Publications

Biochemistry is an essential yet undervalued aspect of soil ecology, especially when analyzing soil C cycling. We assume, based on tradition, intuition or

Phosphorus (P) acquisition in forest ecosystems relies on litter cycling, but foliar P concentrations in beech forests are decreasing. This highlights the urgency to understand how soil microbes adapt to P limitations caused by environmental shifts. In this study, a novel approach combining 33P-wick labeling to trace litter-P recycling with a microbial dilution approach was used to study microbial P cycling associated with microbial biodiversity loss. Sterilized soils, re-inoculated with different dilutions of their native microbial communities were incubated with 33P-labeled beech litter for four weeks. The kinetics of acid phosphatase and the flux of 33P into different soil pools were determined. Carbon (C), nitrogen (N), and P, in soil microbial biomass and in extractable pools (e.g., Presin) were measured. The acid phosphatase activity decreased by 75–92 % with the dilution increase from 10−4 to 10−6 at the P-rich site, indicating a functional loss of P mobilization. The overall acid phosphatase activity was 1-fold higher at the P-deficient site than at the P-rich site, suggesting a high functional redundancy of microbial P mobilization. The recoveries of litter-derived 33P in soil microbial biomass (SMB) and in Presin were 5-fold and 2-fold higher for the P-deficient site than for the P-rich site throughout all dilutions, suggesting that the recycling of litter-P by SMB in the P-deficient soil is highly redundant as an intermediate reservoir. Our study confirms that the high functional redundancy of microbial P acquisition at a P-deficient forest site can maintain pivotal microbial acquisition processes for P uptake.

<p>Grassland management practices, such as grazing with varying animal density and mowing may impact the processes leading to soil organic carbon (SOC) accumulation. Although, they serve similar agricultural purposes, they differ in their effect on plant physiology and their influence on SOC remains uncertain. We hypothesised that both practices affect SOC storage differently due to an altered plant C input and changed growth and physiological response leading consequently to contrasting soil microbial activity.</p><p>Based on this, our experiment included the investigation of three grassland treatments: grazing at two intensities and mowing which are located at the experimental station of SOERE ACBB (Clermont-Ferrand, France). Additionally, we included bare soil and unmanaged abandoned site considering as negative and positive controls, accordingly. The aim of the study was to estimate how grazing and mowing affect SOC chemical characteristics and its link with microbial activity.</p><p>Our results show highest SOC contents under low grazing intensity, whereas SOC content under high grazing intensity was lower and did not differ from abandoned grassland. SOC content under mowing was lowest among all treatments but still higher compared to bare soil. Microbial biomass C (MBC) followed a similar pattern under high grazing intensity and positive control whereas it was similar under mowing and low grazing intensity and lowest under bare soil. Absolute enzyme activities showed a similar tendency as SOC content. However, enzyme activities per MBC resulted in highest values under low grazing intensity and similarly lower values under all other treatments.</p><p>These results demonstrate that microbial parameters responded to management in various ways most probably related to the differences in dung and litter inputs. We suggest that dung input under high grazing intensity increased MBC and consequently compensated for plant removal thus keeping SOC contents increasing. Consequently, grazing at both intensities allows to maintain SOC at similar levels as in absence of management. While on unmanaged land high SOC may be related to absence of harvest, on grazed land it may be related to stimulation of microbial activity due to animal activity. Mowing treatment on the other hand did not allow to increase SOC.</p><p>We conclude that the presence of animals in the system is essential to improve soil heath, biogeochemical cycling, and SOC storage.</p>

Root mucilage modulates soil-plant-water dynamics, but its interactions with microbial community functioning remain poorly understood. The aims of this stu

No abstract is provided for this article.

