Composition and stability of organic matter fractions in soils under different land use regimes

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Fate and stability of dissolved organic carbon in topsoils and subsoils under beech forests

Dissolved organic carbon (DOC) from Oa horizons has been proposed to be an important contributor for subsoil organic carbon stocks. We investigated the fate of DOC by directly injecting a DOC solution from 13C labelled litter into three soil depths at beech forest sites. Fate of injected DOC was quantified with deep drilling soil cores down to 2 m depth, 3 and 17 months after the injection. 27 ± 26% of the injected DOC was retained after 3 months and 17 ± 22% after 17 months. Retained DOC was to 70% found in the first 10 cm below the injection depth and on average higher in the topsoil than in the subsoil. After 17 months DOC in the topsoil was largely lost (− 19%) while DOC in the subsoil did not change much (− 4.4%). Data indicated a high stabilisation of injected DOC in the subsoils with no differences between the sites. Potential mineralisation as revealed by incubation experiments however, was not different between DOC injected in topsoil or subsoils underlining the importance of environmental factors in the subsoil for DOC stabilisation compared to topsoil. We conclude that stability of DOC in subsoil is primary driven by its spatial inaccessibility for microorganisms after matrix flow while site specific properties did not significantly affect stabilisation. Instead, a more fine-textured site promotes the vertical transport of DOC due to a higher abundance of preferential flow paths.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

Vertical partitioning of CO2 production in a forest soil

Large amounts of total organic carbon are temporarily stored in soils, which makes soil respiration one of the major sources of terrestrial CO2 fluxes within the global carbon cycle. More than half of global soil organic carbon (SOC) is stored in subsoils (below 30 cm), which represent a significant carbon (C) pool. Although several studies and models have investigated soil respiration, little is known about the quantitative contribution of subsoils to total soil respiration or about the sources of CO2 production in subsoils. In a 2-year field study in a European beech forest in northern Germany, vertical CO2 concentration profiles were continuously measured at three locations, and CO2 production was quantified in the topsoil and the subsoil. To determine the contribution of fresh litter-derived C to CO2 production in the three soil profiles, an isotopic labelling experiment, using C-13-enriched leaf litter, was performed. Additionally, radiocarbon measurements of CO2 in the soil atmosphere were used to obtain information about the age of the C source in the CO2 production. At the study site, it was found that 90% of total soil respiration was produced in the first 30 cm of the soil profile, where 53% of the SOC stock is stored. Freshly labelled litter inputs in the form of dissolved organic matter were only a minor source for CO2 production below a depth of 10 cm. In the first 2 months after litter application, fresh litter-derived C contributed, on average, 1% at 10 cm depth and 0.1% at 150 cm depth to CO2 in the soil profile. Thereafter, its contribution was less than 0.3% and 0.05% at 10 and 150 cm depths, respectively. Furthermore CO2 in the soil profile had the same modern radiocarbon signature at all depths, indicating that CO2 in the subsoil originated from young C sources despite a radiocarbon age bulk SOC in the subsoil. This suggests that fresh C inputs in subsoils, in the form of roots and root exudates, are rapidly respired, and that other subsoil SOC seems to be relatively stable. The field labelling experiment also revealed a downward diffusion of (CO2)-C-13 in the soil profile against the total CO2 gradient. This isotopic dependency should be taken into account when using labelled C-13 and C-14 isotope data as an age proxy for CO2 sources in the soil

Developing Endemicity of Schistosomiasis, Corsica, France

International audienceUrogenital schistosomiasis was diagnosed in a man from Germany who had never traveled outside Europe. He likely acquired the infection in Corsica, France, but did not swim in the Cavu River, which was linked to a previous outbreak. This case highlights that transmission of schistosomiasis in Corsica is ongoing

Increased microbial anabolism contributes to soil carbon sequestration by mineral fertilization in temperate grasslands

Ecosystem responses to nitrogen (N) additions are manifold and complex, and also affect the carbon (C) cycle. It has been suggested that increased microbial carbon use efficiency (CUE), i.e. growth per C uptake, due to higher N availability potentially increases the stabilization rates of organic inputs to the soil. However, evidence for a direct link between altered microbial anabolism and soil organic C (SOC) stocks is lacking. In this study, unfertilized (control) and NPK-fertilized (NPK) treatments of seven temperate grassland experiments were used to test the hypothesis that fertilizer-induced differences in SOC stocks (ΔSOC) cannot be explained by differences in C input alone, but that microbial anabolism plays an important role in C sequestration. At two experimental sites, microbial CUE and related metabolic parameters was determined using an 18O labeling approach at two different incubation temperatures (10 °C and 20 °C). Fertilization effects on the abundance of Bacteria, Archaea and Fungi were also determined using quantitative PCR targeting the respective rRNA genes. Due to the availability of yield and belowground biomass data, the introductory carbon balance model (ICBM) could be used for all seven sites to estimate the contribution of C input to ΔSOC. A significantly higher microbial growth (+102 ± 6%), lower specific respiration (−16 ± 7%) and thus significantly higher CUE (+53 ± 21%) was found for the NPK treatments, which was consistent across experiments and incubation temperatures and correlated with measured root C:N ratios. Growth (+49 ± 5%) and respiration (+70 ± 9%) were increased by a higher incubation temperature, but this was not the case for CUE. The fungi to bacteria ratio changed significantly from 0.18 ± 0.02 (control) to 0.09 ± 0.02 (NPK). On average, only 77% (51% when excluding one extreme site) of observed ΔSOC was explained by C inputs. The optimized humification coefficient h of the model used to fit the observed ΔSOC was strongly correlated to differences in the root C:N ratio between the control and NPK treatments (R2 = 0.71), thus confirming a link between microbial anabolism and substrate C:N ratio. Furthermore, varying h directly by observed differences in CUE improved the model fit at the two sites investigated. This study provides direct evidence that CUE of soil microbial communities is relevant for SOC sequestration, and its dependency on soil N availability or substrate C:N ratio might allow for its inclusion in models without explicit microbial C pools.</p