Despite some well-documented drawbacks, the acetylene reduction assay (ARA) remains one of the most widespread methods for measuring biological nitrogen (N
2
) 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 N
2
. Based on the biochemistry of the reactions, the theoretical ratio is usually taken to be 3:1. However,
15
N
2
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.
Fine roots balance multiple essential plant functions that ultimately fuel plant productivity. The coordination of root functional traits is multidimensional, meaning that some traits are correlated ...while others can vary independently. This leads to a large variety of adaptive trait combinations and thus functional diversity in plant belowground morphology, anatomy, physiology and symbioses with soil biota. Within forest communities, co‐occurrence of such belowground trait combinations may be especially prevalent in tropical forests, not only due to their immense biodiversity but also because their soils are frequently weathered and limited in phosphorus. Phosphorus occurs in different forms (that differ in their mobility and accessibility) that may select for different trait combinations on relatively small spatial scales. While evidence accumulates of the myriad viable nutrient acquisition strategies on poor tropical soils, they are not well conceptualized. In this study, we outline criteria for the identification and investigation of fine‐root trait syndromes. We then apply these criteria to a case study of tropical tree species adapted to phosphorus‐poor soil and synthesize a range of traits. Five potentially coexisting syndromes are presented that are distinguished by root morphology, mycorrhizal association type, their interactions and implications for fine‐root functioning. We connect the functional variation of these species‐level syndromes in tropical communities to an established phosphorus partitioning hypothesis. The co‐occurrence of fine‐root trait syndromes could have implications for root sampling design, species coexistence, community structure and biogeochemical cycling.
Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm ...damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO2 storage by methane (CH4) production in mangrove sediments. The establishment of non‐native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH4 emissions from ~70 year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434 ± 33 Mg C/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10‐fold (to 4.5 Mg C ha−1 year−1), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH4 emissions from sediments offset ecosystem CO2 storage by only 2%–4%, equivalent to 30–60 Mg CO2‐eq/ha over mangrove lifetime (100 year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals.
Mangroves play an important role in climate change mitigation via coastline protection and carbon burial. Examining recently established, non‐native mangrove stands in Hawai‘i, we show that mangrove forests accrue substantial carbon stocks within several decades after introduction. Mangrove establishment results in a significant net removal of greenhouse gases from the atmosphere and also increases shore height faster than sea level rise, impacts that should be considered during decision‐making for mangrove management.
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.
Arid soils represent a substantial carbonate pool and may participate in surface‐atmosphere CO2 exchange via a diel cycle of carbonate dissolution and exsolution. We used a Keeling plot approach to ...determine the substrate δ13C of CO2 emitted from carbonate‐dominated soils in the Mojave desert and found evidence for a nonrespiratory source that increased with surface temperature. In dry soils at 25–30°C, the CO2 substrate had δ13C values of −19.4 ± 4.2‰, indicative of respiration of organic material (soil organic matter = −23.1 ± 0.8‰). CO2 flux increased with temperature; maximum fluxes occurred above 60°C, where δ13CO2 substrate (−7.2‰ ± 2.8‰) approached soil carbonate values (0.2 ± 0.2‰). In wet soils, CO2 emissions were not temperature dependent, and δ13CO2 substrate was lower in vegetated soils with higher flux rates, higher organic C content, and potential root respiration. These data provide the first direct evidence of CO2 emissions from alkaline desert soils derived from an abiotic source and that diurnal emission patterns are strongly driven by surface temperature.
