Humus Forms, Carbon Stock and Properties of Soil Organic Matter in Forests Formed on Dry Mineral Soils in Latvia

Over the last decades, more attention has been paid to carbon accumulation in soil, more recently, to soil humus forms, as they indicate environmental conditions and state of soil organic matter. There is insufficient information on the impact of soil and forest type on the chemical properties of soil, soil organic matter and humus form. Knowledge about the chemical properties of humus is crucial for modelling C and N accumulation and storage in forest soils. On this account, the aim of this study was to characterize soil humus forms, humus chemical properties and C stock and to determine the spatial distribution correlations between soil humus forms in forests formed on dry mineral soils. We studied humus forms in 44 sampling sites located in different types of forests. Soil samples were collected from genetic O and A (EA) horizons and analysed for organic carbon (CORG) and total nitrogen (NTOT) content and NaOH extractable organic matter. Generalized linear model analysis showed that the distribution of the psammomor and mor humus forms is related to oligotrophic forest types, while glaciogenic and glaciolimnic sediments constitute the main precondition for the occurrence of the mull humus form. The psammomor and mor humus forms have the lowest CORG stock in the topsoil, and more than 75% of the total CORG is accumulated in the O horizon. The mull humus form soils have the highest CORG stock in the mineral topsoil, accumulating 80% of the total topsoil CORG stock. The Ah horizons of the mull humus soils also have a significantly lower CHS-to-CORG ratio.