Nonlinear disruption of ecological interactions in response to nitrogen deposition

Global environmental change (GEC) is affecting species interactions and causing a rapid decline in biodiversity. In this study, I present a new Ecosystem Disruption Index (EDI) to quantify the impacts of simulated nitrogen (N) deposition (0, 10, 20 and 50 kg N ha-1 yr-1 + 6-7 kg N ha-1 yr-1 background) on abiotic and biotic ecological interactions. This comparative index is based on pairwise linear and quadratic regression matrices. These matrices, calculated at the N treatment level, were constructed using a range of abiotic and biotic ecosystem constituents: soil pH, shrub cover, and the first component of several separate principal component analyses using soil fertility data (total carbon and N) and community data (annual plants; microorganisms; biocrusts; edaphic fauna) for a total of seven ecosystem constituents. Four years of N fertilization in a semiarid shrubland completely disrupted the network of ecological interactions, with a greater proportional increase in ecosystem disruption at low-N addition levels. Biotic interactions, particularly those involving microbes, shrubs and edaphic fauna, were more prone to be lost in response to N, whereas interactions involving soil properties were more resilient. In contrast, edaphic fauna was the only group directly affected by N addition, with mites and collembolans increasing their abundance with up to 20 kg N ha-1 yr-1 and then decreasing, which supports the idea of higher-trophic level organisms being more sensitive to disturbance due to more complex links with other ecosystem constituents. Future experimental studies evaluating the impacts of N deposition, and possibly other GEC drivers, on biodiversity and biotic and abiotic interactions may be able to explain results more effectively in the context of ecological networks as a key feature of ecosystem sensitivity

Impacts of altered precipitation, nitrogen deposition and plant competition on a Mediterranean seed bank

Questions: Do increased nitrogen (N) fertilization, altered water supply and increased competition by a nitrophilous plant species alter the establishment of the plant community developed from the soil seed bank present in a semi-arid Mediterranean ecosystem under greenhouse conditions? Does the nitrophilous plant respond in the same way to N addition under field conditions? Location: The kermes oak/rosemary shrubland in the Nature Reserve El Regajal-Mar de Ontígola, central Spain. Methods: In September 2008, native soil seed bank was collected in open areas dominated by rosemary shrubs. Seeds of the nitrophilous Diplotaxis siifolia were collected in early summer 2008. Pots with aliquots of soil were distributed among eight treatments, which resulted from combinations of two levels of N fertilization, water supply and density of D. siifolia in the seed bank. The plant community was left to grow for 3 mo, when chlorophyll fluorescence measurements and harvests were made. In summer 2008, the soil seed bank density of 0 and 50 kg N·ha-1·yr-1 plots was also manipulated in the field by broadcasting seeds to a density of 8000 D. siifolia seeds·m-2. Results: Increased N and competition tended to reduce soil water content in 'high water' pots. High water and N fertilization increased total above- and below-ground plant biomass production. Nitrogen fertilization reduced plant root:shoot ratio, had a negative effect on plant richness and, coupled with 'high competition' and 'high water', reduced forb biomass productivity. The competition treatment decreased chlorophyll fluorescence in grasses. Under field conditions, N fertilization increased D. siifolia density and this effect was dependent on soil phosphorus availability. Conclusions: Our results show that nitrophilous plant species such as D. siifolia can be a threat to local plant communities in the context of increased N deposition and altered precipitation events. In particular, local plant communities from semi-arid Mediterranean climates, and especially the forb element, may suffer more from competition in highly polluted and invaded sites during unusually wet years. Finally, we predict a future increase in the relative dominance of these species in semi-aridMediterranean environments where forb species typically adapted to low-nutrient environments will fail to successfully compete with weeds

Effects of nitrogen deposition on growth and physiology of pleurochaete squarrosa (Brid.) Lindb., a terricolous moss from mediterranean ecosystems

