Jeffrey D. Wood

Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order.

Significance N2O emissions from riverine systems are poorly constrained, giving rise to highly uncertain indirect emission factors that are used in bottom-up inventories. Using a non–steady-state flow-through chamber system, N2O fluxes were measured across a stream order gradient within the US Corn Belt. The results show that N2O emissions scale with the Strahler stream order. This information was used to estimate riverine emissions at the local and regional scales and demonstrates that previous bottom-up inventories based on the Intergovernmental Panel on Climate Change default values have significantly underestimated these indirect emissions. N2O is an important greenhouse gas and the primary stratospheric ozone depleting substance. Its deleterious effects on the environment have prompted appeals to regulate emissions from agriculture, which represents the primary anthropogenic source in the global N2O budget. Successful implementation of mitigation strategies requires robust bottom-up inventories that are based on emission factors (EFs), simulation models, or a combination of the two. Top-down emission estimates, based on tall-tower and aircraft observations, indicate that bottom-up inventories severely underestimate regional and continental scale N2O emissions, implying that EFs may be biased low. Here, we measured N2O emissions from streams within the US Corn Belt using a chamber-based approach and analyzed the data as a function of Strahler stream order (S). N2O fluxes from headwater streams often exceeded 29 nmol N2O-N m−2⋅s−1 and decreased exponentially as a function of S. This relation was used to scale up riverine emissions and to assess the differences between bottom-up and top-down emission inventories at the local to regional scale. We found that the Intergovernmental Panel on Climate Change (IPCC) indirect EF for rivers (EF5r) is underestimated up to ninefold in southern Minnesota, which translates to a total tier 1 agricultural underestimation of N2O emissions by 40%. We show that accounting for zero-order streams as potential N2O hotspots can more than double the agricultural budget. Applying the same analysis to the US Corn Belt demonstrates that the IPCC EF5r underestimation explains the large differences observed between top-down and bottom-up emission estimates.

Population dynamics of Escherichia coli inoculated by irrigation into the phyllosphere of spinach grown under commercial production conditions.

Recent outbreaks of food-borne illnesses associated with the consumption of fresh produce have increased attention on irrigation water as a potential source of pathogen contamination. A better understanding of the behaviour of enteric pathogens introduced into agricultural systems during irrigation will aid in risk assessments and support the development of appropriate farm-level water management practices. For this reason, the survival dynamics of two nalidixic acid resistant strains of Escherichia coli after their spray inoculation into the phyllosphere and soil of field spinach were examined over two growing seasons. E. coli strains NAR, an environmental isolate, and DM3n, a non-pathogenic serotype O157:H7, were applied at rates of 10⁴ to 10⁷ cfu/100ml to the fully developed spinach plants that arose subsequent to the harvesting of their upper leafy portions for commercial purposes (secondary-growth plants). After 72 h, E. coli on spinach were reduced by 3-5 logs. Culturable E. coli were recovered from plants up to 6 days post-inoculation. Survival in soil was greater than in the phyllosphere. Under ambient conditions, the mean 72 h first order decay constant computed by Chicks Law was 0.1 h⁻¹. Although light reduction studies indicated UV irradiation negatively influenced the persistence of E. coli, a simple relationship between UV exposure and phyllosphere E. coli densities could not be established. E. coli introduced to the leafy portions of spinach via spray irrigation displayed rapid declines in their culturability under the open environmental conditions experienced during this study. A 6 day period between the last irrigation and harvest would minimize the risks of E. coli survival in the spinach phyllosphere. E. coli NAR was identified as a possible surrogate for the O157:H7 strain, DM3n.

Relationships between dairy slurry total solids, gas emissions, and surface crusts.

Livestock slurry storages are sources of methane (CH₄), nitrous oxide (NO₂), and ammonia (NH₃) emissions. Total solids (TS) content is an indicator of substrate availability for CH₄ and N₂O production and NH₃ emissions and is related to crust formation, which can affect these gas emissions. The effect of TS on these emissions from pilot-scale slurry storages was quantified from 20 May through 16 Nov. 2010 in Nova Scotia, Canada. Emissions from six dairy slurries with TS ranging from 0.3 to 9.5% were continuously measured using flow-through steady-state chambers. Methane emissions modeled using the USEPA methodology were compared with measured data focusing on emissions when empty storages were filled, and retention times were >30 d with undegraded volatile solids (VS) remaining in the system considered available for CH₄ production (VS carry-over). Surface crusts formed on all the slurries. Only the slurries with TS of 3.2 and 5.8% were covered completely for ∼3 mo. Nitrous oxide contributed <5% of total greenhouse gas emissions for all TS levels. Ammonia and CH₄ emissions increased linearly with TS despite variable crusting, suggesting substrate availability for gas production was more important than crust formation in regulating emissions over long-term storage. Modeled CH₄ emissions were substantially higher than measured data in the first month, and accounting for this could improve overall model performance. Carried-over VS were a CH₄ source in months 2 through 6. The results of this study suggest that substrate availability regulates emissions over long-term storage and that modifying the USEPA model to better describe carbon cycling is warranted.

