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Despite some well-documented draw-backs, the acetylene reduction assay (ARA) remains one of the most widespread methods for measuring biological nitrogen (N₂) fixation (BNF) in symbiotic and free-living niches due to its low cost, simplicity, and high throughput potential. Because ARA measures a proxy reaction (the reduction of acetylene to ethylene by the nitrogenase enzyme), a conversion ratio (‘R ratio’) is required to estimate equivalent fixation of N₂. Based on the biochemistry of the reactions, the theoretical ratio is usually taken to be 3:1. However, ¹⁵N₂ calibrations often generate ratios that deviate considerably from this value. We synthesized calibrated R ratios for terrestrial BNF studies, asking whether values converge on the theoretical ratio and vary across N-fixing niches. From 253 mean values (n = 2,072 samples), we find that some niches (legumes, soil, litter) do center on 3:1, while others fall significantly above (wood, lichen) or below (biocrusts). Moss in particular shows a bimodal distribution that may indicate contributions from alternative nitrogenases. However, almost all niches have very wide distributions (up to 2 orders of magnitude); applying ratio values spanning even the 25th-75th percentile cause BNF rates to vary by a factor of 1.5–2.5, and up to > 8. Despite this, only a minority of studies (~ 30% of 345) perform calibrations, and this proportion has not increased over time. We conclude that high variability precludes the use of theoretical values to obtain accurate BNF estimates via ARA, and that historical data should be considered with appropriate caution. Values should be calibrated directly when the goal is to generate accurate rates or cross-condition comparisons.

Though tropical forest ecosystems are among the largest natural sources of the potent greenhouse gas nitrous oxide (N2O), the spatial distribution of emissions across landscapes is often poorly resolved. Leaf cutter ants (LCA; Atta and Acromyrmex, Myrmicinae) are dominant herbivores throughout Central and SouthAmerica, and influence multiple aspects of forest structure and function. In particular, their foraging creates spatial heterogeneity by concentrating large quantities of organic matter (including nitrogen, N) from the surrounding canopy into their colonies, and ultimately into colony refuse dumps. Here, we demonstrate that refuse piles created by LCA species Atta colombica in tropical rainforests of Costa Rica provide ideal conditions for extremely high rates of N2O production (high microbial biomass, potential denitrification enzyme activity, N content and anoxia) and may represent an unappreciated source of heterogeneity in tropical forest N2O emissions. Average instantaneous refuse pile N2O fluxes surpassed background emissions by more than three orders of magnitude (in some cases exceeding 80 000 mg N2O-N m(-2) h(-1)) and generating fluxes comparable to or greater than those produced by engineered systems such as wastewater treatment tanks. Refuse-concentrating Atta species are ubiquitous in tropical forests, pastures and production ecosystems, and increase density strongly in response to disturbance. As such, LCA colonies may represent an unrecognized greenhouse gas point source throughout the Neotropics.

Information on denitrification (particularly N₂) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrification measurements for a northern hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N₂, N₂O and CO₂ losses and compared these with field measurements of N₂O, NOy, NH₃ and CO₂. Under field-realistic soil moisture and O₂ conditions (average 17.5–20 % O₂, minimum 15 %) incubated soils produced no measurable N₂ flux (detection limit 52.2 µg N m⁻² h⁻¹). Only in a subset of grove soils were fluxes of 70–75 µg N m⁻² h⁻¹ recorded after 102 h of incubation at 5–10 % O₂ following wetting of very dry soils. Making the assumption that potential N₂ production falls just below the detection limit (likely an overestimate given the conditions needed to generate measurable fluxes), N₂ flux rates would fall on the low end of that recorded for a tropical Australian savanna (45–110 µg N m⁻² h⁻¹) under comparable abiotic conditions. Assuming maximum possible production rates, N₂ could comprise <32–76 % of total soil N gas flux following soil wetting in summer. Lack of flux response to soil wetting in winter suggests that cold-season N₂ fluxes are negligible. N₂O fluxes for core incubations were significantly higher than for field chambers; thus it is likely that incubations may overestimate N₂O flux by reducing soil column consumption. Overall, results indicate that soil N₂ fluxes are less dominant in this savanna than in other ecosystems investigated.

