Impacts of increased nitrogen deposition and altered precipitation regimes on soil fertility and functioning in semiarid Mediterranean shrublands

• Soil C:N ratio was affected by the interaction between N fertilization and water supply.
• N fixation is down-regulated by N fertilization.
• Mineral N availability was positively related to the extant N deposition gradient.
• Mg, Fe, Mn and Cu availability were negatively related to the extant N gradient.

We studied the impacts of nitrogen (N) deposition and precipitation gradients on soil chemistry and functioning of calcareous soils from semiarid Mediterranean shrublands. Under greenhouse conditions, N fertilization at 0, 10, 20 and 50 kg N ha−1 yr−1 and low water increased inorganic N. The concentration of extractable cations was higher under low water due to reduced leaching, whereas soil carbon (C) to N ratio was affected by the interaction of N fertilization and water supply, suggesting the role of rainfall in the response of soil C and N storage capacity to N deposition. Soil phosphatase activity increased with N as a consequence of an induced N to phosphorus imbalance, whereas N-fixation was down-regulated by N fertilization, a response attributed to high levels of nitrate. Net N mineralization and nitrification were also reduced by N fertilization. Under field conditions, N availability was positively related to the N deposition gradient (3.98–6.05 kg N ha−1 yr−1). Nutrient availability was primarily and positively related to rainfall and temperature, although N deposition contributed to an overall cation depletion. Finally, we suggest the importance of conducting additional studies on the effects of N deposition and climate change on calcareous soils in Mediterranean ecosystems.