Revisiting Biological Nitrogen Fixation Dynamics in Soybeans

Biological nitrogen (N) fixation is the most relevant process in soybeans (Glycine max L.) to satisfy plant N demand and sustain seed protein formation. Past studies describing N fixation for field-grown soybeans mainly focused on a single point time measurement (mainly toward the end of the season) and on the partial N budget (fixed-N minus seed N removal), overlooking the seasonal pattern of this process. Therefore, this study synthesized field datasets involving multiple temporal measurements during the crop growing season to characterize N fixation dynamics using both fixed-N (kg ha−1) and N derived from the atmosphere [Ndfa (%)] to define: (i) time to the maximum rate of N fixation (β2), (ii) time to the maximum Ndfa (α2), and (iii) the cumulative fixed-N. The main outcomes of this study are that (1) the maximum rate of N fixation was around the beginning of pod formation (R3 stage), (2) time to the maximum Ndfa (%) was after full pod formation (R4), and (3) cumulative fixation was positively associated with the seasonal vapor-pressure deficit (VPD) and growth cycle length but negatively associated with soil clay content, and (4) time to the maximum N fixation rate (β2) was positively impacted by season length and negatively impacted by high temperatures during vegetative growth (but positively for VPD, during the same period). Overall, variation in the timing of the maximum rate of N fixation occurred within a much narrower range of growth stages (R3) than the timing of the maximum Ndfa (%), which varied broadly from flowering (R1) to seed filing (R5–R6) depending on the evaluated studies. From a phenotyping standpoint, N fixation determinations after the R4 growth stage would most likely permit capturing both maximum fixed-N rate and maximum Ndfa (%). Further investigations that more closely screen the interplay between N fixation with soil-plant-environment factors should be pursued.

Effects of Repeated Application of Organic Soil Amendments on Horticultural Soil Physicochemical Properties, Nitrogen Budget and Yield

Application of organic amendments to soil is commonplace in domestic gardening. However, a vast array of materials could be labelled as ‘compost’ by retailers and suppliers. We investigated six different amendments currently used, or available for use, in horticulture: composted bark, composted bracken, spent mushroom compost, composted horse manure, garden waste compost (at two different application rates), and peat. Using a controlled field experiment, we examined the physicochemical differences between the amendments, the subsequent effects on soil characteristics, and resultant yield and biometrics of Lavatera trimiestris. Amended soils resulted in a significantly different multivariate soil environment and N budget when compared to the unamended control. However, the effect on yield and plant biometrics (number of flowers, plant height, etc.) depended on the amendment used. Application of garden compost resulted in up to a five-fold increase in yield. However, there was no significant difference in yields in soils amended with composted bark or peat, when compared to the unamended control. This has implications, as there is increasing pressure to remove peat from products available to domestic gardeners. The variability in the different amendments investigated in our research, in addition to the variable effects on plant growth parameters, suggests that repeated use of a single amendment may not be best practise for gardeners.

Earth’s Nitrogen and Carbon Cycles

Understanding the Earth’s geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth’s N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements”. Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth’s atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth’s surface into the deep Earth. A key question in the flux quantification is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as $\text{NH}_{4}^{+}$ NH 4 + in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of $\text{NH}_{4}^{+}$ NH 4 + -bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth’s mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbonation reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO2. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with C-bearing country rocks, and the resulting release of large quantities of CH4 and/or CO2, has been linked to global warming events.

Spatiotemporal patterns of N₂ fixation in coastal waters derived from rate measurements and remote sensing

Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flos-aquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll a (Chl a) and N2 fixation. We used regression models relating N2 fixation to Chl a, along with remote-sensing-based estimates of Chl a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment–water exchange, and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl a with empirical models relating N2 fixation rates to Chl a.

