Interactions among nitrogen fixation and soil phosphorus acquisition strategies in lowland tropical rain forests

Paradoxically, symbiotic dinitrogen (N₂) fixers are abundant in nitrogen (N)‐rich, phosphorus (P)‐poor lowland tropical rain forests. One hypothesis to explain this pattern states that N₂ fixers have an advantage in acquiring soil P by producing more N‐rich enzymes (phosphatases) that mineralise organic P than non‐N₂ fixers. We assessed soil and root phosphatase activity between fixers and non‐fixers in two lowland tropical rain forest sites, but also addressed the hypothesis that arbuscular mycorrhizal (AM) colonisation (another P acquisition strategy) is greater on fixers than non‐fixers. Root phosphatase activity and AM colonisation were higher for fixers than non‐fixers, and strong correlations between AM colonisation and N₂ fixation at both sites suggest that the N–P interactions mediated by fixers may generally apply across tropical forests. We suggest that phosphatase enzymes and AM fungi enhance the capacity of N₂ fixers to acquire soil P, thus contributing to their high abundance in tropical forests.     

Effects of increased N and P availability on biomass allocation and root carbohydrate reserves differ between N‐fixing and non‐N‐fixing savanna tree seedlings

In mixed tree‐grass ecosystems, tree recruitment is limited by demographic bottlenecks to seedling establishment arising from inter‐ and intra‐life‐form competition, and disturbances such as fire. Enhanced nutrient availability resulting from anthropogenic nitrogen (N) and phosphorus (P) deposition can alter the nature of these bottlenecks by changing seedling growth and biomass allocation patterns, and lead to longer‐term shifts in tree community composition if different plant functional groups respond differently to increased nutrient availability. However, the extent to which tree functional types characteristic of savannas differ in their responses to increased N and P availability remains unclear. We quantified differences in above‐ and belowground biomass, and root carbohydrate contents in seedlings of multiple N‐fixing and non‐N‐fixing tree species characteristic of Indian savanna and dry forest ecosystems in response to experimental N and P additions. These parameters are known to influence the ability of plants to compete, as well as survive and recover from fires. N‐fixers in our study were co‐limited by N and P availability, while non‐N‐fixers were N limited. Although both functional groups increased biomass production following fertilization, non‐N‐fixers were more responsive and showed greater relative increases in biomass with fertilization than N‐fixers. N‐fixers had greater baseline investment in belowground resources and root carbohydrate stocks, and while fertilization reduced root:shoot ratios in both functional groups, root carbohydrate content only reduced with fertilization in non‐N‐fixers. Our results indicate that, even within a given system, plants belonging to different functional groups can be limited by, and respond differentially to, different nutrients, suggesting that long‐term consequences of nutrient deposition are likely to vary across savannas contingent on the relative amounts of N and P being deposited in sites.     

Nitrogen to phosphorus ratios of tree species in response to elevated carbon dioxide and nitrogen addition in subtropical forests

Increased atmospheric carbon dioxide (CO₂) concentrations and nitrogen (N) deposition induced by human activities have greatly influenced the stoichiometry of N and phosphorus (P). We used model forest ecosystems in open‐top chambers to study the effects of elevated CO₂ (ca. 700 μmol mol^?1) alone and together with N addition (100 kg N ha^?1 yr^?1) on N to P (N : P) ratios in leaves, stems and roots of five tree species, including four non‐N₂ fixers and one N₂ fixer, in subtropical China from 2006 to 2009. Elevated CO₂ decreased or had no effects on N : P ratios in plant tissues of tree species. N addition, especially under elevated CO₂, lowered N : P ratios in the N₂ fixer, and this effect was significant in the stems and the roots. However, only one species of the non‐N₂ fixers showed significantly lower N : P ratios under N addition in 2009, and the others were not affected by N addition. The reductions of N : P ratios in response to elevated CO₂ and N addition were mainly associated with the increases in P concentrations. Our results imply that elevated CO₂ and N addition could facilitate tree species to mitigate P limitation by more strongly influencing P dynamics than N in the subtropical forests.     

