Aspects of soil phenolic matter (SPM): An explorative investigation in agricultural, agroforestry, and wood ecosystems

This paper assesses the distribution of soil phenolic matter (SPM) in soil environments from a pedological perspective, with particular emphasis on soil horizonation and its involvement in pedogenic processes.The study was conducted on 15 soil profiles (Andosols, Calcisols, Cambisols, Lixisols, Phaeozems, Podzols, Umbrisols) covering five distinct ecosystems. Although the ecosystems were virtually homogeneous in and of themselves, they represented strongly contrasting climates, geolithology and morphology, land use and vegetation cover (intensive agriculture, chestnut grove, alpine pasture, woody reforestations, and natural woods). Three main SPM fractions were categorised: (i) "total" (SPMt), (ii) "soluble" and/or "labile" (SPMs), and (iii) the fraction with a "high" (SPMh) affinity for the soil body.The SPMh and SPMs contents varied significantly, averaging 2.84±0.52gkg-1 and 0.07±0.02gkg-1, respectively. Thus, these fractions appear to belong to different soil organic matter (SOM) pools. Indeed, the SPMs fraction was characterised by transitional, ephemeral low-weight molecules, which continuously reached the soil through both plant metabolism/decay and SOM degradation and were subsequently transformed into larger materials (humic matter, organo-mineral complexes) and/or lost by leaching and mineralisation. The SPMh fraction is likely involved in pedogenic processes that are primarily driven by the transport/translocation of colloidal phases under the following conditions: (i) in the early stages of horizon differentiation or, more specifically, in transport processes from the overlying A horizons and (ii) in cheluviation phenomena leading to the formation of spodic horizons. Moreover, SPMh also appears to be involved in early "in situ" alteration of cambic horizons. This involvement may result from the strong etching/complexing activity of soluble polyphenols from the overlying horizon, which promotes "in situ" weathering of the parent material.The total amount of SPM in a given environment greatly depended on the environmental conditions, which ranged from 0.02gkg-1 in an intensively cultivated Haplic Cambisol to 13.14gkg-1 in an Andic Cambisol under natural downy oak cover. Additionally, the SPMh/SPMs and SPMt/SOM% ratios decreased significantly from natural woods to intensive agriculture systems and may therefore indicate the extent of different anthropic impacts, such as soil cover/land use, in different ecosystems.