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Excessive phosphorus (P) applications to croplands can contribute to eutrophication of surface waters through surface runoff and subsurface (leaching) losses. We analyzed leaching losses of total dissolved P (TDP) from no-till corn, hybrid poplar (Populus nigra X P. maximowiczii), switchgrass (Panicum virgatum), miscanthus (Miscanthus giganteus), native grasses, and restored prairie, all planted in 2008 on former cropland in Michigan, USA. All crops except corn (13 kg P ha−1 year−1) were grown without P fertilization. Biomass was harvested at the end of each growing season except for poplar. Soil water at 1.2 m depth was sampled weekly to biweekly for TDP determination during March–November 2009–2016 using tension lysimeters. Soil test P (0–25 cm depth) was measured every autumn. Soil water TDP concentrations were usually below levels where eutrophication of surface waters is frequently observed (> 0.02 mg L−1) but often higher than in deep groundwater or nearby streams and lakes. Rates of P leaching, estimated from measured concentrations and modeled drainage, did not differ statistically among cropping systems across years; 7-year cropping system means ranged from 0.035 to 0.072 kg P ha−1 year−1 with large interannual variation. Leached P was positively related to STP, which decreased over the 7 years in all systems. These results indicate that both P-fertilized and unfertilized cropping systems may leach legacy P from past cropland management.

The long-term contribution of nitrification to nitrous oxide (N2 O) emissions from terrestrial ecosystems is poorly known and thus poorly constrained in biogeochemical models. Here, using Bayesian inference to couple 25 years of in situ N2 O flux measurements with site-specific Michaelis-Menten kinetics of nitrification-derived N2 O, we test the relative importance of nitrification-derived N2 O across six cropped and unmanaged ecosystems along a management intensity gradient in the U.S. Midwest. We found that the maximum potential contribution from nitrification to in situ N2 O fluxes was 13%-17% in a conventionally fertilized annual cropping system, 27%-42% in a low-input cover-cropped annual cropping system, and 52%-63% in perennial systems including a late successional deciduous forest. Actual values are likely to be

The active fraction of soil organic carbon is an important component of soil health and often is quickly assessed as the pulse of CO2 released by re-wetting dried soils in short-term (24–72 h) assays. However, soils can lose carbon (C) as they dry and, if soil samples vary in moisture content at sampling, differential C loss during the pre-assay dry-down period may complicate the assay's interpretations. We examined the impact of pre-assay CO2 loss in a long-cultivated agricultural soil at initial moisture contents of 30 %, 50 %, and 70 % water-filled pore space (WFPS). We found that 50 % and 70 % WFPS treatments lost more C during drying than did those in the 30 % WFPS treatment and that dry-down losses led to a 26 %–32 % underestimate of their CO2 pulses. We developed a soil-specific correction factor to account for these initial soil moisture effects. Future C mineralization studies may benefit from similar corrections.

In nitrogen (N)-limited terrestrial ecosystems, plants employ various strategies to acquire and conserve N, including translocation of N in perennial tissues and stimulation of N fixation in roots and soils. Switchgrass (Panicum virgatum) is a genotypically and phenotypically diverse perennial grass with two distinct ecotypes (lowland and upland) and numerous genotypes. It grows well in low-N soils, likely because of its ability to translocate N and to associate with N-fixing microbes, but little is known about variation in these traits among cultivars or even ecotypes. We measured N translocation, N fixation potential in roots and soils, soil net N mineralization, soil net nitrification, and biomass yields in 12 switchgrass cultivars grown in a replicated block experiment in southwestern Michigan, United States. Lowland cultivars had higher yields, rates of N translocation, soil net N mineralization, and N fixation potentials on washed, nonsterile roots, while upland cultivars exhibited higher N fixation potentials in root-free soil. N resorption efficiencies averaged 53 ± 5% (± standard error) for lowland versus 29 ± 3% for upland cultivars. Additionally, there were significant among-cultivar differences for all response variables except mineralization and nitrification, with differences likely explained by cultivar-specific physiologies and microbial communities. The ideal cultivar for biofuels is one that can maintain high yields with minimal fertilizer addition, and there appear to be several cultivars that meet these criteria. In addition, results suggest substantial N cycle differences among cultivars that might be exploited by breeders to create new or improved high-yielding, N-conserving switchgrass lines.

