Successional dynamics of nitrogen fixation and forest growth in regenerating Costa Rican rainforests

Light drives nitrogen fixation in tropical montane cloud forests in Costa Rica

Tropical montane cloud forests are high altitude ecosystems characterized by very high ambient humidity, which favors organisms that depend on the environment for their water status, such as bryophytes and their nitrogen-fixing symbionts. Bryophyte-associated N2 fixation is a major source of new N in several northern environments, but their contributions to the N cycle in other ecosystems is still poorly understood. In this work, we evaluated N2 fixation rates associated with epiphytic bryophytes growing along the stems of pumpwood trees (Cecropia sp.) as well as in surrounding litter and soil from a primary and a secondary cloud forests in the Talamanca Mountain Range, Costa Rica. Nitrogen fixation was significantly higher in substrates from the secondary forest compared to those from the primary forest. Overall, N2 fixation rates associated with epiphytic bryophytes were 57 times those of litter and 270 times what was measured in soil. Further, light intensity was the major factor influencing N2 fixation rates in all substrates. Increased access to light in disturbed cloud forests may therefore favor bryophyte-associated N2 fixation, potentially contributing to the recovery of these ecosystems.

Symbiont plasticity as a driver of plant success

We discuss which plant species are likely to become winners, that is achieve the highest global abundance, in changing landscapes, and whether plant-associated microbes play a determining role. Reduction and fragmentation of natural habitats in historic landscapes have led to the emergence of patchy, hybrid landscapes, and novel landscapes where anthropogenic ecosystems prevail. In patchy landscapes, species with broad niches are favoured. Plasticity in the degree of association with symbiotic microbes may contribute to broader plant niches and optimization of symbiosis costs and benefits, by downregulating symbiosis when it is unnecessary and upregulating it when it is beneficial. Plasticity can also be expressed as the switch from one type of mutualism to another, for example from nutritive to defensive mutualism with increasing soil fertility and the associated increase in parasite load. Upon dispersal, wide mutualistic partner receptivity is another facet of symbiont plasticity that becomes beneficial, because plants are not limited by the availability of specialist partners when arriving at new locations. Thus, under conditions of global change, symbiont plasticity allows plants to optimize the activity of mutualistic relationships, potentially allowing them to become winners by maximizing geographic occupancy and local abundance.

Is there a latitudinal diversity gradient for symbiotic microbes? A case study with sensitive partridge peas

Mutualism is thought to be more prevalent in the tropics than temperate zones and may therefore play an important role in generating and maintaining high species richness found at lower latitudes. However, results on the impact of mutualism on latitudinal diversity gradients are mixed, and few empirical studies sample both temperate and tropical regions. We investigated whether a latitudinal diversity gradient exists in the symbiotic microbial community associated with the legume Chamaecrista nictitans. We sampled bacteria DNA from nodules and the surrounding soil of plant roots across a latitudinal gradient (38.64-8.68 °N). Using 16S rRNA sequence data, we identified many non-rhizobial species within C. nictitans nodules that cannot form nodules or fix nitrogen. Species richness increased towards lower latitudes in the non-rhizobial portion of the nodule community but not in the rhizobial community. The microbe community in the soil did not effectively predict the non-rhizobia community inside nodules, indicating that host selection is important for structuring non-rhizobia communities in nodules. We next factorially manipulated the presence of three non-rhizobia strains in greenhouse experiments and found that co-inoculations of non-rhizobia strains with rhizobia had a marginal effect on nodule number and no effect on plant growth. Our results suggest that these non-rhizobia bacteria are likely commensals-species that benefit from associating with a host but are neutral for host fitness. Overall, our study suggests that temperate C. nictitans plants are more selective in their associations with the non-rhizobia community, potentially due to differences in soil nitrogen across latitude.

