Nitrogen limitation of the microbial biomass in an old-growth forest soil

Abstract

:We assessed whether the microbial biomass of an old-growth mixed-conifer forest soil was N limited by applying the equivalent of 450 kg N/ha as (NH4)2SO4 at a 2-cm depth to mineral soil contained in 10-cm deep microplots. An increase in the size of the soil microbial biomass following N fertilization was used as the test of N limitation in the soil microflora.We estimated the impact of this fertilization treatment on microbial activity and net N transformations by conducting long-term aerobic laboratory incubations of soil, and measuring changes in microbial respiration and inorganic pool sizes. The impact of NH4+ fertilization on autotrophic nitrifiers also was assessed using a nitrification potential assay. Microbial C, N and respiration rate were significantly higher in the 4–10 cm soil layer 6 and 12 months following fertilization. However, in the 0–4 cm layer, these microbial parameters were similar in fertilized and unfertilized soils. Laboratory estimates of net N mineralization rates were higher at both depths and on both dates in fertilized soils. Mean net nitrification rates and nitrification potentials were consistently higher in fertilized soils, but many of these differences were not statistically significant. Our results suggest the biomass and activity of the soil microbial community can be altered by N addition to organically rich, old-growth mineral soils.