Stable hydrogen and carbon isotope ratios of methoxyl groups during plant litter degradation

Abstract

Stable hydrogen and carbon isotope ratios of methoxyl groups (δ(2)Hmethoxyl and δ(13)Cmethoxyl values, respectively) in plant material have been shown to possess characteristic signatures. These isotopic signatures can be used for a variety of applications such as constraining the geographical origin and authenticity of biomaterials. Recently, it has also been suggested that δ(2)Hmethoxyl values of sedimentary organic matter of geological archives might serve as a palaeoclimate/-hydrology proxy. However, deposited organic matter is subject to both biotic and abiotic degradation processes, and therefore an evaluation of their potential impact on the δ(2)Hmethoxyl and δ(13)Cmethoxyl values would allow more reliable interpretations of both isotopic signatures. Here, we investigated this potential influence by exposing foliar litter of five different tree species (Sycamore maple, Mountain ash, European beech, Norway spruce and Scots pine) to natural degradation. The foliar litter was sampled at nine intervals over a 27-month period, and the bulk methoxyl content as well as the δ(2)Hmethoxyl and δ(13)Cmethoxyl values were measured. At the end of the experiment, a loss of the bulk methoxyl in the range of ∼40-70% was measured. Linear regression analysis showed no dependence of δ(2)Hmethoxyl values with methoxyl content for four out of five foliar litter samples studied (R(2) in the range of 0.03 and 0.36, p > .05). On the contrary, the δ(13)Cmethoxyl values showed significant linear correlations for the great majority of the foliar litter samples (R(2) in the range of 0.51 and 0.73, p < .05). The litter species with the greatest methoxyl loss (Mountain ash, Scots pine and Norway spruce) showed the strongest (13)C enrichment, by up to ∼5‰. Since δ(2)Hmethoxyl shows no systematic overall change during the course of degradation, we propose that there is considerable potential for its use as a palaeoclimate proxy for a wide range of geological archives containing, for instance, fossil wood or sedimentary organic matter. Care would need to be taken if δ(13)Cmethoxyl values of degraded organic matter are used for palaeoclimate/-environmental investigations.

Warm season precipitation signal in δ2H values of wood lignin methoxyl groups from high elevation larch trees in Switzerland

“…Their δ 2 H values can be readily measured without isotope fractionation as iodomethane (CH 3 I), which is released upon treatment of the wood with hydroiodic acid . A further advantage is that lignin methoxyl groups of wood are chemically stable and do not isotopically exchange after formation or in the course of biotic and abiotic degradation . Keppler et al compared δ 2 H methoxyl values from trees around the world with corresponding δ 2 H values in meteoric water and found a uniform isotopic fractionation of −216 ± 19‰.…”

“…26 A further advantage is that lignin methoxyl groups of wood are chemically stable and do not isotopically exchange after formation or in the course of biotic and abiotic degradation. 26,27 Keppler et al 25 compared δ 2 H methoxyl values from trees around the world with corresponding δ 2 H values in meteoric water and found a uniform isotopic fractionation of −216 ± 19‰. This value was slightly modified (−213 ± 17‰) by Anhäuser et al 28 Feakins et al 29 applied this method for different tree species in a coastal ecosysthem in Florida and reported that source water δ 2 H values have the greatest influence on δ 2 H methoxyl values.…”

Methyl sulfates as methoxy isotopic reference materials for δ13C and δ2H measurements

“…The results regarding stable carbon isotope analysis might have a substantial impact on the δ 13 C values of plant methoxy groups that have been previously reported . Most of the δ 13 C values of wood methoxy groups that have been measured in the past (~ranging from −20 to −30 mUr) were normalised with CH 3 I reference materials with very negative δ 13 C values (~ −60 to −70 mUr) may need to be corrected by 2–3 mUr towards more negative values.…”

“…The precision ('external precision' including IRMS analysis. The precision of the replicate measurements of δ 13 C values obtained by GC/IRMS is ±0.21 mUr and ±0.40 mUr for HUBG1 and HUBG2, respectively.The results regarding stable carbon isotope analysis might have a substantial impact on the δ 13 C values of plant methoxy groups that have been previously reported 7,12,14,19,25,26,35,51. Most of the δ 13 C values of wood methoxy groups that have been measured in the past (~ranging from −20 to −30 mUr) were normalised with CH 3 I reference materials with very negative δ13 C values (~−60 to −70 mUr) may need to be corrected by 2-3 mUr towards more negative values.…”

Special Issue dedicated to Professor Hanns-Ludwig Schmidt on the occasion of his 85th birthday

“…H.-L. Schmidt has had a most distinguished career in isotope research. It started with the synthesis of radioactive tracer compounds and led to a wide range of fundamental, methodical and applied research on the stable isotope composition of the main elements of the biosphere, principally 13 C/ 12 C, 15 N/ 14 N, 2 H/ 1 H, 18 O/ 16 O and 34 S/ 32 S. That career has been an awe inspiringly long one, 55 years [1] and counting, as is documented by his contribution to a review on the causes and implications of in vivo isotope fractionations [2] in this Special Issue. Young stable isotope researchers may wonder how such a career could have unfolded, what circumstances and coincidences -academic and other -influenced and guided it, and which interactions with colleagues and students accompanied and enriched it.…”