The relationship between N isotopic fractionation within soybean and N2 fixation during soybean development

The contribution of N₂ fixation to overall soybean N uptake has most commonly been quantified by N isotope‐based methods, which rely on isotopic differences in plant N between legumes and non‐fixing reference plants. The choice of non‐fixing reference plants is critical for the accuracy of isotope‐based methods, and mismatched reference plants remain a potential source of error. Accurate estimates of soybean N₂ fixation also require information on N isotopic fractionation within soybean. On the basis of a previous observation of a close correlation between an expression of N fractionation within soybean and the proportion of plant N derived from atmosphere (%Ndfa) determined by ¹⁵N natural abundance, this field study aimed at assessing the relationship between various expressions describing intraplant ¹⁵N or N partitioning and %Ndfa during soybean development. Starting from a late vegetative stage until beginning senescence, the N content and N isotopic composition of shoots, roots and nodules of nodulated and non‐nodulated soybeans was determined at eight different developmental stages. Regression analysis showed that %Ndfa most closely correlated with the difference in the N isotopic composition of shoot N minus that of root including nodule N, and that this relationship was similar to that obtained in a previous multi‐site field study. We therefore consider this expression to hold promise as a means of quantifying %Ndfa independent of a reference plant, which would avoid some of the external sources of error introduced by the use of reference plants in determining %Ndfa.     

Intraspecific variation in herbivore‐induced plant volatiles influences the spatial range of plant–parasitoid interactions

Chemical information influences the behaviour of many animals, thus affecting species interactions. Many animals forage for resources that are heterogeneously distributed in space and time, and have evolved foraging behaviour that utilizes information related to these resources. Herbivore‐induced plant volatiles (HIPVs), emitted by plants upon herbivore attack, provide information on herbivory to various animal species, including parasitoids. Little is known about the spatial scale at which plants attract parasitoids via HIPVs under field conditions and how intraspecific variation in HIPV emission affects this spatial scale. Here, we investigated the spatial scale of parasitoid attraction to two cabbage accessions that differ in relative preference of the parasitoid Cotesia glomerata when plants were damaged by Pieris brassicae caterpillars. Parasitoids were released in a field experiment with plants at distances of up to 60 m from the release site using intervals between plants of 10 or 20 m to assess parasitism rates over time and distance. Additionally, we observed host‐location behaviour of parasitoids in detail in a semi‐field tent experiment with plant spacing up to 8 m. Plant accession strongly affected successful host location in field set‐ups with 10 or 20 m intervals between plants. In the semi‐field set‐up, plant finding success by parasitoids decreased with increasing plant spacing, differed between plant accessions, and was higher for host‐infested plants than for uninfested plants. We demonstrate that parasitoids can be attracted to herbivore‐infested plants over large distances (10 m or 20 m) in the field, and that stronger plant attractiveness via HIPVs increases this distance (up to at least 20 m). Our study indicates that variation in plant traits can affect attraction distance, movement patterns of parasitoids, and ultimately spatial patterns of plant–insect interactions. It is therefore important to consider plant‐trait variation in HIPVs when studying animal foraging behaviour and multi‐trophic interactions in a spatial context.     

Plant characteristics are poor predictors of microsite colonization during the first two years of primary succession

Questions: Do plant characteristics predict microsite colonization in severe habitats dominated by abiotic factors? Specifically, does colonization of microsites differ among shrubs, forbs and grasses or between wind‐ and water‐dispersed plants, non‐native and native plants, or N‐fixing and non‐N‐fixing plants? Location: Kowhai River floodplain, Kaikoura, South Island, New Zealand. Methods: Five microsite characteristics were measured for > 1000 individuals representing 27 colonizing plant species on a two‐year old surface of a primary succession on a New Zealand floodplain. The microsite characteristics included surface contour (convex, concave, or flat), the position of the plant (e.g., upstream, downstream) relative to the closest rock with > 20 cm maximum dimension, the distance to that same rock, the depth of the base of the stem below the surface of a plane resting on the adjacent microrelief, and soil particle size (gravel, pebbles or sand). Results: All plants preferred concave microsites near large rocks relative to systematically placed null points. We found no clear preferences for microhabitats by dispersal mode, native vs. non‐native status, or plants with or without nitrogen‐fixing symbionts, but grasses preferentially colonized fine soil particles. Highly variable responses among species contributed to these results. Better predictability of microsite preference was obtained for individual species than forplants grouped by characteristics. Conclusions Our results suggest that in severe habitats with strong abiotic filters and low microsite availability, such as found in early primary succession, coarse categories of species characteristics are poor predictors of colonization success.     