Comparison of the seasonal and successional variation of asymbiotic and symbiotic nitrogen fixation along a glacial retreat chronosequence

Climate change is resulting in accelerated retreat of glaciers worldwide and much nitrogen-poor debris is left after glacier retreats. Asymbiotic dinitrogen (N2) fixation (ANF) can be considered a 'hidden' source of nitrogen (N) for non-nodulating plants in N limited environments; however, seasonal variation and its relative importance in ecosystem N budgets, especially when compared with nodulating symbiotic N2-fixation (SNF), is not well-understood. In this study, seasonal and successional variations in nodulating SNF and non-nodulating ANF rates (nitrogenase activity) were compared along a glacial retreat chronosequence on the eastern edge of the Tibetan Plateau. Key factors regulating the N2-fixation rates as well as the contribution of ANF and SNF to ecosystem N budget were also examined. Significantly greater nitrogenase activity was observed in nodulating species (0.4-17,820.8 nmol C2H4 g-1 d-1) compared to non-nodulating species (0.0-9.9 nmol C2H4 g-1 d-1) and both peaked in June or July. Seasonal variation in acetylene reduction activity (ARA) rate in plant nodules (nodulating species) and roots (non-nodulating species) was correlated with soil temperature and moisture while ARA in non-nodulating leaves and twigs was correlated with air temperature and humidity. Stand age was not found to be a significant determinant of ARA rates in nodulating or non-nodulating plants. ANF and SNF contributed 0.3-51.5 % and 10.1-77.8 %, respectively, of total ecosystem N input in the successional chronosequence. In this instance, ANF exhibited an increasing trend with successional age while SNF increased only at stages younger than 29 yr and then decreased as succession proceeded. These findings help improve our understanding of ANF activity in non-nodulating plants and N budgets in post glacial primary succession.

The positive effects of mineral-solubilizing microbial inoculants on asymbiotic nitrogen fixation of abandoned mine soils are driven by keystone phylotype

Toward the restoration of the increasing numbers of abandoned mines across China, external-soil spray seeding technologies have become more extensively utilized. However, considerable challenges remain that seriously hamper the effectiveness of these technologies, such as inadequate nutrient availability for plants. Previous studies have shown that mineral-solubilizing microbial inoculants can increase the nodules of legumes. However, their effects on symbiotic nitrogen fixation (SNF), asymbiotic nitrogen fixation (ANF), and diazotrophic communities remain unknown. Further, research into the application of functional microorganisms for the restoration of abandoned mines has been conducted either in greenhouses, or their application in the field has been too brief. Thus, we established a four-year field experiment in an abandoned mine and quantified the SNF, ANF, and diazotrophic communities. To the best of our knowledge, this study is the first to describe the long-term application of specific functional microorganisms for the remediation of abandoned mine sites in the field. We revealed that mineral-solubilizing microbial inoculants significantly increased the soil ANF rate and SNF content. There was no significant correlation between the diazotrophic alpha diversity and soil ANF rate; however, there were strong positive associations between the relative abundance and biodiversity of keystone phylotype (module #5) within ecological clusters and the ANF rate. Molecular ecological networks indicated that microbial inoculants increased network complexity and stability. Moreover, the inoculants significantly enhanced the deterministic ratio of diazotrophic communities. Furthermore, homogeneous selection predominantly mediated the assembly of soil diazotrophic communities. It was concluded that mineral-solubilizing microorganisms played a critical role in maintaining and enhancing nitrogen, which offers a new solution with great potential for the restoration of ecosystems at abandoned mine sites.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China

Background

Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO₃ ^- loss potential following secondary succession are not well understood in karst ecosystems.

Methods

In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and ¹⁵N natural abundance (δ ¹⁵N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ ¹⁵N compositions of soil organic nitrogen (SON) in aggregates and soil to plant ¹⁵N enrichment factor (EF = δ ¹⁵N_leaf -δ ¹⁵N_soil), combined with soil inorganic N contents and δ ¹⁵N compositions were used to indicate the alterations of soil N availability and NO₃ ^-loss potential following secondary succession.

