Dissolved organic matter and its parent organic matter in grass upland soil horizons studied by analytical pyrolysis techniques

Abstract

Summary We have sought to understand the molecular mechanisms by which dissolved organic matter (DOM) forms and soil organic matter (SOM) degrades in upland peaty gley soil under grass. Pyrolysis mass spectrometry (Py‐MS) and pyrolysis gas chromatography mass spectrometry (Py‐GC/MS) were applied to characterize the DOM collected from lysimeters and its parent SOM. The macromolecular organic matter in the litter and fermentation (Lf) horizon of the soil consists primarily of little decomposed lignocellulose from grass, whereas the humus (Oh) horizon is characterized by an accumulation of selectively decomposed lignocellulose material, microbial metabolites and bound fatty acids. The mineral horizon produced a relative enrichment of furan structures derived from microbial reworking of plant polysaccharides but virtually no lignin signals. A series of exceptional long chain C43 to C53 fatty acids with odd over even predominance, probably derived from mycobacteria, were also identified in the Oh horizon. Side‐chain oxidation and shortening, increase of carboxyl functionality and selective removal of syringyl (S) > guaiacyl (G) > p‐hydroxyphenyl (P) lignin units were the main reactions when lignin degraded. Compared with SOM, the DOM shows a large accumulation of more oxidized lignin and aromatic structures, especially those containing carboxylic and dicarboxylic acid functionalities and with shorter side‐chain length. The polysaccharide‐type compounds in the DOM were more modified (greater abundance of furan structures in pyrolysis products), and had significantly lower molecular weight and more diverse polymeric structures than did those in soils. Increased temperature and rainfall appeared to result in greater relative abundance of lignin degradation products and aromatic compounds in DOM.

Variation in litter decomposition-temperature relationships between coniferous and broadleaf forests in Huangshan Mountain, China

“…Probably peroxidase was the most important ligninolytic enzyme excreted by microorganism from the coniferous and broadleaf forest. Increased temperature appeared to result in great lignin degradation in organic matter (Huang et al 1998). Tuomela et al (2000) also found in compost experiment that the elevated temperatures were essential for rapid lignin degradation and the optimum temperature for thermophilic fungi and lignin degradation was 40-50°C.…”

Analysis and behavior of soluble organic nitrogen in forest soils

“…These studies have shown that the heterocyclic N is a significant component of the soil organic matter (e.g., Schulten and Schnitzer 1998), which may not be found from the solid-state 15 N nuclear magnetic resonance (NMR) analysis. Huang et al (1998), using Py-MS and Py-GC-MS to analyze dissolved organic mater from grassland soils, revealed that the dissolved organic matter showed a large accumulation of more oxidized lignin and aromatic structures, especially those containing carboxylic and dicarboxylic acid functionalities and with shorter side-chain length. Pyrolysis techniques have some limitations, including method effects such as cyclization and defunctionalization reactions in pyrolysis and variation in the yields of degradation by-products (Page et al 2002;Templier et al 2005).…”

Dissolved organic carbon chemistry and dynamics in contrasting forest and grassland soils

“…Using analytical pyrolysis, Huang et al (1998) found that DOC was the most oxidized and microbially processed fraction of SOM. Kaiser and Guggenberger (2000) showed that the carbohydrate and lignin profiles of DOC most resembled that of C found in the clay fraction of the mineral soil, which is generally considered to be a highly altered biological reaction product.…”