Albedo can be used to quantify ecosystem and landscape contributions to local and global climate. Such contributions are conventionally expressed as radiative forcing (RF) and global warming impact (GWI). We contextualize our results within landscape carbon production and storage to highlight the importance of changes in albedo for landscape GWI from multiple causes, including net ecosystem production (NEP) and greenhouse gas (GHG) emissions. To examine the spatiotemporal changes in albedo (Δα) in contrasting managed landscapes through calculations of albedo-induced RF (RFΔα) and GWI (GWIΔα) under different climatic conditions. We selected five contrasting landscapes within the Kalamazoo River watershed in southern Michigan USA as proof of concept. The daily MCD43A3 MODIS (V006) product was used to analyze the inter- and intra-annual variations of growing season albedo. In addition, the variations of RFΔα and GWIΔα were computed based on landscape composition and climate. The RFΔα (− 5.6 W m−2) and GWIΔα (− 1.3 CO2eq ha−1 year−1) were high in forest-dominated landscapes, indicating cooling effects and CO2eq mitigation impacts similar to crops. The CO2eq mitigation of cropland-dominated landscapes was on average 52% stronger than forest-dominated landscapes. In the landscape with the highest proportion of forest, under dry and wet conditions CO2eq mitigation was reduced by up to 24% and ~ 30%, respectively; in one cropland-dominated landscape wet conditions reduced CO2eq mitigation by 23%. Findings demonstrate that quantifying spatiotemporal changes in albedo in managed landscapes and under different climatic conditions is essential to understand how landscape modification affects RFΔα and GWIΔα and thereby contributes to ecosystem-level GWI.

Summary A grand challenge facing humanity is how to produce food for a growing population in the face of a changing climate and environmental degradation. Although empirical evidence remains sparse, management strategies that increase environmental sustainability, such as increasing agroecosystem diversity through crop rotations, may also increase resilience to weather extremes without sacrificing yields. We used multilevel regression analyses of long-term crop yield datasets across a continental precipitation gradient to assess how temporal crop diversification affects maize yields in intensively managed grain systems. More diverse rotations increased maize yields over time and across all growing conditions (28.1% on average), including in favorable conditions (22.6%). Notably, more diverse rotations also showed positive effects on yield under unfavorable conditions, whereby yield losses were reduced by 14.0%–89.9% in drought years. Systems approaches to environmental sustainability and yield resilience, such as crop-rotation diversification, are a central component of risk-reduction strategies and should inform the enablement of policies.

Meeting end-of-century global warming targets requires aggressive action on multiple fronts. Recent reports note the futility of addressing mitigation goals without fully engaging the agricultural sector, yet no available assessments combine both nature-based solutions (reforestation, grassland and wetland protection, and agricultural practice change) and cellulosic bioenergy for a single geographic region. Collectively, these solutions might offer a suite of climate, biodiversity, and other benefits greater than either alone. Nature-based solutions are largely constrained by the duration of carbon accrual in soils and forest biomass; each of these carbon pools will eventually saturate. Bioenergy solutions can last indefinitely but carry significant environmental risk if carelessly deployed. We detail a simplified scenario for the United States that illustrates the benefits of combining approaches. We assign a portion of non-forested former cropland to bioenergy sufficient to meet projected mid-century transportation needs, with the remainder assigned to nature-based solutions such as reforestation. Bottom-up mitigation potentials for the aggregate contributions of crop, grazing, forest, and bioenergy lands are assessed by including in a Monte Carlo model conservative ranges for cost-effective local mitigation capacities, together with ranges for (a) areal extents that avoid double counting and include realistic adoption rates and (b) the projected duration of different carbon sinks. The projected duration illustrates the net effect of eventually saturating soil carbon pools in the case of most strategies, and additionally saturating biomass carbon pools in the case of forest management. Results show a conservative end-of-century mitigation capacity of 110 (57-178) Gt CO