What explains variation in the impacts of exotic plant invasions on the nitrogen cycle? A meta‐analysis

Exotic plant invasions can notably alter the nitrogen (N) cycle of ecosystems. However, there is large variation in the magnitude and direction of their impact that remains unexplained. We present a structured meta‐analysis of 100 papers, covering 113 invasive plant species with 345 cases of invasion across the globe and reporting impacts on N cycle‐related metrics. We aim to explain heterogeneity of impacts by considering methodological aspects, properties of the invaded site and phylogenetic and functional characteristics of the invaders and the natives. Overall, plant invasions increased N pools and accelerated fluxes, even when excluding N‐fixing invaders. The impact on N pools depended mainly on functional differences and was greater when the invasive plants and the natives differed in N‐fixation ability, plant height and plant/leaf habit. Furthermore, the impact on N fluxes was related mainly to climate, being greater under warm and moist conditions. Our findings show that more functionally distant invaders occurring in mild climates are causing the strongest alterations to the N cycle.     

Influence of pruning management on P and N distribution and use efficiency by N2 fixing and non-N2 fixing trees used in alley cropping systems

Pruning of hedgerow trees is an important management practice for the successful establishment of an alley cropping system. Although pruning affects biomass production, only meager evidence of this management on distribution of nutrients among the different plant organs after tree regrowth is available. This study examined the effect of pruning on the distribution and use efficiency of N and P in a N₂ fixing leguminous tree species, Gliricidia sepium, and two non-N₂ fixing leguminous tree species, Senna siamea and S. spectabilis, grown in a field on an Alfisol (low in P) at Fashola (Guinea Savanna Zone), Southwestern Nigeria. Four P rates, 0, 20, 40 and 80 kg P ha^–1 as single superphosphate were used and management treatments included pruned versus unpruned plants. The ¹⁵N isotope dilution technique was used to measure N₂ fixation in G. sepium. Partitioning of total P among different plant organs was influenced by plant species and pruning management, but was not affected by P application rates. The distribution of total P in the various plant organs followed that of dry matter yield while N partitioning had a different pattern. Pruned plants distributed about 118% more total P to branches and had a higher physiological P use efficiency (PPUE) than unpruned plants. Leaves were the biggest sink for total N and N allocation in the other plant organs was influenced by plant species and pruning management, G. sepium had relatively more of its total N and P partitioned into roots (about double that of the non-N₂ fixing trees) but had a lower PPUE. Unpruned and pruned G. sepium derived 35 and 54% respectively of their total N from atmospheric N2, with about 54% of the fixed N2 being allocated to leaves and roots. Results showed that N and P pools turned over in the branches during plant regrowth after pruning but the causative factors associated with this phenomenon were not clear.     

Sanctions and mutualism stability: why do rhizobia fix nitrogen?

Why do rhizobia expend resources on fixing N(2) for the benefit of their host plant, when they could use those resources for their own reproduction? We present a series of theoretical models which counter the hypotheses that N(2) fixation is favoured because it (i) increases the exudation of useful resources to related rhizobia in the nearby soil, or (ii) increases plant growth and therefore the resources available for rhizobia growth. Instead, we suggest that appreciable levels of N(2) fixation are only favoured when plants preferentially supply more resources to (or are less likely to senesce) nodules that are fixing more N(2) (termed plant sanctions). The implications for different agricultural practices and mutualism stability in general are discussed.     

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.     

Ecosystem Level Impacts of Invasive Acacia saligna in the South African Fynbos

Recent efforts to clear invasive plants from the fynbos of South Africa forces managers to think about how N₂‐fixing invasives have altered ecosystem processes and the implications of these changes for community development. This study investigated the changes in nitrogen (N) cycling regimes in fynbos with the invasion of Acacia saligna, the effects of clear‐cutting acacia stands on soil microclimate and N cycling, and how altered N resources affected the growth of a weedy grass species. Litterfall, litter quality, soil nutrient pools, and ion exchange resin (IER)‐available soil N were measured in uninvaded fynbos, intact acacia, and cleared acacia stands. In addition, a bioassay experiment was used to ascertain whether the changes in soil nutrient availability associated with acacia would enhance the success of a weedy grass species. Acacia plots had greater amounts of litterfall, which had higher concentrations of N. This led to larger quantities of organic matter, total N, and IER‐available N in the soil. Clearing acacia stands caused changes in soil moisture and temperature, but did not result in differences in IER‐available N. The alteration of N availability by acacias was shown to increase growth rates of the weedy grass Ehrharta calycina, suggesting that secondary invasions by nitrophilous weedy species may occur after clearing N₂‐fixing alien species in the fynbos. It is suggested that managers use controlled burns, the addition of mulch, and the addition of fynbos seed after clearing to lower the levels of available N in the soil and initiate the return of native vegetation.     