We studied the effects of N deposition (0, 10, 20 and 50 kg N ha -1 year-1) 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-1 year-1 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

Effects of nitrogen deposition and soil fertility on cover and physiology of Cladonia foliacea (Huds.) Willd., a lichen of biological soil crusts from Mediterranean Spain

We are fertilizing a thicket with 0, 10, 20 and 50 kg nitrogen (N) ha -1 yr-1 in central Spain. Here we report changes in cover, pigments, pigment ratios and FvFm of the N-tolerant, terricolous, lichen Cladonia foliacea after 1-2 y adding N in order to study its potential as biomarker of atmospheric pollution. Cover tended to increase. Pigments increased with fertilization independently of the dose supplied but only significantly with soil nitrate as covariate. β-carotene/chlorophylls increased with 20-50 kg N ha-1 yr-1 (over the background) and neoxanthin/chlorophylls also increased with N. (Neoxanthin+lutein)/carotene decreased with N when nitrate and pH seasonalities were used as covariates. FvFm showed a critical load above 40 kg N ha-1 yr-1. Water-stress, iron and copper also explained variables of lichen physiology. We conclude that this tolerant lichen could be used as biomarker and that responses to N are complex in heterogeneous Mediterranean-type landscapes

Nitrogen fertilization and water supply affect germination and plant establishment of the soil seed bank present in a semi-arid Mediterranean scrubland

Anthropogenic nitrogen (N) inputs in terrestrial ecosystems are higher than those that occur naturally and have been related to global biodiversity loss and altered ecosystem functioning. However, its effects on Mediterranean-type ecosystems, where production is water-limited and N regulated, remain unclear. We conducted a green-house experiment where we evaluated the effects of four simulated scenarios of N pollution (0, 10, 20 and 50 kg N ha-1 year-1) and two differential water supply regimes on the germination (experiment 1) and early plant establishment (experiment 2) of a seed bank from a semi-arid Mediterranean ecosystem of central Spain. Seed bank density was estimated as 62,374 ± 3,279 seeds m-2. Approximately 99.5% of emerged seeds corresponded to only 14 species of a total of 52, the majority of which were the annual forb Sagina apetala. The responses for N treatments were species-specific, mainly positive or unimodal, with watering treatments having some interactive effects. N and water supply also affected total and specific productivity; the responses found for N treatments were mainly humpback-shaped and an increased water supply had additive effects on community establishment in terms of total plant biomass. This response was linked to forb responsiveness. Contrary to predictions, grass biomass did not change with N supply; however, grass to forb ratio was affected because of changes in the latter. Overall, these experiments suggest a critical load for plant biomass production and conclude that N and water availability and supply can modify germination and plant establishment. This should be taken into account when analysing the effects of global change on the dynamics of plant communities where annuals are dominant or vegetation must establish from seed following a natural or anthropogenic disturbance regime

Impacts of increased nitrogen deposition and altered precipitation regimes on soil fertility and functioning in semiarid Mediterranean shrublands

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 50kgNha-1yr-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 (3.98-6.05kgNha-1yr-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

Spatial distribution and physiology of biological soil crusts from semi-arid central Spain are related to soil chemistry and shrub cover

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 (>0.5 m), 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 (~0.5 m), 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

Spatial distribution of fine root biomass in a remnant Eucalyptus tereticornis woodland in Eastern Australia

