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Virtual Issue Articles: Introduction to a Virtual Special Issue on ecological stoichiometry and global change Amy T. Austin, Peter M. Vitousek Biological stoichiometry of plant production: metabolism, scaling and ecological response to global change J. J. Elser, W. F. Fagan, A. J. Kerkhoff, N. G. Swenson, B. J. Enquist Stoichiometric flexibility as a regulator of carbon and nutrient cycling in terrestrial ecosystems under change Seeta A. Sistla, Joshua P. Schimel Plant stoichiometry at different scales: element concentration patterns reflect environment more than genotype Göran I. Ågren, Martin Weih Are vascular epiphytes nitrogen or phosphorus limited? A study of plant 15N fractionation and foliar N : P stoichiometry with the tank bromeliad Vriesea sanguinolenta Wolfgang Wanek, Gerhard Zotz Resource stoichiometry elucidates the structure and function of arbuscular mycorrhizas across scales Nancy Collins Johnson

Environmental and stoichiometric controls on microbial carbon-use efficiency in soils Stefano Manzoni, Philip Taylor, Andreas Richter, Amilcare Porporato, Göran I. Ågren Litter stoichiometric traits of plant species of high-latitude ecosystems show high responsiveness to global change without causing strong variation in litter decomposition R. Aerts, P. M. van Bodegom, J. H. C. Cornelissen Leaf traits and decomposition in tropical rainforests: revisiting some commonly held views and towards a new hypothesis Stephan Hättenschwiler, Sylvain Coq, Sandra Barantal, Ira Tanya Handa Nitrogen inputs accelerate phosphorus cycling rates across a wide variety of terrestrial ecosystems Alison R. Marklein, Benjamin Z. Houlton Climate change alters stoichiometry of phosphorus and nitrogen in a semiarid grassland Feike A. Dijkstra, Elise Pendall, Jack A. Morgan, Dana M. Blumenthal, Yolima Carrillo, Daniel R. LeCain, Ronald F. Follett, David G. Williams

N : P ratios in terrestrial plants: variation and functional significance Sabine Güsewell Alien and endangered plants in the Brazilian Cerrado exhibit contrasting relationships with vegetation biomass and N : P stoichiometry Luciola S. Lannes, Mercedes M. C. Bustamante, Peter J. Edwards, Harry Olde Venterink

Nutrient limitation on terrestrial plant growth – modeling the interaction between nitrogen and phosphorus Göran I. Ågren, J. Å. Martin Wetterstedt, Magnus F. K. Billberger

Cover image: An overview of the Brazilian Cerrado, courtesy of Luciola S. Lannes.

Stoichiometric patterns in foliar nutrient resorption across multiple scales Sasha C. Reed, Alan R. Townsend, Eric A. Davidson, Cory C. Cleveland

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Introduction Plants and other organisms have a diverse array of strategies with which they maximize growth and survival in a world of limited resources. These limiting resources include light and water, and also essential nutrients that are required for metabolism and growth, but that often are in short enough supply to constrain those vital functions. Ecological stoichiometry focuses on the dynamics and interactions of multiple elements within organisms and the cycling between organisms and their environment. In this Virtual Special Issue (VSI) we bring together a collection of New Phytologist articles focused on ecological stoichiometry and global change that combines the products of the 27th New Phytologist Symposium held in September of 2011, ‘Stoichiometric flexibility in terrestrial ecosystems under global change’ and recent papers published in New Phytologist that address the range of applications of ecological stoichiometry in terrestrial ecosystems.

Tansley review Biological stoichiometry of plant production: metabolism, scaling and ecological response to global change Author for correspondence: J. J. Elser Tel: +1 480 965 9747 Email: j.elser@asu.edu

J. J. Elser, W. F. Fagan, A. J. Kerkhoff, N. G. Swenson, B. J. Enquist

Summary New Phytologist (2010) 186: 593–608 doi: 10.1111/j.1469-8137.2010.03214.x

Key words: biological stoichiometry; carbon; global change; growth rate; metabolic scaling; nitrogen; phosphorus; plant size