Innovative approach for new estimation of NOx snow-source on the Antarctic Plateau

<p>Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the continental interior as well as at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 x 10<sup>6</sup> cm<sup>-3</sup>. It is now well established that such high reactivity of the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NO<sub>x</sub> ≡ NO + NO<sub>2</sub>) produced during the photo-denitrification of the snowpack. Despite the numerous observations collected at various sites during previous campaigns such as ISCAT 1998, 2000, ANTCI, NITE-DC and OPALE, a robust quantification of the NO<sub>x</sub> emissions on a continental scale over Antarctica is still lacking. Only NO emissions were measured during ISCAT and the ratio NO<sub>2</sub>:NO was measured during NITE-DC and OPALE using indirect NO<sub>2</sub> measurements. This leaves significant uncertainties on the snow-air-radiation interaction. To overcome this crucial lack of information, direct NO<sub>2</sub> measurements are needed to estimate the NO<sub>x</sub> flux emissions with reduced uncertainties.</p><p>For the first time, new developed optical instruments based on the IBB-CEAS technique and allowing direct measurement of NO<sub>2</sub> with detection limit of 10 x 10<sup>-12</sup> mol mol<sup>-1</sup>, (1σ), (Barbero et al., 2020) were deployed on the field during the 2019–2020 summer campaign at Dome C (75°06'S, 123°20'E, 3233m a.s.l). They were coupled with new designed dynamic flux chamber experiments. Snows of different ages ranging from newly formed drift snow to 16-20 year-old firn were sampled. Unexpectedly, the same daily average photolysis constant rate of (2.18 ± 0.38) x 10<sup>-8</sup> s<sup>-1</sup> (1σ) was estimated for the different type of snow samples, suggesting that the photolabile nitrate behaves as a single-family source with common photochemical properties. Daily summer NO<sub>x</sub> fluxes were estimated to be (4.4 ± 2.3) x 10<sup>7</sup> molec cm<sup>-2</sup> s<sup>-1</sup>, 10 to 70 times less than what has been estimated in previous studies at Dome C and with uncertainties reduced by a factor up to 30. Using these results, we extrapolated an annual continental snow source NO<sub>x</sub> budget of 0.025 ± 0.013 Tg.N y<sup>-1</sup>, more than three times the N-budget of the stratospheric denitrification estimated to be 0.008 ± 0.003 Tg.N y<sup>-1</sup> for Antarctica (Savarino et al., 2007), making the snowpack source a rather significant source in Antarctica. This innovative approach for the parameterization of nitrate photolysis using flux chamber experiments could  significantly improve future global atmospheric models.</p>

Modeling the Impact of Riparian Hollows on River Corridor Nitrogen Exports

Recent studies in snowmelt-dominated catchments have documented changes in nitrogen (N) retention over time, such as declines in watershed exports of N, though there is a limited understanding of the controlling processes driving these trends. Working in the mountainous headwater East River Colorado watershed, we explored the effects of riparian hollows as N-cycling hotspots and as important small-scale controls on observed watershed trends. Using a modeling-based approach informed by remote sensing and in situ observations, we simulated the N-retention capacity of riparian hollows with seasonal and yearly hydrobiogeochemical perturbations imposed as drivers. We then implemented a scaling approach to quantify the relative contribution of riparian hollows to the total river corridor N budget. We found that riparian hollows primarily serve as N sinks, with N-transformation rates significantly limited by periods of enhanced groundwater upwelling and promoted at the onset of rainfall events. Given these observed hydrologic controls, we expect that the nitrate (NO3-) sink capacity of riparian hollows will increase in magnitude with future climatic perturbations, specifically the shift to more frequent rainfall events and fewer snowmelt events, as projected for many mountainous headwater catchments. Our current estimates suggest that while riparian hollows provision ~5–20% of NO3- to the river network, they functionally act as inhibitors to upland NO3- reaching the stream. Our work linking transient hydrological conditions to numerical biogeochemical simulations is an important step in assessing N-retaining features relative to the watershed N budget and better understanding the role of small-scale features within watersheds.

Spatiotemporal patterns of N₂ fixation in coastal waters derived from rate measurements and remote sensing

Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flosaquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll-a (Chl-a) and N2 fixation. We used regression models relating N2 fixation to Chl-a, along with remote sensing-based estimates of Chl-a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment-water exchange and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl-a with empirical models relating N2 fixation rates to Chl-a.

