title: Biogeochemical indicators of elevated nitrogen deposition in semiarid Mediterranean ecosystems.
abstract: Nitrogen (N) deposition has doubled the natural N inputs received by ecosystems through biological N fixation and is currently a global problem that is affecting the Mediterranean regions. We evaluated the existing relationships between increased atmospheric N deposition and biogeochemical indicators related to soil chemical factors and cryptogam species across semiarid central, southern, and eastern Spain. The cryptogam species studied were the biocrust-forming species Pleurochaete squarrosa (moss) and Cladonia foliacea (lichen). Sampling sites were chosen in Quercus coccifera (kermes oak) shrublands and Pinus halepensis (Aleppo pine) forests to cover a range of inorganic N deposition representative of the levels found in the Iberian Peninsula (between 4.4 and 8.1 kg N ha(-1) year(-1)). We extended the ambient N deposition gradient by including experimental plots to which N had been added for 3 years at rates of 10, 20, and 50 kg N ha(-1) year(-1). Overall, N deposition (extant plus simulated) increased soil inorganic N availability and caused soil acidification. Nitrogen deposition increased phosphomonoesterase (PME) enzyme activity and PME/nitrate reductase (NR) ratio in both species, whereas the NR activity was reduced only in the moss. Responses of PME and NR activities were attributed to an induced N to phosphorus imbalance and to N saturation, respectively. When only considering the ambient N deposition, soil organic C and N contents were positively related to N deposition, a response driven by pine forests. The PME/NR ratios of the moss were better predictors of N deposition rates than PME or NR activities alone in shrublands, whereas no correlation between N deposition and the lichen physiology was observed. We conclude that integrative physiological measurements, such as PME/NR ratios, measured on sensitive species such as P. squarrosa, can provide useful data for national-scale biomonitoring programs, whereas soil acidification and soil C and N storage could be useful as additional corroborating ecosystem indicators of chronic N pollution.
authors.affiliation: Hawkesbury Institute for the Environment, University of Western Sydney, Locked Bag 1797, Penrith, New South Wales, 2751, Australia, R.OchoaHueso@uws.edu.au.
authors.forename: Raúl
authors.initials: R
authors.lastname: Ochoa-Hueso
authors.forename: María
authors.initials: M
authors.lastname: Arróniz-Crespo
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Bowker
authors.forename: Fernando T
authors.initials: FT
authors.lastname: Maestre
authors.forename: M Esther
authors.initials: ME
authors.lastname: Pérez-Corona
authors.forename: Mark R
authors.initials: MR
authors.lastname: Theobald
authors.forename: Marta G
authors.initials: MG
authors.lastname: Vivanco
authors.forename: Esteban
authors.initials: E
authors.lastname: Manrique
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D044002:Bryophyta / Q000737:chemistry; D017753:Ecosystem; D004784:Environmental Monitoring; D008011:Lichens / Q000737:chemistry; D009584:Nitrogen / Q000032:analysis*; D009586:Nitrogen Fixation; D010758:Phosphorus / Q000032:analysis; D028223:Pinus / Q000737:chemistry; D029963:Quercus / Q000737:chemistry; D012987:Soil / Q000737:chemistry; D013030:Spain
chemical_list: D000393:Air Pollutants; D012987:Soil; D010758:Phosphorus; D009584:Nitrogen