Biological stoichiometry theory considers the balance of multiple chemical elements in living systems, whereas metabolic scaling theory considers how size affects metabolic properties from cells to ecosystems. We review recent developments integrating biological stoichiometry and metabolic scaling theories in the context of plant ecology and global change. Although vascular plants exhibit wide variation in foliar carbon : nitrogen : phosphorus ratios, they exhibit a higher degree of ‘stoichiometric homeostasis’ than previously appreciated. Thus, terrestrial carbon : nitrogen : phosphorus stoichiometry will reflect the effects of adjustment to local growth conditions as well as species’ replacements. Plant stoichiometry exhibits size scaling, as foliar nutrient concentration decreases with increasing plant size, especially for phosphorus. Thus, small plants have lower nitrogen : phosphorus ratios. Furthermore, foliar nutrient concentration is reflected in other tissues (root, reproductive, support), permitting the development of empirical models of production that scale from tissue to whole-plant levels. Plant stoichiometry exhibits large-scale macroecological patterns, including stronger latitudinal trends and environmental correlations for phosphorus concentration (relative to nitrogen) and a positive correlation between nutrient concentrations and geographic range size. Given this emerging knowledge of how plant nutrients respond to environmental variables and are connected to size, the effects of global change factors (such as carbon dioxide, temperature, nitrogen deposition) can be better understood.

Research review Stoichiometric flexibility as a regulator of carbon and nutrient cycling in terrestrial ecosystems under change Author for correspondence: Seeta A. Sistla Tel: +1 518 331 8309 Email: sistla@lifesci.ucsb.edu

Seeta A. Sistla, Joshua P. Schimel

Summary New Phytologist (2012) 196: 68–78 doi: 10.1111/j.1469-8137.2012.04234.x

Key words: carbon; global change; scaling; stoichiometric flexibility; terrestrial ecosystems

Ecosystems across the biosphere are subject to rapid changes in elemental balance and climatic regimes. A major force structuring ecological responses to these perturbations lies in the stoichiometric flexibility of systems – the ability to adjust their elemental balance whilst maintaining function. The potential for stoichiometric flexibility underscores the utility of the application of a framework highlighting the constraints and consequences of elemental mass balance and energy cycling in biological systems to address global change phenomena. Improvement in the modeling of ecological responses to disturbance requires the consideration of the stoichiometric flexibility of systems within and across relevant scales. Although a multitude of global change studies over various spatial and temporal scales exist, the explicit consideration of the role played by stoichiometric flexibility in linking micro-scale to macro-scale biogeochemical processes in terrestrial ecosystems remains relatively unexplored. Focusing on terrestrial systems under change, we discuss the mechanisms by which stoichiometric flexibility might be expressed and connected from organisms to ecosystems. We suggest that the transition from the expression of stoichiometric flexibility within individuals to the community and ecosystem scales is a key mechanism regulating the extent to which environmental perturbation may alter ecosystem carbon and nutrient cycling dynamics.

Plant stoichiometry at different scales: element concentration patterns reflect environment more than genotype Göran I. Ågren, Martin Weih

Summary Author for correspondence: Göran I. Ågren Tel: +46 18 672449 Email: goran.agren@slu.se

New Phytologist (2012) 194: 944–952 doi: 10.1111/j.1469-8137.2012.04114.x

Key words: environmental variability; genotypic variability; mineral nutrients; Salix; small-scale variability; within-individual variability

• All plant species require at least 16 elements for their growth and survival but the relative requirements and the variability at different organizational scales is not well understood. • We use a fertiliser experiment with six willow (Salix spp.) genotypes to evaluate a methodology based on Euclidian distances for stoichiometric analysis of the variability in leaf nutrient relations of twelve of those (C, N, P, K, Ca, Mg, Mn, S, Fe, Zn, B, Cu) plus Na and Al. • Differences in availability of the elements in the environment was the major driver of variation. Variability between leaves within a plant or between individuals of the same genotype growing in close proximity was as large as variability between genotypes. • Elements could be grouped by influence on growth: N, P, S and Mn concentrations follow each other and increase with growth rate; K, Ca and Mg uptake follow the increase in biomass; but uptake of Fe, B, Zn and Al seems to be limited. The position of Cu lies between the first two groups. Only for Na is there a difference in element concentrations between genotypes. The three groups of elements can be associated with different biochemical functions.