Low-power, open-path gas sensors enable eddy covariance (EC) flux measurements in remote areas without line power. However, open-path flux measurements are sensitive to fluctuations in air temperature, pressure, and humidity. Laser-based, open-path sensors with the needed sensitivity for trace gases like methane (CH4 ) and nitrous oxide (N2 O) are impacted by additional spectroscopic effects. Corrections for these effects, especially those related to temperature fluctuations, often exceed the flux of gases, leading to large uncertainties in the associated fluxes. For example, the density and spectroscopic corrections arising from temperature fluctuations can be one or two orders of magnitude greater than background N2 O fluxes. Consequently, measuring background fluxes with laser-based, open-path sensors is extremely challenging, particularly for N2 O and gases with similar high-precision requirements. We demonstrate a new laser-based, open-path N2 O sensor and a general approach applicable to other gases that minimizes temperature-related corrections for EC flux measurements. The method identifies absorption lines with spectroscopic effects in the opposite direction of density effects from temperature and, thus, density and spectroscopic effects nearly cancel one another. The new open-path N2 O sensor was tested at a corn (Zea mays L.) field in Southwestern Michigan, United States. The sensor had an optimal precision of 0.1 ppbv at 10 Hz and power consumption of 50 W. Field trials showed that temperature-related corrections were 6% of density corrections, reducing EC random errors by 20-fold compared to previously examined lines. Measured open-path N2 O EC fluxes showed excellent agreement with those made with static chambers (m = 1.0 ± 0.3; r2 = .96). More generally, we identified absorption lines for CO2 and CH4 flux measurements that can reduce the temperature-related corrections by 10-100 times compared to existing open-path sensors. The proposed method provides a new direction for future open-path sensors, facilitating the expansion of accurate EC flux measurements.

PurposeExpected changes in rainfall patterns will affect the timing of N-mineralization and other N transformations, potentially promoting or suppressing biological nitrogen fixation (BNF). We test the hypotheses that BNF is more sensitive to changing rainfall patterns in summit vs. toeslope positions and in till vs. no-till consistent with patterns of soil texture and organic matter.MethodsAt a site in the upper Midwest USA, we measured soybean BNF 15N natural abundance at different landscape positions with and without supplemental rainfall and in till vs. no-till rainfall exclusion shelters to lengthen the dry periods between rainfall events. ResultsSoybean BNF was 41% higher at summit than toeslope positions, consistent with lower soil OM and coarser texture at summits. When precipitation was increased by 20%, BNF decreased at summit positions and was unaffected at toeslope positions. In a separate tillage experiment, with 3-week (but not 2-week) rainfall intervals, %BNF decreased 15% under conventional tillage and increased 14% under no-till. ConclusionsChanging rainfall patterns affected BNF differentially depending on landscape position and tillage in well-drained Alfisols. BNF was greater in summit than in toeslope positions and decreased with added rainfall. BNF under conventional tillage was more sensitive to longer rainfall intervals than was BNF under no-till. Models that incorporate these interactions will be better able to characterize legume crop performance and N use across landscapes and improve global estimates for BNF.

At two sites in the North Central USA (Michigan (KBS) and Wisconsin (ARL)), we evaluated the effect of N fertilization on the yield and quality of five perennial bioenergy feedstock cropping systems: (1) switchgrass (Panicum virgatum L.), (2) giant miscanthus (Miscanthus × giganteus), (3) a native grass mixture (5 species), (4) an early successional field (volunteer herbaceous species), and (5) a restored prairie (18 species). In a randomized complete block design with 5 replicates and 2 split plots, N was applied at 0 and 56 kg ha−1 to split plots for each cropping system from 2010 to 2016. No yield response to N was detected in switchgrass at either location in any year. Giant miscanthus exhibited a positive yield response to N at both sites (11% at KBS and 83% at ARL). Nitrogen fertilizer addition significantly reduced glucose (KBS 12.9 and 13.8 g kg−1 year−1, ARL 11.2 and 9.7 g kg−1 year−1) in the native grass mix and restored prairie systems respectively. Nitrogen fertilizer also reduced xylose at KBS in the switchgrasss, native grass mix, and restored prairie (4.9, 7.5, and 5.0 g kg−1 year−1). At ARL, N fertilization reduced xylose levels in switchgrass, giant miscanthus, and restored prairie (7.4, 6.8, and 6.2 g kg−1 year−1) and increased xylose levels in the early successional system (5.0 g kg−1 year−1).

