Nutrient addition retards decomposition and C immobilization in two wet grasslands

Eutrophication is one of the biggest environmental problems facing wetlands. However, its effect on soil functioning is not yet well understood. We tested the hypothesis that increased nutrient loading into wet grassland ecosystems accelerates soil C and N cycles and decreases microbial immobilization of C and N. Experimental sites were established on two wet grasslands, with either mineral or peaty soils, and fertilized by NPK fertilizer for 3 years. Soils were analyzed for soluble and microbial C and N contents and their transformations, profile of phospholipid fatty acids and number of nirK denitrifiers. Fertilization affected C more than N transformations. Opposite to what was predicted, decomposition was retarded, the soil C cycle was based more on labile C compounds, and the soil was more susceptible to C losses in fertilized versus unfertilized treatments in both soils. Fertilization resulted in lower microbial biomass C and microbial C immobilization and also decreased the activity of lignin-degrading enzymes. Shifts in the composition of the microbial communities led to decreased (1) decomposition of complex organic compounds and (2) immobilization of transformed C. Net nitrification and microbial N immobilization tended to increase in fertilized treatments indicating an acceleration of soil N cycling and losses, but only in the more vulnerable organic soil.