Key Points
δ13CO2 Keeling plots were used to determine CO2 sources from an alkaline calcareous desert soil in situ
Isotopic values show that carbonates serve as a substrate for daytime CO2 emissions from dry soils
Carbonate‐driven emissions increase strongly with surface temperatures up to >60°C
Non-native ungulates (sheep, goats, and pigs) have significant negative impacts on ecosystem biodiversity, structure, and biogeochemical function throughout the Pacific Islands. Elevated nitrogen (N) ...availability associated with ungulate disturbance has been shown to promote the success of resource-exploitive invasive plants. While ungulate removal is a common restoration intervention, evaluations of its efficacy typically focus on vegetation responses, rather than underlying nutrient cycling. We used multiple chronosequences of ungulate exclusion (10–24 years duration) in three Hawaiian ecosystems (montane wet forest, dry forest, and dry shrubland) to determine N cycle recovery by characterizing gross mineralization and nitrification, soil inorganic N concentrations and leaching, N
2
O emissions, and plant tissue δ
15
N. Ungulate removal led to a 1–2 ‰ decline in foliar δ
15
N in most species, consistent with a long-term decrease in N fractionation via ecosystem N losses, or a shift in the relative turnover of N forms. This interpretation was supported by significant (dry forest) or trending (wet forest) increases in mineralization and decreases in nitrification, but conflicts with lack of observed change in inorganic N pool sizes or gaseous losses, and increased leaching in the dry forest. While results could indicate that ungulate invasions do not strongly impact N cycling in the first place (no uninvaded control sites exist in Hawai’i to test this hypothesis), this would be inconsistent with observations from other sites globally. Instead, impacts may be spatially patchy across the landscape, or ungulate invasions (possibly in combination with other disturbances) may have permanently shifted biogeochemical function or decoupled elemental cycles. We conclude that eliminating ungulate disturbance alone may not achieve restoration goals related to N cycling within the timeframe examined here.
Summary
Tropical forests are often characterized by low soil phosphorus (P) availability, suggesting that P limits plant performance. However, how seedlings from different functional types respond to ...soil P availability is poorly known but important for understanding and modeling forest dynamics under changing environmental conditions.
We grew four nitrogen (N)‐fixing Fabaceae and seven diverse non‐N‐fixing tropical dry forest tree species in a shade house under three P fertilization treatments and evaluated carbon (C) allocation responses, P demand, P‐use, investment in P acquisition traits, and correlations among P acquisition traits.
Nitrogen fixers grew larger with increasing P addition in contrast to non‐N fixers, which showed fewer responses in C allocation and P use. Foliar P increased with P addition for both functional types, while P acquisition strategies did not vary among treatments but differed between functional types, with N fixers showing higher root phosphatase activity (RPA) than nonfixers.
Growth responses suggest that N fixers are limited by P, but nonfixers may be limited by other resources. However, regardless of limitation, P acquisition traits such as mycorrhizal colonization and RPA were nonplastic across a steep P gradient. Differential limitation among plant functional types has implications for forest succession and earth system models.
Accurately quantifying rates and patterns of biological nitrogen fixation (BNF) in terrestrial ecosystems is essential to characterize ecological and biogeochemical interactions, identify mechanistic ...controls, improve BNF representation in conceptual and numerical modelling, and forecast nitrogen limitation constraints on future carbon (C) cycling.
While many resources address the technical advantages and limitations of different methods for measuring BNF, less systematic consideration has been given to the broader decisions involved in planning studies, interpreting data, and extrapolating results. Here, we present a conceptual and practical road map to study design, study execution, data analysis and scaling, outlining key considerations at each step.
We address issues including defining N‐fixing niches of interest, identifying important sources of temporal and spatial heterogeneity, designing a sampling scheme (including method selection, measurement conditions, replication, and consideration of hotspots and hot moments), and approaches to analysing, scaling and reporting BNF. We also review the comparability of estimates derived using different approaches in the literature, and provide sample R code for simulating symbiotic BNF data frames and upscaling.
Improving and standardizing study design at each of these stages will improve the accuracy and interpretability of data, define limits of extrapolation, and facilitate broader use of BNF data for downstream applications. We highlight aspects—such as quantifying scales of heterogeneity, statistical approaches for dealing with non‐normality, and consideration of rates versus ecological significance—that are ripe for further development.
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