Seasonal and inter-annual variations of leaf-level photosynthesis and soil respiration in the representative ecosystems of the Okavango Delta, Botswana

Seasonal and inter-annual leaf-level photosynthesis and soil respiration measurements were conducted in representative ecosystems of the Okavango Delta, Botswana, that differ in their long-term soil water content: the permanent swamp, the seasonal floodplain, the rain-fed grassland and the mopane woodland. CO2 fluxes (both leaf-level and soil respiration) at all the sites showed pronounced seasonality with highest and lowest values observed during the wet season and the dry season, respectively. Lowest values of soil respiration at the permanent swamp were a result of the surface flood, while low soil moisture content led to low soil respiration values at the rain-fed grassland and also at the upper lying areas of the seasonal floodplain. Soil temperature alone was a poorly related to soil respiration, while a simple model combining soil water content and soil temperature provided strong correlations with soil respiration. Species dominating wetter habitats showed highest stomatal sensitivity to vapour pressure deficit. Grasses at the seasonal floodplain showed high water use efficiency and strong stomatal limitation of CO2 assimilation as the dry season progressed. During the wet season, high CO2 assimilation among the grasses at the permanent swamp was associated with high nitrogen use efficiency, while at the rain-fed grassland high CO2 assimilation was associated with high leaf nitrogen and leaf phosphorus values. Within each species and across the sites leaf phosphorus appeared to exert stronger influence on leaf-level photosynthesis than leaf nitrogen. Taken together, the results of this study contribute toward understanding the role of environmental factors (soil water content, soil temperature, leaf nutrients) in controlling ecosystem functioning of tropical grassland and savanna ecosystems.