Are vascular epiphytes nitrogen or phosphorus limited? A study of plant 15N fractionation and foliar N : P stoichiometry with the tank bromeliad Vriesea sanguinolenta Wolfgang Wanek, Gerhard Zotz

Summary Author for correspondence: Wolfgang Wanek Tel: +43 1 4277 54254 Email: wolfgang.wanek@univie.ac.at

New Phytologist (2011) 192: 462–470 doi: 10.1111/j.1469-8137.2011.03812.x

Key words: N : P stoichiometry; nutrient limitation; plant 15N fractionation; tank bromeliad; δ15N

• Although there is unambiguous evidence for vascular epiphytic plants to be limited by insufficient water and nutrient supply under natural conditions, it is an open debate whether they are primarily phosphorus (P) or nitrogen (N) limited. • Plant 15N fractionation and foliar N : P stoichiometry of a tank epiphyte (Vriesea sanguinolenta), and its response to combined N–P fertilization, were studied under semi-natural conditions over 334 d to clarify the type of nutrient limitation. • Plants collected in the field and experimental plants with limited nutrient supply showed significant plant 15N fractionation (mean 5‰) and plant N : P ratios of c. 13.5. Higher relative growth rates and declines in plant 15N fractionation (0.5‰) and in foliar N : P ratios to 8.5 in the high N–P treatment indicated that these epiphytes were P limited in situ. The critical foliar N : P ratio was 10.4, as derived from the breakpoint in the relationship between plant 15N fractionation and foliar N : P. • We interpret the widespread 15N depletion of vascular epiphytes relative to their host trees as deriving from 15N fractionation of epiphytes as a result of P limitation. High foliar N : P ratios (> 12) corroborate widespread P limitation (or co-limitation by N and P) of epiphytic bromeliads and, possibly, other epiphyte species.

Tansley review Resource stoichiometry elucidates the structure and function of arbuscular mycorrhizas across scales Author for correspondence: Nancy Collins Johnson Tel: 928-523-6473 Email: nancy.johnson@nau.edu

Nancy Collins Johnson

Summary New Phytologist (2010) 185: 631–647 doi: 10.1111/j.1469-8137.2009.03110.x

Key words: arbuscular mycorrhizas; coadaptation; ecological stoichiometry; functional equilibrium; nitrogen; phosphorus; thresholds; trade balance

Despite the fact that arbuscular mycorrhizal (AM) associations are among the most ancient, abundant and important symbioses in terrestrial ecosystems, there are currently few unifying theories that can be used to help understand the factors that control their structure and function. This review explores how a stoichiometric perspective facilitates integration of three complementary ecological and evolutionary models of mycorrhizal structure and function. AM symbiotic function should be governed by the relative availability of carbon, nitrogen and phosphorus (trade balance model) and allocation to plant and fungal structures should depend on the availabilities of these resources (functional equilibrium model). Moreover, in an evolutionary framework, communities of plants and AM fungi are predicted to adapt to each other and their local soil environment (co-adaptation model). Anthropogenic enrichment of essential resources in the environment is known to impact AM symbioses. A more predictive theory of AM structure and function will help us to better understand how these impacts may influence plant communities and ecosystem properties.

Tansley review N : P ratios in terrestrial plants: variation and functional significance

Author for correspondence: Sabine Güsewell Tel: +00411 632 4307 Fax: +00411 632 1215 Email: sabine.guesewell@env.ethz.ch

Sabine Güsewell

Summary New Phytologist (2004) 164: 243–266 doi: 10.1111/j.1469-8137.2004.01192.x

Key words: critical N : P ratios; ecological stoichiometry; nutrient limitation; plant strategies; species richness; tissue nutrient concentrations

Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios – those of plants whose growth is equally limited by N and P – depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios <10 and >20 often (not always) correspond to N- and P-limited biomass production, as shown by short-term fertilization experiments; however long-term effects of fertilization or effects on individual species can be different. N : P ratios are on average higher in graminoids than in forbs, and in stress-tolerant species compared with ruderals; they correlate negatively with the maximal relative growth rates of species and with their N-indicator values. At vegetation level, N : P ratios often correlate negatively with biomass production; high N : P ratios promote graminoids and stress tolerators relative to other species, whereas relationships with species richness are not consistent. N : P ratios are influenced by global change, increased atmospheric N deposition, and conservation managment.

