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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., (© 2020 by the Ecological Society of America.)

As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N2 fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N2 fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N2 fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of 15N2 into biomass and a novel application of the N2:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N2 fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m−2 d−1), with the highest rates (2.6 mmol N m−2 d−1) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N2 fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m−2 d−1), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N2 fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow. [ABSTRACT FROM AUTHOR]

Nitrogen (N) limitation to net primary production is widespread and influences the responsiveness of ecosystems to many components of global environmental change. Logic and both simple simulation (Vitousek and Fieldin in Biogeochemistry 46: 179–202, 1999) and analytical models (Menge in Ecosystems 14:519–532, 2011) demonstrate that the co-occurrence of losses of N in forms that organisms within an ecosystem cannot control and barriers to biological N fixation (BNF) that keep this process from responding to N deficiency are necessary for the development and persistence of N limitation. Models have focused on the continuous process of leaching losses of dissolved organic N in biologically unavailable forms, but here we use a simple simulation model to show that discontinuous losses of ammonium and nitrate, normally forms of N whose losses organisms can control, can be uncontrollable by organisms and can contribute to N limitation under realistic conditions. These discontinuous losses can be caused by temporal variation in precipitation or by ecosystem-level disturbance like harvest, fire, and windthrow. Temporal variation in precipitation is likely to increase and to become increasingly important in causing N losses as anthropogenic climate change proceeds. We also demonstrate that under the conditions simulated here, differentially intense grazing on N- and P-rich symbiotic N fixers is the most important barrier to the responsiveness of BNF to N deficiency. [ABSTRACT FROM AUTHOR]

Molybdenum (Mo) is a key cofactor in enzymes used for nitrogen (N) fixation and nitrate reduction, and the low availability of Mo can constrain N inputs, affecting ecosystem productivity. Natural atmospheric Mo aerosolization and deposition from sources such as desert dust, sea‐salt spray, and volcanoes can affect ecosystem function across long timescales, but anthropogenic activities such as combustion, motor vehicles, and agricultural dust have accelerated the natural Mo cycle. Here we combined a synthesis of global atmospheric concentration observations and modeling to identify and estimate anthropogenic sources of atmospheric Mo. To project the impact of atmospheric Mo on terrestrial ecosystems, we synthesized soil Mo data and estimated the global distribution of soil Mo using two approaches to calculate turnover times. We estimated global emissions of atmospheric Mo in aerosols (<10 μm in diameter) to be 23 Gg Mo yr−1, with 40%–75% from anthropogenic sources. We approximated that for the top meter of soil, Mo turnover times range between 1,000 and 1,000,000 years. In some industrialized regions, anthropogenic inputs have enhanced Mo deposition 100‐fold, lowering the soil Mo turnover time considerably. Our synthesis of global observational data, modeling, and a mass balance comparison with riverine Mo exports suggest that anthropogenic activity has greatly accelerated the Mo cycle, with potential to influence N‐limited ecosystems. Plain Language Summary: Molybdenum (Mo) is an essential trace element that is, important for terrestrial and aquatic ecosystems, as it is required for biological nitrogen fixation and uptake. Mo is carried in particles to the atmosphere from sources such as desert dust, sea spray, and volcanoes resulting in losses and sources to different ecosystems. Atmospheric Mo deposition is essential on long time scales for soils which have lost Mo due to soil weathering, with consequences for nitrogen cycling. Anthropogenic changes to the Mo cycle from combustion, motor vehicles, and agricultural dust, are likely to be large, and have more than doubled sources of Mo to the atmosphere. Locally, anthropogenic changes to Mo in industrialized regions can represent a 100‐fold increase in deposition, and may affect nitrogen cycling in nitrogen‐limited ecosystems. Key Points: We compiled atmospheric molybdenum (Mo) concentration data and compared observations to a three‐dimensional global atmospheric aerosol modelAnthropogenic activity has likely doubled atmospheric Mo globally, but with regional variationMo turnover time in soils ranges between 1,000 and 1,000,000 years, with the shortest times in dust source regions and industrialized areas [ABSTRACT FROM AUTHOR]