Results

Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ ¹⁵N values of SON also significantly decreased, mainly affected by plant δ ¹⁵N composition and N mineralization. SON content in macro-aggregates and soil NH₄ ⁺ content significantly increased while δ ¹⁵N values of NH₄ ⁺ decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ ¹⁵N values, the EF values, and the (NO₃ ^--N)/(NH₄ ⁺-N) ratio gradually decreased, indicating reduced NO₃ ^- loss following secondary succession.

Conclusions

Soil N availability improves and NO₃ ^- leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

Stable nitrogen and carbon isotope compositions in plant-soil systems under different land-use types in a red soil region, Southeast China

Background

Stable N isotope compositions in plant-soil systems have been widely used to indicate soil N transformation and translocation processes in ecosystems. However, soil N processes and nitrate ( NO 3 - ) loss potential under different land-use types are short of systematic comparison in the red soil region of Southeast China.

Methods

In the present study, the stable N and C isotope compositions (δ ¹⁵N and δ ¹³C) of soil and leaf were analyzed to indicate soil N transformation processes, and the soil to plant ¹⁵N enrichment factor (EF) was used to compare soil NO 3 - loss potential under different land-use types, including an abandoned agricultural land, a natural pure forest without understory, and a natural pure forest with a simple understory.

Results

The foliar δ ¹⁵N value (-0.8‰) in the abandoned agricultural land was greater than those of the forest lands (ranged from -2.2‰ to -10.8‰). In the abandoned agricultural land, δ ¹⁵N values of soil organic nitrogen (SON) increased from 0.8‰ to 5.7‰ and δ ¹³C values of soil organic carbon (SOC) decreased from -22.7‰ to -25.9‰ with increasing soil depth from 0-70 cm, mainly resulting from SON mineralization, soil organic matter (SOM) decomposition, and C₄ plant input. In the soils below 70 cm depth, δ ¹⁵N values of SON (mean 4.9‰) were likely affected by microbial assimilation of ¹⁵N-depleted NO 3 - . The variations in δ ¹⁵N values of soil profiles under the two forests were similar, but the EF values were significant different between the pure forest with a simple understory (-10.0‰) and the forest without understory (-5.5‰).

Conclusions

These results suggest that soil to plant ¹⁵N enrichment factor have a great promise to compare soil NO 3 - loss potential among different ecosystems.

Temperature sensitivity of woody nitrogen fixation across species and growing temperatures

The future of the land carbon sink depends on the availability of nitrogen (N)^1,2 and, specifically, on symbiotic N fixation^3-8, which can rapidly alleviate N limitation. The temperature response of symbiotic N fixation has been hypothesized to explain the global distribution of N-fixing trees^9,10 and is a key part of some terrestrial biosphere models (TBMs)^3,7,8, yet there are few data to constrain the temperature response of symbiotic N fixation. Here we show that optimal temperatures for N fixation in four tree symbioses are in the range 29.0-36.9 °C, well above the 25.2 °C optimum currently used by TBMs. The shape of the response to temperature is also markedly different to the function used by TBMs (asymmetric rather than symmetric). We also show that N fixation acclimates to growing temperature (hence its range of optimal temperatures), particularly in our two tropical symbioses. Surprisingly, optimal temperatures were 5.2 °C higher for N fixation than for photosynthesis, suggesting that plant carbon and N gain are decoupled with respect to temperature. These findings may help explain why N-fixing tree abundance is highest where annual maximum temperatures are >35 °C (ref. ¹⁰) and why N-fixing symbioses evolved during a warm period in the Earth's history^11,12. Everything else being equal, our findings indicate that climate warming will probably increase N fixation, even in tropical ecosystems, in direct contrast to past projections⁸.

