Microorganisms interact with different soil components, such as varying substrates and soil minerals, to drive soil processes and functionality. They can be influenced by the environment, but they themselves can influence their environment by their activities, for example through the production of extracellular enzymes and extracellular polymeric substances (EPS). The formation and stability of aggregates as the backbone of the soil structure, for instance, are thought to be largely influenced by soil microorganisms, or vice versa. There remain, however, open questions as to whether and how microorganisms can influence soil aggregation. While microbes are influencing their environment their interaction with the soil minerals could also change their responses upon adsorption - affecting their influence on soil aggregation. Therefore, the overarching goal of this thesis was to investigate the effect of soil minerals, in particular clay content, on the composition and activity of soil microbial community, with a specific focus on enzyme activities and EPS. Finally, the microbial control of soil aggregation through the influence of substrate availability was explored. In total, two adsorption experiments and two incubation experiments were conducted using soils manipulated experimentally with increasing clay content. The sandy soil was amended with different amounts of soil minerals (i.e. montmorillonite) to achieve a gradient in clay content. For the first incubation experiment, organic substrates differing in decomposability (i.e., starch and cellulose) were added to the soil to stimulate microbial activities and incubated for 80 days. Soil samples from the first incubation experiment were analysed after 0, 3, 10, 20, 40 and 80 days for enzyme activities, microbial community composition, biomass C, EPS-protein and polysaccharide. Additionally, the geometric mean diameter and mean weight diameter of the soil aggregates were determined as measures of aggregate formation and stability, respectively. The first adsorption experiment examined the effect of soil mineral phases on the activities of extracellular enzymes using commercially available extracellular enzymes (α-glucosidase) added to the soil. The second adsorption and incubation experiment investigated the persistence of extracellular enzyme activities (commercially available α-amylase and cellulase) affected by soil minerals. For further insight into how other soil minerals affect extracellular enzymes, kaolinite and goethite in addition to montmorillonite were included in the second adsorption and incubation experiment. The prepared complexes (enzyme + soil and/or soil minerals) from the second adsorption experiment were incubated for 100 days. Further analytical methods include the determination of enzyme activities, microbial biomass C, extraction and quantification of the soil EPS, protein analyses, DNA isolation, DGGE, qPCR and Illumina sequencing. The adsorption experiment showed that extracellular enzyme activities decreased with increasing clay contents. In contrast, such an inhibitory effect on microbial enzyme activity was only observed directly in the incubation experiment after the stimulation of in-situ microorganisms for extracellular enzyme production through substrate addition. Higher extracellular enzyme activities at later incubation days in soils with high clay content suggested an adaptation of the microbial community in response to soil clay content and/ or persistence of extracellular enzymes by adsorption to mineral surfaces. However, the second adsorption experiment showed that the high specific activity and persistence of the enzymes were constrained by the availability of sorption sites. It is therefore reasonable to assume that soil mineral phases support microorganisms in less-sorptive environments by sparing energy on enzyme production, since even a small enzyme release could already propel sufficient activities to degrade target carbon substrates. Starch amendment accelerated respiration and microbial biomass much more than cellulose. While microbial community differed depending on the C substrate (starch or cellulose) added, clay addition had a stronger influence on alpha diversity than substrate addition. Although the production of EPS-protein was closely linked to the provision of additional substrates, the addition of clay minerals resulted in more EPS production than when no additional clay was present. By correlating soil aggregation (stability and formation) with the recorded microbial parameters (that is biomass C, EPS-protein and EPS-polysaccharide), both EPS-protein and EPS-polysaccharide exhibited a significant control on aggregate formation and microbial processes, though, surprisingly, more strongly with high clay content. It was observed that EPS is only a transient compound, which initiates aggregate formation, but clay content plays a more significant role in long-term aggregate stabilization. Overall, this thesis contributed to our knowledge about the interaction of microorganisms with the soil mineral phase and their influence on soil structural stability. The findings established that soil minerals shape the composition and activity of microbial communities. In turn, the microbial production of EPS seems to be more significant for aggregate formation than stability. The results on the effect of soil minerals on extracellular activities provided a paradigm that the persistence of enzyme activities by adsorption does not always hold. Producing EPS might contribute to microbial adaptation that mitigates the negative effect of adsorption on extracellular enzymes. It might also be probable that the EPS become a substance of degradation for the extracellular enzymes. Overall, the results indicated that in clay-rich soils the process leading to extracellular enzyme persistence can be stochastic, depending on multiple factors including sorption sites and substrate availability. Labile organic C clearly plays a role in aggregate formation by supporting EPS production. However, increasing clay content enhanced aggregate stability, promoted the development of distinct microbial communities and increased EPS production. The discrepancy so observed in the contribution of the two EPS parameters, EPS polysaccharide and protein, on soil aggregation points to the need for inclusion of different EPS compositions in future studies relating to soil aggregation.

