Responses of microbial activity to carbon, nitrogen, and phosphorus additions in forest mineral soils differing in organic carbon content

Labile C input to the soil can cause the priming effect (PE) that in turn changes the soil organic C (SOC) content. However, little information is available to predict the magnitude of the PE in different soils, especially under concurrent changes in nutrient inputs. We took advantage of a natural gradient in labile C input in the surroundings of wood ant nests in a temperate coniferous forest which arises through the long-term effects of wood ant foraging on the inputs of honeydew to soil. We collected soils from the surface mineral horizon (high-SOC content) (A horizon) and the subsoil mineral horizon (low-SOC content) (B horizon) at 4 m (low labile C input and higher SOC content) and 70 m (high labile C input and lower SOC content) from four nests. In a 6-month laboratory microcosm experiment, we monitored microbial activity and PE as affected by no nutrient addition (control) or fortnightly additions of labile C alone or in combination with N and/or P (C, CN, CP, CNP). Microbial activity and PE after C addition increased more at 70 m than at 4 m in the B horizon, that is, were higher with a lower SOC content. However, microbial activity and PE in the B horizon were not affected by additions of N and/or P with C. In the A horizon, microbial activity and PE were lower after combined CN addition but increased by combined CP addition relative to C addition alone. In conclusion, labile C inputs had a larger effect on decomposition and PE in low-SOC than high-SOC soils, whereas N and P inputs had greater effects in high-SOC soils than in low-SOC soils. This suggests that low-SOC soils such as those subjected to a high long-term labile C input or those from the subsoil mineral horizon might be more susceptible to increase microbial activity in relation to changes in labile C inputs but less susceptible in relation to changes in N and P inputs relative to high-SOC soils.