Einfluss veränderter Flutungs- und Rotationspraktiken auf mikrobielle Gemeinschaften und ihre Aktivität im Hinblick auf Stickstoff-Fixierung und den Stickstoffzyklus
Zusammenfassung der Projektergebnisse
SP2 analyzed the response of the microbiome towards shifting the flooding regimes by molecular methods, with a focus on nitrogen-cycling microbes. It was anticipated that decrease of flooding and nitrogen fertilization would reduce activity of diazotrophs and change their community structure. Surprisingly, rice root communities expressing nitrogenase genes were rather resistant to these changes, while in maize root-associated nifH transcripts decreased drastically (100-fold) upon N-fertilizer application. Thus, the nitrogen-fixing root community of rice, in contrast to maize, showed a remarkable, unexpected functional stability. According to the insurance hypothesis, more diverse ecosystems are generally considered as more stable, reliable and less prone to break down upon disturbance. In agreement, the diversity of the diazotrophic root-associated population based on nifH DNA sequences was significantly higher at rice roots than at maize roots, and may thus provide a buffer for nitrogenase gene expression. Additional reductionist studies using a cultivated model endophyte Azoarcus olearius corroborated that rice roots appear to provide a more favorable microenvironment for nitrogen fixation than maize at low N-fertilizer doses. Verrucomicrobia constituted a significant part of the rice root community, however culturable members are rare. We succeeded to isolate 4 novel Verrucomicrobia species and genera, and showed for the first time their endophytic lifestyle. In soils, short-term and long term shifts to non-flooded cultivation did not drastically alter the nitrogen-cycling microbiome. Abundance and expression of marker genes for nitrification and nitrogen fixation were similar in maize, flooded and non-flooded rice soils. Only for denitrifiers, abundance or expression of marker genes increased with reduced flooding in rice and maize, suggesting indeed higher availability of nitrate/nitrite under more aerobic conditions. Similar observations were made for rhizosphere soil and roots, as well. Generally, high expression of the denitrification gene nirK was observed, although subproject SP5 did not detect the greenhouse gas N2O at the sampling time, suggesting N- losses going undetected due to further reduction to N2 by NosZ. This was confirmed by very high nosZ expression. The surface of soil cracks formed in response to desiccation in non-flooded systems might cause further N- and C-losses. The crack surface indeed differed in its microbial community from the soil matrix, but showed only slightly elevated microbial N-cycling that might contribute to nitrogen losses. Changes in flooding had a profound effect on the diversity of the soil microbiome. The diversity was highest in flooded rice soil, suggesting long-term management by traditional flooded rice systems leads to the most diverse soil microbiome. Straw addition might help to mitigate N- and C-losses in lessflooded systems. However, it did not significantly affect gene abundance or expression of most N-cycling marker genes. In contrast, probably due the additional carbon input, the microbiome was affected. Bulk soil and rhizosphere soil community composition was drastically changed by straw addition, independent of the flooding regime and plant species grown. The root microbiota itself was hardly changed and plant-specific, underlining the influence of the plant genotype to assemble the root microbiome. In conclusion, altered agricultural management had a strong influence on soil microbial communities, and a minor detectable effect on nitrogen cycling that may lead to N-losses with less flooding. Roots as hotspots for microbial activity appear to provide a more favorable microenvironment for nitrogen fixation in rice than maize at low N-fertilizer doses. This should be taken into consideration in attempts to generate nitrogen-fixing crops, where rice may be a more promising option than maize.
Projektbezogene Publikationen (Auswahl)
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(2015) Roots shaping their microbiome: Global hot spots for microbial activity. Annu Rev Phytopathol 53: 403-424
Reinhold-Hurek, B., W. Bünger, C.S. Burbano, M. Sabale and T. Hurek
