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Indirect nitrous oxide (N 2 O) emissions from streams and rivers are a poorly constrained term in the global N 2 O budget. Current models of riverine N 2 O emissions place a strong focus on denitrification in groundwater and riverine environments as a dominant source of riverine N 2 O, but do not explicitly consider direct N 2 O input from terrestrial ecosystems. Here, we combine N 2 O isotope measurements and spatial stream network modeling to show that terrestrial-aquatic interactions, driven by changing hydrologic connectivity, control the sources and dynamics of riverine N 2 O in a mesoscale river network within the U.S. Corn Belt. We find that N 2 O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N 2 O across the entire river network. The delivery of soil-produced N 2 O to streams was identified as a key mechanism for the high nitrification contribution and potentially accounted for more than 40% of the total riverine emission. This revealed large terrestrial N 2 O input implies an important climate-N 2 O feedback mechanism that may enhance riverine N 2 O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N 2 O emissions may result in significant underestimations.

Changing precipitation has the potential to alter nitrous oxide (N 2 O) emissions from agricultural regions. In this study, we applied the Coupled Model Intercomparison Project Phase 5 end-of-century RCP 8.5 (business as usual) precipitation projections for the U.S. Upper Midwest and examined the effects of mean precipitation changes, characterized by increased early-season rainfall and decreased mid- to late-season rainfall, on N 2 O emissions from a conventionally managed corn (Zea mays L.) cropping system grown in an indoor mesocosm facility over four growing seasons. We also assessed the response of N 2 O emissions to over 1,000 individual rain events. Nitrous oxide emissions were most strongly correlated with water-filled pore space (WFPS) and soil nitrogen (N) status. After rain events, the change in N 2 O emissions, relative to pre-rain emissions, was more likely to be positive when soil NO 3 - was >40 mg N kg -1 soil and soil NH 4 + was >10 mg N kg -1 soil and was more likely to be negative when soil NO 3 - was >40 mg N kg -1 O emissions remained <5 nmol m 4 + was <10 mg N kg -1 soil. Similarly, hourly N 2 O emissions remained <5 nmol m - 2 s -1 when combined NH 4 + + NO 3 - was <20 mg N kg -1 soil or NH 4 + and NO 3 - were <5 and 20 mg N kg -1 soil, respectively. Rain event magnitude did not substantially affect the change in N 2 O flux. Finally, growing-season N 2 O emissions, soil moisture, and inorganic N content were not affected by the future precipitation pattern. Near-optimal soil WFPS combined with soil N concentrations above the identified thresholds favor higher N 2 O emissions., (© 2022 The Authors. Journal of Environmental Quality published by Wiley Periodicals LLC on behalf of American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.)

Nitrous oxide (NO), produced primarily in agricultural soils, is a potent greenhouse gas and is the dominant ozone-depleting substance. Efforts to reduce NO emissions are underway, but mitigation results have been inconsistent. The leguminous perennial kura clover ( M. Bieb.) (KC) can grow side-by-side with cash crops in rotational corn ( L.)-soybean ( L.) systems. With biological nitrogen fixation, KC provides land managers an opportunity to reduce external fertilizer inputs, which may diminish problematic NO emissions. To investigate the effect of a KC living mulch on NO emissions, automated soil chambers coupled to a NO analyzer were used to measure hourly fluxes from April through October in a 2-yr corn-soybean (CS) rotation. Emissions from the KC treatment were significantly greater than those from the conventional CS treatment despite the fact that the KC treatment received substantially less inorganic nitrogen fertilizer. A seasonal tradeoff was observed with the KC treatment wherein emissions before strip-tillage were reduced but were surpassed by high losses after strip-tillage and postanthesis. These results represent the first reported measurements of NO emissions from a KC-based living mulch. The findings cast doubt on the efficacy of KC for mitigating NO loss in CS systems. However, if KC reduces nitrate leaching losses, as has been reported elsewhere, it may result in lower indirect (offsite) NO emissions., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

This study seeks to quantify the roles of soybean and corn plants and the cropland ecosystem in the regional N2O budget of the Upper Midwest, USA. The N2O flux was measured at three scales (plant, the soil-plant ecosystem, and region) using newly designed steady-state flow-through plant chambers, a flux-gradient micrometeorological tower, and continuous tall-tower observatories. Results indicate that the following. (1) N2O fluxes from unfertilized soybean (0.03 ± 0.05 nmol m(-2) s(-1)) and fertilized corn plants (-0.01 ± 0.04 nmol m(-2) s(-1)) were about one magnitude lower than N2O emissions from the soil-plant ecosystem (0.26 nmol m(-2) s(-1) for soybean and 0.95 nmol m(-2) s(-1) for corn), confirming that cropland N2O emissions were mainly from the soil. (2) Fertilization increased the corn plant flux for a short period (about 20 days), and late-season fertilization dramatically increased the soybean plant emissions. (3) The direct N2O emission from cropland accounted for less than 20 % of the regional flux, suggesting a significant influence by other sources and indirect emissions, in the regional N2O budget.