Hydrological Processes published by John Wiley & Sons Ltd Perennial cellulosic crops are promoted for their potential contributions to a sustainable energy future. However, a large-scale perennial bioenergy production requires extensive land use changes through diversion of croplands or conversion of uncultivated lands, with potential implications for local and regional hydrology. To assess the impact of such land use conversions on ecosystem water use, we converted three 22 year-old Conservation Reserve Program (CRP) grasslands and three 50+ year-old conventionally tilled corn-soybean crop fields (AGR) to either no-till continuous maize (corn) or perennial (switchgrass or restored prairie) bioenergy crops. We also maintained one CRP grassland without conversion. We measured evapotranspiration (ET) rates on all fields for 9 years using eddy covariance methods. Results show that: (a) mean growing-season ET rates for perennial crops were similar to the ET rate of the corn they replaced at the previously cultivated (AGR) field but ET rates for perennial crops at CRP fields were 5–9% higher than ET rate for corn on former CRP fields; and (b) mean nongrowing season ET rates for perennial fields were 11–15% lower than those for corn fields, regardless of land use history. On an annual basis, mean ET rates for perennial crops tended to be lower (4–7%) than ET rate of the corn that they replaced at AGR fields but ET rates for perennial crops and corn at CRP fields were similar. Over 9 years, mean ET rates for the same crop across land use histories were remarkably similar for corn, whereas for the perennial crops they were 4–10% higher at former CRP than at former AGR fields, mainly due to differences in growing season ET. Over the 9 years and across all fields, ET returned ~60% of the precipitation back to the atmosphere. These findings suggest that large-scale substitution of perennial bioenergy crops for rainfed corn in mesic landscapes would have little if any (0 to −3%) impact on terrestrial water balances.

Leaching from annual corn (Zea mays L.) crops is a primary source of nitrate (NO3−) pollution of ground and surface waters. Here, we compare NO3− losses from no‐till corn with losses from various alternative perennial cropping systems (switchgrass [Panicum virgatum L.], miscanthus [Miscanthus ×giganteus J.M. Greef & Deuter ex Hodkinson & Renvoiz], a native grass mixture, and restored prairie), as well as hybrid poplar (Populus nigra L. × P. maximowiczii A. Henry ‘NM6’), all grown on a well‐drained soil in Michigan. Soil water was sampled from below the root zone using suction cup samplers during nonfrozen periods (March–November) between 2009 and 2016. Leaching was estimated from NO3− concentrations in soil water and modeled drainage (percolation) rates. Drainage rates were not significantly different among crops, constituting ∼30% of total annual precipitation. Aboveground net primary production (Mg ha−1 yr−1) averaged across the 7 yr was highest in poplar (30.8 ± 1.9 [SE]) followed by miscanthus (23.9 ± 2.4) and corn (20.4 ± 0.9). Volume‐weighted mean NO3− concentrations (mg N L−1) and NO3− leaching (kg ha−1 yr−1) averaged across the 7 yr were 9.2 and 34.1, 2.3 and 5.9, and 3.0 and 7.2, respectively, for corn, perennial grasses and poplar. Approximately 10 to 32% of applied N was lost as NO3− from these crops, with the highest percent losses from poplar (32%) followed by corn (20%). Perennial cropping systems leached considerably more NO3− in first few years after planting, but over 7 yr they lost much less NO3− than corn. Perennial crops may therefore help ameliorate NO3− pollution in agricultural landscapes even if they receive modest N fertilization.

To create a comprehensive view of ecosystem resource use, we integrated parallel resource use efficiency observations into a multiple-resource use efficiency (mRUE) framework using a dynamic factor analysis model. Results from 56 site-years of eddy covariance data and mRUE factors for a site in the US Midwest show temporal dynamics and coherence (using Pearson’s R) among resources are associated with interannual variation in precipitation. Loading factors are derived from mRUE observations and quantify how strongly data are connected to the underlying ecosystem state. Water and light resource use loading factors are coherent at annual timescales (Pearson’s R of 0.86), whereas declining patterns of carbon use efficiency loading factors highlight the ecosystem’s trade-off between carbon uptake and respiration during the growing season. At annual and monthly timescales, influence decreases from ~ 85 to ~ 65% for loading factors for carbon use, while influence of light use loading factors peaks to ~ 60% at growing season timescales. Quantifying variation in ecosystem function provides novel insights into the temporal dynamics of changing importance of multiple resources to ecosystem function.

Loss of reactive nitrogen (N) from agricultural fields in the U.S. Midwest is a principal cause of the persistent hypoxic zone in the Gulf of Mexico. We used eight years of high resolution satellite imagery, field boundaries, crop data layers, and yield stability classes to estimate the proportion of N fertilizer removed in harvest (NUE) versus left as surplus N in 8 million corn (Zea mays) fields at subfield resolutions of 30 × 30 m (0.09 ha) across 30 million ha of 10 Midwest states. On average, 26% of subfields in the region could be classified as stable low yield, 28% as unstable (low yield some years, high others), and 46% as stable high yield. NUE varied from 48% in stable low yield areas to 88% in stable high yield areas. We estimate regional average N losses of 1.12 (0.64–1.67) Tg N y−1 from stable and unstable low yield areas, corresponding to USD 485 (267–702) million dollars of fertilizer value, 79 (45–113) TJ of energy, and greenhouse gas emissions of 6.8 (3.4–10.1) MMT CO2 equivalents. Matching N fertilizer rates to crop yield stability classes could reduce regional reactive N losses substantially with no impact on crop yields, thereby enhancing the sustainability of corn-based cropping systems.

