Solid State 13C NMR Spectroscopic Study of Soil Organic Matter in a Forest Ecosystem Subjected to Prescribed Burning and Thinning.

Prescribed burning and thinning are gaining popularity as low-cost forest protection measures. Such field management practices could alter the properties of soil organic matter (SOM), especially the humic substances. Surface soil samples from the Bankhead National Forest, Alabama, containing field plots subjected to thinning and burning treatment were used to investigate the possible heat-induced structural changes associated with SOM. The OM in the whole soils, humic substances and density-based fractions was characterized using elemental analysis and solid state cross polarization magic angle spinning nuclear magnetic resonance (CPMAS ¹³C NMR) spectroscopy. Our data revealed that certain thermally induced changes in SOM fractions occurred during the burning process. We attributed the structural differences in humic (HA) and fulvic (FA) acid fractions from burnt only and thin/burn treatment sites to complete combustion of ground vegetation in burnt only sites relative to thin/burn sites containing partially burnt or non-charred plant debris. The changes were involved mainly with alkyl C, O-alkyl C and carbohydrate functional groups, implying that most prominent reactions occurring during the low-intensity burning process involved dehydrogenation, de-oxygenation, and decarboxylation. Burning and thinning treatment may affect the distribution and composition of free and occluded particulate OM fractions. The limited structural changes in the SOM fractions suggested that the prescribed burning and thinning treatment had a low impact on changing the composition of the organic C forms in the soils, indicating that low intensity fire treatment in this ecosystem would not severely affect any major structural changes in OM forms.