| dc.description.abstract | Greenhouse gas (GHG) fluxes from tropical deforestation and regrowth associated with global climate and land-use change are highly uncertain components of the contemporary carbon budget, due in part to the lack of spatially explicit and consistent information on changes in forest cover. Quantifying mass and energy exchanges within tropical forests, such Karura Forest which is contiguous with the city of Nairobi, is essential for understanding their role in the global carbon budget and how they will respond to perturbations in climate. Understanding the processes responsible for net sources and sinks of GHGs such as methane (CRt) and nitrous oxide (N20) would help in better predictions of future concentrations and the rate and extent of Climate Change.
This study was carried out in Karura Forest and the objectives were (1) to asses the effect of forest type on CIiJ and N20 fluxes, (2) to evaluate the effect of seasons (wet and dry) and soil water content on the terrestrial sink and source of CH4 and N20 and (3) to determine the relationship between terrestrial N20 and CH4 fluxes, and the rates of soil net mineralization and nitrification in Karura forest.
Gas fluxes were measured monthly using chamber techniques for a period of 10 months (March - December, 2008) from a natural forest stand and three different exotic plantations (Eucalyptus saligna, Cuppressus torulosa and Araucaria cunningham species). Gas analysis was done on a Shimadzu gas chromatograph (GC) fitted with an electron capture detector (ECD) for the N20 and a flame ionization detector (FID) for the CH4 detection and a 6Ft long-steel packed Porapak Q analytical column. Soil water content, inorganic N stocks and net N mineralization and nitrification rates were also determined.
The forest was a net consumer (negative fluxes) of CH4 both during the wet and dry months. The CRt fluxes were significantly influenced both by the seasons (Feat = 9.44, Ftab = <0.001) and also between the species types (Feat = 4.61, Ftab =0.004). There was more uptake during the wet months compared to the dry ones. The annual uptake ofCH4 was highest in the natural forest at -4.77±0.14 while the exotics were averagely equal consumers: E. saligna (-3.75±0.14), C. torulosa (-3.61±0.13) and A. cunningham at - 3.23±0.11 Kg CRt halyr'. Most of the uptake was experienced during the wet months with the natural forest leading at -1.85±0.21 and A. cunningham the least at -1.25±0.13 mg CH4 m-2 d'. There was a net emission (positive fluxes) of N20 from the forest.
The N20 fluxes were very significantly different between the seasons (Feal = 6.98, Ftab < 0.001) and also between the species type (Feal = 6.22, Ftab < 0.001). The influence of the interaction between the seasons and species type on the fluxes was also very significant (Feal = 2.47, Ftab < 0.001). The natural forest was the highest emitter of N20 at 0.37±0.03, followed by E. saligna (0.24±0.01), A. cunningham (0.20±0.02) and lowest in the C. torulosa stand at 0.18±0.01 KgN ha-1yr-l. Most of the emmisions were experienced during the wet months with the natural forest leading at 14.7 ng Ncmid' and the exotics averagely equal at 8 ng Ncm'
The percent water field pore space (%WFPS) had a significant influence on the Cf4 fluxes (Feal = 2.64, = 0.112); but not on the N20 fluxes ((Feal = 0.04, Ftab = 0.85). Overall the N03 pool increased during the wet months while the NH4 pool decreased. The A. 'cunningham stand had the lowest pool of N stocks and the natural forest, the highest. The E. saligna stand had the highest pool ofN03-N both in the dry (17.9mgNlKg soil) and wet months (22mgN/Kg soil) while the natural forest stand had the highest NH4-N pool; 17.8mgN/Kg soil in the dry months but only 6.7mgN/Kg soil in the wet months.
Net-N mineralization rates were not influenced by season (Feal = 0.21, Ftab = 0.644) but there was influence from the species types (Feal = 1.66, Ftab = 0.028). Net-N nitrification rates were greatly influenced by season (Feal = 11.57, Ftab = 0.001) and also by the species type (Feal = 6.69, Ftab < 0.001).
A relationship was observed between Net-N mineralization rates and N20 fluxes (~ = 0.48, Feal = 13, Ftab = 0.003) but no relationship with the Cf4 fluxes (Feal = 0.009, Ftab = 0.924). There was an influence of Net-N Nitrification rates on N20 fluxes ((r2=0.28, Feal = 5.45, Ftab, = 0.03) and also on the CH4 fluxes (Feal = 1.18, Ftab = 0.29).
This study showed that total soil C, C: N ratio of litter, soil moisture, Inorganic N stocks and their rates of mineralization, influences N20 and CH4 fluxes. It also showed that the forest generally consumes C~ but is net emitter ofN20. Greater N20 emissions and CH4 consumptions were measured from the natural forest as compared to the exotics. The undisturbed natural forest stands are able to consume more CH4 and emit more N20 compared to the commercially managed (disturbed) exotic stands. The findings of this study will be important in improving forest management practices and decisions to help reduce greenhouse gas emissions in Kenya and elsewhere.
The study recommends forest management practices that emphasize on the overall balance of needs versus environmental conservation. Natural regeneration of a (natural) forest should be encouraged instead of reforestation efforts which often go for exotic species. There is also need to integrate data from various sources to map and monitor forest resources.
This way a long-term and sustainable system of forest conservation and management can be realized.
There is need for further research to investigate the effect of age of a forest stand (and different species) on terrestrial gas fluxes, to help especially in dealing with commercial plantations. This will assist forest management decisions especially with regard to tree species selection when planting | en_US |