Microorganisms metabolize soil organic carbon (C) as a source of energy and biosynthetic precursors. Conventional metabolic flux analysis (MFA), coupled to 13C-labelling, can reconstruct C allocation through central metabolic pathways, but only reflects mass flow and not the thermodynamics of metabolism. We coupled metabolic energetics (13C), mass flow (18O) and calorespirometry in soil using an optimal set of isotopomer tracers. Fifteen position-specific or uniformly 13C-labelled isotopomers - four for alanine, seven glucose, and four glutamic acid – were added to a Luvisol, and substrate-derived 13CO2 fluxes along with microbial use efficiencies (CUE and SUE) were quantified as well as heat dissipation via isothermal microcalorimetry. Our results demonstrate that the temporal dynamics of catabolic CO2 release resemble that of heat dissipation, with both peaking approximately 18 h after substrate addition, irrespective of whether the tracer enters the central metabolic pathway at the monosaccharide level (glucose), at the pyruvate level (alanine) or the citric acid cycle (glutamic acid). This indicates that heat dissipation during the growth phase was strongly dominated by the microbial metabolic processes. Heat dissipation declined disproportionally compared to C mineralization after multiplicative growth, resulting in a lower calorespirometric ratio. Substrate-derived microbial biomass C (13C-MBC) pools showed that amino acids were incorporated, and retained in the biomass with intensive recycling, whereas glucose gets taken up, incorporated but ongoingly consumed, which leads to the peak biomass. This suggests that sugar may be a good tracer for metabolism. Glucose isotopomer utilization indicated dominance of the pentose phosphate and Entner Douderoff pathways over glycolysis, suggesting high activity of fast-growing organisms with considerable C allocation to anabolism. While calorespirometric ratio declined stepwise from 2426 kJ mol-1 , SUE and EUE were close to 100% during initial stage after the addition and declined when substance respiration started. In contrast, neither 18O-water- nor 13C-MFA-based CUE were altered by substrate supply, indicating that exogeneous substrate did not alter the microbial utilization and microbial quick regulation. Therefore, substrate mixtures do not induce a major shift in metabolic pathways during growing on them, leaving overall CUE largely unaffected. This study shows that the heat dissipation of growing microbial communities under high C supply is closely linked to their catabolic CO2 release. Consumption of easily-available carbon does not alter CUE (i.e. metabolic and physiological state of the soil microbiome), but strongly reduces SUE and EUE during ongoing substrate use. We furthermore demonstrated that coupled MFA and calorespirometry provides a powerful tool to understand in-situ microbial C and energy use in soils.

Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisition of mineral phosphorus by plants. However, the role of LMWOAs in mobilizing organic phosphorus, which is the predominant phosphorus form in at least half of the world's ecosystems, especially in humid climates, is unclear. The mechanisms of phosphorus mobilization by LMWOAs depend on climate, mainly precipitation, and shape the phosphorus nutrition strategies of plants. We disentangled the impact of roots and associated microorganisms on mechanisms of phosphorus cycling mediated by LMWOAs by studying soils along an ecosystem-sequence (ecosequence) from arid shrubland (~70 mm yr−1), and Mediterranean woodland (~370 mm yr−1) to humid-temperate forest (~1470 mm yr−1). Phosphorus speciation in soil was examined by X-ray absorption near edge structure analysis (XANES). LMWOAs were quantified as biological rock-weathering and organic phosphorus mobilization agents and compared to kinetics of acid phosphatase as a proxy for organic phosphorus mineralization. Calcium-bound phosphorus in topsoils decreased from 126 mg kg−1 in the arid shrubland, to 19 mg kg−1 in the Mediterranean woodland and was undetectable in the humid-temperate forest. In contrast, organic phosphorus in topsoils in close root proximity (0–2 mm distance to roots) was absent in the arid shrubland but raised to 220 mg kg−1 in the Mediterranean woodland and to 291 mg kg-1 in the humid-temperate forest. The organic phosphorus content in topsoils was 1.6 to 2.4 times higher in close root proximity (0–2 mm distance to roots) compared to bulk soil (4–6 mm distance to roots) in the Mediterranean woodland and humid-temperate forest, showing intensive phosphorus bioaccumulation in the rhizosphere. Redundancy analysis (RDA) revealed that LMWOAs were explained by the content of hydroxyapatite and variscite phosphorus-species in the arid shrubland, indicating that LMWOAs contribute to mineral weathering in this soil. LMWOA contents, phosphatase activity, and microbial biomass carbon correlated strongly with organic phosphorus in the humid-temperate forest soil, which implies a high relevance of LMWOAs for organic phosphorus recycling. In the Mediterranean woodland soil, however, oxalic acid correlated with organic phosphorus in the topsoil (suggesting phosphorus recycling), whereas in the subsoil malic and citric acid were correlated with primary and secondary phosphorus minerals (implying mineral weathering). We conclude that phosphorus acquisition and cycling depend strongly on climate and that the functions of LMWOAs in the rhizosphere change fundamentally along the precipitation gradient. In the arid shrubland LMWOAs facilitate biochemical weathering (rock eating), while in the humid-temperate forest their functions change towards supporting organic phosphorus recycling (vegetarian).

Dataset and code for the publication: Intracellular carbon storage by microorganisms is an overlooked pathway of biomass growth<br> Mason-Jones, K., Breidenbach, A., Dyckmans, J., Banfield, C.C., Dippold, M.A.<br> Nature Communications<br> 2023<br> <br> Article DOI: 10.1038/s41467-023-37713-4