Nuclear magnetic resonance relaxometry to characterise the decomposition degree of peat soils

Abstract

An adequate response to the ecological challenges associated with the traditional peatland management and corresponding site-specific measures require spatial information on soil properties and functions, most of which are related to the degree of peat decomposition. Our laboratory study tests the expectation that NMR relaxometry provides simple and rapidly available proxies characterising the decomposition state of peat. We observe that the mean NMR relaxation time is correlated with established soil physical parameters quantifying peat decomposition: water absorption index, unit water content, bulk density and von Post index. The higher the decomposition degree, the faster is the NMR relaxation, which mainly results from a decreasing pore space. Correlation maps between the T₁ and T₂ relaxation times identify hydrogel-like effects in weakly decomposed peat that vanishes if the material decomposes under aerobic conditions. T₁/T₂ ratios of more than ten are observed for peat material with cellular components in contrast to earthified topsoil peat with ratios of less than two. Our attempt to transfer the NMR relaxation data to estimates of water retention functions is partially successful. However, our results also indicate that the relaxation mechanisms in peat are not only controlled by pore sizes. We observe increasing surface relaxivities with increasing decomposition degree, which is most likely the result of a chemical transformation of the pore surface that alters its paramagnetic properties. The magnitude of this increase is significantly higher for T₁ than for T₂, because the interaction of water molecules and pore surface affects the corresponding NMR relaxation mechanisms differently.