Das Ziel dieser Arbeit war es, die Auswirkungen verschiedener Bodennutzungssysteme auf ausgewählte physiko-chemische Bodeneigenschaften und pflanzenbauliche Parameter auf sandigen Böden auf den Dauerfeldversuchsflächen in Berlin-Dahlem und Dedelow zu untersuchen. Ein erster Arbeitsschwerpunkt am Standort Berlin-Dahlem bestand darin, die Wirkungen verschiedener Bodenbearbeitungstiefen (17 cm und 28 cm) in Abhängigkeit der drei Faktorstufen - mit und ohne Kalkdüngung, mit und ohne Stallmistdüngung, flache und tiefe Bodenbearbeitung - zu ermitteln. Im zweiten Arbeitsschwerpunkt wurden in einer fünffeldrigen Fruchtfolge am Standort Dedelow Vergleiche verschiedener Verfahren der konventionellen und konservierenden Bodenbearbeitung und deren Effekte untersucht. Die Durchdringungswiderstandmessungen in Berlin-Dahlem zeigten nach langjähriger Bodenbearbeitung höhere Bodenfestigkeitswerte in der tieferen Bodenbearbeitungsvariante als in der flachen Variante. Die wasserstabilen Aggregate in Berlin-Dahlem wiesen in der flachen Bodenbearbeitung höhere Werte auf. Die ermittelten Wasserspeicherkurven in Berlin-Dahlem zeigten in der flachen Bodenbearbeitungsvariante für die ausgewählten Parzellen in 10-15 cm Bodentiefe im Vergleich zu der tieferen Variante tendenziell höhere nutzbare Feldkapazitätswerte. In Dedelow zeigten die Wasserretentionskurven unter verschiedenen Bodenbearbeitungsverfahren unregelmäßige Streuungen auf. Die höchsten Gesamtkohlenstoffgehaltswerte wurden in der flachen Bodenbearbeitung unter dem Prüfglied „mit Kalk- und mit Stallmist“ gedüngten Parzellen erzielt. In Dedelow zeigten die Gesamtkohlenstoffgehalte zwischen konservierender und konventioneller Bodenbearbeitung in 10-15 cm Bodentiefe kaum Unterschiede. Der Einfluss von Düngung und Bodenbearbeitungstiefen auf den Kornertrag bei Hafer war mit einem Ertrag von 29,2 dt ha-1 am stärksten am Standort Dedelow in der tiefen Bodenbearbeitung unter dem Prüfglied „mit Kalk- und ohne Stallmistdüngung“. / On the basis of various long-term field trials it was the goal to investigate the influence of different tillage systems on soil physical, soil chemical and crop parameters on sandy loamy locations in Berlin-Dahlem and Dedelow. The first focus of the work in Berlin-Dahlem was to investigate the effects between different tillage depths (17 cm and 28 cm) and the selected factors “with and without liming”, “with and without manure fertilization” and “shallow and deep tillage”. The second focus was on a five-field crop rotation site in Dedelow to compare different methods of conventional and conservation tillage and their effects. The penetration resistance values in Berlin-Dahlem showed after long-time tillage in the deep soil tillage system higher compactions compared to shallow tillage systems. The soil aggregate stability values in Berlin-Dahlem showed in the shallow tillage higher values. The obtained soil water retention curves in Berlin-Dahlem showed an increase of the available water capacity for the selected plots in the shallow soil tillage system compared to the deep tillage system in the 10 - 15 cm soil depth. The soil water retention curves showed irregular variations under the different tillage systems in Dedelow. The highest total carbon content were observed in the shallow tillage system in the "with lime and with farmyard manure" treatments. In general, however, the total carbon content predominate nearly the same amounts in 10 - 15 cm soil depth in both conservation and conventional tillage systems in Dedelow. The influence of fertilization and tillage depth on grain yield in oats showed the maximum value as 29,2 dt ha-1 in the deep tillage system in the "with lime and without farmyard manure" treatments.

