A procedure of determining carbon-13 composition in soil organic carbon on an Isotope Ratio Mass-Spectrometer

In this study, a procedure of determining the 13C isotope composition ([13C]/[12C]) in soil organic carbon (SOC) using an isotope ratio mass spectrometer (IRMS) was developed. The procedure would be a useful approach in the studies on carbon sequestration that is of great concern among environmentalists worldwide nowadays. The procedure includes: drying, crushing, sifting and removing carbonate in soil samples before the analysis on the mass spectrometer. Results showed that the developed procedure gained a good repeatability of 0.21 ‰. The accuracy of the procedure waschecked by analyzing a surrogate soil sample, a mixture of soil with known δ13CSOC and IAEA-CH-3 cellulose standard.

Differences in the Levels of the Selected Phytoestrogens and Stable Isotopes in Organic vs. Conventional Hops and Beer

Xanthohumol (XN), isoxanthohumol (IX) and 8-prenylnaringenin (8-PN) are important prenylflavonoids present in hops with potential beneficial properties. In this study, we examined differences in the content of XN, IX and 8-PN in hops and beer produced under organic and conventional production regimes. A An ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS) method for analysing XN, IX and 8-PN in hops and beer was developed and validated, with LOQ ranging from 0.5 to 10 ng/mL. Finally, we examined 15N/14N and 12C/13C isotope ratios in the hops and beer using isotope ratio mass spectrometry (IRMS). The results show no statistically significant difference in the content of the selected prenylflavonoids between organic and conventionally produced hops and beer—in the whole sample group, as well as between the matched pairs. Stable isotope analysis indicated that only δ15N values are statistically higher in organically produced hops and beer. However, the differentiation according to the type of production could not be made solely based on the δ15N signature, but it could be used to provide supporting evidence.

Carbon Availability and Nitrogen Mineralization Control Denitrification Rates and Product Stoichiometry during Initial Maize Litter Decomposition

Returning crop residues to agricultural fields can accelerate nutrient turnover and increase N2O and NO emissions. Increased microbial respiration may lead to formation of local hotspots with anoxic or microoxic conditions promoting denitrification. To investigate the effect of litter quality on CO2, NO, N2O, and N2 emissions, we conducted a laboratory incubation study in a controlled atmosphere (He/O2, or pure He) with different maize litter types (Zea mays L., young leaves and roots, straw). We applied the N2O isotopocule mapping approach to distinguish between N2O emitting processes and partitioned the CO2 efflux into litter- and soil organic matter (SOM)-derived CO2 based on the natural 13C isotope abundances. Maize litter increased total and SOM derived CO2 emissions leading to a positive priming effect. Although C turnover was high, NO and N2O fluxes were low under oxic conditions as high O2 diffusivity limited denitrification. In the first week, nitrification contributed to NO emissions, which increased with increasing net N mineralization. Isotopocule mapping indicated that bacterial processes dominated N2O formation in litter-amended soil in the beginning of the incubation experiment with a subsequent shift towards fungal denitrification. With onset of anoxic incubation conditions after 47 days, N fluxes strongly increased, and heterotrophic bacterial denitrification became the main source of N2O. The N2O/(N2O+N2) ratio decreased with increasing litter C:N ratio and Corg:NO3− ratio in soil, confirming that the ratio of available C:N is a major control of denitrification product stoichiometry.

Diel Variation of Hydrochemistry and Carbon Flux in the Banzhai River, SW China

This study was undertaken in the Banzhai, a small groundwater-fed stream flowing over carbonate karst terrain in the southwest of Guizhou, China. To assess the biogeochemical processes behind carbon fluxes and sinks and calculate the end-member contribution to the geological carbon sink, samples were collected at a 2-h sampling interval during a two-day period, and the diel-variation of δ13CDIC, δ13CPOC, and C/N were analyzed. During the sampling period, temperature, pH, electrical conductivity (EC), dissolved oxygen (Do), and chlorophyll were measured at a 15-min interval using in situ sensors. The results showed that (1) the hydro-chemical variations reflected the photosynthesis of subaquatic plants and degassing. These processes likely turned a part of HCO3- to organic carbon, which subsequently precipitated. (2) The 13C isotope ratios indicated a varying intensity of photosynthesis and degassing during the HCO3- migration process. Moreover, subaquatic plants changed their metabolic pathway from C3 to C4 carbon fixation due to the lack of CO2 in the water and utilized HCO3- in the water as their carbon source. (3) The net carbon sink flux was 1784.54 kg CO2, where carbonate weathering, the biological carbon pump, and weathering of silicate rocks accounted for 85.80 %, 13.64 %, and 0.56 %, respectively. (4) In the Banzhai ground river, the DIC during the migration process was mainly lost through degassing, allogenic acid, and consumption by photosynthesis of subaquatic plants.

13C Isotope Labelling to Follow the Flux of Photorespiratory Intermediates

Measuring the carbon flux through metabolic pathways in intact illuminated leaves remains challenging because of, e.g., isotopic dilution by endogenous metabolites, the impossibility to reach isotopic steady state, and the occurrence of multiple pools. In the case of photorespiratory intermediates, our knowledge of the partitioning between photorespiratory recycling, storage, and utilization by other pathways is thus rather limited. There has been some controversy as to whether photorespiratory glycine and serine may not be recycled, thus changing the apparent stoichiometric coefficient between photorespiratory O2 fixation and CO2 release. We describe here an isotopic method to trace the fates of glycine, serine and glycerate, taking advantage of positional 13C content with NMR and isotopic analyses by LC–MS. This technique is well-adapted to show that the proportion of glycerate, serine and glycine molecules escaping photorespiratory recycling is very small.