Browsing by Subject "Soil microbiology"

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Function and composition of the soil microbial community in calcareous grassland exposed to elevated atmospheric carbon dioxide
(2003) Ebersberger, Diana; Kandeler, Ellen
Terrestrial ecosystems generally respond to rising atmospheric carbon dioxide (CO2) concentrations with increased net primary productivity and increased water use efficiency. This may change the amount and quality of organic substances entering the soil and fuelling microbial metabolism. Soil microorganisms and their activity might also be affected by increased soil moisture at elevated CO2. This thesis was designed to analyse the response of the soil microbial community in a species-rich calcareous grassland in the Swiss Jura Mountains, which had been exposed to ambient and elevated CO2 concentrations (365 and 600 ppm) for six growing seasons. In the first study, laboratory incubation experiments were conducted to explore the relationship between litter quality under elevated carbon dioxide and enzymes involved in carbon cycling. Naturally senescent, mixed litter from the long-term field experiment was incubated with soil material for 10, 30 and 60 days. Soil samples were then obtained close to the litter layer using a microtome cutting device. Litter and soil samples were analysed for invertase and xylanase activity. The lower litter quality produced under elevated CO2, i.e. wider C/N ratio, yielded lower invertase and xylanase activities of litter. Litter addition stimulated activities in adjacent soil. Invertase activities of adjacent soil were not affected by litter quality, while soil xylanase activity was higher in soil compartments adjacent to litter from elevated CO2 plots. The reduced enzyme activities of litter produced under elevated CO2 can slow decomposition, at least during the initial stages. Since the effects of litter quality on enzyme activities in adjacent soil were small, we conclude that CO2-induced belowground C-inputs (e.g. increased root mass) and altered moisture conditions are more important controls of enzyme activities than altered litter quality. In the second study, functional diversity of the soil microbial community was assessed by analysing N-mineralisation and activities of enzymes of the C-, N-, P- and S-cycle of soil samples taken in spring and summer 1999, in the 6th season of CO2 exposure. In spring, N-mineralisation increased significantly by 30% at elevated CO2, while there was no significant difference between treatments in summer. The response of soil enzymes to CO2 enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly, by 32% and 21%, respectively. In summer, activity differences between CO2 treatments were greatest in the case of urease and protease (+21% and +17% at elevated CO2). The significant stimulation of N-mineralisation and enzyme activities at elevated CO2 was probably caused by higher soil moisture and/or increased root biomass. In the third study, soil microbial community structure of soil samples taken in spring and summer 1999 was analysed by means of PLFA profiles and 16S rDNA fingerprints obtained by PCR-DGGE. PLFA profiles were not affected by elevated CO2. Ordination analysis of DNA fingerprints revealed a significant relation between CO2 enrichment and variation in DNA fingerprints. This variation must be attributed to low intensity bands because dominant bands did not differ between treatments. Diversity of the bacterial community (number of bands in DNA fingerprints and Shannon indices) was not affected. The observed minute, but significant changes in the structure of the soil bacterial community might be caused by changes in the quality of rhizodeposits at elevated CO2. These could either result from altered rhizodeposition of individual plants or from altered species composition of the calcareous grassland.The 4th part of the thesis compiles data on soil microorganisms, soil fauna, soil structure and nitrogen cycle of calcareous grassland after CO2 exposure for six growing seasons. Microbial biomass, soil basal respiration and the metabolic quotient were not altered significantly. PLFA analysis revealed no significant shift in the ratio of fungi to bacteria. Protozoans, bacterivorous and fungivorous nematodes, acarians, collembolans, and root-feeding nematodes were not affected by elevated CO2. Total nematode numbers averaged slightly lower (-16%) and nematode mass was significantly reduced (by 43%) due to fewer large-diameter nematodes classified as omnivorous and predacious. CO2 exposure resulted in a shift towards smaller aggregate sizes; this was caused by higher soil moisture. Reduced aggregate sizes result in reduced pore neck diameters. This can confine the locomotion of large-diameter nematodes and possibly accounts for their decrease. The CO2 enrichment also affected the nitrogen cycle. N stocks in living plants and surface litter increased, but N in soil organic matter and microorganisms remained unaltered. N mineralisation increased considerably, but microbial N did not differ between treatments, indicating that net N immobilization rates were unaltered.
Soil microorganisms as hidden miners of phosphorus in soils under different cover crop and tillage treatments
(2022) Hallama, Moritz; Kandeler, Ellen
Phosphorus (P) is one of the most limiting plant nutrients for agricultural production. The soil microbial community plays a key role in nutrient cycling, affecting access of roots to P, as well as mobilization and mineralization of organic P (Porg). This thesis aimed to better understand the potential of cover crops to enhance plant-soil-microbe interactions to improve the availability of P. This dissertation consists of a meta-analysis of and two field experiments. The used methods showed that microbial P, the activity of P-cycling enzymes and PLFAs increased under cover crops, indicating an enhanced potential for organic P cycling. Gram- positive and Gram-negative bacteria, and to a lesser extent also arbuscular mycorrhizal fungi, increased their abundance with cover crops. However, saprotrophic fungi could benefit most from the substrate input derived from cover crop roots or litter. Enzyme-stable Porg shifted towards pools of a greater lability in the active soil compartments (rhizosheath and detritusphere). The effects of agricultural management, such as cover crop species choice and tillage, were detectable, but weaker compared to the effect of the presence of cover crops. With the obtained results, the research aims of this thesis could be successfully addressed. We were able to confirm that cover crops have the potential to improve main crops’ access to P. Furthermore, we presented and discussed three pathways of P benefit. In the plant biomass pathway, P is cycled through cover crop biomass and becomes available for the main crop upon litter decomposition. The microbial enhancement pathway describes how the cover crop’s interaction with soil microbes increases their abundance and activity, thereby increasing the availability of Porg. Some cover crop species seem to be capable of utilizing a biochemical modification pathway, where changes in the sorption capacity of the soil result in a greater quantity of plant-available phosphate. However, the latter pathway was apparently not important in the crop rotations used in our field experiments. The data also allowed us to characterize ways in which plant-soil-microbe interactions under cover crops affected the relationship of soil microbial functions to the enzymatic availability of Porg pools. Cover crops increased the abundance and activity of microbes, especially fungi, as well as microbial P. This enhancement in P-cycling potential shifted Porg toward pools of greater availability to added enzymes. However, the relation between enzymes and Porg pools is complex and is possibly affected by soil P composition and other site characteristics, indicating the need for further research in this area. Finally, we elucidated how the choice of cover crop species and agricultural management can shift the relative importance of the pathways for the P benefit of the main crop, while site-specific management allows farmers to adapt to local conditions and to optimize the functions of their agroecosystems. In conclusion, our results indicate that the pathways of cover crop derived P benefit take place simultaneously. We confirmed the potential of cover crop biomass for the cycling of P, and we suggest that our observed increases in the availability of soil Porg are related to microbial abundance and activity. The interactions of cover cropping and tillage indicate also that P benefit can be optimized by management decisions. Finally, these new insights into soil phosphorus cycling in agroecosystems have the potential to support further development of more sustainable agricultural systems.