Investigating soil organic matter dynamics in topsoil and subsoil by burying isotopically labelled litter substrate for four years

Subsoils have gained increasing attention due its slower soil organic carbon (SOC) turnover than in topsoil. Thus, subsoil with low content of mineral associated organic matter but a large number of exchange sites on mineral surfaces represents the potential to accumulate and sequester carbon (C). Generally, it has been assumed that the carbon turnover mechanism in topsoil and subsoil is influenced by similar environmental factors, with the difference of a lower C content in subsoil. In contrast, diverse abiotic variables prevalent in subsoils, like low temperature, high moisture, nutrient availability, etc., have been shown to imply different processes influencing C turnover in subsoils. Therefore, differences in processes and factors affecting SOC turnover in topsoil and subsoil are incompletely identified and understood.

Our objective is to investigate whether C decomposition and stabilisation mechanisms in topsoil and subsoil differ given the same added substrate content, as well as how it responds to increasing substrate C content. To assess these questions, a long-term (total duration 20-year) field incubation experiment was conducted at three different locations with varied soil textures in which soil was mixed and labelled with isotopically (13C) enriched beech litter substrate with different C contents of 8, 16, 32, and 64 g substrate kg^-1 in topsoil (10 cm) and 2, 4, 8, and 16 g substrate kg^-1 in subsoil (60 cm), filled in mesocosms, and buried. Soil samples were collected after one, two, and four years. Soil was fractionated into particulate organic matter (>20µm) and mineral-associated organic matter (<20µm) to find out how carbon is stabilised in these fractions, and stable C isotopes were measured. Our results indicate that the decomposition of the identical litter substrate strongly depends on the soil depth. The results of four years of buried field-microcosms will be presented, which will shed more light on differences in mechanisms responsible for SOC dynamics and the fate of litter substrate into different SOC pools of topsoil and subsoil.