Multiscale analyses of solar‐induced florescence and gross primary production

Solar induced fluorescence (SIF) has shown great promise for probing spatiotemporal variations in terrestrial gross primary production (GPP), the largest component flux of the global carbon cycle. However, scale mismatches between SIF and ground-based GPP have posed challenges towards fully exploiting these data. We used SIF obtained at high spatial sampling rates and resolution by NASAs Orbiting Carbon Observatory (OCO-2) satellite to elucidate GPP-SIF relationships across space and time in the US Corn Belt. Strong linear scaling functions (R2 ≥ 0.79) that were consistent across instantaneous to monthly timescales were obtained for corn ecosystems, and for a heterogeneous landscape based on tall tower observations. Although the slope of the corn function was ~56% higher than for the landscape, SIF was similar for corn (C4) and soybean (C3). Taken together, there is strong observational evidence showing robust linear GPP-SIF scaling that is sensitive to plant physiology but insensitive to the spatial or temporal scale.

Multi-scale analyses reveal robust relations between solar induced fluorescence and gross primary production

Solar induced fluorescence (SIF) has shown great promise for probing spatiotemporal variations in terrestrial gross primary production (GPP), the largest component flux of the global carbon cycle. However, scale mismatches between SIF and ground-based GPP have posed challenges towards fully exploiting these data. We used SIF obtained at high spatial sampling rates and resolution by NASAs Orbiting Carbon Observatory (OCO-2) satellite to elucidate GPP-SIF relationships across space and time in the US Corn Belt. Strong linear scaling functions (R2 ≥ 0.79) that were consistent across instantaneous to monthly timescales were obtained for corn ecosystems, and for a heterogeneous landscape based on tall tower observations. Although the slope of the corn function was ~56% higher than for the landscape, SIF was similar for corn (C4) and soybean (C3). Taken together, there is strong observational evidence showing robust linear GPP-SIF scaling that is sensitive to plant physiology but insensitive to the spatial or temporal scale.

Simulating crop phenology in the Community Land Model and its impact on energy and carbon fluxes

A reasonable representation of crop phenology and biophysical processes in land surface models is necessary to accurately simulate energy, water, and carbon budgets at the field, regional, and global scales. However, the evaluation of crop models that can be coupled to Earth system models is relatively rare. Here we evaluated two such models (CLM4-Crop and CLM3.5-CornSoy), both implemented within the Community Land Model (CLM) framework, at two AmeriFlux corn-soybean sites to assess their ability to simulate phenology, energy, and carbon fluxes. Our results indicated that the accuracy of net ecosystem exchange and gross primary production simulations was intimately connected to the phenology simulations. The CLM4-Crop model consistently overestimated early growing season leaf area index, causing an overestimation of gross primary production, to such an extent that the model simulated a carbon sink instead of the measured carbon source for corn. The CLM3.5-CornSoy-simulated leaf area index (LAI), energy, and carbon fluxes showed stronger correlations with observations compared to CLM4-Crop. Net radiation was biased high in both models and was especially pronounced for soybeans. This was primarily caused by the positive LAI bias, which led to a positive net long-wave radiation bias. CLM4-Crop underestimated soil water content during midgrowing season in all soil layers at the two sites, which caused unrealistic water stress, especially for soybean. Future work regarding the mechanisms that drive early growing season phenology and soil water dynamics is needed to better represent crops including their net radiation balance, energy partitioning, and carbon cycle processes.