Tropical forest fires have become more common due to interactions between deforestation, land clearing, and drought. Forest recovery following fires may be limited by nitrogen. Biological nitrogen fixation (BNF) is the main pathway for new nitrogen (N) to enter most ecosystems, but BNF may be constrained by other nutrients, such as molybdenum and phosphorus, which are required for the process. We studied the role of BNF 7 years into the recovery of southeastern Amazon tropical forests that were burned experimentally either annually or every 3 years between 2004 and 2010. We hypothesized that, compared with unburned primary forests, BNF in burned forests would increase due to the depletion of N in the aboveground biomass pool and that soil concentrations of molybdenum and phosphorus from ash inputs would increase, reducing their potential constraints on BNF. Despite the fires depleting about half the aboveground N pool and rapid rates of recovery in leaf biomass and litterfall production, we found low rates of both free-living and symbiotic BNF. Higher concentrations of molybdenum and phosphorus in the mineral soils of the burned forests indicated that these elements were likely not limiting BNF. Our results suggest that despite the N demand for regrowth post-fire, substantial N stored in soils likely downregulates BNF. Overall, we found surprisingly low BNF rates (< 1.2 kg N ha−1 y−1) in this region of the Amazon, which contrasts with the traditional paradigm that (1) tropical forests fix large quantities of N and (2) that BNF increases after forest disturbance. [ABSTRACT FROM AUTHOR]

The success of filamentous nitrogen (N)-fixing cyanobacteria in productive, transiently N-limited freshwaters reflects, in large part, their ability to produce and sustain the activities of specialized N-fixing heterocyst cells. Heterocyst production is variable, and the responses of planktonic cyanobacterial blooms to N limitation differ markedly among systems. Temporal variations in cyanobacteria colony size may determine both heterocyst production and sensitivity to top-down control by zooplankton grazers. We promoted the development of cyanobacterial blooms through phosphorus additions and trophic manipulations in freshwater ponds, to test the role of colony size structure in regulating N-fixer bloom development. The in situ growth and heterocyst production of Anabaena spp. were strongly linked to variations in colonial filament size. Anabaena spp. initially recruited to the water column as short and poorly heterocysted filaments, exhibiting low (mean = 0.09 d-1) rates of population growth. The growth rate increased by more than fourfold (mean = 0.39 d-1) with the onset of colony elongation and heterocyst production, which resulted in rapid seasonal build-ups of cyanobacterial cells (>105 cells ml-1). Size-dependent growth was also important in determining the outcome of zooplankton-cyanobacteria interactions. In microcosm experiments, zooplankton consumers directly grazed on Anabaena spp. colonies, reduced the mean filament size, and reduced the efficiency with which heterocysts fixed nitrogen. These results suggest that colony size is a fundamental mechanism that link cellular physiological constraints to variations in trophic controls and the responses of aquatic systems to N limitation. [ABSTRACT FROM AUTHOR]

Many types of ecosystems have little or no N2 fixation even when nitrogen (N) is strongly limiting to primary production. Estuaries generally fit this pattern. In contrast to lakes, where blooms of N2-fixing cyanobacteria are often sufficient to alleviate N deficits relative to phosphorus (P) availability, planktonic N2 fixation is unimportant in most N-limited estuaries. Heterocystic cyanobacteria capable of N2 fixation are seldom observed in estuaries where the salinity exceeds 8–10 ppt, and blooms have never been reported in such estuaries in North America. However, we provided conditions in estuarine mesocosms (salinity over 27 ppt) that allowed heterocystic cyanobacteria to grow and fix N2 when zooplankton populations were kept low. Grazing by macrozooplankton at population densities encountered in estuaries strongly suppressed cyanobacterial populations and N2 fixation. The cyanobacteria grew more slowly than observed in fresh waters, at least in part due to the inhibitory effect of sulfate (SO4 2−), and this slow rate of growth increased their vulnerability to grazing. We conclude that interactions between physiological (bottom–up) factors that slow the growth rate of cyanobacteria and ecological (top–down) factors such as grazing are likely to be important regulators excluding planktonic N2 fixation from most Temperate Zone estuaries. [ABSTRACT FROM AUTHOR]

The net anthropogenic nitrogen input (NANI) approach is a simple quasi-mass-balance that estimates the human-induced nitrogen inputs to a watershed. Across a wide range of watersheds, NANI has been shown to be a good predictor of riverine nitrogen export. In this paper, we review various methodologies proposed for NANI estimation since its first introduction and evaluate alternative calculations suggested by previous literature. Our work is the first study in which a consistent NANI calculation method is applied across the U.S. watersheds and tested against available riverine N flux estimates. Among the tested methodologies, yield-based estimation of agricultural N fixation (instead of crop area-based) made the largest difference, especially in some Mississippi watersheds where the tile drainage was a significant factor reducing watershed N retention. Across the U.S. watersheds, NANI was particularly sensitive to farm N fertilizer application, cattle N consumption, N fixation by soybeans and alfalfa, and N yield by corn, soybeans, and pasture, although their relative importance varied among different regions. [ABSTRACT FROM AUTHOR]