Nitrogen fertilization decreases forest soil fungal and bacterial biomass in three long-term experiments

We examined the effects of N fertilization on forest soil fungal and bacterial biomass at three long-term experiments in New England (Harvard Forest, MA; Mt. Ascutney, VT; Bear Brook, ME). At Harvard Forest, chronic N fertilization has decreased organic soil microbial biomass C (MBC) by an average of 54% and substrate induced respiration (SIR) was decreased by an average of 45% in hardwood stands. In the pine stand, organic soil MBC was decreased by 40% and SIR decreased by an average of 35%. The fungal:bacterial activity ratio was also decreased in the hardwood stands from an average of 1.5 in the control plot to 1.0 in the High-N plot, and in the pine stands from 1.9 in control plot to 1.0 in the High-N stand. At Mt. Ascutney, MBC was reduced by an average of 59% and SIR by 52% in the High N plots relative to the unfertilized plots, and the fungal:bacterial activity ratio was only slightly decreased. The Bear Brook watershed is in an earlier stage of N saturation (Stage 0–1) and did not exhibit significant fertilization effects on microbial biomass. Across all three sites, MBC and SIR had negative relationships with total N inputs in both mineral soils and organic soils, though the effect was much stronger in organic soils. Both MBC and SIR were positively correlated with dissolved organic C, total soil C, and bulk soil C:N ratios. These results are consistent with the N saturation hypothesis, but do not indicate a strong role for microbial N immobilization in preventing N loss.