Alleviating Nitrogen Limitation in Mediterranean Maquis Vegetation Leads to Ecological Degradation

Abstract

Soils are being degraded at an alarming rate and thereby also crucial ecosystem goods and services. Nitrogen (N) enrichment is a major driver of this degradation. While the negative impacts of N enrichment on vegetation are well known globally, those on various ecological interactions, and on ecosystem functioning, remain largely unknown. Because Mediterranean ecosystems are N limited, they are good model systems for evaluating how N enrichment impacts not only vegetation but also ecological partnerships and ecosystem functioning. Using a 7￢ﾀﾐyear N￢ﾀﾐmanipulation (dose and form) field experiment running in a Mediterranean Basin maquis located in a region with naturally low ambient N deposition (<4ￂﾠkgￂﾠNￂﾠha￢ﾈﾒ1ￂﾠy￢ﾈﾒ1), we assessed the impacts of the N additions on (i) the dominant plant species (photosynthetic N￢ﾀﾐuse efficiency); (ii) plant￢ﾀﾓsoil ecological partnerships with ectomycorrhiza and N￢ﾀﾐfixing bacteria; and (iii) ecosystem degradation (plant￢ﾀﾓsoil cover, biological mineral weathering and soil N fixation). N additions significantly disrupted plant￢ﾀﾓsoil cover, plant￢ﾀﾓsoil biotic interactions, and ecosystem functioning compared with ambient N deposition conditions. However, the higher the ammonium dose (alone or with nitrate), the more drastic these disruptions were. We report a critical threshold at 20￢ﾀﾓ40ￂﾠkg ammonium ha￢ﾈﾒ1ￂﾠy￢ﾈﾒ1 whereby severe ecosystem degradation can be expected. These observations are critical to help explain the mechanisms behind ecosystem degradation, to describe the collective loss of organisms and multifunction in the landscape, and to predict potential fragmentation of Mediterranean maquis under conditions of unrelieved N enrichment. Copyright ￂﾩ 2017 John Wiley & Sons, Ltd.