No abstract

Biochar has been frequently suggested as an amendment to improve soil quality and mitigate climate change. To investigate the optimal management of nitrogen (N) fertilization, we examined the combined effect of biochar and N fertilizer on plant N uptake and NO emissions in a cereal rotation system in a randomized two-factorial field experiment on a sandy loam soil in Brandenburg, Germany. The biochar treatment received 10Mgha wood-derived biochar in September 2012. Four levels of N fertilizer, corresponding to 0, 50%, 100%, 130% of the recommended fertilizer level, were applied in winter wheat (Triticum aestivum L.)) and winter rye (Secale cereal L.) in 2013 and 2014 followed by the catch crop oil radish (Raphanus sativus L. var. oleiformis). Biomass and N uptake of winter wheat and winter rye were significantly affected by the level of N fertilizer but not by biochar. For N uptake of oil radish an interaction effect was observed for biochar and N fertilizer. Without applied fertilizer, 39% higher N uptake was found in the presence of biochar, accompanied by higher soil NH content and elevated cumulative CO emissions. At 130% of the recommended fertilizer level, 16% lower N uptake and lower cumulative NO emissions were found in the biochar-mediated treatment. No significant change in abundance of microbial groups and nosZ gene were observed. Our results highlight that biochar can have a greenhouse gas mitigation effect at high levels of N supply and may stimulate nutrient uptake when no N is supplied.

Verticillium dahliae induced wilt disease in strawberry can cause severe yield losses and thus lead to inevitable negative economic impacts. Inoculation of plants with non-pathogenic strains of Verticillium was conducted as a biologic control agent (BCA) according to the concept that preoccupation of the ecologic niche rendered strawberry plants immune to infection with soil-borne pathogenic Verticillium. This concept was tested for economic viability in a field trial under commercial conditions. Results were reported for 2 years of field trials under practice conditions in two locations in Brandenburg, Germany. Inoculation was shown to have a positive effect of 20% of plants, while 30% of plants remain unaffected and of equally high vitality. However, 50%-60% of plants were impacted negatively, showing severe wilt symptoms up to total loss. The characteristic progression of wilt symptoms suggested an infestation caused by Phytophtora sp. and other pathogens. Further results showed that the main factor of the side effects was caused by different qualities of plant material in interaction to the inoculation with the BCA and only to a minor extent depended on pre-infestation of soils. We conclude that specific conditions, such as certified plant material or soil analysis for other pathogens besides Verticillium, avoided these side-effects relevant for commercial farming.

Abstract. Beside physico-chemical interactions between particulate organic matter (POM), mineral particles and dissolved molecules, microbial biofilms are an important factor of aggregate stability as a proxy of soil quality. Soil primary particles are linked by the highly viscous extracellular biofilm matrix known as extracellular polymeric substance (EPS). Matrix composition depends on the community of biofilm producing species and environmental conditions affecting gene expression. This work investigates the influence of microbial biodiversity on soil aggregate stability under controlled environmental conditions. We hypothesized that the formation of different microbial populations would cause different aggregate stabilities. Therefor a sterile sandy agricultural soil with pyrochar amendment from pine wood was incubated for 76 days in pF-bioreactors. One variant was inoculated with a soil extract, whereas the other one was infected by airborne microbes. A control soil remained unincubated. During the incubation, soil samples were taken for taxon-specific qPCR to determine the abundance of eubacteria, fungi, archaea, acidobacteria, actinobacteria, α-proteobacteria and β-proteobacteria. After incubation soil aggregates were separated for aggregate stability measurement by ultrasonication, density fractionation and SOC analysis. As the eubacterial populations of both incubated variants reach a similar level after 49 days, the variant inoculated with the living soil extract shows a much higher fungal population compared to the air-born variant. Within the eubacterial population acidobacteria and β-proteobacteria differ significantly in their abundance between the variants. Although the variants show a strongly significant difference in eubacterial/fungal population structure, there are only marginal differences in aggregate stability.

Abstract. Apart from physico-chemical interactions between soil components, microbial life is assumed to be an important factor of soil structure forming processes. Bacterial exudates, the entanglement by fungal hypae and bacterial pseudomycelia as well as fungal glomalin are supposed to provide the occlusion of particulate organic matter (POM) through aggregation of soil particles. This work investigates the resilience of POM occlusion in face of different microbial communities under controlled environmental conditions. We hypothesized that the formation of different communities would cause different grades of POM occlusion. For this purpose samples of a sterile sandy agricultural soil were incubated for 76 days in bioreactors. Particles of pyrochar from pine wood were added as POM analogue. One variant was inoculated with a native soil extract, whereas the control was infected by airborne microbes. A second control soil remained non-incubated. During the incubation, soil samples were taken for taxon-specific qPCR to determine the abundance of Eubacteria, Fungi, Archaea, Acidobacteria, Actinobacteria, α-Proteobacteria and β-Proteobacteria. After the incubation soil aggregates (100–2000 μm) were collected by sieving and disaggregated using ultrasound to subject the released POM to an analysis of organic carbon (OC). Our results show, that the eubacterial DNA of both incubated variants reached a similar concentration after 51 days. However, the structural composition of the two communities was completely different. The soil-born variant was dominated by Acidobacteria, Actinobacteria and an additional fungal population, whereas the air-born variant mainly contained β-Proteobacteria. Both variants showed a strong occlusion of POM into aggregates during the incubation. Yet, despite the different population structure, there were only marginal differences in the release of POM along with the successive destruction of soil aggregates by ultrasonication. This leads to the tentative assumption that POM occlusion in agricultural soils could be resilient in face of changing microbial communities.