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Rationale: Many bacteria synthesize carbon (C) and energy storage compounds, including water‐insoluble polyester lipids composed mainly or entirely of poly(3‐hydroxybutyrate) (PHB). Despite the potential significance of C and energy storage for microbial life and C cycling, few measurements of PHB in soil have been reported. Methods: A new protocol was implemented, based on an earlier sediment extraction and derivatization procedure, with quantification by gas chromatography/mass spectrometry (GC/MS) and 13C‐isotopic analysis by GC/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Results: The PHB content was 4.3 μg C g−1 in an agricultural soil and 1.2 μg C g−1 in a forest topsoil. This was an order of magnitude more PHB than obtained by the existing extraction method, suggesting that native PHB in soil has been previously underestimated. Addition of glucose increased the PHB content by 135% and 1,215% over 5 days, with the largest increase in the relatively nutrient‐poor forest soil. In the agricultural soil, 68% of the increase was derived from added 13C‐labeled glucose, confirming synthesis of PHB from glucose for the first time in soil. Conclusions: The presence and responsiveness of PHB in both these contrasting soils show that PHB could provide a useful indicator of bacterial nutritional status and unbalanced growth. Microbial storage could be important to C and nutrient cycling and be a widespread strategy in the life of soil bacteria. The presented method offers new insight into the significance of this compound in soil. [ABSTRACT FROM AUTHOR]

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 C 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 C 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 C 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 C 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. [ABSTRACT FROM AUTHOR]

Background and aims: Biopores as microbial hotspots provide additional nutrients to crops - but only if their roots grow within the biopores. Such reuse has never been quantified as pre-crop-specific biopores are hardly differentiated from the multitude of pre-existing biopores. Quantification requires e.g. radionuclide labelling of pre-crops (Cs, to label their biopores) and main crops (C, to detect new roots). Preliminary testing was performed on simulated biopore reuse: both nuclides given to the same plant were excreted into the same rhizosphere. Methods: Cichorium intybus (cv. Puna) and Medicago sativa (cv. Planet) were each sequentially labelled via the leaves with Cs and CO. β-signals were visualised by imaging of horizontal soil cuts - with and without shielding off the weaker C. Results: Both species allocated 7.1-9.4% of the Cs and 21-63% of the C below ground. The first image gave both activities; while the second gave only Cs. Subtracting the second from the first image gave the C distribution, resulting in successful separation of the signals. Thus, separate spatial representations of the roots were obtained. Main root locations by Cs and C showed a very high spatial overlap coefficient (> 0.95). Conclusions: Biopore reuse quantification likely becomes feasible with this sequential labelling and shielding approach. [ABSTRACT FROM AUTHOR]