Fate of CuO-derived lignin oxidation products during plant combustion: Application to the evaluation of char input to soil organic matter
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Three suites of synthetic chars, produced from honey mesquite, cordgrass and loblolly pine under controlled combustion conditions, served as model materials for diverse natural chars originating from combustion of angiosperm/gymnosperm and woody/non-woody plants. The lignin oxidation products (LOPs), quantified using the alkaline cupric oxide (CuO) oxidation method, were used to study the impact of combustion on lignins and their commonly used parameters. Our results show that combustion can greatly decrease the yield of the eight major lignin phenols (vanillyl, syringyl and cinnamyl phenols) with no lignin phenols detected in any synthetic char produced at 400 C. With increasing combustion temperature, the value of the syringyl/vanillyl phenols ratio (S/V) of angiosperm char initially increased but then declined dramatically when the temperature reached 200-250 C. The pattern of change may relate to the thermal alteration of the plant cell ultrastructure. The cinnamyl/vanillyl phenols ratio value (C/V) of a non-woody plant, cordgrass, also showed a similar two-stage change with increasing combustion temperature. Combustion duration also caused a similar effect on the yield and signatures of lignin phenols, showing that the combustion severity (temperature and/or duration) has great influence on LOP yield and signature. The acid/aldehyde ratio of vanillyl phenols [(Ad/Al)v] and syringyl phenols [(Ad/Al)s] increased with increasing combustion temperature and duration and reached a maximum value at 300-350 C, regardless of plant species. The highly elevated acid/aldehyde ratio value reached in some cases exceeded the reported values of humic and fulvic acids extracted from soils and sediments. Furthermore, the ratio of 3,5-dihydroxybenzoic acid to vanillyl phenols (3,5Bd/V), a soil humification indicator, increased significantly during combustion at 250-350 C in char samples from all the plant species. Our results imply that changes in LOP signatures in chars can serve as potential indicators of their formation temperature. Simulations using a two end member mixing model showed that different char samples, with different degrees of lignin alteration, could have a substantial effect on the biomarker signatures of soil organic matter (SOM). The shifts in lignin signatures in SOM vary widely, depending on the SOM characteristics (e.g. soil litter vs. mineral soil) and the proportion of char in the mixture. Overall, the input of high temperature char tends to dilute the lignin signal in SOM and may lead to underestimation of vascular plant OM input to environmental matrices. Our observations demonstrate that, in addition to photochemical degradation, thermal alteration is an important abiotic lignin degradation process. 2008 Elsevier Ltd. All rights reserved.
