Fate of N additions in a multiple resource-limited Mediterranean oak savanna

Abstract

Mediterranean oak savannas, such as Spanish dehesas, are multiple resource-limited ecosystems found in semiarid regions which are key contributors to interannual variability of the global carbon (C) budget. Interactions between nitrogen (N) and phosphorus (P) cycles are expected to play a major role in overall ecosystem function as anthropogenic N deposition shifts ecosystems from N to P limitation, leaving unknown how increased N availability might influence C uptake. Therefore, the fate of N additions in dehesas is important for understanding global C cycling. Using a 15N tracer experiment within fertilized (N or N + P) plots of a Holm oak dehesa, we tested the effects of ecosystem spatial heterogeneity (habitat), P addition, and time on the fate of added N. We expected that open pasture areas would retain more of the added N in biological components due to greater N limitation, that the addition of P would enhance N retention in biological components relative to N alone, and that added N would shift from being within the microbial biomass immediately after addition to being predominantly within plants at the beginning of the following growing season. We found that open pasture plots with N only had the greatest label recovery seven months after the start of the experiment, supporting the idea that open pasture was more N-limited than under-canopy areas. However, soil was the largest sink for added N, regardless of habitat, treatment, or time. Our results suggest that abiotic fixation of N may play an important role in modifying the effects of N deposition in dehesas.