Root mucilage modulates soil-plant-water dynamics, but its interactions with microbial community functioning remain poorly understood. The aims of this study were to estimate (I) the impacts of mucilage and soil water content on the microbial community composition and (II) the mucilage consumption by individual microbial groups. C4 root mucilage from maize (at 40 and 200 μg C per gram dry soil, corresponding to 10 and 50% of soil microbial biomass, respectively) was added in single pulses to a C3 soil at two moisture levels: optimum (80% of water-holding capacity (WHC)) and drought (30% of WHC). After 15 days of incubation, the microbial community composition was studied by phospholipid fatty acids (PLFA) analysis and incorporation of mucilage-derived 13C into individual microbial groups was determined by compound-specific isotope analysis. Microbial community composition remained largely unaffected by mucilage addition but was affected by moisture. Whereas an increase in water content reduced mucilage 13C recovery in PLFA for the low-dose mucilage amendment from 19 to 9%, it had no effect under the high-dose amendment (11–12%). This suggests that the role of mucilage for microbial functioning is especially pronounced under drought conditions. The fungal PLFA 18:2ω6,9 was present only under drought conditions, and fungi profited in their mucilage C utilisation from the lower competitiveness of many bacterial groups under drought. In this study, Gram-negatives (G−, characterised by PLFA 18:1ω9c, 18:1ω7c, 16:1ω7c and cy17:0) showed the highest mucilage-derived 13C in PLFA, especially at the high-dose amendment, suggesting them to be the major decomposers of mucilage, especially when the availability of this C source is high. Gram-positives (G+) included different sub-groups with distinct responses to moisture: G+ 1 (a15:0) were only competitive for mucilage C under drought, whereas G+ 3 (i17:0) were only able to utilise mucilage-derived C under optimal moisture conditions. During the 15-day incubation, they built up more than 40% of their membranes from mucilage-derived C, suggesting that in the case of high availability, mucilage can act as an important C source for this microbial group. However, under drought, G− 1 and fungi were incorporating the most mucilage C into their membranes (approx. 20% of PLFA-C). The observation that, for some groups, the high-dose mucilage amendments under drought led to higher 13C incorporation into PLFA than under optimum moisture suggests that mucilage can compensate drought effects for particular microbial groups. Thus, mucilage may not only act as a C source for microorganisms but may also mitigate drought effects for specific rhizosphere microbial groups.
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We highly acknowledge DAAD and Alexander von Humboldt Foundation (AvH) for funding MAA and MS. Research was funded by DFG project “Mucilage: the hydraulic bridge between roots and soil” (CA 921/2-1), DFG KU 1184/29-1, INST 186/1006-1 /P and the Robert-Bosch Foundation in the framework of the Robert Bosch Junior Professorship to MD. We would like to thank the Centre for Stable Isotope Research and Analysis, Goettingen, for IRMS measurements and δ13C determinations. We would also like to thank the editor and two anonymous reviewers for their critical comments which improved the quality of the manuscript.
Ahmed MA, Kroener E, Holz M, Zarebanadkouki M, Carminati A (2014) Mucilage exudation facilitates root water uptake in dry soils. Funct Plant Biol 41:1129–1137. https//doi:10.1071/FP13330
Ahmed MA, Holz M, Woche SK, Bachmann J and Carminati A (2015) Effect of soil drying on mucilage exudation and its water repellency: a new method to collect mucilage. J Plant Nutr Soil Sci 178: 821–824. https//doi:10.1002/jpln.201500177
Ahmed MA, Zarebanadkouki M, Ahmadi K, Kroener E, Kostka S, Kaestner A, Carminati A (2017) Engineering rhizosphere hydraulics: pathways to improve plant adaptation to drought. Vadose Zone J. https://doi.org/10.2136/vzj2016.09.0090
Brax M, Buchmann C, Schaumann GE (2017) Biohydrogel induced soil–water interactions: how to untangle the gel effect? A review. J Plant Nutr Soil Scin 180:121–141. https://doi.org/10.1002/jpln.201600453
Frostegård Å, Tunlid A, Bååth E (1993) Phospholipid fatty acid composition, biomass, and activity of microbial communities from two soil types experimentally exposed to different heavy metals. Appl Environ Microbiol 59:3605–3617PubMedPubMedCentralGoogle Scholar
Heuer H, Krsek M, Baker P, Smalla K, Wellington EM (1997) Analysis of actinomycete communities by specific amplification of genes encoding 16S rRNA and gel-electrophoretic separation in denaturing gradients. Appl Environ Microbiol 63:3233–3241PubMedPubMedCentralGoogle Scholar
Schimel JP, Scott WJ, Killham K (1989) Changes in cytoplasmic carbon and nitrogen pools in a soil bacterium and a fungus in response to salt stress. Appl Environ Microbiol 55:1635–1637PubMedPubMedCentralGoogle Scholar
York LM, Carminati A, Mooney SJ, Ritz K, Bennett MJ (2016) The holistic rhizosphere: integrating zones, processes, and semantics in the soil influenced by roots. J Exp bot 67:3629–3643. https://doi.org/10.1093/jxb/erw108
Yuan H, Zhu Z, Liu S, Ge T, Jing H, Li B, Liu Q, Lynn TM, Wu J, Kuzyakov Y (2016) Microbial utilization of rice root exudates: 13C labeling and PLFA composition. Biol Fertil Soils 52:615–627. https://doi.org/10.1007/s00374-016-1101-0
Zickenrott I-M, Woche SK, Bachmann J, Ahmed MA, Vetterlein D (2016) An efficient method for the collection of root mucilage from different plant species—a case study on the effect of mucilage on soil water repellency. J Plant Nutr Soil Sci 179:294–302. https://doi.org/10.1002/jpln.201500511