Contrasting effects of nitrogen addition on soil respiration in two Mediterranean ecosystems

Increased atmospheric nitrogen (N) deposition is known to alter ecosystem carbon source-sink dynamics through changes in soil CO2 fluxes. However, a limited number of experiments have been conducted to assess the effects of realistic N deposition in the Mediterranean Basin, and none of them have explored the effects of N addition on soil respiration (R-s). To fill this gap, we assessed the effects of N supply on Rs dynamics in the following two Mediterranean sites: Capo Caccia (Italy), where 30 kg ha(-1) year(-1) was supplied for 3 years, and El Regajal (Spain), where plots were treated with 10, 20, or 50 kg N ha(-1) year(-1) for 8 years. Results show a complex, non-linear response of soil respiration (R-s) to N additions with Rs overall increasing at Capo Caccia and decreasing at El Regajal. This suggests that the response of Rs to N addition depends on dose and duration of N supply, and the existence of a threshold above which the N introduced in the ecosystem can affect the ecosystem's functioning. Soil cover and seasonality of precipitations also play a key role in determining the effects of N on Rs as shown by the different responses observed across seasons and in bare soil vs. the soil under canopy of the dominant species. These results show how increasing rates of N addition may influence soil C dynamics in semiarid ecosystems in the Mediterranean Basin and represent a valuable contribution for the understanding and the protection of Mediterranean ecosystems.