Alien and endangered plants in the Brazilian Cerrado exhibit contrasting relationships with vegetation biomass and N : P stoichiometry Luciola S. Lannes, Mercedes M. C. Bustamante, Peter J. Edwards, Harry Olde Venterink

Summary Author for correspondence: Luciola S. Lannes Tel: +41 76 405 2686 Email: luciola.lannes@env.ethz.ch

New Phytologist (2012) 196: 816–823 doi: 10.1111/j.1469-8137.2012.04363.x

Key words: biomass; Brachiaria decumbens; Cerrado; Melinis minutiflora; N : P ratio; nutrients; plant invasion; savanna

• Although endangered and alien invasive plants are commonly assumed to persist under different environmental conditions, surprisingly few studies have investigated whether this is the case. We examined how endangered and alien species are distributed in relation to community biomass and N : P ratio in the above-ground community biomass in savanna vegetation in the Brazilian Cerrado. • For 60 plots, we related the occurrence of endangered (Red List) and alien invasive species to plant species richness, vegetation biomass and N : P ratio, and soil variables. • Endangered plants occurred mainly in plots with relatively low above-ground biomass and high N : P ratios, whereas alien invasive species occurred in plots with intermediate to high biomass and low N : P ratios. Occurrences of endangered or alien plants were unrelated to extractable N and P concentrations in the soil. • These contrasting distributions in the Cerrado imply that alien species only pose a threat to endangered species if they are able to invade sites occupied by these species and increase the above-ground biomass and/or decrease the N : P ratio of the vegetation. We found some evidence that alien species do increase above-ground community biomass in the Cerrado, but their possible effect on N : P stoichiometry requires further study.

Nutrient limitation on terrestrial plant growth – modeling the interaction between nitrogen and phosphorus Göran I. Ågren, J. Å. Martin Wetterstedt, Magnus F. K. Billberger

Summary Author for correspondence: Göran I. Ågren Tel: +46 18 672449 Email: goran.agren@slu.se

New Phytologist (2012) 194: 953–960 doi: 10.1111/j.1469-8137.2012.04116.x

Key words: carbon; co-limitation; Liebig’s law; model; multiple limitation hypothesis (MLH); nitrogen; phosphorus; plant growth

• Growth of plants in terrestrial ecosystems is often limited by the availability of nitrogen (N) or phosphorous (P) Liebig’s law of the minimum states that the nutrient in least supply relative to the plant’s requirement will limit the plant’s growth. An alternative to the law of the minimum is the multiple limitation hypothesis (MLH) which states that plants adjust their growth patterns such that they are limited by several resources simultaneously. • We use a simple model of plant growth and nutrient uptake to explore the consequences for the plant’s relative growth rate of letting plants invest differentially in N and P uptake. • We find a smooth transition between limiting elements, in contrast to the strict transition in Liebig’s law of the minimum. At N : P supply ratios where the two elements simultaneously limit growth, an increase in either of the nutrients will increase the growth rate because more resources can be allocated towards the limiting element, as suggested by the multiple limitation hypothesis. However, the further the supply ratio deviates from these supply rates, the more the plants will follow the law of the minimum. • Liebig’s law of the minimum will in many cases be a useful first-order approximation.

Stoichiometric patterns in foliar nutrient resorption across multiple scales Sasha C. Reed, Alan R. Townsend, Eric A. Davidson, Cory C. Cleveland

Summary Author for correspondence: Sasha C. Reed Tel: +1 435 210 4824 Email: screed@usgs.gov

New Phytologist (2012) 196: 173–180 doi: 10.1111/j.1469-8137.2012.04249.x

Key words: forest succession; net primary production; nitrogen; nutrient limitation; phosphorus; resorption; stoichiometry; tropical rain forest

• Nutrient resorption is a fundamental process through which plants withdraw nutrients from leaves before abscission. Nutrient resorption patterns have the potential to reflect gradients in plant nutrient limitation and to affect a suite of terrestrial ecosystem functions. • Here, we used a stoichiometric approach to assess patterns in foliar resorption at a variety of scales, specifically exploring how N : P resorption ratios relate to presumed variation in N and/or P limitation and possible relationships between N : P resorption ratios and soil nutrient availability. • N : P resorption ratios varied significantly at the global scale, increasing with latitude and decreasing with mean annual temperature and precipitation. In general, tropical sites (absolute latitudes < 23°26′) had N : P resorption ratios of < 1, and plants growing on highly weathered tropical soils maintained the lowest N : P resorption ratios. Resorption ratios also varied with forest age along an Amazonian forest regeneration chronosequence and among species in a diverse Costa Rican rain forest. • These results suggest that variations in N : P resorption stoichiometry offer insight into nutrient cycling and limitation at a variety of spatial scales, complementing other metrics of plant nutrient biogeochemistry. The extent to which the stoichiometric flexibility of resorption will help regulate terrestrial responses to global change merits further investigation.