Carbon cost of plant nitrogen acquisition: global carbon cycle impact from an improved plant nitrogen cycle in the Community Land Model

Plants typically expend a significant portion of their available carbon (C) on nutrient acquisition – C that could otherwise support growth. However, given that most global terrestrial biosphere models (TBMs) do not include the C cost of nutrient acquisition, these models fail to represent current and future constraints to the land C sink. Here, we integrated a plant productivity‐optimized nutrient acquisition model – the Fixation and Uptake of Nitrogen Model – into one of the most widely used TBMs, the Community Land Model. Global plant nitrogen (N) uptake is dynamically simulated in the coupled model based on the C costs of N acquisition from mycorrhizal roots, nonmycorrhizal roots, N‐fixing microbes, and retranslocation (from senescing leaves). We find that at the global scale, plants spend 2.4 Pg C yr^?1 to acquire 1.0 Pg N yr^?1, and that the C cost of N acquisition leads to a downregulation of global net primary production (NPP) by 13%. Mycorrhizal uptake represented the dominant pathway by which N is acquired, accounting for ~66% of the N uptake by plants. Notably, roots associating with arbuscular mycorrhizal (AM) fungi – generally considered for their role in phosphorus (P) acquisition – are estimated to be the primary source of global plant N uptake owing to the dominance of AM‐associated plants in mid‐ and low‐latitude biomes. Overall, our coupled model improves the representations of NPP downregulation globally and generates spatially explicit patterns of belowground C allocation, soil N uptake, and N retranslocation at the global scale. Such model improvements are critical for predicting how plant responses to altered N availability (owing to N deposition, rising atmospheric CO₂, and warming temperatures) may impact the land C sink.     

Mammal Browsers and Rainfall Affect Acacia Leaf Nutrient Content,Defense, and Growth in South African Savannas

Five sets of herbivore exclosures situated in mesic and semi‐arid savannas in Hluhluwe‐iMfolozi Park, South Africa were used to investigate the effects of mammal browsers and savanna type on plant traits relating to leaf nutrient content, defense, and growth in seven Acacia species. Mostly, browsing did not significantly affect leaf nutrient content but for a few species (i.e., increasing foliar N and P, decreasing C/N, and total polyphenols). Browser effects on structural defenses tended to be more pronounced than for leaf nutrient content and chemical defenses, particularly for semi‐arid species, resulting in longer, thicker, and denser spines, and a lower bite size index on browsed plants for most semi‐arid species. Browsing had no significant effect on growth rates for all species. Secondly, we investigated the effect of savanna type (mesic vs. semi‐arid) on the same set of plant traits and growth rates. A trade‐off in defense strategy was evident where mesic species had lower quality leaves and invested more heavily in growth and chemical defenses, while semi‐arid species generally had higher nutrient content leaves and invested more in structural defenses and higher levels of ramification. These findings suggest that the previously documented trade‐off in plant growth, resprouting ability and architecture between herbivore versus fire‐adapted savanna woody species can possibly be extended to include browse quality and defense type.     

Facilitation‐ vs. competition‐driven succession: the key role of resource‐ratio

Symbiotic nitrogen (N)‐fixing plants are abundant during primary succession, as typical bedrocks lack available N. In turn, fixed N accumulates in soils through biomass turnover and recycling, favouring more nitrophilous organisms. Yet, it is unclear how this facilitation mechanism interacts with competition for other limiting nutrients such as phosphorus (P) and how this affects succession. Here, we introduce a resource‐explicit, community assembly model of N‐fixing species and analyze successional trajectories along resource availability gradients using contemporary niche theory. We show that facilitation‐driven succession occurs under low N and high enough P availabilities, and is characterised by autogenic ecosystem development and relatively ordered trajectories. We show that late facilitation‐driven succession is sensitive to catastrophic shifts, highlighting the need to invoke other mechanisms to explain ecosystem stability near the climax. Put together with competition‐driven succession, these results lead to an enriched version of Tilman's resource‐ratio theory of succession.     

