Effect of crop rotation and rice straw mulching on microbial communities involved in carbon conversion processes
Final Report Abstract
The cultivation of rice is linked to huge water consumption and high methane emissions. Crop rotation between paddy rice in the wet season and an upland crop such as maize in the dry season is a strategy to counteract these problems. However, introduction of upland crops in paddy fields often leads to carbon and nitrogen loss due to desiccation crack formation. The application of rice straw can reduce crack formation and serves as fertilizer. In this project, the soil microbial responses to rice-maize crop rotation and rice straw application in the soil and rhizosphere of maize were evaluated. Our first study revealed that crop rotation as well as straw application lead to changes in microbial community composition, whereby the effect of crop rotation was stronger than the immediate response of the microbiota to straw application. Specific bacterial and fungal taxa were identified that responded to these alterations in the bulk soil as well as in the rhizosphere. Responses in the microbial community were linked to the reduced flooding, leading to a decrease in the relative abundance of anaerobic bacteria, as well as to the cultivated crop, leading to the enrichment of plant-host specific symbionts, endophytes and pathogens. A second study aimed at the identification of the straw-degrading microorganisms. It turned out that straw degradation underwent a clear temporal succession. In the initial phase, fast growing bacteria became labelled, followed by the labelling of fungi, known to degrade more complex carbon compounds. Straw degradation was at all points in time performed by aerobic microorganisms as known from oxic soils, though with little diversity concerning the fungal players. Anaerobic microorganisms were not involved, which is remarkable, because the soils under rice-maize crop rotation host bacterial communities that are still quite similar to those of paddy soils under rice-rice rotation, and an involvement of anaerobic microorganisms in straw degradation even under oxic incubation conditions has been reported for paddy soils. Thus, our data suggest that methane production is effectively suppressed when rice straw is returned to the soil during the period of upland cropping. A comparative analysis of the straw degradation process in the rhizosphere versus bulk soil revealed that the microbial straw degradation process differs to some extent between these two compartments. This indicates that microorganisms specifically enriched in the rhizosphere profit from straw as additional carbon source. The size of the labelled microbial population was smaller in the rhizosphere than in bulk soil, pointing to a preferential utilization of plant root derived carbon by microorganisms in the rhizosphere. Nevertheless, straw application modulates root exudate metabolism in the rhizosphere, as evident from our finding that the presence of rice straw as additional carbon source reduces the uptake of root exudates by rhizosphere microorganisms. The impact of the desiccation cracks formed during the dry season in paddy soils under crop rotation had only a minor impact on the soil microbial community. We did not observe significant changes in bacterial community composition or abundance, although crack surfaces are considered as a microbial hot spot in soil. However, an increased microbial activity was observed in crack surface soil, which may lead to a higher loss of organic carbon. Taken together, the work of this project demonstrated that the altered agricultural management regimes affect soil microbial communities and their activities in paddy soils specifically in dependence on the localization of the microorganisms in the soil matrix or in microbial hot spots such as the rhizosphere or preferential flow paths.
Publications
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(2017) Characterization of the first rice paddy cluster I isolate, Methyloterricola oryzae gen. nov., sp. nov. and amended description of Methylomagnum ishizawai. Int. J. Syst. Evol. Microbiol. 67: 4507-4514
Sarah Maarastawi, the PhD student funded by this project, contributed to the study: Frindte, K., Maarastawi, S.A., Lipski, A., Hamacher, J., Knief, C.
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(2018) Crop rotation and straw application impact microbial communities in Italian and Philippine soils and the rhizosphere of Zea mays. Front. Microbiol. 9: 1295
Maarastawi, S. A., Frindte, K., Linnartz, M., Knief, C.
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(2018) Temporal dynamics and compartment specific rice straw degradation in bulk soil and the rhizosphere of maize. Soil Biol. Biochem. 127: 220-212
Maarastawi, S. A., Frindte, K., Geer, R. Kröber, E., Knief, C.
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(2019) Preferential flow pathways in paddy rice soils as hot spots for nutrient cycling. Geoderma 337: 549-606
Fuhrmann I. Maarastawi S., Neumann J., Amelung W., Frindte K., Knief C., Lehndorff E., Wassmann R., Siemens J.
