Factors controlling microbial biomass in soils of Mt. Kilimanjaro

The contents of organic matter and microorganisms in soils are sensitive parameters to evaluate soil quality. In general, high contents in organic and microbial carbon (Corg, Cmic) are related to high soil fertility. Especially in the tropics, climate and land use have strong direct effects on basic soil parameters and soil microbial biomass. Furthermore, alterations of the basic soil parameters (e.g. in contents, stocks or distribution) also entail changes of soil microorganisms. Although these major factors and their controlling effect on soil microbial biomass are known, detailed studies including soils of multiple major ecosystems of a region are scarce, especially in Africa. Various climates can be observed on the African continent and the projected climate change is assumed to have regionally diverging effects. Furthermore, at the expense of natural ecosystems, the high population growth in Africa leads to an increasing demand for agricultural land and high rates of land-use change. To evaluate the soils' environmental services and its vulnerability, it is crucial to understand these changes and their effects on soil microbial biomass. Africa's highest mountain, the Kilimanjaro, offers outstanding potential to investigate Corg and Cmic in soils of various climates, ecosystems and land-use types. Hence, the overall purpose of this investigation was to examine factors controlling microbial biomass in soils of Mt. Kilimanjaro. The work was conducted on the southern slopes of Mt. Kilimanjaro and covered an climate/elevation gradient from 950 to 3880 m a.s.l.. Up to twelve ecosystems that developed in different climates were investigated, classified into natural (6 plots), semi-natural by men slightly affected habitats (3) and agriculturally managed sites (3). Basic soil parameters and soil microbial biomass were measured in multiple depths and different spatial scales. Sampling was conducted between January 2011 and October 2013. The effects of climate and land use on basic soil parameters and soil microbial biomass were statistically analyzed and compared. Elevation distinctly influences temperature and precipitation along the slopes of Mt. Kilimanjaro and thus was used to investigate the climatic effect on soil microbial biomass. Along the investigated elevation gradient, Corg and Cmic contents in soils were found to follow a hump-shaped distribution with a maximum in the consistently warm and humid forest ecosystems at elevations between 2000 m and 3000 m a.s.l.. In addition, the forest belt exhibited also the strongest negative trends in C contents with depth. Cmic stocks did not show a direct correlation to climate. However, Corg stocks varied depending on water availability, temperature and net primary production (NPP) along the elevation gradient. In ecosystems with a dry character at low elevations, soil Corg stocks increased by 3 kg m-2 and in the consistently humid ecosystems at higher elevations by 1 kg m-2 per 1000 m in elevation, respectively. The variability in soil Cmic contents during the climatic transition phase from dry-to-wet season was more pronounced at low elevations / dry climate and lead to a decrease of Cmic when the rain season began. At similar elevations, the pronounced contrast between climatic seasons also resulted in a high increase in CO2 efflux and the specific metabolic quotient (qsCO2) under increased precipitation. Contents and stocks of Corg and Cmic were altered by human influences on the ecosystems, which was revealed by the investigation of two land use conversions typical for the region and on a similar elevation/climatic level. Compared with Corg, the contents and stocks of Cmic were stronger decreased by the negative effect of intensive land use. This effect was especially distinct in the upper soil horizons. Intensive agricultural management also resulted in an up to 3-fold decrease of the substrate available for microbial growth (Cmic:Corg ratio). In addition, soil CO2 efflux and qsCO2 in soils of agricultural fields were up to four times higher than in the soils of less disturbed ecosystems. A detailed heterogeneity study in the savannah ecosystem revealed small-scale patterns of basic soil parameters and soil microbial biomass. Corg, N, Cmic and Nmic contents varied several-fold between and within 15 x 15 m plots in the same area and climate. Basic soil parameters (e.g. Corg, N) are controlled by factors such as climate, vegetation and relief even on a small scale. Because of their correlation to soil microbial biomass, such basic soil parameters provide additional information for multivariate prediction techniques and are able to increase the calculation's accuracy. Climate, land use and NPP were identified as the main drivers affecting the microbial biomass in soils of Mt. Kilimanjaro. Climate and land use are independent of each other but both distinctly influence the NPP and/or vegetation of an ecosystem. The consistent humid climates in ecosystems above 2000 m a.s.l. supports high NPP, high contents and stocks of Corg and Cmic as well as increased substrate availability in soil. The soils of natural ecosystems in the National Park (> 2000 m a.s.l.) and of slightly disturbed ecosystems at low elevations are characterized by effective, closed nutrient and C cycles in a steady-state, but are nevertheless highly vulnerable to the negative consequences of land-use change. The agricultural land use in the densely populated areas of Mt. Kilimanjaro negatively alters important and sensitive parameters within the C cycle. The observed reduction in Corg, Cmic, available substrate and microbial efficiency leads to the assumption of an open C cycle entailing a further decline in the contents and stocks of Corg and Cmic with increased land use.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Pabst, Holger

Contributors dc:contributor

• Kuzyakov, Yakov

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2061/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2061