Effects of salt stress and rhizobial inoculation on growth and nitrogen fixation of three peanut cultivars

Increasing soil salinity represents a major constraint for agriculture in arid and semi‐arid lands, where mineral nitrogen (N) deficiency is also a frequent characteristic of soils. Biological N fixation by legumes may constitute a sustainable alternative to chemical fertilisation in salinity‐affected areas, provided that adapted cultivars and inoculants are available. Here, the performance of three peanut cultivars nodulated with two different rhizobial strains that differ in their salt tolerance was evaluated under moderately saline water irrigation and compared with that of N‐fertilised plants. Shoot weight was used as an indicator of yield. Under non‐saline conditions, higher yields were obtained using N fertilisation rather than inoculation for all the varieties tested. However, under salt stress, the yield of inoculated plants became comparable to that of N‐fertilised plants, with minor differences depending on the peanut cultivar and rhizobial strain. Our results indicate that N fixation might represent an economical, competitive and environmentally friendly choice with respect to mineral N fertilisation for peanut cultivation under moderate saline conditions.     

A consortium of non‐rhizobial endophytic microbes from Typha angustifolia functions as probiotic in rice and improves nitrogen metabolism

• Endophytic microbes isolated from plants growing in nutrient‐deficient environments often possess properties that improve nutrition of agriculturally important plants. A consortium of non‐rhizobial endophytic microbes isolated from a macrophyte Typha angustifolia growing in the marginal wetlands associated with a Uranium mine was characterized for their beneficial effect on rice and the mechanisms of growth promotion were investigated.
• The microbes were identified and characterized for their potential plant growth promoting (PGP) properties. Effect of these microbes on nitrogen (N)‐metabolism of rice was tested as Typha endophytes were predominantly (N)‐fixing. Relative N‐use efficiency and expression of genes involved in N‐uptake and assimilation were investigated in treated plants.
• Evidence of horizontal gene transfer (HGT) of dinitrogen reductase gene was observed within the consortium from a Pseudomonas stutzeri strain. The consortium behaved as plant probiotic and showed substantial growth benefits to Typha, their natural host as well as to rice. Typha endophytes colonized rice endosphere significantly increasing biomass, shoot length and chlorophyll content in rice plants both under N‐sufficient and N‐deficient conditions. N‐uptake and assimilation genes were upregulated in plants treated with the endophytes even after three weeks post infection.
• Our results suggested, HGT of nitrogen‐fixation trait to be highly prevalent among endophytes isolated from nutrient‐poor habitats of the uranium mine. A long‐term nitrogen deficiency response in the treated plants was elicited by the consortium improving N‐uptake, assimilation and relative N‐use efficiency of rice plants. This appeared to be at least one of the main strategies of plant growth promotion.

     

Nitrogen‐fixing tree abundance in higher‐latitude North America is not constrained by diversity

The rarity of nitrogen (N)‐fixing trees in frequently N‐limited higher‐latitude (here, > 35°) forests is a central biogeochemical paradox. One hypothesis for their rarity is that evolutionary constraints limit N‐fixing tree diversity, preventing N‐fixing species from filling available niches in higher‐latitude forests. Here, we test this hypothesis using data from the USA and Mexico. N‐fixing trees comprise only a slightly smaller fraction of taxa at higher vs. lower latitudes (8% vs. 11% of genera), despite 11‐fold lower abundance (1.2% vs. 12.7% of basal area). Furthermore, N‐fixing trees are abundant but belong to few species on tropical islands, suggesting that low absolute diversity does not limit their abundance. Rhizobial taxa dominate N‐fixing tree richness at lower latitudes, whereas actinorhizal species do at higher latitudes. Our results suggest that low diversity does not explain N‐fixing trees' rarity in higher‐latitude forests. Therefore, N limitation in higher‐latitude forests likely results from ecological constraints on N fixation.     

Nitrogen fixation does not axiomatically lead to phosphorus limitation in aquatic ecosystems

A long‐standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N₂‐fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32‐year field data set on eutrophic, floating‐plant dominated ecosystems to show that the growth of non‐N₂‐fixing species can become N limited under high P and low N inputs, even in the presence of N₂ fixing species. N₂‐fixers typically require higher P concentrations than non‐N₂‐fixers to persist. Hence, the N₂ fixers cannot deplete the P concentration enough for the non‐N₂‐fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over‐enriched aquatic ecosystems.     