Research review Environmental and stoichiometric controls on microbial carbon-use efficiency in soils

Author for correspondence: Stefano Manzoni Tel: +1 919 6605467 Email: stefano.manzoni@duke.edu

Stefano Manzoni, Philip Taylor, Andreas Richter, Amilcare Porporato, GĂśran I. Ă&#x2026;gren

Summary New Phytologist (2012) 196: 79â&#x20AC;&#x201C;91 doi: 10.1111/j.1469-8137.2012.04225.x

Key words: biogeochemical model; carbon-use efficiency (CUE); microbial stoichiometry; nutrient limitation; soil moisture; temperature

Carbon (C) metabolism is at the core of ecosystem function. Decomposers play a critical role in this metabolism as they drive soil C cycle by mineralizing organic matter to CO2. Their growth depends on the carbon-use efficiency (CUE), defined as the ratio of growth over C uptake. By definition, high CUE promotes growth and possibly C stabilization in soils, while low CUE favors respiration. Despite the importance of this variable, flexibility in CUE for terrestrial decomposers is still poorly characterized and is not represented in most biogeochemical models. Here, we synthesize the theoretical and empirical basis of changes in CUE across aquatic and terrestrial ecosystems, highlighting common patterns and hypothesizing changes in CUE under future climates. Both theoretical considerations and empirical evidence from aquatic organisms indicate that CUE decreases as temperature increases and nutrient availability decreases. More limited evidence shows a similar sensitivity of CUE to temperature and nutrient availability in terrestrial decomposers. Increasing CUE with improved nutrient availability might explain observed declines in respiration from fertilized stands, while decreased CUE with increasing temperature and plant C : N ratios might decrease soil C storage. Current biogeochemical models could be improved by accounting for these CUE responses along environmental and stoichiometric gradients.

Litter stoichiometric traits of plant species of high-latitude ecosystems show high responsiveness to global change without causing strong variation in litter decomposition R. Aerts, P. M. van Bodegom, J. H. C. Cornelissen

Summary Author for correspondence: R. Aerts Tel: +31 205987211 Email: rien.aerts@falw.vu.nl

New Phytologist (2012) 196: 181–188 doi: 10.1111/j.1469-8137.2012.04256.x

Key words: global change; highlatitude ecosystems; litter decomposition; litter stoichiometry; Plant Economics Spectrum

• High-latitude ecosystems are important carbon accumulators, mainly as a result of low decomposition rates of litter and soil organic matter. We investigated whether global change impacts on litter decomposition rates are constrained by litter stoichiometry. • Thereto, we investigated the interspecific natural variation in litter stoichiometric traits (LSTs) in high-latitude ecosystems, and compared it with climate change-induced LST variation measured in the Meeting of Litters (MOL) experiment. This experiment includes leaf litters originating from 33 circumpolar and high-altitude global change experiments. Two-year decomposition rates of litters from these experiments were measured earlier in two common litter beds in sub-Arctic Sweden. • Response ratios of LSTs in plants of high-latitude ecosystems in the global change treatments showed a three-fold variation, and this was in the same range as the natural variation among species. However, response ratios of decomposition were about an order of magnitude lower than those of litter carbon/nitrogen ratios. • This implies that litter stoichiometry does not constrain the response of plant litter decomposition to global change. We suggest that responsiveness is rather constrained by the less responsive traits of the Plant Economics Spectrum of litter decomposability, such as lignin and dry matter content and specific leaf area.

Research review Leaf traits and decomposition in tropical rainforests: revisiting some commonly held views and towards a new hypothesis Author for correspondence: S. Hättenschwiler Tel: +33 467 61 22 36 Email: stephan.hattenschwiler@cefe.cnrs.fr

Stephan Hättenschwiler, Sylvain Coq, Sandra Barantal, Ira Tanya Handa

Summary New Phytologist (2011) 189: 950â&#x20AC;&#x201C;965 doi: 10.1111/j.1469-8137.2010.03483.x

Key words: energy starvation; French Guiana; litter quality; mycorrhizas; nutrient cycling; nutrient limitation; phosphorus; soil fauna

