Linking carbon and hydrological cycles in Lubigi wetland; Kampala - Uganda
The response of cyperus papyrus (papyrus) to environmental changes affect the potential for the plant carbon sequestration and transpiration. This study was conducted in Lubigi wetland; Kampala – Uganda from Sept 2010 to Dec 2012 to understand the ecophysiological process that affect carbon transformations and transpiration in papyrus wetlands. The effects of environmental changes on papyrus wetlands carbon sequestration and transpiration and the link between the two processes, growth of aerial shoot characterized as total dry matter change (biomass), the rate of change of dry matter (Net Primary Productivity - NPP), the rate of increase of biomass per unit size (Relative Growth Rate – RGR) and plant water loss were estimated. Spline growth analysis program was used to calculate RGR from biomass value. Papyrus biomass partitioning of the dry matter to the aerial shoot components, culm recruitment, mature and senescence culms were also investigated. Estimates of dissolved organic carbon (DOC) retention within or release from the wetland, in situ measurements of water level, pH and temperature variations were made. Transpiration was measured using diffusion porometer. Climate variables were recorded using weather station, installed in wetland. DOC concentrations in wetland water samples was analyzed using organic carbon analyzer. The overall relationships between culm-girth and biomass, and that between the wetland environmental conditions with papyrus productivity and stomatal conductance were determined. Papyrus culm density significantly increased from 16.1 to 35.93 culms m-2, exhibiting significant changes in both culm recruitment and senescence. Estimated RGR was 1.04g g-1 d-1. The RGR of developing culm was controlled by rate of change in leaf weight ratio (LWR) between 41 to 156 days of growth. Using culm density of 27 m-2, productivity range was 16.74 to 37.37 g m-2 d-1. Seasonal aerial biomass and productivity was not significantly different (p = 0.983) and P = 0.099 respectively). Vapour pressure deficit (VPD), photosynthetically active radiation (PAR) and air temperature did not significantly influence productivity during dry and wet seasons (P = 0.882 and p = 0.409 respectively). Except for the dry season that accounted for 26.1% of NPP variance, the wet season change was independent (0%) of weather variables. Water level fluctuations above 11.78 cm significantly reduced total culm density (p = 0.00). Seasonal DOC concentrations within the wetland were significantly different (p = 0.01). Water physio-chemical variables (temperature, pH and water level) were not significantly associated with DOC concentrations in the wetland r2 = 0.00, p = 0.832). The inflow and outflow DOC concentrations in the streams ranged between 5.99 – 7.48 mg 1 -1 and 5.88 – 10.6 mg 1-1 during dry season, and 6.24 – 8.48 mg 1 -1 and 6.20 – 23.65 mg 1 -1 during wet season respectively. DOC loss was not detected during wet season but detectable loss of carbon from the wetland occurred during dry season. The wet season stomatal conductance were significantly higher compared to the dry season (p = 0.00). Stomatal conductance were highest early in the day and it declined as the day progressed, but stomata were consistently more open in the wet than dry season. The measured weather variables (PAR, air temperature and VPD) significantly accounted for 69.4% (p = 0.032) and 87.5% (p = 0.002) changes in stomatal conductance during the wet and dry seasons respectively, but during the dry season, the stomata were more sensitive to the changes in vapour pressure deficit (r2 = 0.604, p = 0.025) than in the wet season ( r2 = 0.389, p = 0.132), closing sharply as the vapour pressure deficit increased. In conclusion, dry matter produced per unit of water transpired by the papyrus during wet and dry seasons were 0.003 and 0.004 respectively, implying that stomatal closure during the dry season did not reduce photosynthesis. Growth measurements also suggest that increasing temperatures may result in substantial decrease in carbon sequestration and also increased DOC export. Water level fluctuations above 11.78 cm lower culm density and the potential effect is on the canopy size that directly influence photosynthetically active radiation trapping and assimilation. An investigation on the nutrient use of efficiency, papyrus hydraulic retention and the evaluation of total organic carbon (particulate organic carbon – POC and DOC) in wetlands biogeochemical processes and should be integrated in the overall assessment of carbon and hydrological cycles.