Misconceptions and practical problems in the use of 15N soil enrichment techniques for estimating N2 fixation

The ¹⁵N methods are potentially accurate for measuring N₂ fixation in plants. The only problem with those methods is, how to ensure that the ¹⁵N/¹⁴N ratio in the plant accurately reflects the integrated ¹⁵N/¹⁴N ratio (R) in soil which is variable in time and with soil depth. However, the consequences of using an inappropriate reference plant vary with the level of N₂ fixation and the conditions under which the study was made. For example, the errors introduced into the values of N₂ fixation are higher at low levels of fixation, and decrease with increasing rates of fixation. At very high N₂ fixation rates, the errors are often insignificant. Also, the magnitude of error is proportional to the rate of decline of the ¹⁵N/¹⁴N ratio with time. Since N₂ fixation in most plants would be expected to below 60%, the question of how to select a good reference plant is still pertinent. In this paper, we have discussed some of the criteria to adopt in selecting reference plants, e.g. how to ensure that the reference plant is not fixing N₂, is absorbing most of its N from the same zone as the fixing plant, and in the same pattern with time, etc. In addition, we have discussed ¹⁵N labelling materials and methods that are likely to minimize any errors even when the fixing and reference plants don't match well in certain important criteria. The use of slow release ¹⁵N fertilizer or ¹⁵N labelled plant materials results in slow changes in the ¹⁵N/¹⁴N ratio of soil, and is strongly recommended. Where ¹⁵N inorganic fertilizers are used, the application of the fertilizer in small splits at various intervals is recommended over a one-time application. The problem with the reference crop, which has sometimes discouraged potential users of the ¹⁵N methods, is surmountable, as discussed in this paper.     

Variation in modern pollen from tropical evergreen forests and the monsoon seasonality gradient in SW India

Abstract. This study analyses the pollen signature of tropical lowland forests (< 1000 m a.s.l.) in the Asian monsoon climate. Its aim is to investigate how well the pollen data can reproduce the vegetation patterns in tropical India, and how the variations in the pollen composition are related to the gradient of decreasing plant moisture availability (measured by the ratio of actual over equilibrium evapotranspiration) that is associated with the strong seasonality of precipitation that characterizes the monsoon climate regime. We used canonical correspondence analysis (CCA) to relate the variations in the pollen composition of 71 surface soil samples from evergreen and semi‐evergreen forests distributed along the western coast of south India (8° 48’ N‐15° 08’ N), with the climate characteristics of the sampling sites. We show that variations in plant moisture availability strongly determine variations in the pollen composition; for example evergreen and semi‐evergreen forests can be distinguished on the basis of their pollen assemblages. Variations in the mean temperature of the coldest month associated with elevation also determine distinct pollen assemblages; for example evergreen forests above 800 m a.s.l. present different pollen signatures than those below this altitude/temperature limit. Variations in the relative abundance of some pollen taxa are strongly related to plant moisture availability and taxa indicators of climate can be identified. Hence, modern pollen assemblages from tropical forests in south India carry considerable information about vegetation patterns and climate. Paleoclimatic changes, notably in the monsoon season, could be quantified.     

MALDI mass spectrometry‐assisted molecular imaging of metabolites during nitrogen fixation in the Medicago truncatula–Sinorhizobium meliloti symbiosis

Symbiotic associations between leguminous plants and nitrogen‐fixing rhizobia culminate in the formation of specialized organs called root nodules, in which the rhizobia fix atmospheric nitrogen and transfer it to the plant. Efficient biological nitrogen fixation depends on metabolites produced by and exchanged between both partners. The Medicago truncatula–Sinorhizobium meliloti association is an excellent model for dissecting this nitrogen‐fixing symbiosis because of the availability of genetic information for both symbiotic partners. Here, we employed a powerful imaging technique – matrix‐assisted laser desorption/ionization (MALDI)/mass spectrometric imaging (MSI) – to study metabolite distribution in roots and root nodules of M. truncatula during nitrogen fixation. The combination of an efficient, novel MALDI matrix [1,8–bis(dimethyl‐amino) naphthalene, DMAN] with a conventional matrix 2,5–dihydroxybenzoic acid (DHB) allowed detection of a large array of organic acids, amino acids, sugars, lipids, flavonoids and their conjugates with improved coverage. Ion density maps of representative metabolites are presented and correlated with the nitrogen fixation process. We demonstrate differences in metabolite distribution between roots and nodules, and also between fixing and non‐fixing nodules produced by plant and bacterial mutants. Our study highlights the benefits of using MSI for detecting differences in metabolite distributions in plant biology.     

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.