Biological soil crusts on initial soils: organic carbon dynamics and chemistry under temperate climatic conditions

Abstract. Numerous studies have been carried out on the community structure and diversity of biological soil crusts (BSCs) as well as their important functions on ecosystem processes. However, the amount of BSC-derived organic carbon (OC) input into soils and its chemical composition under natural conditions has rarely been investigated. In this study, different development stages of algae- and moss-dominated BSCs were investigated on a~natural (<17 yr old BSCs) and experimental sand dune (<4 yr old BSCs) in northeastern Germany. We determined the OC accumulation in BSC-layers and the BSC-derived OC input into the underlying substrates for bulk materials and fractions <63 μm. The chemical composition of OC was characterized by applying solid-state ¹³C NMR spectroscopy and analysis of the carbohydrate-C signature.¹⁴C contents were used to assess the origin and dynamic of OC in BSCs and underlying substrates. Our results indicated a rapid BSC establishment and development from algae- to moss-dominated BSCs within only 4 yr under this temperate climate. The distribution of BSC types was presumably controlled by the surface stability according to the position in the slope. We found no evidence that soil properties influenced the BSC distribution on both sand dunes. ¹⁴C contents clearly indicated the existence of two OC pools in BSCs and substrates, recent BSC-derived OC and lignite-derived "old" OC (biologically refractory). The input of recent BSC-derived OC strongly decreased the mean residence time of total OC. The downward translocation of OC into the underlying substrates was only found for moss-dominated BSCs at the natural sand dune which may accelerate soil formation at these spots. BSC-derived OC mainly comprised O-alkyl C (carbohydrate-C) and to a lesser extent also alkyl C and N-alkyl C in varying compositions. Accumulation of alkyl C was only detected in BSCs at the experimental dune which may induce a~lower water solubility of BSC-derived extracellular polymeric substances when compared to BSCs at the natural sand dune indicating that hydrological effects of BSCs on soils depend on the chemical composition of the extracellular polymeric substances.