Nitrous oxide (N2O) is a major greenhouse gas and cultivated soils are the most important anthropogenic source. N2O production and consumption are known to occur at depths below the A or Ap horizon, but their magnitude in situ is largely unknown. At a site in SW Michigan, USA, we measured N2O concentrations at different soil depths and used diffusivity models to examine the importance of depth-specific production and consumption. We also tested the influence of crop and management practices on subsurface N2O production in (1) till versus no-till, (2) a nitrogen fertilizer gradient, and (3) perennial crops including successional vegetation. N2O concentrations below 20 cm exceeded atmospheric concentrations by up to 900 times, and profile concentrations increased markedly with depth except immediately after fertilization when production was intense in the surface horizon, and in winter, when surface emissions were blocked by ice. Diffusivity analysis showed that N2O production at depth was especially important in annual crops, accounting for over 50% of total N2O production when crops were fertilized at recommended rates. At nitrogen fertilizer rates exceeding crop need, subsurface N2O production contributed 25–35% of total surface emissions. Dry conditions deepened the maximum depth of N2O production. Tillage did not. In systems with perennial vegetation, subsurface N2O production contributed less than 20% to total surface emissions. Results suggest that the fraction of total N2O produced in subsurface horizons can be substantial in annual crops, is low under perennial vegetation, appears to be largely controlled by subsurface nitrogen and moisture, and is insensitive to tillage.

Perennial grasses can assimilate nitrogen (N) fixed by non-nodulating bacteria living in the rhizosphere and the plant's own tissues, but many details of associative N fixation (ANF) remain unknown, including ANF's contribution to grass N nutrition, the exact location of fixation, and composition of the associated microbial community. We examined ANF in switchgrass (Panicum virgatum L.), a North American perennial grass, using 15N-enriched N2 isotopic tracer additions in a combination of in vitro, greenhouse, and field experiments to estimate how much N is assimilated, where fixation takes place, and the likely N-fixing taxa present. Using in vitro incubations, we documented fixation in root-free rhizosphere soil and on root surfaces, with average rates of 3.8 μg N g root−1 d−1 on roots and 0.81 μg N g soil−1 d−1 in soil. In greenhouse transplants, N fixation occurred only in the early growing season, but in the field, fixation was irregularly detectable throughout the 3-month growing season. Soil, leaves, stems, and roots all contained diazotrophs and incorporated fixed N2. Metagenomic analysis suggested that microbial communities were distinct among tissue types and influenced by N fertilizer application. A diverse array of microbes inhabiting the rhizosphere, and possibly aboveground tissues, appear to be episodically contributing fixed N to switchgrass.

Nitrogen (N) is an essential but generally limiting nutrient for biological systems. Development of the Haber-Bosch industrial process for ammonia synthesis helped to relieve N limitation of agricultural production, fueling the Green Revolution and reducing hunger. However, the massive use of industrial N fertilizer has doubled the N moving through the global N cycle with dramatic environmental consequences that threaten planetary health. Thus, there is an urgent need to reduce losses of reactive N from agriculture, while ensuring sufficient N inputs for food security. Here we review current knowledge related to N use efficiency (NUE) in agriculture and identify research opportunities in the areas of agronomy, plant breeding, biological N fixation (BNF), soil N cycling, and modeling to achieve responsible, sustainable use of N in agriculture. Amongst these opportunities, improved agricultural practices that synchronize crop N demand with soil N availability are low-hanging fruit. Crop breeding that targets root and shoot physiological processes will likely increase N uptake and utilization of soil N, while breeding for BNF effectiveness in legumes will enhance overall system NUE. Likewise, engineering of novel N-fixing symbioses in non-legumes could reduce the need for chemical fertilizers in agroecosystems but is a much longer-term goal. The use of simulation modeling to conceptualize the complex, interwoven processes that affect agroecosystem NUE, along with multi-objective optimization, will also accelerate NUE gains.