Interactive effects of N:P ratios and light on nitrogen‐fixer abundance

Nitrogen‐fixers can contribute significant amounts of nitrogen (N) and impact ecosystem functioning in diverse aquatic and terrestrial ecosystems. What determines N‐fixer abundance still remains poorly understood. Here we experimentally investigate major environmental controls on the abundance of N‐fixers: nitrogen to phosphorus (N:P) ratio and light. We grew a N‐fixer, cyanobacterium Anabaena flos‐aquae, in a multispecies community of freshwater phytoplankton in replicated factorial design treatments with two N:P ratios and two light levels. We show that low N:P ratios promote the dominance of the N‐fixer in the community, but only under high light. Under low light, N:P ratio did not have a significant effect on the abundance of the N‐fixer. N fixation occurred at low N:P only and increased with increasing light. In contrast, the density of non N‐fixing cyanobacteria did not depend on N:P ratios. Green algae dominated under high N:P and high light only, exhibiting the opposite pattern of dominance to N‐fixers. These results are consistent with patterns observed in nature and help explain the N‐fixer distribution along the environmental gradients of nutrients and light.     

Small traits with big consequences: how seed traits of nitrogen‐fixing plants might influence ecosystem nutrient cycling

Symbiotic nitrogen (N)‐fixing plants have important effects on the biogeochemical processes of the sites they inhabit, but their ability to reach these sites is determined by the dispersal of their seeds. Differences in seed size and dispersal vectors of N‐fixing and non‐fixing plants could influence the spatial and temporal distributions of N fixers, and thus could have important impacts on biogeochemical cycling. Using seed mass, dispersal vector, and biome data retrieved from online public databases, we ask if there are systematic differences in seed mass and dispersal vectors between N‐fixing and non‐fixing plants. We demonstrate that rhizobial N fixers tend to have larger seeds that are more likely to be biotically dispersed than seeds of non‐fixers, whereas actinorhizal N‐fixing trees tend to have small, abiotically dispersed seeds. We then synthesize existing evidence from the literature to draw links between these dispersal traits and the spatio–temporal patterns of N fixers, as well as their biogeochemical effects on terrestrial ecosystems. Using this literature, we argue that the spatio–temporal distributions of N fixers are influenced by their seed dispersal characteristics, and that these distribution patterns have important effects on the total amount of N fixed at a site and the timing of N inputs during processes such as succession.     

Taxonomic identity determines N2 fixation by canopy trees across lowland tropical forests

Legumes capable of fixing atmospheric N₂ are abundant and diverse in many tropical forests, but the factors determining ecological patterns in fixation are unresolved. A long‐standing idea is that fixation depends on soil nutrients (N, P or Mo), but recent evidence shows that fixation may also differ among N₂‐fixing species. We sampled canopy‐height trees across five species and one species group of N₂‐fixers along a landscape P gradient, and manipulated P and Mo to seedlings in a shadehouse. Our results identify taxonomy as the major determinant of fixation, with P (and possibly Mo) only influencing fixation following tree‐fall disturbances. While 44% of trees did not fix N₂, other trees fixed at high rates, with two species functioning as superfixers across the landscape. Our results raise the possibility that fixation is determined by biodiversity, evolutionary history and species–specific traits (tree growth rate, canopy stature and response to disturbance) in the tropical biome.     

Controlling Cyanobacterial Blooms in Hypertrophic Lake Taihu,China: Will Nitrogen Reductions Cause Replacement of Non-N2 Fixing by N2 Fixing Taxa?

Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China’s third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N₂ fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N₂ fixing with N₂ fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N₂ fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N₂ fixing with N₂ fixing cyanobacteria, and N₂ fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N₂ fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N₂ fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.     

PHYSIOLOGICAL DIFFERENCES BETWEEN TWO SPECIES OF CYANOBACTERIA IN RELATION TO PHOSPHORUS LIMITATION1

In order to gain a better understanding of the factors responsible for changes in cyanobacterial species composition in the Alberche River (central Spain), which is characterized by spatiotemporal N and P variability, we undertook P‐limitation studies on two isolated N₂‐fixing species that are well represented in the river: Calothrix elenkinii (Kosinskaja) UAM 225 and Nostoc punctiforme (Kützing) UAM 220. We examined differences in phosphorus‐storage capacity, phosphate uptake, and phosphatase activity between the two isolates grown under N₂‐fixing conditions. Although they shared a similar pattern of phosphate uptake, C. elenkinii cultures had a greater phosphorus storage capacity and greater phosphatase activity than those of N. punctiforme. The former showed a stronger response when cultured under low P concentrations. The two species showed specific differences in the rates of intracellular polyphosphate granule utilization and phosphatase activity. Our results suggest that C. elenkinii might be particularly well suited to river environments in which nutrient concentrations are generally low, but which experience episodic pulses of high P supply (inorganic and/or organic), favoring “luxury uptake” and storage strategies.     

