Effects of simulated nitrogen deposition on soil phosphorus availability and element composition in a phosphorus-limited Mediterranean shrubland

Understanding soil biogeochemical responses to increased nitrogen (N) deposition is essential for assessing and mitigating the impacts of global environmental change on terrestrial ecosystems. However, the effects of N addition on key soil elements—those critical for ecosystem productivity and pollution dynamics—remain insufficiently understood, particularly in vulnerable Mediterranean ecosystems characterized by phosphorus (P) and/or water limitations. Therefore, we examined the effects of simulated N deposition on extractable total P (ETP) and its organic and inorganic fractions, alongside the extractable concentrations of 12 mineral elements in a P-limited Mediterranean shrubland. Over the study period, N addition notably increased ETP and its fractions, while decreasing iron (Fe) concentrations. The increase in extractable P is likely attributable to enhanced activity of P-mobilizing enzymes, coupled with a decline in plant photosynthetic activity and nutrient uptake capacity during colder seasons. The reduction in extractable Fe may be linked to lower activity of carbon (C)- and N-mobilizing enzymes under N addition. Our findings suggest that N accumulation initially triggers ecosystem mobilization of less available nutrients, potentially alleviating existing P limitations. However, the observed shifts in enzyme activity—from N or C to P mobilization—indicate that the ecosystem’s capacity to mobilize P may become saturated under prolonged N deposition, ultimately intensifying P limitation and favoring P-efficient and acid-tolerant species over time.