The active fraction of soil organic carbon (SOC) is an important component of soil health and often is quickly assessed as the amount of CO2 released by re-wetting dried soils in short-term (24–72 h) assays. However, soils can lose carbon (C) as they dry and if soil samples vary in moisture content at sampling, differential C loss during the pre-assay dry-down period may complicate interpretations of C availability. We examined pre-assay CO2 loss and its influence on apparent C availability in the same soil at initial moisture contents of 30, 50, and 70 % water-filled pore space (WFPS). We found that 50 and 70 % WFPS treatments lost more C during drying than those in the 30 % WFPS treatment, which led to a 26–32 % underestimate of C availability in wetter soil. We developed a soil-specific correction factor to account for these initial soil moisture effects. Future C mineralization studies may benefit from similar corrections.

Nitrous oxide (N2 O) emissions are highly episodic in response to nitrogen additions and changes in soil moisture. Automated gas sampling provides the necessary high temporal frequency to capture these emission events in real time, ensuring the development of accurate N2 O inventories and effective mitigation strategies to reduce global warming. This paper outlines the design and operational considerations of automated chamber systems including chamber design and deployment, frequency of gas sampling, and options in terms of the analysis of gas samples. The basic hardware and software requirements for automated chambers are described, including the major challenges and obstacles in their implementation and operation in a wide range of environments. Detailed descriptions are provided of automated systems that have been deployed to assess the impacts of agronomy on the emissions of N2 O and other significant greenhouse gases. This information will assist researchers across the world in the successful deployment and operation of automated N2 O chamber systems.

Nitrous oxide (N

Groundwater irrigation of cropland is expanding worldwide with poorly known implications for climate change. This study compares experimental measurements of the net global warming impact of a rainfed versus a groundwater-irrigated corn (maize)-soybean-wheat, no-till cropping system in the Midwest US, the region that produces the majority of U.S. corn and soybean. Irrigation significantly increased soil organic carbon (C) storage in the upper 25 cm, but not by enough to make up for the CO2 -equivalent (CO2 e) costs of fossil fuel power, soil emissions of nitrous oxide (N2 O), and degassing of supersaturated CO2 and N2 O from the groundwater. A rainfed reference system had a net mitigating effect of -13.9 (±31) g CO2 e m-2 year-1 , but with irrigation at an average rate for the region, the irrigated system contributed to global warming with net greenhouse gas (GHG) emissions of 27.1 (±32) g CO2 e m-2 year-1 . Compared to the rainfed system, the irrigated system had 45% more GHG emissions and 7% more C sequestration. The irrigation-associated increase in soil N2 O and fossil fuel emissions contributed 18% and 9%, respectively, to the system's total emissions in an average irrigation year. Groundwater degassing of CO2 and N2 O are missing components of previous assessments of the GHG cost of groundwater irrigation; together they were 4% of the irrigated system's total emissions. The irrigated system's net impact normalized by crop yield (GHG intensity) was +0.04 (±0.006) kg CO2 e kg-1 yield, close to that of the rainfed system, which was -0.03 (±0.002) kg CO2 e kg-1 yield. Thus, the increased crop yield resulting from irrigation can ameliorate overall GHG emissions if intensification by irrigation prevents land conversion emissions elsewhere, although the expansion of irrigation risks depletion of local water resources.

Nitrous oxide (N2O) is a potent greenhouse gas and major component of the net global warming potential of bioenergy feedstock cropping systems. Numerous environmental factors influence soil N2O production, making direct correlation difficult to any one factor of N2O fluxes under field conditions. We instead employed quantile regression to evaluate whether soil temperature, water‐filled pore space (WFPS), and concentrations of soil nitrate (NO3−) and ammonium (NH4+) determined upper bounds for soil N2O flux magnitudes. We collected data over 6 years from a range of bioenergy feedstock cropping systems including no‐till grain crops, perennial warm‐season grasses, hybrid poplar, and polycultures of tallgrass prairie species each with and without nitrogen (N) addition grown at two sites. The upper bounds for soil N2O fluxes had a significant and positive correlation with all four environmental factors, although relatively large fluxes were still possible at minimal values for nearly all factors. The correlation with NH4+ was generally weaker, suggesting it is less important than NO3− in driving large fluxes. Quantile regression slopes were generally lower for unfertilized perennials than for other systems, but this may have resulted from a perpetual state of nitrogen limitation, which prevented other factors from being clear constraints. This framework suggests efforts to reduce concentrations of NO3− in the soil may be effective at reducing high‐intensity periods—”hot moments”—of N2O production.