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Soil CH4fluxes are driven by CH4-producing and -consuming microorganisms that determine whether soils are sources or sinks of this potent greenhouse gas. To date, a comprehensive understanding of underlying microbiome dynamics has rarely been obtained in situ. Using quantitative metatranscriptomics, we aimed to link CH4-cycling microbiomes to net surface CH4fluxes throughout a year in two grassland soils. CH4fluxes were highly dynamic: both soils were net CH4sources in autumn and winter and sinks in spring and summer, respectively. Correspondingly, methanogen mRNA abundances per gram soil correlated well with CH4fluxes. Methanotroph to methanogen mRNA ratios were higher in spring and summer, when the soils acted as net CH4sinks. CH4uptake was associated with an increased proportion of USCa and ? pmoAand pmoA2 transcripts. We assume that methanogen transcript abundance may be useful to approximate changes in net surface CH4emissions from grassland soils. High methanotroph to methanogen ratios would indicate CH4sink properties. Our study links for the first time the seasonal transcriptional dynamics of CH4-cycling soil microbiomes to gas fluxes in situ. It suggests mRNA transcript abundances as promising indicators of dynamic ecosystem-level processes.

Soil CH4fluxes are driven by CH4-producing and -consuming microorganisms that determine whether soils are sources or sinks of this potent greenhouse gas. To date, a comprehensive understanding of underlying microbiome dynamics has rarely been obtained in situ. Using quantitative metatranscriptomics, we aimed to link CH4-cycling microbiomes to net surface CH4fluxes throughout a year in two grassland soils. CH4fluxes were highly dynamic: both soils were net CH4sources in autumn and winter and sinks in spring and summer, respectively. Correspondingly, methanogen mRNA abundances per gram soil correlated well with CH4fluxes. Methanotroph to methanogen mRNA ratios were higher in spring and summer, when the soils acted as net CH4sinks. CH4uptake was associated with an increased proportion of USCα and γ pmoAand pmoA2 transcripts. We assume that methanogen transcript abundance may be useful to approximate changes in net surface CH4emissions from grassland soils. High methanotroph to methanogen ratios would indicate CH4sink properties. Our study links for the first time the seasonal transcriptional dynamics of CH4-cycling soil microbiomes to gas fluxes in situ. It suggests mRNA transcript abundances as promising indicators of dynamic ecosystem-level processes.

Refined conversion factors for soil fungal biomarkers are proposed.High interspecific variability is present in all fungal biomarkers.A modeling approach supports the validity of biomarker estimates in diverse soils.ITS1 copies vary strongly, but are fungal-specific with least phylogenetic bias.A combination of fungal biomarkers will reveal soil fungal physiology and activity.

Regional direct field measurements of potential response of soil organiccarbon SOC and total nitrogen TN stocks to different land uses is of great importance for the development of bestmanagement practices with respect to SOC and TN accumulation in soils of BadenWürttemberg, SW Germany. This study was conducted to evaluate the effects of plow tillage PT, reduced tillage RT, and grassland GL on soil organicC and TN stocks in surface soils. Soils samples from 13 sites of adjacent paired PT, RT, and GL were taken at depths of 0–5, 5–10, and 10–20 cm throughout BadenWürttemberg. Averaged over the 13 sites, GL had significantly higher SOC concentrations in the 0–5 cm layer and significantly higher TN concentrations in the 0–5 and 5–10 cm layers, compared with soils under PT. Soils under RT tended to have higher SOC concentrations in the 0–5 and 5–10 cm layers and higher TN concentrations at all the three depths, compared with soils under PT. Grassland contained significantly higher SOC and TN stocks than soils under PT in the 0–5 and 5–10 cm layers. Soils under RT tended to have higher SOC and TN stocks than soils under PT in all the three layers. Grassland contained 44.5 14.75 Mg C ha–1 more SOC and 43.5 1.58 Mg N ha–1 more TN stocks than soils under PT, and soils under RT contained 7.3 2.7 Mg C ha–1 more SOC and 7.9 0.29 Mg N ha–1 more TN stocks than soils under PT in the 0–20 cm depth interval, respectively. We estimated that GL accumulated an average 1.27 Mg C ha–1 y–1and 0.129 Mg N ha–1 y–1more than soils under PT over an average period of 11 y, whereas soils under RT accumulated 0.32 Mg C ha–1 y–1and 0.033 Mg N ha–1 y–1relative to soils under PT in the top 20 cm, respectively. The outcomes of this study are significant for the assessment of C and N sequestration forecasting of landuse management associated with grassland and reduced tillage in SW Germany.

We review recent experimental results on the role of soil biota in stabilizing or destabilizing soil organic matter (SOM). Specifically, we analyze how the differential substrate utilization of the various decomposer organisms contributes to a decorrelation of chemical stability, residence time, and carbon (C) age of organic substrates.Along soil depth profiles, a mismatch of C allocation and abundance of decomposer organisms is consistently observed, revealing that a relevant proportion of soil C is not subjected to efficient decomposition. Results from recent field and laboratory experiments suggest that (1) bacterial utilization of labile carbon compounds is limited by short-distance transport processes and, therefore, can take place deep in the soil under conditions of effective local diffusion or convection. In contrast, (2) fungal utilization of phenolic substrates, including lignin, appears to be restricted to the upper soil layer due to the requirement for oxygen of the enzymatic reaction involved. (3) Carbon of any age is utilized by soil microorganisms, and microbial C is recycled in the microbial food web. Due to stoichiometric requirements of their metabolism, (4) soil animals tend to reduce the C concentration of SOM disproportionally, until it reaches a threshold level.The reviewed investigations provide new and quantitative evidence that different soil C pools underlie divergent biological constraints of decomposition. The specialization of decomposers towards different substrates and microhabitats leads to a relatively longer persistence of virtually all kinds of organic substrates in the nonpreferred soil spaces. We therefore propose to direct future research explicitly towards such biologically nonpreferred areas where decomposition rates are slow, or where decomposition is frequently interrupted, in order to assess the potential for long-term preservation of C in the soil.

Abstract: The effect of the endogeic earthworm species Octolasion tyrtaeum (Savigny) on decomposition of uniformly 14C-labelled lignin (lignocellulose) was studied in microcosms with upper mineral soil (Ah-horizon) from two forests on limestone, representing different stages of succession, a beech- and an ash-tree-dominated forest. Microcosms with and without lower mineral soil (Bw-horizon) were set-up; one O. tyrtaeum was added to half of them. It was hypothesised that endogeic earthworms stabilise lignin and the organic matter of the upper mineral soil by mixing with lower mineral soil of low C content. Cumulative C mineralization was increased by earthworms and by the addition of lower mineral soil. Effects of the lower mineral soil were more pronounced in the beech than in the ash forest. Cumulative mineralization of lignin was strongly increased by earthworms, but only in the beech soil (+24.6%). Earthworms predominantly colonized the upper mineral soil; mixing of the upper and lower mineral soils was low. The presence of lower mineral soil did not reduce the rates of decomposition of organic matter and lignin; however, the earthworm-mediated increase in mineralization was less pronounced in treatments with (+8.6%) than in those without (+14.1%) lower mineral soil. These results indicate that the mixing of organic matter with C-unsaturated lower mineral soil by endogeic earthworms reduced microbial decomposition of organic matter in earthworm casts.