Long-term effects of grazing, liming and nutrient fertilization on the nitrifying community of grassland soils

Human-managed grasslands receive significant inputs of fertilizing materials, which can greatly influence soil biological processes associated with the cycling of nitrogen (N), including microbial nitrification. Here we specifically address how soil ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) respond to 23 years of different management practices in a permanent grassland experiment. We found that AOB amoA gene copy numbers were significantly higher in limed soils (associated with greater pH values) whereas AOA amoA gene copy numbers were higher in grazed grasslands. The chronic addition of inorganic N fertilizer in the form of ammonium nitrate (either applied alone or in combination with other macro nutrients) greatly contributed to increase AOB abundance. Our study brings evidence of how soil-nitrifying communities can differently respond to the long-term effect of animal (i.e. rabbit) grazing and to repeated applications of nutrient (e.g. NH4NO3) and non-nutrient (i.e. CaCO3) fertilizing materials.