Light, nitrogen supply, and neighboring plants dictate costs and benefits of nitrogen fixation for seedlings of a tropical nitrogen-fixing tree

The ability to fix nitrogen may confer a competitive advantage or disadvantage to symbiotic nitrogen-fixing plants depending on the availability of soil nitrogen and energy to fuel fixation. Understanding these costs and benefits of nitrogen fixation is critical to predicting ecosystem dynamics and nutrient cycling. We grew inoculated (with symbiotic bacteria) and uninoculated seedlings of Pentaclethra macroloba (a nitrogen-fixing tree species) both in isolation and with Virola koschnyi (a nonfixing species) under gradients of light and soil nitrogen to assess how the ability to fix nitrogen and fixation activity affect growth, biomass allocation, and responses to neighboring plants. Inoculation itself did not provide a growth advantage to nitrogen fixers, regardless of nitrogen limitation status. Higher nitrogen fixation rates increased biomass growth similarly for nitrogen-limited and nitrogen-saturated fixers. Nodule production was offset by reduced fine-root biomass for inoculated nitrogen fixers, resulting in no change in total belowground allocation associated with nitrogen fixation. Under nitrogen-limited conditions, inoculated nitrogen fixers partially downregulated fixation in the presence of a nonfixing neighbor. These results suggest that nitrogen fixation can provide a growth advantage, even under nitrogen-saturated conditions, and that nitrogen fixers may reduce fixation rates to minimize facilitation of neighbors.

Light fuels while nitrogen suppresses symbiotic nitrogen fixation hotspots in neotropical canopy gap seedlings

Mature neotropical lowland forests have relatively lower symbiotic nitrogen fixation (SNF) rates compared with secondary forests. Canopy gap formation may create transient SNF hotspots in mature forests that increase overall SNF rates in these ecosystems, as canopy gaps are pervasive across the landscape and increasing in frequency. However, what environmental conditions are driving SNF upregulation in canopy gaps is unknown. In a field experiment to test these potential environmental controls on SNF, we grew 540 neotropical nitrogen-fixing legume seedlings (Pentaclethra macroloba, Zygia longifolia, and Stryphnodendron microstachyum) under manipulated light and soil nitrogen availability in canopy gaps and intact forests at La Selva Biological Station, Costa Rica. Seedling biomass, nodule biomass, and SNF (g N seedling^-1  h^-1 ) were 4-, 17- and 42-fold higher, respectively, in canopy gaps than in the intact forest. Nitrogen additions decreased SNF, but light had a stronger positive effect. Upregulation of SNF in canopy gaps was driven by increased plant growth and not a disproportionate increased SNF allocation. These data provide evidence that canopy gap SNF hotspots are driven, in part, by light availability, demonstrating a potential driver of SNF spatial heterogeneity. This further suggests that canopy gap dynamics are important for understanding the biogeochemistry of neotropical forests.

Diversity and Variation of Asymbiotic Nitrogen-Fixing Microorganisms in Alpine Grasslands on the Tibetan Plateau

Asymbiotic nitrogen-fixing (ANF) bacteria contribute a substantial amount of nitrogen in ecosystems, especially in those with low symbiotic nitrogen fixation (SNF) capability. Degradation of alpine grassland is widespread on the Tibetan Plateau and sown grassland has become one of the main strategies for grassland restoration. However, the diversity and community structure of ANF bacteria in different grassland types remain unknown. The aim of this study was to fill this gap. Soil samples were obtained from 39 grassland plots selected from three counties in the eastern Tibetan Plateau. The plots were classified as natural grassland (NG), sown grassland (SG), lightly degraded grassland (LDG), and severely degraded grassland (SDG). ANF microbial communities of the four grassland types were compared at the level of community and species diversity by 16S rRNA high-throughput sequencing technology. The phylum Proteobacteria accounted for >72% of the ANF bacteria. The community structures of soil ANF bacteria differed significantly (p < 0.01) among grassland types. We concluded that: (1) planting gramineous forage could possibly mitigate the decrease in diversity of soil ANF bacteria caused by grassland degradation; and (2) the diversity of soil ANF bacteria in alpine grassland of the Tibetan Plateau is closely related to grassland degradation and restoration.

