A. Hofmann

University of Zurich, Zürich, Zurich, Switzerland

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Publications (6)6.41 Total impact

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    ABSTRACT: The dynamics of soil organic matter are a key factor in controlling the terrestrial carbon cycle. Compound specific stable carbon isotope analysis has given new insight in to the stability of individual organic molecules in soil. For lignin, one of the major plant compounds, available data suggest the existence of both a labile (turnover time
    Organic Geochemistry - ORG GEOCHEM. 01/2010; 41(11):1219-1224.
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    ABSTRACT: A frequently cited assumption is that lignin in soils should be relatively stable due to its recalcitrant chemical structure. In recent years, this view has been challenged by new analytical techniques that use both lignin-specific biomarker molecules and compound specific isotope analysis. Applying these techniques to long-term field experiments with natural carbon isotopic labelling (C3-C4 vegetation change), it could be shown that the dynamics of lignin in soils are more complex and cannot be explained by its recalcitrant structure alone. In particular, there seem to be both a stable and a labile lignin pool in soils. As for soil organic carbon in general, interactions with the mineral phase have been suggested to be involved in the stabilization of lignin in soils. The present study focuses on the stable pool and tries to answer the following questions: (I) Which soil fractions contain most lignin? (II) Is the stable pool related to a particular soil fraction? (III) Can differences in lignin content between soil fractions be explained by properties of the mineral phase (surface area, type of minerals)? We used a combined density and aggregate size fractionation of an agricultural soil before and after it had been naturally 13C-labelled by 18 years of maize cropping. We identified old lignin deriving from the time before maize cropping by compound-specific isotope analysis of lignin-derived phenolic biomarkers. In the studied soil, we found a large proportion of lignin in coarse heavy fraction, suggesting inclusion in macroaggregates. However, isotope data indicated that lignin in this fraction was less stable in the long-term than lignin in light fractions. A potential explanation might be that some lignin-containing cell structures (e.g. thick cell walls) are recalcitrant enough to persist for decades even under intensive cropping. Due to intensive cropping before the isotopic labelling started, the light fractions would already be enriched in these structures, whereas in aggregates, also more decomposable structures may have been preserved. Aggregate turnover would then liberate part of these easily decomposable lignin structures resulting in the observed decomposition pattern. Therefore we suggest that inclusion in aggregates is particularly important for stabilization of the more easily decomposable lignin fraction in soil.
    04/2009;
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    ABSTRACT: Lignin has long been considered a relatively stable component of soil organic matter. However, recent studies suggest that lignin may turn over within years to decades in arable soil. Here we analysed lignin concentrations in an 18-year field experiment under continuous silage maize, where two soils were sampled at six points in time. Our objectives were to examine the long-term dynamics of (i) lignin derived from a previous C3-vegetation and (ii) lignin derived from maize, as influenced by two levels of maize biomass input. Total lignin concentrations in soil were quantified by gas chromatography of lignin cupric oxide oxidation products. Compound-specific 13C isotope analysis allowed discrimination between C3-derived lignin and maize-derived lignin. Degradation dynamics of C3-derived lignin were independent of biomass input level, suggesting that priming did not affect soil lignin concentrations. After 18 years, approximately two-thirds of the initial C3-derived lignin remained in the soils, whereas, on average, 10% of the recent maize-derived lignin input was retained. We suggest that lignin is effectively stabilized in these arable soils, although the mechanisms involved remain unclear.
    European Journal of Soil Science 02/2009; 60(2):250 - 257. · 2.65 Impact Factor
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    ABSTRACT: Retardation of soil organic carbon (SOC) decay after nitrogen addition to litter or soil has been suggested in several recent studies and has been attributed to a retardation in lignin decay. With our study we tested the long-term effect of mineral nitrogen fertilization on the decay of the SOC component lignin in arable soil. To achieve this, we tracked 13C-labeled lignin and SOC in an arable soil that is part of a 36-year field experiment with two mineral nitrogen fertilization levels. We could show that nitrogen fertilization neither retarded nor enhanced the decay of old SOC or lignin over a period of 36 years, proposing that decay of lignin was less sensitive to nitrogen fertilization than previously suggested. However, for fresh biomass there were indications that lignin decay might have been enhanced by nitrogen fertilization, whereas decay of SOC was unaffected. A retardation of SOC decay due to nitrogen addition, as found in other experiments, can therefore only be explained by effects on lignin decay, if lignin was actually measured.
    Biogeosciences Discussions 01/2009; 6(1):1657-1675.
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    ABSTRACT: Retardation of soil organic carbon (SOC) decay after nitrogen addition to litter or soil has been suggested in several recent studies and has been attributed to a retardation in lignin decay. With our study we tested the long-term effect of mineral fertilization (N+P) on the decay of the SOC component lignin in arable soil. To achieve this, we tracked 13C-labeled lignin and SOC in an arable soil that is part of a 36-year field experiment (conversion from C3 to C4 crops) with two mineral fertilization levels. We could show that fertilization neither retarded nor enhanced the decay of old SOC or lignin over a period of 36 years, proposing that decay of lignin was less sensitive to fertilization than previously suggested. However, for new, C4-derived lignin there were indications that decay might have been enhanced by the fertilization treatment, whereas decay of new SOC was unaffected.
    Biogeosciences 01/2009; 6(7):1139-1148. · 3.75 Impact Factor
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    ABSTRACT: Retardation of soil organic carbon (SOC) decay after nitrogen addition to litter or soil has been suggested in several recent studies and has been attributed to a retardation in lignin decay. With our study we tested the long-term effect of mineral fertilization (N+P) on the decay of the SOC component lignin in arable soil. To achieve this, we tracked <sup>13</sup>C-labeled lignin and SOC in an arable soil that is part of a 36-year field experiment (conversion from C<sub>3</sub> to C<sub>4</sub> crops) with two mineral fertilization levels. We could show that fertilization neither retarded nor enhanced the decay of old SOC or lignin over a period of 36 years, proposing that decay of lignin was less sensitive to fertilization than previously suggested. However, for new, C<sub>4</sub>-derived lignin there were indications that decay might have been enhanced by the fertilization treatment, whereas decay of new SOC was unaffected.
    Biogeosciences. 01/2009;