Drivers of the microbial metabolic quotient across global grasslands

Aim: The microbial metabolic quotient (MMQ; mg CO2-C/mg MBC/h), defined as the amount of microbial CO2 respired (MR; mg CO2-C/kg soil/h) per unit of microbial biomass C (MBC; mg C/kg soil), is a key parameter for understanding the microbial regulation of the carbon (C) cycle, including soil C sequestration. Here, we experimentally tested hypotheses about the individual and interactive effects of multiple nutrient addition (nitrogen + phosphorus + potassium + micronutrients) and herbivore exclusion on MR, MBC and MMQ across 23 sites (five continents). Our sites encompassed a wide range of edaphoclimatic conditions; thus, we assessed which edaphoclimatic variables affected MMQ the most and how they interacted with our treatments. Location: Australia, Asia, Europe, North/South America. Time period: 2015–2016. Major taxa: Soil microbes. Methods: Soils were collected from plots with established experimental treatments. MR was assessed in a 5-week laboratory incubation without glucose addition, MBC via substrate-induced respiration. MMQ was calculated as MR/MBC and corrected for soil temperatures (MMQsoil). Using linear mixed effects models (LMMs) and structural equation models (SEMs), we analysed how edaphoclimatic characteristics and treatments interactively affected MMQsoil. Results: MMQsoil was higher in locations with higher mean annual temperature, lower water holding capacity and lower soil organic C concentration, but did not respond to our treatments across sites as neither MR nor MBC changed. We attributed this relative homeostasis to our treatments to the modulating influence of edaphoclimatic variables. For example, herbivore exclusion, regardless of fertilization, led to greater MMQsoil only at sites with lower soil organic C (< 1.7%). Main conclusions: Our results pinpoint the main variables related to MMQsoil across grasslands and emphasize the importance of the local edaphoclimatic conditions in controlling the response of the C cycle to anthropogenic stressors. By testing hypotheses about MMQsoil across global edaphoclimatic gradients, this work also helps to align the conflicting results of prior studies

Temporal rarity is a better predictor of local extinction risk than spatial rarity

Spatial rarity is often used to predict extinction risk, but rarity can also occur temporally. Perhaps more relevant in the context of global change is whether a species is core to a community (persistent) or transient (intermittently present), with transient species often susceptible to human activities that reduce niche space. Using 5–12 yr of data on 1,447 plant species from 49 grasslands on five continents, we show that local abundance and species persistence under ambient conditions are both effective predictors of local extinction risk following experimental exclusion of grazers or addition of nutrients; persistence was a more powerful predictor than local abundance. While perturbations increased the risk of exclusion for low persistence and abundance species, transient but abundant species were also highly likely to be excluded from a perturbed plot relative to ambient conditions. Moreover, low persistence and low abundance species that were not excluded from perturbed plots tended to have a modest increase in abundance following perturbance. Last, even core species with high abundances had large decreases in persistence and increased losses in perturbed plots, threatening the long-term stability of these grasslands. Our results demonstrate that expanding the concept of rarity to include temporal dynamics, in addition to local abundance, more effectively predicts extinction risk in response to environmental change than either rarity axis predicts alone.Fil: Wilfahrt, Peter A.. University of Minnesota; Estados UnidosFil: Asmus, Ashley L.. University of Minnesota; Estados UnidosFil: Seabloom, Eric. University of Minnesota; Estados UnidosFil: Henning, Jeremiah A.. University of Minnesota; Estados UnidosFil: Adler, Peter. State University of Utah; Estados UnidosFil: Arnillas, Carlos A.. University of Toronto Scarborough; CanadáFil: Bakker, Jonathan. University of Washington; Estados UnidosFil: Biederman, Lori. University of Iowa; Estados UnidosFil: Brudvig, Lars A.. Michigan State University; Estados UnidosFil: Cadotte, Marc W.. University of Toronto Scarborough; CanadáFil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Eskelinen, Anu. German Centre for Integrative Biodiversity Research; AlemaniaFil: Firn, Jennifer. University of Queensland; AustraliaFil: Harpole, W. Stanley. German Centre for Integrative Biodiversity Research; Alemania. Helmholtz Centre for Environmental Research; Alemania. Martin Luther University Halle-Wittenberg; AlemaniaFil: Hautier, Yann. Utrecht University; Países BajosFil: Kirkman, Kevin P.. University of KwaZulu-Natal; SudáfricaFil: Komatsu, Kimberly J.. Smithsonian Environmental Research Center; Estados UnidosFil: Laungani, Ramesh. Doane University; Estados UnidosFil: MacDougall, Andrew. University of Guelph; CanadáFil: McCulley, Rebecca L.. University of Kentucky; Estados UnidosFil: Moore, Joslin L.. Monash University; AustraliaFil: Morgan, John W.. La Trobe University; AustraliaFil: Mortensen, Brent. Benedictine College; Estados UnidosFil: Ochoa Hueso, Raul. Universidad de Cádiz; EspañaFil: Ohlert, Timothy. University of New Mexico; Estados UnidosFil: Power, Sally A.. University of Western Sydney; AustraliaFil: Price, Jodi. Charles Sturt University; AustraliaFil: Risch, Anita C.. Swiss Federal Institute for Forest, Snow and Landscape Research; SuizaFil: Schuetz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research; SuizaFil: Shoemaker, Lauren. University of Wyoming; Estados UnidosFil: Stevens, Carly. Lancaster University; Reino UnidoFil: Strauss, Alexander T.. University of Minnesota; Estados Unidos. University of Georgia; Estados UnidosFil: Tognetti, Pedro Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: Virtanen, Risto. University of Oulu; FinlandiaFil: Borer, Elizabeth. University of Minnesota; Estados Unido