Asymbiotic nitrogen fixation in the phyllosphere of the Amazon forest: Changing nitrogen cycle paradigms

Biological nitrogen fixation is a key process for the maintenance of natural ecosystems productivity. In tropical forests, the contribution of asymbiotic nitrogen fixation (ANF) to the nitrogen (N) input has been underestimated, even though few studies have shown that ANF may be as important as symbiotic nitrogen fixation in such environments. The inputs and abiotic modulators of ANF in the Amazon forest are not completely understood. Here, we determined ANF rates and estimated the N inputs from ANF in the phyllosphere, litter and rhizospheric soil of nine tree species in the Amazon forest over time, including an extreme drought period induced by the El Niño-Southern Oscillation. Our data showed that ANF rates in the phyllosphere were 2.8- and 17.6-fold higher than in the litter and rhizospheric soil, respectively, and was highly dependent on tree taxon. Sampling time was the major factor modulating ANF in all forest compartments. At the driest period, ANF rates were approximately 1.8-fold and 13.1-fold higher than at periods with higher rainfall, before and after the extreme drought period, respectively. Tree species was a key modulator of ANF in the phyllosphere, as well as N and Vanadium concentrations. Carbon, molybdenum and vanadium concentrations were significant modulators of ANF in the litter. Based on ANF rates at the three sampling times, we estimated that the N input in the Amazon forest through ANF in the phyllosphere, litter and rhizospheric soil, was between 0.459 and 0.714 kg N ha^-1 yr^-1. Our results highlight the importance of ANF in the phyllosphere for the N input in the Amazon forest, and suggest that changes in the patterns of ANF driven by large scale climatic events may impact total N inputs and likely alter forest productivity.

Comparable canopy and soil free-living nitrogen fixation rates in a lowland tropical forest

Biological nitrogen fixation (BNF) is a fundamental part of nitrogen cycling in tropical forests, yet little is known about the contribution made by free-living nitrogen fixers inhabiting the often-extensive forest canopy. We used the acetylene reduction assay, calibrated with ¹⁵N₂, to measure free-living BNF on forest canopy leaves, vascular epiphytes, bryophytes and canopy soil, as well as on the forest floor in leaf litter and soil. We used a combination of calculated and published component densities to upscale free-living BNF rates to the forest level. We found that bryophytes and leaves situated in the canopy in particular displayed high mass-based rates of free-living BNF. Additionally, we calculated that nearly 2 kg of nitrogen enters the forest ecosystem through free-living BNF every year, 40% of which was fixed by the various canopy components. Our results reveal that in the studied tropical lowland forest a large part of the nitrogen input through free-living BNF stems from the canopy, but also that the total nitrogen inputs by free-living BNF are lower than previously thought and comparable to the inputs of reactive nitrogen by atmospheric deposition.

Biological nitrogen fixation across major biomes in Latin America: Patterns and global change effects

Biological nitrogen fixation (BNF) supports terrestrial primary productivity and plays key roles in mediating human-induced changes in global nitrogen (N) and carbon cycling. However, there are still critical uncertainties in our understanding of the amount of BNF occurring across terrestrial ecosystems, and of how terrestrial BNF will respond to global change. We synthesized BNF data from Latin America, a region reported to sustain some of the highest BNF rates on Earth, but that is underrepresented in previous data syntheses. We used meta-analysis and modeling approaches to estimate BNF rates across Latin America's major biomes and to evaluate the potential effects of increased N deposition and land-use change on these rates. Unmanaged tropical and subtropical moist forests sustained observed and predicted total BNF rates of 10 ± 1 and 14 ± 1 kg N ha^-1 y^-1, respectively, supporting the hypothesis that these forests sustain lower BNF rates than previously thought. Free-living BNF accounted for two-thirds of the total BNF in these forests. Despite an average 30% reduction of free-living BNF in response to experimental N-addition, our results suggest free-living BNF rate responses to current and projected N deposition across tropical and subtropical moist forests are small. In contrast, the conversion of unmanaged ecosystems to crop and pasture lands increased BNF rates across all terrestrial biomes, mostly in savannas, grasslands, and dry forests, increasing BNF rates 2-fold. The information obtained here provides a more comprehensive understanding of BNF patterns for Latin America.