Biological nitrogen fixation (BNF) is the main natural source of new nitrogen inputs in terrestrial ecosystems, supporting terrestrial productivity, carbon uptake, and other Earth system processes. We assembled a comprehensive global dataset of field measurements of BNF in all major N-fixing niches across natural terrestrial biomes derived from the analysis of 376 BNF studies. The dataset comprises 32 variables, including site location, biome type, N-fixing niche, sampling year, quantification method, BNF rate (kg N ha −1 y −1 ), the percentage of nitrogen derived from the atmosphere (%N dfa ), N fixer or N-fixing substrate abundance, BNF rate per unit of N fixer abundance, and species identity. Overall, the dataset combines 1,207 BNF rates for trees, shrubs, herbs, soil, leaf litter, woody litter, dead wood, mosses, lichens, and biocrusts, 152 herb %N dfa values, 1,005 measurements of N fixer or N-fixing substrate abundance, and 762 BNF rates per unit of N fixer abundance for a total of 424 species across 66 countries. This dataset facilitates synthesis, meta-analysis, upscaling, and model benchmarking of BNF fluxes at multiple spatial scales.

This article is a Commentary on Bourgeois et al. 223: 1784–1794.

Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600-1,300 mm annual rainfall) to semi-arid woodland (600-250 mm). Aridity resulted in increased 'openness' of N cycling, indicated by increasing δ¹⁵Nsoil and nitrate: ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ¹⁵Nroot xylem sap provides a more direct measure of plant N acquisition than δ¹⁵N foliage. We found highly variable offsets between δ¹⁵Nfoliage and δ¹⁵Nroot xylem sap, both between taxa at a single site (1.3-3.4 %o) and within taxa across sites (0.8-3.4 %o). As a result, δ¹⁵Nfoliage overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ¹⁵Nroot xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ¹⁵Nroot xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ¹⁵Nsoil and/or decreasing mycorrhizal colonisation with aridity. We propose that δ¹⁵Nroot xylem sap is a more direct indicator of N source than δ¹⁵Nfoliage, with calibration required before it could be applied to quantify BNF.

Plant element stoichiometry and stoichiometric flexibility strongly regulate ecosystem responses to global change. Here, we tested three potential mechanistic drivers (climate, soil nutrients, and plant taxonomy) of both using paired foliar and soil nutrient data from terrestrial forested National Ecological Observatory Network sites across the USA. We found that broad patterns of foliar nitrogen (N) and foliar phosphorus (P) are explained by different mechanisms. Plant taxonomy was an important control over all foliar nutrient stoichiometries and concentrations, especially foliar N, which was dominantly related to taxonomy and did not vary across climate or soil gradients. Despite a lack of site-level correlations between N and environment variables, foliar N exhibited intraspecific flexibility, with numerous species-specific correlations between foliar N and various environmental factors, demonstrating the variable spatial and temporal scales on which foliar chemistry and stoichiometric flexibility can manifest. In addition to plant taxonomy, foliar P and N:P ratios were also linked to soil nutrient status (extractable P) and climate, especially actual evapotranspiration rates. Our findings highlight the myriad factors that influence foliar chemistry and show that broad patterns cannot be explained by a single consistent mechanism. Furthermore, differing controls over foliar N versus P suggests that each may be sensitive to global change drivers on distinct spatial and temporal scales, potentially resulting in altered ecosystem N:P ratios that have implications for processes ranging from productivity to carbon sequestration.

Biological nitrogen fixation represents the largest natural flux of new nitrogen (N) into terrestrial ecosystems, providing a critical N source to support net primary productivity of both natural and agricultural systems. When they are common, symbiotic associations between plants and bacteria can add more than 100 kg N ha−1 y−1 to ecosystems. Yet, these associations are uncommon in many terrestrial ecosystems. In most cases, N inputs derive from more cryptic sources, including mutualistic and/or free-living microorganisms in soil, plant litter, decomposing roots and wood, lichens, insects, and mosses, among others. Unfortunately, large gaps remain in the understanding of cryptic N fixation. We conducted a literature review to explore rates, patterns, and controls of cryptic N fixation in both unmanaged and agricultural ecosystems. Our analysis indicates that, as is common with N fixation, rates are highly variable across most cryptic niches, with N inputs in any particular cryptic niche ranging from near zero to more than 20 kg ha−1 y−1. Such large variation underscores the need for more comprehensive measurements of N fixation by organisms not in symbiotic relationships with vascular plants in terrestrial ecosystems, as well as identifying the factors that govern cryptic N fixation rates. We highlight several challenges, opportunities, and priorities in this important research area, and we propose a conceptual model that posits an interacting hierarchy of biophysical and biogeochemical controls over N fixation that should generate valuable new hypotheses and research.

