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

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

2017

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 R ₛ 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 R ₛ overall increasing at Capo Caccia and decreasing at El Regajal. This suggests that the response of R ₛ 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 R ₛ 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.

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