| dc.description.abstracteng | Ecosystem functions of tropical mountain ecosystems and their ability to provide ecosystem services are particularly threatened by the combined impact of climate and land-use change. Carbon and nutrient cycling are fundamental ecosystem function that control C storage and pools, provide plant nutrients and regulate microbial and faunal activity. Soils, as the linkage between abiotic and biotic components of an ecosystem, are strongly affected by changes in these cycles. To understand the impacts of climate and land-use changes on biodiversity and associated ecosystem services and stability on Mt. Kilimanjaro, detailed understanding and description of the current biotic and abiotic controls on ecosystem soil C and nutrient fluxes are needed. Therefore, this research described and quantified cycles of C and major nutrients (N, P, K, Ca, Mg, Mn, Na, S and Si) on pedon and stand scale along a 3400 m elevation gradient and across three stages of land-use intensity. The first objective was to assess the effects of land-use change and climatic variation along the elevation gradient, on litter fall, litter quality, litter decomposition, and C stabilization in soil. The second objective was to use qualitative indicators (composition of soil organic matter and microbial communities) to relate pool changes to the underlying processes. The third objective was to link spatial variability and characteristics of the aboveground biomass to belowground pools and processes under contrasting climatic conditions in alpine and colline ecosystems.
Twelve research sites (0.25 - 1 ha) were selected between 800 and 4200 m a.s.l., representing natural forests, savanna and alpine vegetation as well as traditional subsistence and plantation farming. Litterfall was measured every two weeks over one year and inputs of C, macro and micronutrients was calculated for a subset of these sites. Decomposition rates of native and standardized (TBI) litter were quantified and TBI indices for decomposition and C stabilization were used to assess seasonal variabilities. Annual patterns of litterfall and decomposition were closely related to rainfall seasonality and temperature. Leaf litterfall contributed 60-75% to total litterfall and decreased from 1900 to 2900 m a.s.l. Within the same elevation range, annual litter decomposition decreased by about 25%. Further decrease of decomposition rates in (sub-) alpine ecosystems indicated a strong decline of productivity and turnover at 2900 m and above. Maxima of decomposition rates occurred between 1900 and 2500 m and were linked to the seasonal homogeneity of temperature and moisture availability. At this elevation, litterfall, decomposition rates and C stabilization showed the least seasonal variation. Ecosystems below 1900 m were subjected to pronounced seasonal moisture limitation. Particularly C stabilization in savanna (950 m) was up to 23 times higher during the rainy season compared to the dry season. Above 2900 m, seasonality increased again with lower annual precipitation and greater temperature limitation during cold seasons. Land-use change from natural forests to agroforestry systems increased litter macronutrient content and deposition (N, P, K), thus enhancing biogeochemical cycles. Carbon stabilization in these ecosystems and in the colline zone was reduced by about 30% by land-use intensification. Soil microbes in these ecosystems were less efficient in soil organic matter (SOM) decomposition but at the same time more demanding for new C sources.
Topsoil samples (0-10 cm) were analyzed for C and N content, pH, microbial biomarkers and soil organic matter chemical composition (py-GC/MS). Total phospholipid-derived fatty acids (PLFA) content increased with elevation until Ocotea forest (2100 m), reaching a maximum of 2100 nmol g-1 soil, followed by a decrease in (sub-) alpine ecosystems. Gram-negative bacteria abundance, accounting for 25-40% of total PLFAs, mainly determined this trend. Changes in the composition of microbial communities along the slopes of Mt. Kilimanjaro are a result of this climatic optimum and the consequent niche differentiation of certain groups. With increasing elevation and the harsh environmental conditions in the alpine zone above 4000 m (low temperature, low soil C and N contents), gram-positive bacteria are replaced by fungi. These variations were indirectly dependent on climatic factors, and mainly explained by changes in vegetation composition and soil parameters. Pyrolysis fractions (>280°C) quantitatively dominated the soil organic matter composition. The contribution of volatile compounds in SOM increased with elevation, indicating an increase of easily available SOM components. However, the increase of total SOM content at mid elevation is mainly determined by a more stable C pool (i.e. bound alkanes/-enes/-ols).
Two intensive research campaigns were conducted in alpine Helichrysum and colline savanna ecosystems. Three different vegetation cover types in Helichrysum were characterized. For each cover type, soil C and N pools, gross N turnover and diurnal greenhouse gas fluxes were measured, On the savanna plain, six trees were selected (legume Acacia nilotica and non-legume Balanites aegyptiaca) and crown area was distinguished from open area. Carbon, N and δ13C in plant biomass and soil, soil C and N pools, water content, available nutrients, cation exchange capacity, temperature, pH, as well as root biomass and greenhouse-gas exchange were measured for each cover type. Shrub-covered patches in Helichrysum ecosystem had between 60% and 170% higher soil C and N compared to low-vegetation patches. Higher amounts of aboveground litter promoted microbial growth, soil C stabilization and competition for N. This led to higher substrate availability and microbial biomass, and consequently higher respiration rates. Under savanna trees, soil C and N content, microbial biomass and N availability were about 40% higher than in open area. δ13C values in soil under the crown shifted towards the signal of tree leaves, suggesting that tree litterfall contributes 15% to SOM. These inputs increased microbial carbon use efficiency under the trees due to narrower C:N ratios compared to C4-grass litter. Wide C:N ratios require microorganisms to dispose of the C surplus via increased respiration to achieve their optimum C:N stoichiometry. Therefore, CO2 efflux was 15% higher in grassland than under the trees.
Ecosystems at mid elevation (~2000 m) represent the interception zone of optimal moisture and temperature conditions throughout the year. High litter inputs and fast turnover control the C sequestration in these ecosystems, while climatic restraints on decomposition limit the C turnover in soils at lower (drought) and higher elevation (low temperatures). Soil organic matter chemistry in Mt. Kilimanjaro forests is strongly dependent on a precipitation and temperature equilibrium. High ecosystem productivity at mid-elevations leads to increased amounts of volatile compounds but at the same time increases stabile carbon pools. Land-use intensification decreases stabilization of new C inputs through higher microbial C demand and turnover. This increases C and nutrient cycles in agricultural compared to natural ecosystems. The variability of vegetation cover types controls substrate availability in Helichrysum and savanna ecosystems. Two contrasting processes control the effects on CO2 fluxes in both ecosystems: Carbon mineralization at Helichrysum sites is enhanced by higher substrate availability under vegetated patches. In contrast, dry season C fluxes in savanna are related to the litter substrate quality and microbial C-use efficiency. | de |