Browsing by Subject "Streuabbau"

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Substrate availability affects abundance and function of soil microorganisms in the detritusphere
(2008) Poll, Christian; Kandeler, Ellen
Plant litter is the major source of soil organic carbon (SOC). Its decomposition plays a pivotal role in nutrient recycling and influences ecosystem functioning and structure. Soil microorganisms are the main protagonists of litter decomposition. Among other factors, their activity is controlled by the physicochemical conditions of the soil. This interaction is strongly influenced by the soil structure, resulting in a heterogeneous distribution of microorganisms, substrates and physicochemical conditions at the small-scale. Due to this heterogeneity, microhabitats differ in their decomposition rate of organic C. Considering microhabitat diversity is therefore important for understanding C turnover. In the detritusphere, plant litter closely interacts with the soil by releasing soluble C into the adjacent soil and providing new sites for microorganisms. The abundant readily available substrates characterise the detritusphere as a hot spot of microbial activity and C turnover. Despite the important role of this microhabitat, the interaction of physicochemical conditions with soil microorganisms remains unclear. This thesis was designed to clarify the effect of litter C transport on the spatial and temporal availability of substrates and therefore on microbial abundance and activity in the detritusphere. This goal was addressed in three studies. The first study focused on the influence of solute transport conditions on microbial activity and substrate utilisation by the microbial community. In two 2-week microcosm experiments, diffusion and convection were considered as transport mechanisms; both mechanisms were studied at two different water contents. The second study aimed to identify temporal patterns of diffusive solute transport and microbial activity at two water contents during an 84-day incubation. Both studies emphasised the important role of fungi in the detritusphere. The third study therefore identified fungi that benefit from freshly added litter. The three studies combined classical soil biological methods and modern techniques. Analysis of microbial biomass, ergosterol content, CO2 production, and enzyme activities provided general information on the mineralisation of litter C as well as on microbial activity and abundance. A convective-diffusive solute transport model with a first-order decay was used to interpret enzyme activity profiles. This allowed the underlying factors determining the spatial dimension of the detritusphere to be identified. By adding plant residues with a different 13C signature than the SOC, it was possible to quantify the transport of litter C into different C pools. The incorporation litter C into different microbial groups, for example, was traced by coupling of phospholipid fatty acid (PLFA) extraction with 13C analysis. Fungal species were identified by constructing clone libraries based on 18S rDNA and subsequent sequencing. The results of the first study indicated that the transport rate of soluble substrates determines the spatial dimension of the detritusphere, with an enlarged detritusphere after convective versus diffusive transport. The isotopic ratios of bacterial and fungal PLFAs differed under both transport mechanisms, indicating different substrate utilisation strategies: bacteria relied on the small-scale transport of substrates, whereas fungi assimilated new C directly in the litter layer. Water content affected only diffusive C transport and modified the temporal pattern of microbial activity by enhancing transport at higher soil water content. The expected chronological order of C transport, microbial growth and enzyme release was verified in the second and third study. During the first two weeks, mainly easily available and soluble litter compounds were mineralised and transported into the adjacent soil. After this initial phase, depolymerisation of complex litter compounds started. During the initial phase, enhanced C transport induced greater microbial biomass and activity, and increased fungal diversity. During the later phase, however, substrate availability and microbial activity were reduced. Measurements of microbial biomass C and ergosterol indicated that the initial phase was dominated by bacterial r strategists, whereas fungal K strategists dominated the later phase. Sequencing of fungal 18S rDNA detected a shift in the fungal community during the initial phase, pointing to growth of pioneer colonisers, especially Mortierellaceae. These fungi do not produce ergosterol and therefore were not detected by the ergosterol measurements. Accordingly, the r strategists consist of both bacteria and fungi. During the later phase, the fungal community was dominated by the cellulose-degrading fungus Trichocladium asperum. Based on these results, the original concept was modified and a two-phase conceptual model of litter C turnover and microbial response in the detritusphere was developed. In conclusion, this thesis yields new insight into litter decomposition at the small-scale. Combining classical methods with modern techniques enabled the development of a conceptual model of litter C turnover and microbial response in the detritusphere. This provides a useful basis for future studies addressing, for example, the impact of global change on the interaction of decomposition and soil microorganisms.