Spectroscopic investigation of soil organic matter composition for shelterbelt agroforestry systems

While the role of agroforestry systems in increasing soil organic matter (SOM) storage has been studied, insufficient information is available on their effect on the chemical composition of SOM. The objective of this study was to determine the carbon (C) functional group chemistry of SOM for shelterbelts and compare it to the adjacent agricultural fields by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Carbon K-edge X-ray absorption near edge structure (XANES) spectroscopies. ATR-FTIR spectral analysis indicated larger proportions of conjugated carboxylic and aromatic C groups for hybrid poplar, white spruce and caragana shelterbelts, phenolic C for hybrid poplar and Manitoba maple shelterbelts and aliphatic and aromatic C for Manitoba maple shelterbelts compared to the adjacent agricultural fields. Polysaccharide, ether and alcoholic C functional groups were generally lower for shelterbelts compared to agricultural fields, with the exception of hybrid poplar species. Analysis by C K-edge XANES spectroscopy on a subset of soils showed the accumulation of aromatic C, ketones and carbohydrates in the surface soil layer (0-5 cm) for the shelterbelts compared to agricultural fields. Pearson correlation analysis indicated that the majority of SOM added under the shelterbelts was in the form of plant-derived aromatic, phenolic and carboxylic C groups. The results of this study suggested that the initial composition of litter and its decomposition rate had a strong influence on the composition of SOM under the shelterbelts. The higher proportion of processed forms of SOM, such as ketones, indicated that the SOM for shelterbelts was at a more advanced stage of decomposition compared to agricultural fields; likely due to the surface deposition of litter under shelterbelts.