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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 year). 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 year. 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. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition is a major threat to the semiarid Mediterranean ecosystems. We simulated a gradient of N deposition (0, 10, 20 and 50 kg N ha year + 6.4 kg N ha year ambient deposition) in a Mediterranean shrubland from central Spain. In autumn 2011 (after 4 years of experimental duration), soil cores were taken to extract the soil fauna. Acari (45.54%) and Collembola (44.00%) were the most represented taxonomical groups, and their abundance was negatively related to soil pH. Simulated N deposition had an impact on the total number of individuals in soil as well as on Collembola and Pauropoda abundance. Collembola abundance increased with N loads up to 20 kg N ha year and then decreased. This response was attributed to soil acidification (between 0 and 20 kg N ha year) and increased soil ammonium (between 20 and 50 kg N ha year). Pauropoda were favoured by additions of 50 kg N ha year, and it was the only taxonomical group whose abundance was exclusively related to N deposition, suggesting their potential as bioindicators. Contrary to predictions, there was a negative relationship between soil faunal abundance and plant diversity. In conclusion, soil faunal communities from semiarid Mediterranean ecosystems in central Spain seem to be primarily influenced by soil chemistry (mainly pH) but are also susceptible to increased N deposition. The main drivers of change under increased N deposition scenarios seem to be soil acidification and increased ammonium in soils where nitrate is the dominant mineral N form. [ABSTRACT FROM AUTHOR]

We studied the effects of N deposition (0, 10, 20 and 50 kg N ha year) on cover and physiology of Pleurochaete squarrosa, a terricolous moss from semiarid Mediterranean ecosystems. We also investigated the effects of N fertilization under competition with vascular plants or under water stress. Under greenhouse conditions, vascular plant competition reduced moss cover, and there was a significant interaction between N and competition. Water stress reduced moss cover under high and low competition conditions. Nitrogen fertilization increased moss cover irrespectively of the N dose supplied at low competition conditions. Under field conditions, N deposition affected moss physiology but not cover. Most of the physiological variables analyzed responded to N deposition, although the response of some of them was saturated with only 10 kg N ha year over the background (nitrate reductase; phosphomonoesterase; tissue N and K). The response of indicators such as chlorophyll a and lutein contents did not show any evidence of saturation, which probably makes them the best candidates in monitoring programs. Based on the data provided, the applicability of the phosphomonoesterase can also be considered. In addition, the importance of taking into account the existence of superimposed environmental gradients (such as those in soil mineral N content) interacting with the response of P. squarrosa to predict impacts of N deposition has been demonstrated. Therefore, detailed soil surveys and integrative physiological evaluations will be required to produce a significantly better picture of the effects of N deposition on Mediterranean ecosystems along extant N deposition gradients. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: N deposition is currently affecting nutrient cycling in terrestrial ecosystems. We studied the effects of four years of N application (0, 10, 20 and 50kgNha−1 year−1 +background deposition) on soil chemistry and fertility in a semiarid shrubland in central Spain. Soil pH and nutrient availability fluctuated seasonally. The inorganic-N fraction in soil was dominated by nitrate, as expected in calcareous soils. N application increased inorganic N availability in soil. There was a negative correlation between N application and soil K+ availability and pH, measured as the % change after four years. Soil N and C storage (evaluated as the % change) slightly increased after four years. Our data suggest that, in the short-term, the seasonality of nutrients overwhelm any chemical alteration related to N deposition. However, the potential implication of continuous N addition on soil chemistry in the long-term is not well understood. [Copyright &y& Elsevier]

Abstract: Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen (Cladonia foliacea) and one moss (Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems. [Copyright &y& Elsevier]