Die bei der Biogasproduktion anfallenden Gärreste werden als organische Dünger im Pflanzenbau eingesetzt. Infolge des Vergärungsprozesses unterscheiden sich Gärreste von herkömmlichen organischen Düngern. Daraus könnten sich spezifische positive oder negative Wirkungen auf Bodenfruchtbarkeit und Pflanzen ergeben, die Gegenstand dieser Arbeit sind. In einem sechsjährigen Feldversuch (Energiepflanzenfruchtfolge, sandiger Boden) wur- den die Düngeeffekte von fünf verschiedenen Gärresten mit denen von Rindergülle, Stallmist und mineralischem Dünger (KAS) verglichen. Zur umfassenden Charakterisie- rung der Bodenfruchtbarkeit erfolgten Untersuchungen aus den Bereichen Bodenchemie, -physik, -mikrobiologie und Pflanzenbau, wobei die Humusdynamik einen Schwerpunkt bildete. Die Mengenkalkulation der Düngung basierte auf der gleichen Gabe organi- schem C mit einem N-Ausgleich durch KAS. Es ergaben sich für Gärreste gegenüber KAS überwiegend positive Effekte auf Hu- musgehalt, Aggregatstabilität und bakterielle Aktivität und Diversität. Diese Effekte waren denen von Rindergülle und Stallmist ähnlich. Möglicherweise besteht aber durch Gärrestdüngung ein erhöhtes Risiko von P- und K-Überschüssen sowie N-Verlusten. Eine erhöhte Benetzungshemmung wurde nicht gefunden. Die mikrobielle Gemeinschaft des Bodens zeigte eine spezifische Beeinflussung durch die Gärreste in der funktionellen, jedoch nicht in der genetischen Struktur. Die N-Verfügbarkeit war deutlich geringer als die von KAS, wodurch sich für Grünroggen Mindererträge ergaben. Für Silomais und Sorghum fand sich bei kombinierter Anwendung mit KAS ein zu KAS ähnliches Ertragsniveau. Gärreste haben analog zu herkömmlichen organischen Düngern bei effizientem Einsatz das Potential zu KAS vergleichbare Erträge zu erzielen und gleichzeitig die Bodenfrucht- barkeit zu fördern. Diese Schlussfolgerung ist im Kontext der spezifischen Standort- und Versuchsbedingungen zu sehen und sollte durch eine längere Versuchszeit abgesichert werden. / Digestates arising during biogas production are used as organic fertilizers in agriculture. Due to the fermentation process, digestates have different properties from traditional organic fertilizers. This may result in specific positive or negative effects on soil fertility and plants, which are subject of this work. In a six year field experiment (energy crop rotation, sandy soil) fertilizer effects of five different digestates were compared with cattle slurry, farm yard manure and mineral fertilizer (KAS). For a comprehensive characterization of soil fertility effects, methods from the fields of soil chemistry, physics, microbiology and plant science were applied. One special focus of these investigations was humus dynamics. Manure doses were based on the same dose of organic carbon, resulting different N-doses were compensated by KAS. Compared with KAS, digestates showed predominantly positive effects on humus con- tent, soil aggregate stability and bacterial activity and diversity. These effects of di- gestates were similar to those of cattle slurry and farm yard manure. There may be an increased risk of P and K oversupply and N losses as a result of digestate fertilization. An increased soil water repellency was not found. The soil bacterial community was specifically influenced by digestates regarding its functional, but not regarding its genetic structure. Saprotrophic fungy were reduced by some digestates in comparison to KAS. N availability was considerably lower than of KAS, resulting in yield depressions of forage rye. For silage maize and sorghum, digestates combined with KAS equaled the yield level of KAS. If efficiently applied, digestates have like other organic fertilizers the potential to produce equal yields as KAS whilst enhancing soil fertility. This conclusion is restricted to the specific conditions of experimental site and design and must be verified during a longer experimental period.