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Lignin dynamics in arable soils as determined by 13C natural abundance


Hofmann, A. Lignin dynamics in arable soils as determined by 13C natural abundance. 2009, University of Zurich, Faculty of Science.

Abstract

Lignin is the second most abundant polymer in nature after the polysaccharides cellulose and
hemicellulose. It is a main component in plant cell walls, where it has stabilizing and protective
functions. Because of its abundance in plant material, lignin can also be found in the decomposition
product organic matter in soils and sediments. Due to its complex phenolic structure and the fact
that only specialized fungi can decompose it, lignin has long been referred to as a very stable
macromolecule that contributes to stabilized organic matter. This older belief of lignin as a
refractory molecule, which is selectively preserved, has been much challenged recently. Lignin has
been found to turn over faster than bulk organic matter and turnover times of <1 to 38 years have
been proposed.
The main objective of this thesis was to quantitatively describe lignin decomposition in long-term
field experiments in order to validate the proposed turnover times. Other important objectives were
to test the effect of arable management practices (i) mineral fertilization or (ii) biomass
incorporation on decomposition and to provide evidence on soil fractions that retain lignin.
To track the decomposition and retention of lignin quantitatively over decades, labeling with natural
carbon isotopic abundance was taken advantage of in an 18-year and a 36-year continuous maize
field experiment. Labeling is based on the different 13C to 12C ratios in plants with different
photosynthetic pathways. Conversion from C3 vegetation (e.g. wheat) to C4 vegetation (e.g. maize)
induces labeling of the soil organic matter. Lignin was extracted from archived soil samples by
alkaline cupric oxide oxidation, which is an established method for soils and sediments. The
oxidation products, lignin-specific monomers, were quantified using gas chromatography and the
stable carbon isotopic composition was analyzed by isotope ratio mass spectrometry.
Decomposition of C3-derived lignin in soil organic matter could best be described by doubleexponential
decay dynamics for the studied experiments. The fast pool had a turnover time of 3
years, the slow pool of 90 years. The results suggest that turnover might not be as fast as
proposed recently from other experiments. Interpretation is however still limited because data for
time periods of longer than 30 years is scarce. Mineral fertilization did not retard lignin
decomposition in the long-term in the studied 36-year experiment. Due to the complexity of the
agro-ecosystem the results differed from earlier controlled lab studies, proposing that fertilization
might have contradicting effects in the field. Biomass incorporation naturally increased the total
amount of SOC and lignin in the soils, but had no priming effect on initial C3-derived lignin,
suggesting that these lignin moieties might have been stabilized in soil. In fact, in both long-term
field experiments after 18 or 36 years still at least 60 or 40 % of the initial C3-derived lignin was
detectable. A fractionation study for the 18-year experiment indicated lignin might have been
retained in the coarse particulate organic matter fraction or in the free silt fraction. The silt fraction
had been proposed earlier as a possible fraction for lignin stabilization, suggesting mineral-organic
interactions. The retention in free particulate organic matter could also be related to interaction with
minerals, because organic matter was protected with a mineral crust, as in early stages of
aggregation.
From this study it can be concluded that lignin decomposes within decades in soil. However, it
seems that a portion of the lignin is to a certain extent stabilized in soil, most likely through a form
of protection by soil minerals. The decomposition dynamics could not be influenced by
management, suggesting that in the long-term (decades) complex ecosystem feedbacks might
outweigh distinct priming effects found in short-term studies (months to years).
Proposed research perspectives are the compound-specific investigation of long-term field
experiments (also other land uses, e.g. grassland or forest), where fertilization effects on soil
organic carbon decomposition had been shown previously in order to find out if lignin is involved in
the slow decomposition. Another direction of further study could be to explore the mechanisms of
lignin retention over decades, which could be assessed e.g. in controlled labeling experiments with
aggregates.

