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

A Hofmann
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
more » ... 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.
doi:10.5167/uzh-31029 fatcat:gfmglxrtujafrcpzhzjnwngo4i