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130 PARALLEL SESSION HALL H IV | FREE CONTRIBUTIONSPARALLEL SESSION HALL H IV | FREE CONTRIBUTIONS131SOIL CO 2 EFFLUX IN AN AFROMONTANE FOREST OF ETHIOPIA ASDRIVEN BY SEASONALITY AND TREE SPECIESYonas Yohannes 1,2 , Olga Shibistova 1,3 , Asferachew Abate 1 , MasreshaFetene 4 , Georg Guggenberger 1TREE-GRASS COMPETITION IN SAVANNA DEPENDING ON ORGANICAND MINERAL NUTRIENTS1Institute of Soil Science, Leibniz Universität Hannover, Hannover, DE; 2 ForestryResearch Center, Ethiopian Institute of Agricultural Research, Addis Ababa, ET;3VN Sukachev Institute of Forest, SB-RAS, Akademgorodok, Krasnoyarsk, RU;4Department of Biology, Addis Ababa University, , Addis Ababa, ETJudith Sitters 1 , Anneke Valk 2 , Milena Holmgren 2 , Peter Edwards 1 , HarryOlde Venterink 11ETH Zurich, Zurich, CH, judith.sitters@env.ethz.ch, 2 Resource Ecology Group,Wageningen University, Wageningen, The NetherlandsTHURSDAY 10:15 Hall H IVVariability of soil CO 2 efflux strongly depends on soil temperature, soilmoisture and plant phenology. Separating the effects of these factors is criticalto understand the belowground carbon dynamics of forest ecosystem. InEthiopia with its unreliable seasonal rainfall, variability of soil CO 2 efflux maybe particularly associated with seasonal variation. In this study, soil respirationwas measured in nine plots under the canopies of three indigenous trees(Croton macrostachys, Podocarpus falcatus and Prunus africana) growingin an Afromontane forest of south-eastern Ethiopia. Our objectives were toinvestigate seasonal and diurnal variation in soil CO 2 flux rate as a function ofsoil temperature and soil moisture, and to investigate the impact of tree speciescomposition on soil respiration. Results showed that soil respiration displayedstrong seasonal patterns, being lower during dry periods and higher duringwet periods. The dependence of soil respiration on soil moisture under thethree tree species explained about 50% of the seasonal variability. The relationfollowed a Gaussian function, and indicated a decrease in soil respiration at soilvolumetric water contents exceeding a threshold of about 30%. Under moremoist conditions soil respiration is tentatively limited by low oxygen supply.On a diurnal basis temperature dependency was observed, but not during dryperiods when plant and soil microbial activities were restrained by moisturedeficiency. Tree species influenced soil respiration, and there was a significantinteraction effect of tree species and soil moisture on soil CO 2 efflux variability.During wet (and cloudy) period, when shade tolerant late successional P.falcatus is having a physiological advantage, soil respiration under this treespecies exceeded that under the other two species. In contrast, soil CO 2 effluxrates under light demanding pioneer C. macrostachys appeared to be leastsensitive to dry (but sunny) conditions. This is probably related to the relativelyhigher carbon assimilation rates and associated root respiration. We concludethat besides the anticipated changes in precipitation pattern in Ethiopia anyanthropogenic disturbance fostering the pioneer species may alter the futureecosystem carbon balance by its impact on soil respiration.Tree-grass co-existence in savannas is affected by competition for nutrients.Grasses appear superior competitors, but only inorganic nutrients were studied,not organic forms like dung. Particularly legume trees may be better able to useorganic-P than grasses because of a higher phosphatase activity. We studiedcompetition between a grass (Panicum infestum) and a tree seedling, of eithera legume (Acacia nilotica or Albizia lebbek) or non-legume (Garcinia livingstoniior Keetia zanzibarica). Plants grew in pots with soil for 11 weeks, with mineralnutrients, dung, or no fertilizer. In contrast to previous experiments, the grassdid not reduce tree growth. It benefit from competing with trees in the control,but not when fertilized with dung or mineral nutrients. Phosphatase activitywas highest for the legumes, intermediate for the grass and lowest for theother trees. Dung and mineral nutrients had a similar effect on phosphatase.So, higher phosphatase activity of the legume trees did not improve theirperformance under organic-nutrient supply. Whether this depends on time orinterspecific differences in nutrient uptake needs further study.THURSDAY 10:30 Hall H IVgtöSociety for Tropical Ecology | Gesellschaft für Tropenökologie e.V. Status and future of tropical biodiversity | Frankfurt, 21 - 24 February 2011gtö