Community structures of N2‐fixing bacteria associated with the phyllosphere of a Holm oak forest and their response to drought

Biological nitrogen (N) fixation is a key pathway in terrestrial ecosystems and is therefore critical for understanding the responses of ecosystems to global environmental changes. The free‐living diazotrophic community is distributed along the canopy‐to‐soil profile, but the ecological significance of epiphyllic N₂ fixers, despite their functional relevance, on plant foliar surfaces remains very poorly understood compared with the N₂‐fixing community in forest litter and soils. We assessed the community structure of N₂ fixers and overall bacteria by genetic fingerprinting (t‐RFLP) to explore the seasonal successional patterns of the microbial community in the natural phyllosphere of a Holm oak (Quercus ilex) forest submitted to 12‐year field experiment of rain exclusion mimicking the conditions of drought projected for the coming decades. Leaves of Holm oak were analysed in different seasons over a period of 1.5 years. The bacterial community of the phyllosphere did not correspond to the surrounding soil biome in the same area. These analyses provided field evidence for the presence of free‐living diazotrophs associated with the tissues of leaves of Holm oak, the dominant tree species of many Mediterranean forests. The results also revealed that the community composition is affected seasonally and inter‐annually by the environment, and that the composition shifts in response to climate change. Drought treatment increased the richness of the epiphyllic microbial community, especially during the summer. These changes were associated with higher C:N ratios of leaves observed in response to drought in semiarid areas. This epiphyllic microbiota that can potentially fix N₂ extends the capacity of plants to adapt to the environment.     

Bottom-up rather than top-down processes regulate the abundance and activity of nitrogen fixing plants in two Connecticut old-field ecosystems

The maintenance of nitrogen limitation in terrestrial ecosystems remains a central paradox in biogeochemistry. Although plants that form a symbiotic association with nitrogen fixing bacteria should be at a competitive advantage over non-fixing plant species in N limited environments, N₂ fixing plants are uncommon in most mid- to high-latitude ecosystems. Theory and observation suggest that preferential grazing on N-rich tissues by herbivores, resource limitations to growth, reproduction and N₂ fixation, and temperature limitations to the activity of the N₂ fixing enzyme nitrogenase, explain the rarity of N₂ fixing plants. These ideas, however, have never been confronted by multifactor experiments in the field. In a 3 year field experiment, we found that the abundance, growth, reproductive output and fraction of plant-N derived from N₂ fixation in temperate, old-field ecosystems was constrained by the availability of phosphorus (P). Although the availability of light was crucial to the performance of old-field N₂ fixing plants, the largest gains in biomass and the rate of N₂ fixation were observed in the plots fertilized with P. By contrast, herbivory had no effect on the abundance, biomass and activity of N₂ fixing plants and inconsistent effects on foliar nitrogen concentrations (opposing directions, depending upon year), suggesting that herbivores do not affect the ecology of N₂ fixing plants in old field ecosystems, at least not over the course of 3 years. Together with a recent study demonstrating that C limitation explains the absence of N₂ fixing trees in temperate forests our analysis suggests that stand replacing disturbances shift the limitation on the abundance and activity of N₂ fixing plants from P early in secondary succession to light later in succession, as the forest canopy closes and incident light levels decline precipitously.     

Cellular interactions: lessons from the nitrogen‐fixing cyanobacteria

Marine nitrogen‐fixing cyanobacteria play a central role in the open‐ocean microbial community by providing fixed nitrogen (N) to the ocean from atmospheric dinitrogen (N₂) gas. Once thought to be dominated by one genus of cyanobacteria, Trichodesmium, it is now clear that marine N₂‐fixing cyanobacteria in the open ocean are more diverse, include several previously unknown symbionts, and are geographically more widespread than expected. The next challenge is to understand the ecological implications of this genetic and phenotypic diversity for global oceanic N cycling. One intriguing aspect of the cyanobacterial N₂ fixers ecology is the range of cellular interactions they engage in, either with cells of their own species or with photosynthetic protists. From organelle‐like integration with the host cell to a free‐living existence, N₂‐fixing cyanobacteria represent the range of types of interactions that occur among microbes in the open ocean. Here, we review what is known about the cellular interactions carried out by marine N₂‐fixing cyanobacteria and where future work can help. Discoveries related to the functional roles of these specialized cells in food webs and the microbial community will improve how we interpret their distribution and abundance patterns and contributions to global N and carbon (C) cycles.     