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

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 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 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

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

European semiarid Mediterranean ecosystems are sensitive to nitrogen deposition:impacts on plant communities and root phosphatase activity

Nitrogen (N) deposition is predicted to impact on the structure and functioning of Mediterranean ecosystems. In this study, we measured plant species composition, production and root phosphatase activity in a field experiment in which N (0, 10, 20 and 50 kg N ha(-1) year(-1)) was added since October 2007 to a semiarid shrubland in central Spain. The characteristically dominant annual forb element responded negatively to N after similar to 2.5 and similar to 3.5 years. In contrast, the nitrophilous element (mainly crucifers) increased with N after similar to 2.5 and similar to 5.5 years, a response controlled by between-year variations in rainfall and the heterogeneous distribution of P availability. We also described a hierarchy of factors driving the structure and composition of the plant community: soil fertility was the most important driver, whereas calcareousness/acidity of soils and shrub cover played a secondary role; finally, N deposition contributed to explain a smaller fraction of the total variance, and its effects were predominantly negative, which was attributed to ammonium toxicity. Root phosphatase activity of three species was not responsive to N after similar to 2.5 years but there was a negative relationship with soil P in two of them. We conclude that increased N deposition in semiarid Mediterranean ecosystems of Europe can contribute to cause a shift in plant communities associated with an increase in the nitrophilous element and with a decline in abundance of various forb species adapted to the local conditions

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

Decoupling of nutrient cycles in a Eucalyptus

Elevated atmospheric [CO2] (eCO2) is currently altering nutrient cycling and availability in ecosystems worldwide. If the availabilities and turnover rates of macro‐ and micronutrients are differentially affected, then nutrient cycles may become out of sync (i.e., decoupled). We evaluated the impacts of 3 years of eCO2 (550 µmol CO2/mol) on the availability, stability and coupling of eleven essential macro and micronutrients in a mature, P‐limited Eucalyptus woodland from Eastern Australia (EucFACE). Despite increases in N and P availability in the first 18 months of study, nutrient availabilities and their stoichiometric ratios were unaffected by eCO2 across the study period (26–57 months of experimental duration). In contrast, the stability of nutrient availability increased under eCO2, which was concomitant with a reduction in the degree of biogeochemical coupling. Synthesis. We demonstrate that macro‐ and micro‐nutrient cycles can quickly become out of sync (i.e., decoupled) under eCO2 in a low‐nutrient Australian eucalypt woodland, despite the lack of consistent effects on nutrient availability; such decoupling of nutrient cycles may have unpredicted consequences in terms of ecosystem functioning under the widely assumed positive relationship between biogeochemical coupling and ecosystem functioning