Priming Effects in the Rhizosphere of Maize: Mechanisms and Field Relevance
Final Report Abstract
Providing root exudates as energy source for microorganisms, plants alter the microbial activity in the rhizosphere. Thereby, they influence the rate of native soil organic matter (SOM) decomposition in the soil surrounding their roots. Changes in SOM decomposition induced by rhizodeposits, especially exudates, are termed rhizosphere priming effects (RPE). RPEs have widely been observed in numerous studies under controlled conditions, but due to technical obstacles, field studies are still rare. This project focused on the relevance of RPEs in agroecosystems and on their underlying mechanisms. We hypothesized that RPEs strongly depend on i) the quantity of primer, and ii) the mineral N status of the soil. To test these hypotheses we conducted several field and laboratory experiments based on isotope technologies and the measurement of microbial parameters. In the field, a C3-to-C4 vegetation change was applied to partition soil CO2 efflux into its sources (SOM- and root-derived CO2). RPEs were calculated as changes in SOM-derived CO2 in planted compared to unplanted soil. We could show that RPEs are measureable and relevant in agroecosystems where they accelerated the decomposition of SOM by up to 126% compared to a bare fallow. This enhanced SOM decomposition was attributed to an increase in microbial biomass and extracellular enzyme activities (EEA) confirming the microbial activation hypothesis. Our data further revealed that EEAs are driven by root activity and hence, by plant phenological stage and rooting depth. In the rhizosphere, a habitat characterized by a limitation in mineral nitrogen (N), organisms may benefit from the release of available N caused by positive RPEs. To investigate the coupling between C and N cycles, gross N mineralization rates were determined (15N dilution approach). Our data showed a strong positive correlation between SOM decomposition and gross N mineralization underpinning the tight coupling and beneficial effects of RPEs in terms of N availability. In accordance to that, this project further revealed that the magnitude of RPEs strongly depends on the mineral N availability in the soil. Nitrogen fertilization substantially reduced RPEs in the field. A lower root activity after N fertilization presumably decreased root exudation. Lower exudation and higher N availability may have reduced RPEs compared to non-fertilized maize fields. Conversely, without N addition microorganisms are more dependent on SOM decomposition for N mining. These findings further pointed to the N mining hypothesis as a main explanation of RPEs in the field. The quality and quantity of root exudates largely depend on root morphology. Under controlled conditions, we investigated the link between root hairs, a critical part of the entire root morphology, and the RPE. Higher rhizodeposition caused positive RPEs (69% increase) of a wild type barley at tillering, while a mutant without root hairs produced negative RPEs (28% decline). At the head emergence stage, when nutrients were scarce, both barley types produced positive RPEs (72% and 209% increase for the wild type and the mutant, respectively) and microbial biomass increased compared to tillering. Moreover, extracellular enzymes responsible for the decomposition of stable SOM had higher activities in cases of positive RPEs. Overall, these results showed that root hairs play an important role in regulating RPEs. As demonstrated by the previously mentioned experiments, C input into the rhizosphere is the main driver for RPEs. Since rhizodeposition is linked to photosynthesis, aboveground processes largely contribute to C cycling in the rhizosphere. We designed and tested a novel setup for high-resolution measurements of RPEs based on 13C-CO2 continuous labeling and laser technology to measure the isotopic composition of soil CO2 allowing to calculate RPEs. High-resolution measurements of priming will deliver further information on the tight coupling of assimilation and C turnover in the rhizosphere. In summary, this project has led to a more comprehensive understanding of RPEs in arable soils. We showed that RPEs are relevant in the field, with SOM decomposition more than twice as high in the presence of plants compared to bare fallow soil. The magnitude of RPEs largely depends on the N availability in soil. Nitrogen fertilization substantially reduced SOM decomposition. Our studies revealed that microbial activation and SOM decomposition for N mining are the prevailing mechanisms of RPEs in this arable field.
Publications
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(2016) Maize rhizosphere priming: field estimates using 13C natural abundance. Plant and Soil 409, 87-97
Kumar A., Kuzyakov Y., Pausch J.
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(2016) Rhizosphere priming of barley with and without root hairs. Soil Biology and Biochemistry 100, 74-82
Pausch J., Kühnel A., Forbush K., Loeppmann S., Kuzyakov Y., Cheng W.
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(2017) Effects of maize roots on aggregate stability and enzyme activities in soil. Geoderma 306, 50-57
Kumar A., Dorodnikov M., Splettstößer T., Kuzyakov Y., Pausch J.
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(2018) Maize phenology alters the distribution of enzyme activities in soil: Field estimates. Applied Soil Ecology 125, 233-239
Kumar A., Shahbaz M., Blagodatskaya E., Kuzyakov Y., Pausch J.
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(2018) Plant-microbial interactions in the rhizosphere: root mediated changes in microbial activity and soil organic matter turnover. Dissertation. University of Bayreuth
Kumar A.