The effect of simulated herbivory on growth and nutrient status of focal and neighbouring early successional woody plant species

Defoliation through herbivory is well known to affect target plants and their associated belowground properties, but the response of plants and their soil environment to defoliation of their neighbours is less well understood. We performed a controlled shade‐house experiment involving three plant species that colonize New Zealand floodplains during primary succession, i.e. a palatable N₂‐fixing shrub (Carmichaelia odorata), a palatable deciduous small tree (Fuchsia excorticata) and a less palatable evergreen tree (Weinmannia racemosa). All species were grown in large pots for 40 months both singly and in two species pairs, and either one or both of the species grown in pairs were clipped to simulate herbivory. Responses of growth and foliar nutrient status to clipping varied strongly among species, with Carmichaelia having the largest response and Fuchsia having the smallest. Carmichaelia also enhanced soil microbial biomass and activity, and foliar N concentrations of Weinmannia. However, this did not translate to a net positive effect; instead Carmichaelia competitively reduced growth and foliar P concentrations of both other species. Most effects of Carmichaelia on the soil microflora, and growth and nutrient status of its neighbours, disappeared when Carmichaelia was clipped. Further, the effect of clipping Carmichaelia had a stronger impact on growth, soil activity and nutrient status of the other two species than did the clipping of those species. These results contradict expectations that N₂‐fixing plants should promote growth of other species in pioneer communities or that defoliation of N₂‐fixers exacerbate positive effects; in our study, defoliation of Carmichaelia merely mitigated the negative effects that it had on other species. They also suggest that interplay of competition and differential herbivory among coexisting plants has important implications for soil microflora and processes, relative nutrient acquisition and stoichiometry of coexisting plant species, and potentially plant community development.     

Benthic N2 fixation in coral reefs and the potential effects of human‐induced environmental change

Tropical coral reefs are among the most productive and diverse ecosystems, despite being surrounded by ocean waters where nutrients are in short supply. Benthic dinitrogen (N₂) fixation is a significant internal source of “new” nitrogen (N) in reef ecosystems, but related information appears to be sparse. Here, we review the current state (and gaps) of knowledge on N₂ fixation associated with coral reef organisms and their ecosystems. By summarizing the existing literature, we show that benthic N₂ fixation is an omnipresent process in tropical reef environments. Highest N₂ fixation rates are detected in reef‐associated cyanobacterial mats and sea grass meadows, clearly showing the significance of these functional groups, if present, to the input of new N in reef ecosystems. Nonetheless, key benthic organisms such as hard corals also importantly contribute to benthic N₂ fixation in the reef. Given the usually high coral coverage of healthy reef systems, these results indicate that benthic symbiotic associations may be more important than previously thought. In fact, mutualisms between carbon (C) and N₂ fixers have likely evolved that may enable reef communities to mitigate N limitation. We then explore the potential effects of the increasing human interferences on the process of benthic reef N₂ fixation via changes in diazotrophic populations, enzymatic activities, or availability of benthic substrates favorable to these microorganisms. Current knowledge indicates positive effects of ocean acidification, warming, and deoxygenation and negative effects of increased ultraviolet radiation on the amount of N fixed in coral reefs. Eutrophication may either boost or suppress N₂ fixation, depending on the nutrient becoming limiting. As N₂ fixation appears to play a fundamental role in nutrient‐limited reef ecosystems, these assumptions need to be expanded and confirmed by future research efforts addressing the knowledge gaps identified in this review.     

Impact of nitrogen deposition on forest and lake food webs in nitrogen‐limited environments

Increased reactive nitrogen (N_r) deposition has raised the amount of N available to organisms and has greatly altered the transfer of energy through food webs, with major consequences for trophic dynamics. The aim of this review was to: (i) clarify the direct and indirect effects of N_r deposition on forest and lake food webs in N‐limited biomes, (ii) compare and contrast how aquatic and terrestrial systems respond to increased N_r deposition, and (iii) identify how the nutrient pathways within and between ecosystems change in response to N_r deposition. We present that N_r deposition releases primary producers from N limitation in both forest and lake ecosystems and raises plants' N content which in turn benefits herbivores with high N requirements. Such trophic effects are coupled with a general decrease in biodiversity caused by different N‐use efficiencies; slow‐growing species with low rates of N turnover are replaced by fast‐growing species with high rates of N turnover. In contrast, N_r deposition diminishes below‐ground production in forests, due to a range of mechanisms that reduce microbial biomass, and decreases lake benthic productivity by switching herbivore growth from N to phosphorus (P) limitation, and by intensifying P limitation of benthic fish. The flow of nutrients between ecosystems is expected to change with increasing N_r deposition. Due to higher litter production and more intense precipitation, more terrestrial matter will enter lakes. This will benefit bacteria and will in turn boost the microbial food web. Additionally, N_r deposition promotes emergent insects, which subsidize the terrestrial food web as prey for insectivores or by dying and decomposing on land. So far, most studies have examined N_r‐deposition effects on the food web base, whereas our review highlights that changes at the base of food webs substantially impact higher trophic levels and therefore food web structure and functioning.     