Reach-Scale Model of Aquatic Vegetation Quantifies N Fate in a Bedrock-Controlled Karst Agroecosystem Stream

In-stream fate of nutrients in karst agroecosystems remains poorly understood. The significance of these streams is recognized given spring/surface water confluences have been identified as hotspots for biogeochemical transformations. In slow-moving streams high in dissolved inorganic nutrients, benthic and floating aquatic macrophytes are recognized to proliferate and drastically impact nutrient fate; however, models that quantify coupled interactions between these pools are limited. We present a reach-scale modeling framework of nitrogen dynamics in bedrock-controlled streams that accounts for coupled interactions between hydrology, hydraulics, and biotic dynamics and is validated using a multi-year, biweekly dataset. A fluvial N budget with uncertainty was developed to quantify transformation dynamics for the dissolved inorganic nitrogen (DIN) pool using a GLUE-like modeling framework, and scenario analyses were run to test for model function over variable environmental conditions. Results from a 10,000 run uncertainty analysis yielded 195 acceptable parameter sets for the calibration period (2000–2002), 47 of which were acceptable for the validation period (2003) (Nash-Sutcliffe Efficiency (NSE) > 0.65; percent bias (PBIAS) < ±15), with significantly different posterior parameter spaces for parameters including denitrification coefficients and duckweed growth factors. The posterior solution space yielded model runs with differing biomass controls on DIN, including both algae and duckweed, but suggested duckweed denitrifies at a rate that would place the bedrock agroecosystem stream on the high-end of rates reported in the literature, contradicting the existing paradigm about bedrock streams. We discuss broader implications for watershed-scale water quality modeling and implementation strategies of management practices for karst agroecosystems, particularly with respect to stream restoration.

Joint analyses of nitrate transit time distributions and legacy effects in catchments with contrasting physical settings in Germany

&lt;p&gt;Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate), posing a global problem. Although measures have been implemented to reduce N inputs, they have rarely led to decreasing riverine nitrate concentrations and loads. This limited response to the measures can either be caused by the accumulation of organic N in the soils (biogeochemical legacy) &amp;#8211;or by long travel times (TTs) of inorganic N to the streams (hydrological legacy). Both legacy types determine the temporal dimension of catchment response on the one hand and the quantitative dimension on the other hand.&lt;/p&gt;&lt;p&gt;Here we analyze several decades of N input, water quality and discharge observations from 62 catchments in 8 federal states in Germany. The selection of catchments represents a wide range of land use, geology and soils, topography and hydroclimate. In an input-output assessment, N input from atmospheric deposition, waste water treatment and agriculture is compared with riverine N concentrations (nitrate-N) as N output. We assess jointly the N budget and the effective TTs of N through the soil and groundwater compartments. In combination with long-term trajectories of the C&amp;#8211;Q relationships, we evaluate the potential for and the characteristics of an N legacy.&lt;/p&gt;&lt;p&gt;Our data-driven approach shows a mean legacy of 73 % (spanning 0 &amp;#8211; 90 %), cumulating to a total missing mass of 4270 kg N/ha a. Log-normal distributed TTs have a mean of 6 years (0.8 &amp;#8211; 34 years) with an R&lt;sup&gt;2&lt;/sup&gt; of 89 % between the convolved N input and N output. Due to the chemostatic export regime (mean CV&lt;sub&gt;C&lt;/sub&gt;/CV&lt;sub&gt;Q&lt;/sub&gt;: 0.36 &lt; 0.5) and relatively short TTs in most of the catchments, the biogeochemical legacy seems to dominate the catchment responses nowadays. Further analyses aim to investigate the controlling parameters determining the N time lags and legacies type. A correlation analyses hint to topographic parameters, mainly slope and topographic wetness index, as main controls of the legacy i.e. that flat catchments have the tendency to higher legacies or retention.&lt;/p&gt;&lt;p&gt;Legacies of almost &amp;#190; of the N input pose a challenge to the limited denitrification potential of soils and aquifers or indicate a massive N accumulation in the catchment. Latter can cause elevated N concentrations for the next decades explaining at the same time a limited response to measures. The dominant biogeochemical legacy suggests that management needs to address both a longer-term reduction of N inputs and shorter-term mitigation of past high N loads by favoring denitrification.&lt;/p&gt;