Sequestration of soil organic matter in broadleaf and coniferous forests in soil at various stages of pedogenesis

Soil contains three times more carbon (C) than the atmosphere and biosphere combined. It therefore represents an important tool for removal of carbon dioxide (CO₂) from the atmosphere and its long term sequestration. However, in the current climate crisis, C that is already stored in soil reservoir represents a potential threat because different fractions of C in soil have different stability against the rising global temperatures. C is stored in soil in the form of soil organic matter (SOM) which is a mixture of many different organic components. Based on different properties we can divide SOM into two pools. The first pool is represented by small fragments of dead biomass referred to as free particulate organic matter (FPOM). The other pool is comprised of organic matter, usually chemically transformed or converted to microbial necromass, which is in various ways associated with soil mineral matrix. This pool is referred to as mineral associated organic matter (MAOM). It is assumed that MAOM becomes C saturated during soil development because it is limited by the amount of available mineral surfaces. FPOM, on the other hand, does not saturate and can therefore play an important role in later stages of soil development. Beside this some OM is stored in organic horizons in forest floor (Oe layer), which we expect will have similar pattern as FPOM. However, there is scarcity of studies that examine this assumption. In this work we studied the hypothesis that soils in different stages of development will differ in the amount of C stored in FPOM and MAOM fractions. On top of that, we assumed that this difference will be affected by the dominant tree species growing on the soil and the effect of tree species and soil age will not always be additive. We tested this hypothesis by analyzing C storage in soil and amount of C in FPOM and MAOM using two types of soils - recultivated spoil heap (immature soil) and forest soil in the surrounding area (mature soil). Plots with only one type of tree species (spruce or alder) in 3 replications were present on each of these soil types. Our results show that different tree species have different effects on the amount of C stored in mineral soil and Oe layer in immature and mature soils. In mineral soil more, C was sequestered under alder on recultivated heap, while in the surrounding area no difference between tree species was found. In Oe layer more C was sequestered under spruce in both types of soils. Soils did not differ in the amounts of FPOM and MAOM present in soil, but they did differ in the amount of C stored in these fractions. In young soil, MAOM fraction stored more C under alder than spruce. For mature soil the opposite was true. For FPOM fraction no significant effect of tree species or soil age was found but in young soil higher C storage in FPOM was found under alder compared to spruce.