N and P Cycling in Tanzanian Humid Savanna: Influence of Herbivores, Fire, and N2-Fixation

Availabilities of nitrogen (N) and phosphorus (P) have a strong influence on plant growth and the species composition of savannas, but it is not clear how these availabilities depend on factors such as fire, N₂-fixation, and activities of wild herbivores and cattle. We quantified soil N and P availabilities in various ways (extractable pools, mineralization, resin adsorption) along vegetation gradients within a recently abandoned cattle ranch and a former game reserve in Tanzania (both areas now part of the Saadani National Park). We also assessed annual N and P balances to evaluate how long-term availabilities of N and P are affected by large herbivores, symbiotic N₂-fixation, and fire. The results show that cattle ranching led to a spatial re-distribution of nutrients, with the local accumulation of P being stronger and more persistent than that of N. In the former game reserve, intensively grazed patches of short grass tended to have elevated soil N and P availabilities; however, because quantities of nutrients removed through grazing exceeded returns in dung and urine, the nutrient balances of these patches were negative. In dense Acacia stands, N₂-fixation increased N availability and caused a net annual N input. Fire was the major cause for nutrient losses from tallgrass savanna, and estimated N inputs from the atmosphere and symbiotic N₂-fixation were insufficient to compensate for these losses. Our results call into question the common assumption that N budgets in annually burned savanna are balanced; rather, these ecosystems are a mosaic of patches with both N enrichment and impoverishment, which vary according to the vegetation type.     

Non‐additive effects of litter mixing are suppressed in a nutrient‐enriched stream

Investigations of how species compositional changes interact with other aspects of global change, such as nutrient mobilization, to affect ecosystem processes are currently lacking. Many studies have shown that mixed species plant litters exhibit non‐additive effects on ecosystem functions in terrestrial and aquatic systems. Using a full‐factorial design of three leaf litter species with distinct initial chemistries (carbon:nitrogen; C:N) and breakdown rates (Liriodendron tulipifera, Acer rubrum and Rhododendron maximum), we tested for additive and non‐additive effects of litter species mixing on breakdown in southeastern US streams with and without added nutrients (N and phosphorus). We found a non‐additive (antagonistic) effect of litter mixing on breakdown rates under reference conditions but not when nutrient levels were elevated. Differential responses among single‐species litters to nutrient enrichment contributed to this result. Antagonistic litter mixing effects on breakdown were consistent with trends in litter C:N, which were higher for mixtures than for single species, suggesting lower microbial colonization on mixtures. Nutrient enrichment lowered C:N and had the greatest effect on the lowest‐ (R. maximum) and the least effect on the highest‐quality litter species (L. tulipifera), resulting in lower interspecific variation in C:N. Detritivore abundance was correlated with litter C:N in the reference stream, potentially contributing to variation in breakdown rates. In the nutrient‐enriched stream, detritivore abundance was higher for all litter and was unrelated to C:N. Thus, non‐additive effects of litter mixing were suppressed by elevated streamwater nutrients, which increased nutrient content of all litter, reduced variation in C:N among litter species and increased detritivore abundance. Nutrients reduced interspecific variation among plant litters, the base of important food web pathways in aquatic ecosystems, affecting predicted mixed‐species breakdown rates. More generally, world‐wide mobilization of nutrients may similarly modify other effects of biodiversity on ecosystem processes.     