Continuous measurement of soil NO emissions is needed to constrain NO budget and emission factors. Here, we describe the performance of a low-power Teledyne NO analyzer and automated chamber system, powered by wind and solar, that can continuously measure soil NO emissions. Laboratory testing of the analyzer revealed significant temperature sensitivity, causing zero drift of -10.6 nmol mol °C. However, temperature-induced span drift was negligible, so the associated error in flux measurement for a typical chamber sampling period was on the order of 0.016 nmol m s. The 1-Hz precision of the analyzer over a 10-min averaging interval, after wavelet decomposition, was 1.5 nmol mol, equal to that of a tunable diode laser NO analyzer. The solar/wind hybrid power system performed well during summer, but system failures increased in frequency in spring and fall, usually at night. Although increased battery storage capacity would decrease down time, supplemental power from additional sources may be needed to continuously run the system during spring and fall. The hourly flux data were numerically subsampled at weekly intervals to assess the accuracy of integrated estimates derived from manually sampling static chambers. Weekly sampling was simulated for each of the five weekdays and for various times during each day. For each weekday, the cumulative N emissions estimate using only morning measurements was similar (within 15%) to the estimate using only afternoon measurements. Often, weekly sampling partially or completely missed large episodic NO emissions that continuous automated chamber measurements captured, causing weekly measurements to underestimate cumulative N emissions for 9 of the 10 sampling scenarios., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

Restoration of wetland hydrology can produce ecological benefits but may have unintended consequences. We examined effects of altered water level on release of dissolved reactive phosphorus (DRP) and greenhouse gases (GHG) in soil cores from a marsh being evaluated for restoration. We also measured field concentrations of DRP and other constituents in wetland porewater. Intact cores from a sampling location with higher Fe and lower calcium carbonate (CaCO(3)) contents released more DRP than another location, and displayed higher DRP under completely saturated compared to partly drained conditions. Porewater samples collected from the high-Fe location also contained higher DRP levels. Chemical data suggest that redox-driven reactions largely controlled DRP levels at the high-Fe site, while CaCO(3) adsorption was more important at the low-Fe site. Over the long term, water table elevation may attenuate P draining from the wetland due to decreased mineralization. However, such measures may increase P release in the short term. Raising the water level in soil cores resulted in decreased nitrous oxide (N(2)O) emissions, increased methane (CH(4)) emissions, and an overall increase in total global warming potential (GWP). The proportion of total GWP contributed by N(2)O decreased from 14% to < or = 1% as water level was raised, while the proportion contributed by CH(4) increased from 10 to 20% to 60 to 80%. Restoration of hydrology in the Rice Lake wetland has the potential to affect both local water quality and global air quality. These combined effects complicate the cost-to-benefit analysis of such wetland restoration efforts.

Atmospheric ammonia (NH3) has increased dramatically as a consequence of the production of synthetic nitrogen (N) fertilizer and proliferation of intensive livestock systems. It is a chemical of environmental concern as it readily reacts with atmospheric acids to produce fine particulate matter and indirectly contributes to nitrous oxide (N2O) emissions. Here, we present the first tall tower observations of NH3 within the U.S. Corn Belt for the period April 2017 through December 2018. Hourly average NH3 mixing ratios were measured at 100 and 56 m above the ground surface and fluxes were estimated using a modified gradient approach. The highest NH3 mixing ratios (>30 nmol mol−1) occurred during early spring and late fall, coinciding with the timing of fertilizer application within the region and the occurrence of warm air temperatures. Net ecosystem NH3 exchange was greatest in spring and fall with peak emissions of about +50 nmol m−2 s−l. Annual NH3 emissions estimated using state‐of‐the‐art inventories ranged from 0.6 to 1.4 × the mean annual gross tall tower fluxes (+2.1 nmol m−2 s−1). If the tall tower observations are representative of the Upper Midwest and broader U.S. Corn Belt regions, the annual gross emissions were +720 Gg NH3‐N y−1 and +1,340 Gg NH3‐N y−1, respectively. Finally, considering the N2O budget over the same region, we estimated total reactive N emissions (i.e., N2O + NH3) of approximately 1,790 Gg N y−1 from the U.S. Corn Belt, representing ~23% of the current annual new N input. Key Points: Observations of NH3 from a tall tower (100 m) provide insights regarding regional emissions from the US Corn BeltAnnual NH3 emissions estimated using state‐of‐the‐art inventories were 0.6 to 1.4‐fold compared to the tall tower flux densityAnnual gross emission estimates for the Upper Midwest and broader US Corn Belt regions were +720 Gg NH3‐N y−1 and +1340 Gg NH3‐N y−1 [ABSTRACT FROM AUTHOR]

