Seasonal variation of carbon fluxes in a sparse savanna in semi arid Sudan

<p>Abstract</p> <p>Background</p> <p>Large spatial, seasonal and annual variability of major drivers of the carbon cycle (precipitation, temperature, fire regime and nutrient availability) are common in the Sahel region. This causes large variability in net ecosystem exchange and in vegetation productivity, the subsistence basis for a major part of the rural population in Sahel. This study compares the 2005 dry and wet season fluxes of CO₂for a grass land/sparse savanna site in semi arid Sudan and relates these fluxes to water availability and incoming photosynthetic photon flux density (PPFD). Data from this site could complement the current sparse observation network in Africa, a continent where climatic change could significantly impact the future and which constitute a weak link in our understanding of the global carbon cycle.</p> <p>Results</p> <p>The dry season (represented by Julian day 35–46, February 2005) was characterized by low soil moisture availability, low evapotranspiration and a high vapor pressure deficit. The mean daily NEE (net ecosystem exchange, Eq. 1) was -14.7 mmol d^-1for the 12 day period (negative numbers denote sinks, i.e. flux from the atmosphere to the biosphere). The water use efficiency (WUE) was 1.6 mmol CO₂mol H₂O^-1and the light use efficiency (LUE) was 0.95 mmol CO₂mol PPFD^-1. Photosynthesis is a weak, but linear function of PPFD. The wet season (represented by Julian day 266–273, September 2005) was, compared to the dry season, characterized by slightly higher soil moisture availability, higher evapotranspiration and a slightly lower vapor pressure deficit. The mean daily NEE was -152 mmol d^-1for the 8 day period. The WUE was lower, 0.97 mmol CO₂mol H₂O^-1and the LUE was higher, 7.2 <it>μ</it>mol CO₂mmol PPFD^-1during the wet season compared to the dry season. During the wet season photosynthesis increases with PPFD to about 1600 <it>μ</it>mol m^-2s^-1and then levels off.</p> <p>Conclusion</p> <p>Based on data collected during two short periods, the studied ecosystem was a sink of carbon both during the dry and wet season 2005. The small sink during the dry season is surprising and similar dry season sinks have not to our knowledge been reported from other similar savanna ecosystems and could have potential management implications for agroforestry. A strong response of NEE versus small changes in plant available soil water content was found. Collection and analysis of flux data for several consecutive years including variations in precipitation, available soil moisture and labile soil carbon are needed for understanding the year to year variation of the carbon budget of this grass land/sparse savanna site in semi arid Sudan.</p

Groundnut variety introductions and breeding objectives in the sandy rainlands of the Sudan

Thirteen introduced and one local varieties of groundnuts (Arachis hypo- gaea) were evaluated comparatively and individually in the Sand Sheet and Pediplain soils in 1963-1965. The effect of added N and P fertilizer on each variety was also studied. All varieties did better on the Pediplain than on the Sand Sheet soils due to the better moisture retention properties of the former. The amount of seasonal rainfall affected greatly the yields and the magnitude of the response to fertilizers. The very low yields at Umm Higlig in 1965 suggested that fertilizer application could hardly be economical in seasons of poorly distributed rainfall. Although fertilizers increased markedly vegetative growth this was not always reflected in the yield of pods due probably to the lack of sufficient soil moisture at the reproductive stage. The upright bunch varieties seemed more adaptable to the area's short rainy season than the spreading bunch and runner varieties due to the earliness of the former. The runner varieties showed a greater resistance, long period of seed dormancy and high oil content appeared very important objectives for future breeding in the area under study.Tsangarakis C. Z., Gerakis P. A. Groundnut variety introductions and breeding objectives in the sandy rainlands of the Sudan. In: Journal d'agriculture tropicale et de botanique appliquée, vol. 16, n°6-8, Juin-juillet-août 1969. pp. 368-376