Shifts in symbiotic associations in plants capable of forming multiple root symbioses across a long‐term soil chronosequence

Changes in soil nutrient availability during long‐term ecosystem development influence the relative abundances of plant species with different nutrient‐acquisition strategies. These changes in strategies are observed at the community level, but whether they also occur within individual species remains unknown. Plant species forming multiple root symbioses with arbuscular mycorrhizal (AM) fungi, ectomycorrhizal (ECM) fungi, and nitrogen‐(N) fixing microorganisms provide valuable model systems to examine edaphic controls on symbioses related to nutrient acquisition, while simultaneously controlling for plant host identity. We grew two co‐occurring species, Acacia rostellifera (N₂‐fixing and dual AM and ECM symbioses) and Melaleuca systena (AM and ECM dual symbioses), in three soils of contrasting ages (c. 0.1, 1, and 120 ka) collected along a long‐term dune chronosequence in southwestern Australia. The soils differ in the type and strength of nutrient limitation, with primary productivity being limited by N (0.1 ka), co‐limited by N and phosphorus (P) (1 ka), and by P (120 ka). We hypothesized that (i) within‐species root colonization shifts from AM to ECM with increasing soil age, and that (ii) nodulation declines with increasing soil age, reflecting the shift from N to P limitation along the chronosequence. In both species, we observed a shift from AM to ECM root colonization with increasing soil age. In addition, nodulation in A. rostellifera declined with increasing soil age, consistent with a shift from N to P limitation. Shifts from AM to ECM root colonization reflect strengthening P limitation and an increasing proportion of total soil P in organic forms in older soils. This might occur because ECM fungi can access organic P via extracellular phosphatases, while AM fungi do not use organic P. Our results show that plants can shift their resource allocation to different root symbionts depending on nutrient availability during ecosystem development.     

Signatures of nutrient limitation and co‐limitation: responses of autotroph internal nutrient concentrations to nitrogen and phosphorus additions

Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and it is therefore important to understand how these nutrients, independently or in combination, influence the growth and nutrient content of primary producers. Using meta‐analysis of 118 field and laboratory experiments in freshwater, marine and terrestrial ecosystems, we tested hypotheses about co‐limitation of N and P by comparing the effects of adding N alone, P alone, and both N and P together on internal N (e.g. %N, C:N) and P (e.g. %P, C:P) concentrations in autotroph communities. In particular, we tested the following predictions. First, if only one nutrient was limiting, addition of that nutrient should decrease the concentration of the other nutrient, but addition of the non‐limiting nutrient would have no effect on the internal concentration of the limiting nutrient. If community co‐limitation was occurring then addition of either nutrient should result in a decrease in the internal concentration of the other nutrient. Community co‐limitation could also result in no change – or even an increase – in N concentrations in response to P addition if P stimulated growth of N fixers. Finally, if biochemically dependent co‐limitation was occurring, addition of a limiting nutrient would not decrease, and could even increase, the concentration of the other, co‐limited nutrient. We found no general evidence for the decrease in the internal concentration of one nutrient due to addition of another nutrient. The one exception to this overall pattern was marine systems, where N addition decreased internal P concentrations. In contrast, P addition increased internal N concentrations across all experiments, consistent with co‐limitation. These results have important implications for understanding the roles that N and P play in controlling producer growth and internal nutrient accumulation as well as for managing the effects of nutrient enrichment in ecosystems. Synthesis On a global scale, humans have doubled nitrogen (N) inputs and quadrupled phosphorus (P) inputs relative to pre‐industrial levels. N and P fertilization influences autotroph internal nutrient concentrations and ratios and thereby affects a variety of community and ecosystem processes, including decomposition and consumer population dynamics. It is therefore critical to understand the effects of nutrient additions on the growth and nutrient concentrations of primary producers. We used meta‐analysis to evaluate the responses of autotroph internal N and P concentrations to additions of N, P, and N+P and make inferences about limitation and co‐limitation of N and P across marine, terrestrial, and freshwater ecosystems. We found little evidence for single‐nutrient limitation, highlighting the fact that multiple nutrients generally limit primary production.     

Regulation of phosphate uptake reveals cyanobacterial bloom resilience to shifting N:P ratios

相似文献   

Facultative nitrogen fixation by canopy legumes in a lowland tropical forest

Symbiotic dinitrogen (N₂) fixation is often invoked to explain the N richness of tropical forests as ostensibly N₂-fixing trees can be a major component of the community. Such arguments assume N₂ fixers are fixing N when present. However, in laboratory experiments, legumes consistently reduce N₂ fixation in response to increased soil N availability. These contrasting views of N₂ fixation as either obligate or facultative have drastically different implications for the N cycle of tropical forests. We tested these models by directly measuring N₂-fixing root nodules and nitrogenase activity of individual canopy-dominant legume trees (Inga sp.) across several lowland forest types. Fixation was substantial in disturbed forests and some gaps but near zero in the high N soils of mature forest. Our findings suggest that canopy legumes closely regulate N₂ fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.