Abstract: Indirect nitrous oxide (N2O) emissions from drainage ditches and headwater streams are poorly constrained. Few studies have monitored stream N2O emissions and fewer modeling studies have been conducted to simulate stream N2O emissions. In this study, we developed direct and indirect N2O emission modules and a corresponding calibration module for use in the Soil and Water Assessment Tool (SWAT) model, and implemented the expanded SWAT model (termed SWAT‐N2O) to a representative fourth‐stream‐order catchment (210 km2) and six first‐order stream catchments (0.22–1.83 km2) in southeastern Minnesota. We simulated the spatial and temporal fluctuations of the indirect emissions from streams, identified emission “hot spots” and “hot moments,” and diagnosed the correlations between direct and indirect emissions. We showed that zero‐order streams and first‐order streams could contribute 0.034–0.066 and 0.011 nmol N2O m−2 s−1 (expressed on the basis of unit catchment area) to the total surface emissions, respectively. Emissions from zero‐order and first‐order streams equal 24–41% of direct emissions from soil, which may explain the emission gap between calculations using top‐down and bottom‐up methods. Clear spatial patterns were identified for both direct and indirect emissions and their spatial variations were negatively correlated. Our results suggest that the IPCC N2O emission factor for streams in the Corn Belt should be increased by 3.2–5.7 times. Increasing precipitation and streamflow in the Corn Belt may potentially increase frequencies of soil anoxic conditions and nitrate leaching to streams, and subsequently increase N2O emissions from both soils and streams. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) has a global warming potential that is 300 times that of carbon dioxide on a 100-y timescale, and is of major importance for stratospheric ozone depletion. The climate sensitivity of N2O emissions is poorly known, which makes it difficult to project how changing fertilizer use and climate will impact radiative forcing and the ozone layer. Analysis of 6 y of hourly N2O mixing ratios from a very tall tower within the US Corn Belt--one of the most intensive agricultural regions of the world--combined with inverse modeling, shows large interannual variability in N2O emissions (316 Gg N2O-N⋅y-1 to 585 Gg N2O-N⋅y-1). This implies that the regional emission factor is highly sensitive to climate. In the warmest year and spring (2012) of the observational period, the emission factor was 7.5%, nearly double that of previous reports. Indirect emissions associated with runoff and leaching dominated the interannual variability of total emissions. Under current trends in climate and anthropogenic N use, we project a strong positive feedback to warmer and wetter conditions and unabated growth of regional N2O emissions that will exceed 600 Gg N2O-N⋅y-1, on average, by 2050. This increasing emission trend in the US Corn Belt may represent a harbinger of intensifying N2O emissions from other agricultural regions. Such feedbacks will pose a major challenge to the Paris Agreement, which requires large N2O emission mitigation efforts to achieve its goals. [ABSTRACT FROM AUTHOR]

Chamber-based estimates of soil-to-atmosphere nitrous oxide (N2O) gas flux tend to underestimate actual emission rates due to inherently nonlinear time series data. In theory, this limitation can be minimized by adjusting measurement conditions to reduce nonlinearity and/or by using flux-calculation (FC) schemes that account for the so-called "chamber effect." The current study utilizes gas transport theory and stochastic analysis to evaluate accuracy and precision of N2O flux determinations under specific soil and chamber conditions. The analysis demonstrates chat measures taken to increase the absolute accuracy of flux estimates, including shorter deployment times, larger chamber heights, and nonlinear FC schemes, will also increase the variance in flux estimates to an extent that depends on errors associated with sampling techniques and analytical instrument performance. These effects, in the absence of any actual variation in fluxes, can generate coefficients of variation ranging from 3 to 70% depending on measurement conditions. It is also shown that nonlinear FC schemes are prone to generating positively skewed distributions. These effects decrease confidence in N2O flux estimates and inhibit the detection of differences arising from experimental factors. In general, a linear FC scheme will be more likely to detect relative differences in fluxes, although less accurate in absolute terms than nonlinear schemes. The techniques described here have been codified into an accessible spreadsheet-based tool for evaluating accuracy and precision trade-offs under specific measurement conditions. [ABSTRACT FROM AUTHOR]