Molybdenum, phosphorus, and pH do not constrain nitrogen fixation in a tropical forest in the southeastern Amazon

High rates of biological nitrogen fixation (BNF) are commonly reported for tropical forests, but most studies have been conducted in regions that receive substantial inputs of molybdenum (Mo) from atmospheric dust and sea-salt aerosols. Even in these regions, the low availability of Mo can constrain free-living BNF catalyzed by heterotrophic bacteria and archaea. We hypothesized that in regions where atmospheric inputs of Mo are low and soils are highly weathered, such as the southeastern Amazon, Mo would constrain BNF. We also hypothesized that the high soil acidity, characteristic of the Amazon Basin, would further constrain Mo availability and therefore soil BNF. We conducted two field experiments across the wet and dry seasons, adding Mo, phosphorus (P), and lime alone and in combination to the forest floor in the southeastern Amazon. We sampled soils and litter immediately, and then weeks and months after the applications, and measured Mo and P availability through resin extractions and BNF with the acetylene reduction assay. The experimental additions of Mo and P increased their availability and the lime increased soil pH. While the combination of Mo and P increased BNF at some time points, BNF rates did not increase strongly or consistently across the study as a whole, suggesting that Mo, P, and soil pH are not the dominant controls over BNF. In a separate short-term laboratory experiment, BNF did not respond strongly to Mo and P even when labile carbon was added. We postulate that high nitrogen (N) availability in this area of the Amazon, as indicated by the stoichiometry of soils and vegetation and the high nitrate soil stocks, likely suppresses BNF at this site. These patterns may also extend across highly weathered soils with high N availability in other topographically stable regions of the tropics.

Soil nitrogen concentration mediates the relationship between leguminous trees and neighbor diversity in tropical forests

Legumes provide an essential service to ecosystems by capturing nitrogen from the atmosphere and delivering it to the soil, where it may then be available to other plants. However, this facilitation by legumes has not been widely studied in global tropical forests. Demographic data from 11 large forest plots (16–60 ha) ranging from 5.25° S to 29.25° N latitude show that within forests, leguminous trees have a larger effect on neighbor diversity than non-legumes. Where soil nitrogen is high, most legume species have higher neighbor diversity than non-legumes. Where soil nitrogen is low, most legumes have lower neighbor diversity than non-legumes. No facilitation effect on neighbor basal area was observed in either high or low soil N conditions. The legume–soil nitrogen positive feedback that promotes tree diversity has both theoretical implications for understanding species coexistence in diverse forests, and practical implications for the utilization of legumes in forest restoration.

Xu et al. examine the effect of leguminous trees on neighbor diversity across 11 plots in tropical forests around the world, and find that in high soil nitrogen conditions, most legume species have higher neighbor diversity than non-legumes, and vice versa where soil nitrogen is low. Their results have practical implications for the utilization of legumes in forest restoration.

More Than a Functional Group: Diversity within the Legume–Rhizobia Mutualism and Its Relationship with Ecosystem Function