In forests, the majority of fine roots are located within the upper soil horizons, and fine root biomass decreases with depth. We evaluated spatial patterns in the distribution of fine root biomass and determined relationships with soil properties and vegetation structure in a Eucalyptus tereticornis woodland in East Australia. Fine root biomass (0–50 cm depth) was 678 (± 96.9) g m−2 and decreased exponentially with depth. Total fine root biomass was positively related to aboveground herbaceous biomass and increased with increasing proximity to larger trees, reflecting contributions from both herbaceous understorey plants and mature trees. Plants produced more fine roots in soil patches with lower organic matter content, possibly as a functional response to increase acquisition of essential nutrients in more nutrient-depleted soils. Aboveground plant attributes were more important predictors of fine roots in the shallowest layer, while water availability was a stronger predictor of fine root biomass in deeper layers, likely reflecting the harsh climatic conditions prior to sampling. Fine roots represent an important gap in many ecosystem models despite being key for biogeochemical cycling. Here, we showed that the spatial patterns of fine root biomass can be inferred from soil and vegetation characteristics across remnant Australian Eucalyptus woodlands

Comparison of trends in habitat and resource selection by the Spanish Festoon, Zerynthia rumina, and the whole butterfly community in a semi-arid Mediterranean ecosystem

Butterfly community and single species based approaches were taken to establish conservation priorities within a nature reserve in Central Spain. In this study, patch type (sclerophyllous, halophilous, or disturbed), potential herbaceous nectar availability, potential woody plant nectar availability, total nectar availability, and two approximations to plant diversity (herbaceous and woody plant diversity) were evaluated as variables that account for adult butterfly density. Butterfly communities in the reserve, which consist mostly of generalist species, were denser in relatively wet areas dominated by halophilous vegetation. Diversity did not significantly vary between ecologically different transects. Total nectar availability correlated with higher butterfly densities within both undisturbed and disturbed areas, which could be primarily explained by the lack of water typical of semi-arid Mediterranean climates, where fresh, nectariferous vegetation is scarce. Woody plants were also found to be important sources of nectar and shelter. In the dryer sclerophyllous sites, adult butterfly density was best explained by herbaceous plant diversity, suggesting better quality of available resources. The endangered specialist Zerynthia rumina (L.) (Lepidoptera: Papilionidae) was only present at the sclerophyllous sites. Its density was very low in all sampled transects, excluding one relatively isolated transect with high larval host-plant density. In contrast to the community-based approach, density of Z. rumina adults is better explained by the density of its larval host-plant than by nectar availability, a trend previously described for other sedentary species. Management strategies for protecting insect-rich areas should consider the specific ecological requirements of endangered species

Species richness effects on ecosystem multifunctionality depend on evenness, composition and spatial pattern

Recent studies have suggested that the simultaneous maintenance of multiple ecosystem functions (multifunctionality) is positively supported by species richness. However, little is known regarding the relative importance of other community attributes (e.g. spatial pattern, species evenness) as drivers of multifunctionality. We conducted two microcosm experiments using model biological soil crust communities dominated by lichens to: (i) evaluate the joint effects and relative importance of changes in species composition, spatial pattern (clumped and random distribution of lichens), evenness (maximal and low evenness) and richness (from two to eight species) on soil functions related to nutrient cycling (β-glucosidase, urease and acid phosphatase enzymes, in situ N availability, total N, organic C, and N fixation), and (ii) assess how these community attributes affect multifunctionality. Species richness, composition and spatial pattern affected multiple ecosystem functions (e.g. organic C, total N, N availability, β-glucosidase activity), albeit the magnitude and direction of their effects varied with the particular function, experiment and soil depth considered. Changes in species composition had effects on organic C, total N and the activity of β-glucosidase. Significant species richnessÃevenness and spatial patternÃevenness interactions were found when analysing functions such as organic C, total N and the activity of phosphatase. The probability of sustaining multiple ecosystem functions increased with species richness, but this effect was largely modulated by attributes such as species evenness, composition and spatial pattern. Overall, we found that model communities with high species richness, random spatial pattern and low evenness increased multifunctionality. Synthesis. Our results illustrate how different community attributes have a diverse impact on ecosystem functions related to nutrient cycling, and provide new experimental evidence illustrating the importance of the spatial pattern of organisms on ecosystem functioning. They also indicate that species richness is not the only biotic driver of multifunctionality, and that particular combinations of community attributes may be required to maximize it