Changes in organic carbon in topsoil and subsoil and microbial community composition caused by repeated additions of organic amendments and N fertilisation in a long-term field experiment in Sweden

The effects of 13 years of biennial application of four organic amendments (compost, green manure, farmyard manure and sewage sludge) on soil organic carbon (SOC) stocks, microbial biomass and community structure were compared with those of bare fallow, an unfertilised control and N-fertilised treatments. The experiment was conducted on a clay soil in western Sweden, in a randomised block design with four replicates. The crops grown were spring barley and oats. Changes in SOC mass were estimated by the ‘equivalent soil mass’ concept. Carbon inputs from crops were calculated from grain yields using linear allometric functions. The decomposition rate of soil organic matter was derived from a bare fallow treatment. Humification coefficients, defined here as the fraction of C input entering the SOC pool, for the organic amendments and crop residues were estimated by fitting a single-pool first order model to the final SOC stocks. The humification coefficient was highest for domestic waste compost (0.9) and lowest for above-ground crop residues (0.19). Crop yields were highest in sewage sludge-amended soil, probably due to favourable physical soil conditions, as indicated by lower bulk density (1.27–1.30gcm−3) than in the other treatments (1.32–1.38gcm−3). Despite addition of relatively high amounts of organic N, yields in other treatments receiving organic amendments were significantly lower than in those receiving mineral N fertiliser. In particular, the compost material was found to be highly recalcitrant, as indicated by its low C/N ratio (10) and low crop yield. Comparison of the correlation between phospholipid fatty acid (PLFA) concentration and SOC showed that compost supported relatively less microbial biomass than the other substrates tested. An important finding was that differences in SOC between treatments were significant to 40cm depth and that up to 27% of the SOC changes observed to 40cm depth occurred in the upper subsoil (25–40cm). Thus, SOC changes below ploughing depth should be considered in SOC balance studies.