Spatial and temporal patterns in soil organic carbon, microbial biomass and activity under different land-use types in a long-term soil-monitoring network

Preservation of soil organic carbon (SOC) requires knowledge concerning the quantity and quality of both the SOC and the SOC-decomposing microbial community. In northern Germany, this information is assessed as part of Schleswig-Holstein's long-term soil-monitoring programme, in which topsoils have been analysed since 1995. In this study, we evaluated long-term data from Schleswig-Holstein's monitoring sites and compared the content of SOC and microbial biomass carbon (MBC) among croplands, grasslands and forests. We distinguished the microbial fractions as total MBC (chloroform fumigation extraction method) and glucose-responsive MBC, and we used the MBC/SOC ratio and soil basal respiration (SBR) to obtain a qualitative view of the SOC and its turnover potential. Additionally, we performed a temporal comparison for SOC and MBC between the periods 1995-2002 and 2005-2015. The results showed that median SOC content is the highest in forest soils (41 g kg(-1)), followed by grasslands (33 g kg(-1)), and croplands (12 g kg(-1)). Different ascending orders occurred for the total MBC (cropland < forest < grassland), however, and for the glucose-responsive MBC (forest < cropland < grassland), indicating differences in SOC quality and the microbial community structure. The different ratios between MBC and SOC showed that SOC in croplands and grasslands is characterised as easily degradable, but more stable in grasslands under no-fill conditions. Forest SOC appears almost non-degradable, and thus is most stable in the long term. We concluded that land-use-specific carbon characteristics, including the functional microbial community structure, are beneficial for evaluation of SOC preservation potential. From the temporal perspective, a significant increase (45-114 %) was observable for the total MBC on cropland and grassland sites, whereas the changes in SOC were not significant. The remarkable growth of the total MBC in relation to the glucose-responsive MBC indicates shifts in the microbial community towards a K-strategy metabolism.