Seminar: Georg Dittmann

We investigated the role of easily available carbon sources, which mimic root exudates, and existing soil organic matter, which provides energy and organic forms of nitrogen, on the fixation of atmospheric dinitrogen (N₂) by “free-living” diazotrophs. Enhancing diazotrophic nitrogen fixation as a replacement for mineral N fertilisers would reduce losses of mineral N to groundwater and radiation absorbing gaseous forms of N to the atmosphere, and would hence lead to sustainable food production. In addition, diazotrophic N₂ fixation may overcome nutrient limitation following elevated CO₂ concentrations. Therefore, we performed ¹⁵N₂ incubation experiments using gradients of soil organic matter (SOM) and easily available carbon. We analysed the total ¹⁵N uptake using isotope ratio mass spectrometry and the soil lipidome, which is a proxy for soil microbial community functioning, using ultra-high-pressure liquid chromatography coupled with high-resolution tandem mass spectrometry (UHPLC-HRMS/MS) and feature-based molecular network analysis. Our results showed a high potential for diazotrophic N fixation increasing the d¹⁵N values of the soil by 1227 ‰, which correspond to a potential uptake of 157 kg N per hectare over the main plant exudation period of 60 days. The input of easily available carbon significantly increased the fixation of labelled nitrogen by 535 ‰ corresponding to a total potential N fixation of 69 kg N ha^-160d^-1. Simultaneously, we observed the biosynthesis of microbial storage lipids (Triacylglycerols) and distinct preference of nitrogen-free over nitrogen-containing lipids, which suggests stress related membrane adaptations/substitutions by major parts of the microbial community. In contrast, the uptake of labelled nitrogen was decreased by 369 ‰ in soils with a high content of soil organic matter, corresponding to a total uptake of 47 kg N ha^-160d^-1. Simultaneously, the higher SOM content was positively related to the presence of membrane-associated lipids, indicating growth of cellular biomass and thus a larger microbial community. Our results imply that increased root exudation, caused by elevated CO₂ concentrations, increases the potential for “free-living” diazotrophic N fixation in SOM poor, but not in organic matter/N-rich soils. Thus, sustainable management strategies that make use of the full potential of free livening Nitrogen fixers may lead to a more sustainable food production without additional N fertilizers.