest consumes more CH4, and emits more N2O, in the wet season. Greater N2O emission and CH4 consumption was measured from the natural forest as compared to the exotic plantations.These findings will help improve forest management practices and decisions to reduce greenhouse gas emissions in Kenya.



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