Changes in soil organic matter and soil humic substances following the afforestation of former agricultural lands in the boreal-nemoral ecotone (Latvia)

Abstract

Abandonment of agricultural land is a widespread process in Northern Europe, which is associated with changes in content and distribution of organic matter in soil, including C stocks. There is insufficient information on changes in soil organic matter properties during afforestation of these lands. The aim of the study was to determine and describe the influence of afforestation on organic carbon (CORG) content and properties of soil organic matter (SOM) in mineral topsoil in the boreo-nemoral ecotone in Latvia. We studied soils in 4 model territories that represented abandoned farmlands and territories where mixed or coniferous forests were established at different times on former agricultural lands. Soil samples from mineral topsoil (genetic A or AE horizons) were obtained during the summer period in two model territories (11 sampling sites) with loamy texture soil, and two model territories with higher sand content of soil (9 sampling sites). The soil samples were analysed for organic carbon (CORG) and total nitrogen (NTOT) content. Soil alkali (NaOH) extracts were used to determine properties of soil organic matter – alkali extractable carbon (CHS) content, composition and humification index (HIX) of soil humic substances (HS) and spectroscopic (UV–Vis, fluorescence emission spectra, FTIR) properties of the soil humic acid fraction (HA). CORG content gradually increased with forest land age. Gradual decrease in the humic acid (HA) to fulvic acid (FA) ratio (CHA/CFA) after land-use change indicated that SOM was transported to mineral topsoil mainly through leaching of the soluble FA fraction. FTIR spectra revealed that the HA fraction from all studied soils had relatively similar chemical structure and that SOM in mineral topsoil was degraded to relatively similar compounds. However, HAs extracted from forest soils had higher abundance of –OH groups, more aromatic CC structures, and relatively lower abundance of NH groups in amides. UV–Vis and fluorescence emission spectroscopic analysis of HA fraction along with PCA analysis showed that soil properties also may have affected chemical properties of soil organic matter during the afforestation. Molecular weight of the HA fraction gradually increased with age of the forest land in sandy soils while there were no significant changes in sandy loam soils. Fluorescence emission analysis also showed that HAs extracted from EAh horizons in Podzol and Anthrosol soils in old forest lands had the highest aromaticity.The results suggest that sandy soils have the most dynamic changes in SOM properties after afforestation. Therefore, further studies should focus on whether and how these changes affect SOM fate in soils, physical stabilisation with mineral soil and SOM leaching into subsoil.