Anna Gunina

Improved δ13C analysis of amino sugars in soil by ion chromatography–oxidation–isotope ratio mass spectrometry

RATIONALE Amino sugars build up microbial cell walls and are important components of soil organic matter. To evaluate their sources and turnover, δ(13)C analysis of soil-derived amino sugars by liquid chromatography was recently suggested. However, amino sugar δ(13)C determination remains challenging due to (1) a strong matrix effect, (2) CO2 -binding by alkaline eluents, and (3) strongly different chromatographic behavior and concentrations of basic and acidic amino sugars. To overcome these difficulties we established an ion chromatography-oxidation-isotope ratio mass spectrometry method to improve and facilitate soil amino sugar analysis. METHODS After acid hydrolysis of soil samples, the extract was purified from salts and other components impeding chromatographic resolution. The amino sugar concentrations and δ(13)C values were determined by coupling an ion chromatograph to an isotope ratio mass spectrometer. The accuracy and precision of quantification and δ(13)C determination were assessed. RESULTS Internal standards enabled correction for losses during analysis, with a relative standard deviation <6%. The higher magnitude peaks of basic than of acidic amino sugars required an amount-dependent correction of δ(13)C values. This correction improved the accuracy of the determination of δ(13)C values to <1.5‰ and the precision to <0.5‰ for basic and acidic amino sugars in a single run. CONCLUSIONS This method enables parallel quantification and δ(13)C determination of basic and acidic amino sugars in a single chromatogram due to the advantages of coupling an ion chromatograph to the isotope ratio mass spectrometer. Small adjustments of sample amount and injection volume are necessary to optimize precision and accuracy for individual soils.

Initial changes in soil properties and carbon sequestration potential under monocultures and short-rotation alley coppices with poplar and willow after three years of plantation

Initial changes in soil structure and C stocks were studied under short-rotation coppices (SRC) planted on former cropland near Göttingen, Central Germany. Plantations were established either as monocultures with willow (Willow-SRC) or poplar (Poplar-SRC), or as an agroforestry system with willow strips and grassland alleys in between (Willow-AF). A neighbouring cropland served as a control. Three sampling campaigns were applied in this study. The first sampling was conducted at a fine scale to reveal the differences in soil C with depth (i.e. 0-3, 3-6, 6-9, 9-12, 12-15, 15-20, 20-30cm). Here, results indicated the main differences between plantations in 0-3, 3-20 and 20-30cm layers. These soil depths were therefore chosen for the second sampling campaign to reveal differences in aggregate composition, C accumulation in aggregates and density fraction, and microbial biomass carbon (MBC) between plantations. Furthermore, quality of soil organic matter and amount of C mineralised by microorganisms were estimated by an incubation experiment. Results here indicated two times higher CO2 emissions from the top layer than from the lower layers under SRCs, as well as higher MBC in SRCs (490-788.7μgCg-1) than in cropland (266.4μgCg-1). The results of the third sampling on the texture of respective soil horizons indicated a significant correlation (R2=78%) of soil clay to C at 0-3cm depth. It was concluded that aggregation and C in microbial biomass and free light fractions were the first indicators of soil quality improvement after conversion of arable land to SRC plantations.