Proper estimates of decomposition are essential for tropical forests, given their key role in the global carbon (C) cycle. However, the current paradigm for litter decomposition is insufficient to account for recent observations and may limit model predictions for highly diverse tropical ecosystems. In light of recent findings from a nutrient-poor Amazonian rainforest, we revisit the commonly held views that: litter traits are a mere legacy of live leaf traits; nitrogen (N) and lignin are the key litter traits controlling decomposition; and favourable climatic conditions result in rapid decomposition in tropical forests. Substantial interspecific variation in litter phosphorus (P) was found to be unrelated to variation in green leaves. Litter nutrients explained no variation in decomposition, which instead was controlled primarily by nonlignin litter C compounds at low concentrations with important soil fauna effects. Despite nearoptimal climatic conditions, tropical litter decomposition proceeded more slowly than in a climatically less favourable temperate forest. We suggest that slow decomposition in the studied rainforest results from a syndrome of poor litter C quality beyond a simple lignin control, enforcing energy starvation of decomposers. We hypothesize that the litter trait syndrome in nutrient-poor tropical rainforests may have evolved to increase plant access to limiting nutrients via mycorrhizal associations.

Nitrogen inputs accelerate phosphorus cycling rates across a wide variety of terrestrial ecosystems Alison R. Marklein, Benjamin Z. Houlton

Summary Author for correspondence: Alison Marklein Tel: +1 530 752 1491 Email: armarklein@ucdavis.edu

New Phytologist (2010) 193: 696–704 doi: 10.1111/j.1469-8137.2011.03967.x

Key words: extracellular enzyme; meta-analysis; nitrogen; nitrogen deposition; nutrient availability; nutrient limitation; phosphatase; phosphorus

• Biologically essential elements – especially nitrogen (N) and phosphorus (P) – constrain plant growth and microbial functioning; however, human activities are drastically altering the magnitude and pattern of such nutrient limitations on land. Here we examine interactions between N and P cycles of P mineralizing enzyme activities (phosphatase enzymes) across a wide variety of terrestrial biomes. • We synthesized results from 34 separate studies and used meta-analysis to evaluate phosphatase activity with N, P, or N×P fertilization. • Our results show that N fertilization enhances phosphatase activity, from the tropics to the extra-tropics, both on plant roots and in bulk soils. By contrast, P fertilization strongly suppresses rates of phosphatase activity. • These results imply that phosphatase enzymes are strongly responsive to changes in local nutrient cycle conditions. We also show that plant phosphatases respond more strongly to fertilization than soil phosphatases. The tight coupling between N and P provides a mechanism for recent observations of N and P co-limitation on land. Moreover, our results suggest that terrestrial plants and microbes can allocate excess N to phosphatase enzymes, thus delaying the onset of single P limitation to plant productivity as can occur via human modifications to the global N cycle.

Climate change alters stoichiometry of phosphorus and nitrogen in a semiarid grassland Feike A. Dijkstra, Elise Pendall, Jack A. Morgan, Dana M. Blumenthal, Yolima Carrillo, Daniel R. LeCain, Ronald F. Follett, David G. Williams

Summary Author for correspondence: Feike A. Dijkstra Tel: +61 2 8627 1122 Email: feike.dijkstra@sydney.edu.au

New Phytologist (2012) 196: 807–815 doi: 10.1111/j.1469-8137.2012.04349.x

Key words: elevated carbon dioxide; grasslands; homeostasis; N : P stoichiometry; nutrient availability; PHACE; soil moisture; temperature

• Nitrogen (N) and phosphorus (P) are essential nutrients for primary producers and decomposers in terrestrial ecosystems. Although climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, the impacts of climate change on the relative availability of N with respect to P remain highly uncertain. • In a semiarid grassland in Wyoming, USA, we studied the effects of atmospheric CO2 enrichment (to 600 ppmv) and warming (1.5/3.0°C above ambient temperature during the day/night) on plant, microbial and available soil pools of N and P. • Elevated CO2 increased P availability to plants and microbes relative to that of N, whereas warming reduced P availability relative to N. Across years and treatments, plant N : P ratios varied between 5 and 18 and were inversely related to soil moisture. • Our results indicate that soil moisture is important in controlling P supply from inorganic sources, causing reduced P relative to N availability during dry periods. Both wetter soil conditions under elevated CO2 and drier conditions with warming can further alter N : P. Although warming may alleviate N constraints under elevated CO2, warming and drought can exacerbate P constraints on plant growth and microbial activity in this semiarid grassland.

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