Heterogeneity of peat decomposition under rice cultivation on the Pacific coast, Japan

Peat decomposition has micro-scale heterogeneity because the rate of decomposition differs between micro-local conditions related to pore distribution, botanical composition, and differences in plant parts. Therefore, the heterogeneity of peat decomposition degree in micro-scale can be a good indicator for further understanding of peat decomposition process. However, previous studies using “bulk analysis” have denoted peat heterogeneity as merely chemical structural diversity, and did not examine the heterogeneity of peat constituents at a microscopic level. This study aimed to obtain peat heterogeneity in the micro-scale as a new indicator of peat decomposition and conducted “heterogeneity analysis” as follows: (1) analyzing a decomposition index of individual peat micro-fragments using spectral data from Fourier transform infrared (FTIR) micro-spectroscopy and principal component analysis (PCA) for specific absorption intensities, and (2) determining the distribution of peat decomposition index as an indicator of the heterogeneity of peaty soils. Twelve peaty horizons were sampled from three sites on coastal or alluvial plains. These horizons were buried under mineral soils used for rice cultivation. We applied PCA to 775 infrared (IR) spectra obtained from observed peat micro-fragments and calculated the micro-fragment decomposition degree (MDD) which was the first principal component score explained by a decrease in CH single bonds and a relative increase in polymerized aromatic CC double bonds. The distribution range of MDD demonstrated that heterogeneity of decomposition degree tends to be large in mid-decomposed peat. Moreover, the bimodal distribution pattern of MDD represented heterogeneous state formed by mineral inter-bedding in peat. Therefore, “heterogeneity analysis” could effectively represent the intrinsic heterogeneity of peat decomposition degree without losing the microscopic diversity.