Processes of belowground carbon cycling in the plant-mycorrhiza-soil system as affected by silvicultural management
Final Report Abstract
One important aspect in tiie functional ecology of natural and plantation forests is element cycling between atinosphere, plant, and soil. Carbon cycling affects many key processes from the stand level to the global scale. Complementing to the works on aboveground biomass production, and in order to identify preferable options for soil carbon management, we are addressing the belowground component of carbon cycling. The studies are carried out in the natural forest, with focus on different functional types of trees; Croton macrostachyus, Podocarpus falcatus, and Prunus Africana. In the silvicultural stands Cupressus lusitanica is comparatively investigated in an unmanaged plot and in a plot under intensive promotion. Three major fields have been focused on by a suite of modern and complementary techniques: (1) transfer of organic carbon to soil, (2) turnover of organic carbon in the soil, and (3) the output of carbon through soil respiration. The major experiment relies on whole tree 13C labelling of in total 7 study frees and the respective controls and chasing the label in leaves, phloem sap, roots and associated mycorrhizal fungi and bacteria, soil and soil fractions, and soil CO2 efflux during a one year period. As an independent method, the 14C approach is used for the same ecosystem compartments. In addition, the soil CO2 efflux is monitored. The results show that the organic carbon allocation from recently assimilated carbon to soil via phloem sap is very fast. The pioneer C. macrostachyus has a particular fast transfer, whereas P. falcatus is more conservative. This mirrors the larger photosynthetic uptake rate and higher transpiration. Cupressus lusitanica behaves similar to P. falcatus, reflecting the gymnosperm coniferous genetical traits. Silvicultural management did not influence the time course of the transport of recently assimilated carbon to the soil (and back to the atmosphere). Soil CO2 efflux in the natural forest and in the C. lusitanica stands is primarily controlled by the soil moisture with higher CO2 efflux rates during the moist periods. Besides, the soil CO2 efflux appears to respond to functional types of trees. Under C. macrostachyus the decrease in soil CO2 efflux at drier conditions is not so pronounced as under the other tree species. Considering the high photosynthetic activity of C. macrostachyus during the dry periods and the fast allocation of recently assimilated carbon to the roots and the soil CO2 efflux, this observation is probably related to the smaller decline in the autotrophic part of the soil CO2 efflux. Hence, this would not affect the soil organic carbon pool. In the C. lusitanica stands, thinning and intensive promotion of potential crop trees leads to a higher soil CO2 efflux. Comparable patterns of 13C in the soil CO2 efflux under the labelled unmanaged and managed frees and the higher soil temperature in the managed plot suggest that this additional CO2 is derived from heterotrophic respiration, i.e. mineralization of soil organic matter. The field work is almost terminated. Within the extension phase it is planned to finish the δ13C analysis of compounds of the autotrophic continuum, i.e. roots, mycorrhiza fungi, and associated bacteria for the one year chasing period and to model the results obtained. In addition, the results gained from the whole tree 13C labelling should be linked to information based on the 14C approach.
Publications
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2003. Changes in soil organic carbon, nitrogen and sulphur stocks due to the conversion of natural forest into tree plantations (Pinus patula and Eucalyptus globulus) in the highlands of Ethiopia. World Resources Review 15, 462-482
Ashagrie Y., Zech W., Guggenberger G., Teketay D.
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2005. Transformation of a Podocarpus falcatus dominated natural forest into a plantation of Eucalyptus globulus: C, N and S dynamics from particle and aggregate size fractions, Munessa, Ethiopia. Agriculture Ecosystems and Environment 106, 89-98
Ashagrie Y., Zech W., Guggenberger G.
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2006. Signal improvement in elemental analyzer/continuous flow isotope ratio mass spectrometry for samples with low sulfur contents using a pre-concentration technique for on-line concentration adjustment. Rapid Communications in Mass Spectrometry 20, 1679-1682
Fritzsche F., Tichomirowa M.
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2006. Soil-plant hydrology of indigenous and exotic trees in an Ethiopian montane forest. Tree Physiology 26, 1043-1054
Fritzsche F., Abate A., Fetene M., Beck E., Weise S., Guggenberger G.
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2007. Soil aggregation, and total and particulate organic matter following conversion of native forests to continuous cultivation in Ethiopia. Soil and Tillage Research 94, 101-109
Ashagrie Y., Zech W., Guggenberger G. & Mamo T.
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2007. Soils of the Main Ethiopian Rift Valley: a transect study. Catena 70, 209-219
Fritzsche F., Zech W., Guggenberger G.