1. The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P) acquisition have important implications for plant growth, species distributions, and responses to CO₂ fertilization under global change, especially in P-poor tropical ecosystems. Currently, it is unclear whether investment in strategies that enhance plant acquisition (arbuscular mycorrhizal, AM; colonization or root phosphatase activity, RPA) are determined primarily by phylogeny, or whether these strategies differ among N₂-fixing legumes and nonfixing plants as a result of differing N availability. 2. We hypothesized that plant N status, which can vary widely independent of N fixation, correlates with investment in acquisition, because: (a) N and concentrations scale in plant tissue indicative of coupled demand and (b) plants with more N may have more resources available to allocate to acquisition strategies. 3. We grew seedlings of eight tropical tree species from three families (including three N₂-fixing and one nonfixing legume) under greenhouse conditions in native forest soil for four months. Species represented almost the full range of foliar N observed in tropical trees. 4. Neither foliar N nor P concentrations correlated with investment in P acquisition. Across all species, we found an inverse relationship between investment in AM colonization and RPA, but this trade-off was unrelated to foliar N or P and did not differ between functional types (i.e., N₂ fixers vs. nonfixers). 5. Within legumes (family Fabaceae), two strategies were evident that were unrelated to fixation status. High-fixing Inga and nonfixing Dialium displayed high foliar N and P concentrations and greater proportional investment in RPA versus AM, while lower fixing Ormosia species were characterized by lower foliar nutrient concentrations and proportionally more investment in AM. 6. Synthesis. Investment in P acquisition strategies in tropical trees is not dependen on foliar N or functional group, but instead may be controlled in part by resource trade-offs. High diversity in nutrient strategies between related species cautions again the use of simple functional groupings to draw conclusions about nutrient acquisition in tropical trees.

High rates of land conversion and land use change have vastly increased the proportion of secondary forest in the lowland tropics relative to mature forest. As secondary forests recover following abandonment, nitrogen (N) and phosphorus (P) must be present in sufficient quantities to sustain high rates of net primary production and to replenish the nutrients lost during land use prior to secondary forest establishment. Biogeochemical theory and results from individual studies suggest that N can recuperate during secondary forest recovery, especially relative to P. Here, we synthesized 23 metrics of N and P in soil and plants from 45 secondary forest chronosequences located in the wet tropics to empirically explore (1) whether there is a consistent change in nutrients and nutrient cycling processes during secondary succession in the biome; (2) which metrics of N and P in soil and plants recuperate most consistently; (3) if the recuperation of nutrients during succession approaches similar nutrient concentrations and fluxes as those in mature forest in ∼100 yr following the initiation of succession; and (4) whether site characteristics, including disturbance history, climate, and soil order are significantly related to nutrient recuperation. During secondary forest succession, nine metrics of N and/or P cycling changed consistently and substantially. In most sites, N concentrations and fluxes in both plants and soil increased during secondary succession, and total P concentrations increased in surface soil. Changes in nutrient concentrations and nutrient cycling processes during secondary succession were similar whether mature forest was included or excluded from the analysis, indicating that nutrient recuperation in secondary forest leads to biogeochemical conditions that are similar to those of mature forest. Further, of the N and P metrics that recuperated, only soil total P and foliar δ15N were strongly influenced by site characteristics like climate, soils, or disturbance history. Predictable nutrient recuperation across a diverse and productive ecosystem may support future forest growth and could provide a means to quantify successful restoration of ecosystem function in secondary tropical forest beyond biomass or species composition.