Abstract

Lignin is the second most abundant polymer in nature after the polysaccharides cellulose and
hemicellulose. It is a main component in plant cell walls, where it has stabilizing and protective
functions. Because of its abundance in plant material, lignin can also be found in the decomposition
product organic matter in soils and sediments. Due to its complex phenolic structure and the fact
that only specialized fungi can decompose it, lignin has long been referred to as a very stable
macromolecule that contributes to stabilized organic matter. This older belief of lignin as a
refractory molecule, which is selectively preserved, has been much challenged recently. Lignin has
been found to turn over faster than bulk organic matter and turnover times of <1 to 38 years have
been proposed.
The main objective of this thesis was to quantitatively describe lignin decomposition in long-term
field experiments in order to validate the proposed turnover times. Other important objectives were
to test the effect of arable management practices (i) mineral fertilization or (ii) biomass
incorporation on decomposition and to provide evidence on soil fractions that retain lignin.
To track the decomposition and retention of lignin quantitatively over decades, labeling with natural
carbon isotopic abundance was taken advantage of in an 18-year and a 36-year continuous maize
field experiment. Labeling is based on the different 13C to 12C ratios in plants with different
photosynthetic pathways. Conversion from C3 vegetation (e.g. wheat) to C4 vegetation (e.g. maize)
induces labeling of the soil organic matter. Lignin was extracted from archived soil samples by
alkaline cupric oxide oxidation, which is an established method for soils and sediments. The
oxidation products, lignin-specific monomers, were quantified using gas chromatography and the
stable carbon isotopic composition was analyzed by isotope ratio mass spectrometry.
Decomposition of C3-derived lignin in soil organic matter could best be described by doubleexponential
decay dynamics for the studied experiments. The fast pool had a turnover time of 3
years, the slow pool of 90 years. The results suggest that turnover might not be as fast as
proposed recently from other experiments. Interpretation is however still limited because data for
time periods of longer than 30 years is scarce. Mineral fertilization did not retard lignin
decomposition in the long-term in the studied 36-year experiment. Due to the complexity of the
agro-ecosystem the results differed from earlier controlled lab studies, proposing that fertilization
might have contradicting effects in the field. Biomass incorporation naturally increased the total
amount of SOC and lignin in the soils, but had no priming effect on initial C3-derived lignin,
suggesting that these lignin moieties might have been stabilized in soil. In fact, in both long-term
field experiments after 18 or 36 years still at least 60 or 40 % of the initial C3-derived lignin was
detectable. A fractionation study for the 18-year experiment indicated lignin might have been
retained in the coarse particulate organic matter fraction or in the free silt fraction. The silt fraction
had been proposed earlier as a possible fraction for lignin stabilization, suggesting mineral-organic
interactions. The retention in free particulate organic matter could also be related to interaction with
minerals, because organic matter was protected with a mineral crust, as in early stages of
aggregation.
From this study it can be concluded that lignin decomposes within decades in soil. However, it
seems that a portion of the lignin is to a certain extent stabilized in soil, most likely through a form
of protection by soil minerals. The decomposition dynamics could not be influenced by
management, suggesting that in the long-term (decades) complex ecosystem feedbacks might
outweigh distinct priming effects found in short-term studies (months to years).
Proposed research perspectives are the compound-specific investigation of long-term field
experiments (also other land uses, e.g. grassland or forest), where fertilization effects on soil
organic carbon decomposition had been shown previously in order to find out if lignin is involved in
the slow decomposition. Another direction of further study could be to explore the mechanisms of
lignin retention over decades, which could be assessed e.g. in controlled labeling experiments with
aggregates.

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Additional indexing

Item Type:Dissertation
Referees:Schmidt M W I, Heim A, Miltner A
Communities & Collections:07 Faculty of Science > Institute of Geography
Dewey Decimal Classification:910 Geography & travel
Language:English
Date:2009
Deposited On:19 Feb 2010 09:28
Last Modified:05 Apr 2016 13:56
Number of Pages:108

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