5th Swiss Geoscience Meeting, Geneva 2007
Lignin degradation dynamics in arable soils as
measured by natural 13C abundance
Hofmann Anett*, Heim Alexander*, Abiven Samuel*, Christensen Bent T.**,
Miltner Anja***, Richnow Hans H. *** & Schmidt, Michael W.I.*
* University of Zurich, Department of Geography, Winterthurerstrasse 190, CH-8057
Zürich (anett.hofmann@geo.uzh.ch)
**University of Aarhus, Faculty of Agricultural Sciences, Blichers Allé 20, DK-8830 Tjele
***Helmholtz Centre for Environmental Research - UFZ, Division Biogeochemistry/
Technology, Permoserstrasse 15, D-04318 Leipzig
Lignin is a component of soil organic matter that has long been considered as
very recalcitrant against biodegradation and has therefore been regarded as
contributing to the stable soil organic carbon pool. However, recent studies
have shown that lignin might turn over within decades in arable soils (Dignac et
al. 2005; Heim & Schmidt 2007). Long-term field experiments in the range of
decades that could validate the proposed turnover times and dynamics are rare.
The objective of our study was to evaluate lignin concentrations in order to
describe the dynamics of lignin degradation in arable soils.
We analyzed archived soil samples of two different soil types from an 18-year
semi-field experiment where high and low amounts of maize harvest residues
(maize = C4-vegetation) are incorporated annually into a soil previously
cropped with C3-plants, e.g. barley or wheat. Because C4-plants have a higher
13C natural abundance compared to C3 plants, the vegetation conversion
introduces a natural label to soil organic carbon and its components. This label
allows tracking the degradation of older, C3-derived lignin over time. Soil lignin
concentrations were quantified by gas chromatography of lignin cupric oxide
oxidation products and natural 13C abundance was measured by gas
chromatography-combustion-isotope ratio mass spectrometry (Goñi & Eglinton
1996).
Results show that after 18 years 65% of old C3-derived lignin was still retained
in the soils, independent of the crop residue incorporation level and soil type
(Figure 1). Concentrations of C3-derived lignin decreased faster during the first
decade of the experiment than during the second, suggesting that lignin in soils
could be divided into a labile (=fast decaying) lignin pool and a more stable
lignin pool with a slower turnover.
From the high retention of old C3-derived lignin moieties in the investigated
arable soils we conclude an effective stabilization of lignin by the mechanisms
of chemical recalcitrance, physical protection in aggregates or interaction with
mineral surfaces (von Lützow et al. 2006).
5th Swiss Geoscience Meeting, Geneva 2007
Figure 1. About two thirds of the initial concentration of old lignin was still
retained by both soil types 18 years after the start of the experiment. This
finding suggests effective stabilization mechanisms for lignin. Soil type and
levels of crop residue incorporation had no influence on the degradation
dynamics of lignin. The biexponential curve was fitted by solving for the least
square difference assuming a rate constant of 0.0047 a-1 for the stable pool
(Lobe et al. 2002). Labile pool: 34 %, klabile pool 0.17; stable pool: 66 %.
REFERENCES
Dignac, M.-F., Bahri, H., Rumpel, C., Rasse, D.P., Bardoux, G., Balesdent, J.,
Girardin, C., Chenu, C., Mariotti, A. 2005. Carbon-13 natural abundance as a
tool to study the dynamics of lignin monomers in soil: an appraisal at the
Closeaux experimental field (France). Geoderma 128, 3-7.
Goñi, M.A., Eglinton, T.I. 1996. Stable carbon isotopic analyses of lignin-derived
CuO oxidation products by isotope ratio monitoring-gas chromatography-mass
spectrometry (irm-GC-MS). Organic Geochemistry 24, 601-615.
Heim, A. & Schmidt, M.W.I. 2007. Lignin turnover in arable soil and grassland
analysed with two different labelling approaches. European Journal of Soil
Science 58, 599-608
Lobe, I., Du Preez, C.C., Amelung, W. 2002. Influence of prolonged arable
cropping on lignin compounds in sandy soils of the South African Highveld.
European Journal of Soil Science 53, 553-562.
von Lützow, M., Kögel-Knabner, I., Ekschmitt, K., Matzner, E., Guggenberger,
G., 2006. Stabilization of organic matter in temperate soils: mechanisms and
their relevance under different soil conditions. European Journal of Soil Science
57, 426-445.