Lignin turnover in an agricultural field: from plant residues to soil‐protected fractions

Abstract

Lignin has long been suspected to be a major source of stable carbon in soils, notably because of the recalcitrant nature of its polyphenolic structure relative to other families of plant molecules. However, lignin turnover studies have produced conflicting results, most of them suggesting that large proportions of plant-residue lignin decompose within a year of incorporation into soils. Here, we propose a tworeservoir model where lignin in undecomposed plant residue (Lp) can either reach soil fractions where it is somewhat protected from further decomposition (Ls) or is transformed to non-lignin products. Model calibration data were obtained through compound-specific 13 C isotopic analyses conducted in a zero-to 9-year chronosequence of maize monoculture after wheat in a temperate loam soil of the Paris basin. Lignin was quantified by CuO oxidation as VSC-lignin, i.e. the sum of vanillil-(V), syringyl-(S) and coumaryl-type (C) phenols. Model calibrations indicate that Lp has a turnover rate faster than 1 year and that 92% is mineralized as CO 2 or transformed into other non-lignin products, while only 8% reaches the Ls fraction. Estimated turnover rate of the Ls fraction was 0.05 years À1 . The model also suggested that about half of Lp was not measured because it had been excluded from the samples in the process of sieving at 5 mm. In conclusion, the model indicates that chemical recalcitrance alone is not sufficient to explain VSC-lignin turnover in soils, and that, functionally, the most relevant mechanism appears to be the transfer of VSC-lignin molecules and fragments from decomposing plant tissues to soil-protected fractions.

How relevant is recalcitrance for the stabilization of organic matter in soils?

“…4). These two seemingly contrasting observations can be reconciled if two lignin pools are assumed with similar parameters as determined by Rasse et al (2006) at Les Closeaux. The existence of a large pool (approx.…”

“…A recent study by Rasse et al (2006) found that observed changes in lignin isotope ratio can be explained by a twocompartment model. The model assumes an unprotected lignin pool and a stabilized pool.…”

“…92% of the lignin and has a turnover time of <1 y, while the latter contains only 8% of the soil lignin pool, but contains lignin with a longer mean residence time in the soil (18 y in the study by Rasse et al [2006]). Consequently, virtually all lignin detected in soil samples represents this slow pool as lignin in the fast pool turns over too fast to significantly accumulate in the soil.…”

“…However, independent methods have confirmed these results (Derrien et al, 2006). The two-pool lignin model of Rasse et al (2006) cannot be applied to carbohydrates and proteins as the pyrolysis products from plant material and soil differ in the chemical composition indicating their in situ formation. As possible explanations for the observed residence times related to the slow pool, physicochemical protection and biological recycling of organic compounds in soils remain (Gleixner et al, 2001).…”

Storage and stability of organic matter and fossil carbon in a Luvisol and Phaeozem with continuous maize cropping: A synthesis

“…Apparently there was a minor contribution of fresh plant material to this SOM (Nierop et al, 2005). And indeed, it has also been reported for similar soil types that lignin rapidly decomposes and does not contribute as much to SOM as other, potentially more labile compounds may (Dignac et al, 2005;Rasse et al, 2006;Heim and Schmidt, 2007). The differences in natural 13 C abundances allowed for the calculation of turnover times for the pyrolysis products.…”