A Long Term Assessment of Phosphorus Treatment by a Constructed Wetland Receiving Dairy Wastewater

Constructed wetlands are cost effective wastewater treatment systems being increasingly used in agriculture. Efficient phosphorus (P) treatment by wetlands can however, be a challenge due to slow removal mechanisms. As a result, extended contact times are often required to achieve desired treatment. There are also concerns related to the long-term sustaina bility of P treatment. Therefore, a five year dataset from a surface flow constructed wetland located in Bible Hill Nova Scotia, Canada was examined to: 1) determine the effects of continued loading on P treatment and 2) evaluate hydrological impacts on P treatment. The wetland was intensively monitored year-round from November, 2000 through April, 2005 for total P (TP) and soluble reactive P(SRP). Dairy wastewater (milkhouse wash water and liquid manure) was loaded at 1.5 ± 1.0 kg ha−1 d−1 for TP and 1.0 ± 0.9 kg ha−1 d−1 for SRP. Mass reductions for the entire monitoring period were 53.7% and 52.7% for TP and SRP, respectively. Soils in this wetland appeared to have a sustained P adsorption capacity, with treatment being largely influenced by hydrology and fluctuations in wastewater loading rates. Linear regression of monthly TP and SRP mass reductions, with monthly outflow volumes resulted in decreased mass reductions with increased outflow. Monthly mass reductions were > 50% when corresponding outflows were < 100 mm. When monthly outflow exceeded 100 mm, however, mass reductions became highly variable. To maintain effective P management by constructed wetlands, the use of approaches that prevent high external hydrological loadings are recommended.

Partitioning N2O emissions within the U.S. Corn Belt using an inverse modeling approach

Nitrous oxide (N2O) emissions within the US Corn Belt have been previously estimated to be 200–900% larger than predictions from emission inventories, implying that one or more source categories in bottom-up approaches are underestimated. Here we interpret hourly N2O concentrations measured during 2010 and 2011 at a tall tower using a time-inverted transport model and a scale factor Bayesian inverse method to simultaneously constrain direct and indirect agricultural emissions. The optimization revealed that both agricultural source categories were underestimated by the Intergovernmental Panel on Climate Change (IPCC) inventory approach. However, the magnitude of the discrepancies differed substantially, ranging from 42 to 58% and from 200 to 525% for direct and indirect components, respectively. Optimized agricultural N2O budgets for the Corn Belt were 319 ± 184 (total), 188 ± 66 (direct), and 131 ± 118 Gg N yr−1 (indirect) in 2010, versus 471 ± 326, 198 ± 80, and 273 ± 246 Gg N yr−1 in 2011. We attribute the interannual differences to varying moisture conditions, with increased precipitation in 2011 amplifying emissions. We found that indirect emissions represented 41–58% of the total agricultural budget, a considerably larger portion than the 25–30% predicted in bottom-up inventories, further highlighting the need for improved constraints on this source category. These findings further support the hypothesis that indirect emissions are presently underestimated in bottom-up inventories. Based on our results, we suggest an indirect emission factor for runoff and leaching ranging from 0.014 to 0.035 for the Corn Belt, which represents an upward adjustment of 1.9–4.6 times relative to the IPCC and is in agreement with recent bottom-up field studies.

Nitrous oxide emissions are enhanced in a warmer and wetter world

Significance N2O has 300 times the global warming potential of CO2 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. Here, atmospheric inverse analyses reveal that direct and indirect N2O emissions from the US Corn Belt are highly sensitive to perturbations in temperature and precipitation. We combine top-down constraints on these emissions with a land surface model to evaluate the climate feedback on N2O emissions. Our results show that, as the world becomes warmer and wetter, such feedbacks will pose a major challenge to N2O mitigation efforts. 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.

EVALUATION OF A DIFFUSED AIR AERATION SYSTEM FOR A CONSTRUCTED WETLAND RECEIVING DAIRY WASTEWATER

Treatment systems that utilize natural processes, such as constructed wetlands, offer a sustainable and economical alternative to conventional wastewater treatment. Dairy farm wastewater was loaded into two similar (~100 m2 each) surface flow treatment wetlands in Bible Hill, Nova Scotia, Canada, at an average rate of 65 kg BOD5 ha-1 d-1. A diffused air aeration system was installed in one of the wetlands based on a design oxygen requirement of 3.52 kg O2 d-1. Wetland treatment performance was evaluated over 20 months (September 2002 to April 2004), during which time the aeration system operated for 13 months. The wetlands were monitored intensively for dissolved oxygen (DO), five-day biochemical oxygen demand (BOD5), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH3-N), nitrate-nitrogen (NO3--N), total phosphorus (TP), and Escherichia coli. Both the aerated and non-aerated wetlands provided effective year-round wastewater treatment. Artificial aeration significantly increased TKN and NH3-N mass reductions (P < 0.001). Aeration did not significantly affect the removal of BOD5, TSS, NO3--N, TP, and E. coli. The data suggest that the benefits of wetland aeration are not great enough to warrant its widespread adoption for small-scale agricultural systems.