Studies of biodiversity and ecosystem function (BEF) have long focused on the role of nitrogen (N)-fixing legumes as a functional group that occupies a distinct and important niche relative to other plants. Because of their relationship with N-fixing rhizobial bacteria, these legumes access a different pool of N than other plants and therefore directly contribute to increases in productivity and N-cycling. Despite their recognized importance in the BEF literature, the field has not moved far beyond investigating the presence/absence of the legume functional group in species mixtures. Here, we synthesize existing information on how the diversity (species richness and functional diversity) of both legumes and the rhizobia that they host impact ecosystem functions, such as nitrogen fixation and primary productivity. We also discuss the often-overlooked reciprocal direction of the BEF relationship, whereby ecosystem function can influence legume and rhizobial diversity. We focus on BEF mechanisms of selection, complementarity, facilitation, competitive interference, and dilution effects to explain how diversity in the legume–rhizobia mutualism can have either positive or negative effects on ecosystem function—mechanisms that can operate at scales from rhizobial communities affecting individual legume functions to legume communities affecting landscape-scale ecosystem functions. To fully understand the relationship between biodiversity and ecosystem function, we must incorporate the full diversity of this mutualism and its reciprocal relationship with ecosystem function into our evolving BEF framework.

Substrate stoichiometry determines nitrogen fixation throughout succession in southern Chinese forests

The traditional view holds that biological nitrogen (N) fixation often peaks in early- or mid-successional ecosystems and declines throughout succession based on the hypothesis that soil N richness and/or phosphorus (P) depletion become disadvantageous to N fixers. This view, however, fails to support the observation that N fixers can remain active in many old-growth forests despite the presence of N-rich and/or P-limiting soils. Here, we found unexpected increases in N fixation rates in the soil, forest floor, and moss throughout three successional forests and along six age-gradient forests in southern China. We further found that the variation in N fixation was controlled by substrate carbon(C) : N and C : (N : P) stoichiometry rather than by substrate N or P. Our findings highlight the utility of ecological stoichiometry in illuminating the mechanisms that couple forest succession and N cycling.

Nitrogen-fixing and non-fixing trees differ in leaf chemistry and defence but not herbivory in a lowland Costa Rican rain forest

Nitrogen-fixing plants provide critical nitrogen inputs that support the high productivity of tropical forests, but our understanding of the ecology of nitrogen fixers – and especially their interactions with herbivores – remains incomplete. Herbivores may interact differently with nitrogen fixers vs. non-fixers due to differences in leaf nitrogen content and herbivore defence strategies. To examine these potential differences, our study compared leaf carbon, nitrogen, toughness, chemical defence and herbivory for four nitrogen-fixing tree species (Inga oerstediana, Inga sapindoides, Inga thibaudiana and Pentaclethra macroloba) and three non-fixing species (Anaxagorea crassipetala, Casearia arborea and Dipteryx panamensis) in a lowland tropical rain forest. Leaf chemical defence, not nutritional content, was the primary driver of herbivore damage among our species. Even though nitrogen fixers exhibited 21.1% higher leaf nitrogen content, 20.1% lower C:N ratios and 15.4% lower leaf toughness than non-fixers, we found no differences in herbivory or chemical defence between these two plant groups. Our results do not support the common hypotheses that nitrogen fixers experience preferential herbivory or that they produce more nitrogen-rich defensive compounds than non-fixers. Rather, these findings suggest strong species-specific differences in plant–herbivore relationships among both nitrogen-fixing and non-fixing tropical trees.

Symbiotic N fixation is sufficient to support net aboveground biomass accumulation in a humid tropical forest

Symbiotic nitrogen (N) fixation has been shown to support carbon storage in young regenerating tropical forests, but N-fixing trees can also be strong competitors with non-fixing trees, making it unclear which mechanism drives long term patterns in biomass accretion. Many tropical forests have excess N, but factors such as rising atmospheric CO₂ or selective cutting practices might induce additional N demand. Here we combine decades of stem inventory data, in-situ measures of symbiotic N fixation, and simulations of N demand to evaluate demographic and biogeochemical controls on biomass dynamics in legume-rich lowland forests of Trinidad. We document sustained net biomass accumulation and high rates of N fixation in these forests, regardless of the timing of selective timber harvests, including an old growth stand. The biomass accumulation was explained by growth of non-fixing trees, not N-fixing trees, but the total amount of symbiotic N fixation was sufficient to account for most of net above ground N demands, suggesting that N-fixers could contribute to the long-term C sink in these forests via fertilizing non-fixers.