Sources and degradability of organic matter revealed by 14C analysis

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The organic matter stored in subsoils has been recognized as a large organic carbon stock that needs to be considered in the global terrestrial OC pool. Until now the composition and dynamics of subsoil are not well understood and it remains unclear, which processes control the strong increase in the radiocarbon age with soil depth. The research unit 1806 thus uses a broad set of methods to identify mechanisms leading to the accumulation and stabilization of organic carbon in subsoils. This subproject uses radiocarbon and organic-geochemical analyses to differentiate different organic sources in subsoil horizons and to determine their degradability. During the first project phase we determined a close correlation of the distribution of roots with the amount of organic carbon and its radiocarbon age. However, the abundance of roots - the major source of fresh organic carbon to subsoils - decreased considerably with soil depth. Radiocarbon contents of subsoil horizon were up to 9000 years BP suggesting a large and increasing contribution of very old, potentially geogenic carbon with soil depth. In contrast, the carbon dioxide respired from several hundred to thousand year old subsoil horizons had modern radiocarbon contents suggesting that fresh/young organic compounds are present and are degraded preferentially. During the second phase of this project our aim is to a) identify and characterize the old organic matter fraction and b) to evaluate the degradability of organic matter in subsoils in more detail. We will analyse different refractory organic carbon pools and establish and apply novel methods to radiocarbon date CO2 in subsoil horizons and microbial membrane lipids. This will give us information on stable and labile organic substrates in subsoils and their degradability.