Carbon dioxide exchange and biomass productivity of the herbaceous layer of a managed tropical humid savanna ecosystem in western Kenya

Abstract

Aims Humid savannas, as a result of high precipitation amounts, are highly productive. They are also hotspots for land use change and potential sources of carbon dioxide (CO 2) due to the large soil carbon (C) stocks. Understanding how ecosystem CO 2 exchange is influenced by changes arising from agricultural land use is vital in future management of these ecosystems and in responding to the ongoing shifts in management and climate. The aim of this study was to identify how ecosystem CO2 exchange and biomass productivity of the herbaceous layer of a humid savanna in Kenya respond to current management practices. Methods We used flux chambers to quantify CO2 fluxes, while monthly harvests were undertaken to determine biomass development of the herbaceous layer of three sites that were (i) fenced to exclude livestock grazing, (ii) subjected to grazing by livestock and (iii) abandoned after being cultivated for maize production and also open to grazing by livestock. Important findings The peak aboveground biomass ranged between 380 and 1449 g m −2 and biomass production was significantly (P < 0.05) lower in the grazed and abandoned plots. The maximum gross primary production (GPP) and net ecosystem CO 2 exchange (NEE) ranged between 21.8 ± 1.3 to 32.5 ± 2.7 and −9.6 ± 0.7 to −17.9 ± 4.8 µmol m −2 s −1 , respectively. Seasonal NEE fluctuations ranged between 10 and 21 µmol m −2 s −1 , while spatial (among sites) differences ranged between 2 and 10 µmol m −2 s −1 . Ecosystem respiration (Reco) fluctuated between 5 and 10 µmol m −2 s −1 during the growing season. Reco was, however, not significantly different among the sites. Unlike in other similar ecosystems where ecosystem respiration is determined by the ambient temperature, we did not find any relationship between Reco and temperature in this savanna. Instead, soil moisture accounted for 38–88% of the spatial and seasonal fluctuations in ecosystem CO2 fluxes and aboveground biomass production. Management influenced the maximum GPP and NEE rates through modification of soil moisture, plant species composition and aboveground biomass. We concluded that soil moisture is the key determinant of ecosystem CO2 exchange and productivity in this tropical savanna. Management, however, significantly modifies C fluxes and productivity through its influence on soil moisture, plant species composition and aboveground green biomass and should be taken into consideration in future similar studies.