Browsing by Subject "Nitrous Oxide"

Now showing 1 - 3 of 3
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    Assessing Agricultural Nitrous Oxide Emissions and Hot Moments Using Mesocosm Simulations
    (2021-02) Miller, Lee
    Nitrous oxide (N2O) is a major greenhouse gas and the leading stratospheric ozone depleting substance emitted today. Effective mitigation strategies from agricultural soils, the largest anthropogenic source of N2O, have remained elusive due to the occurrence of difficult to predict “hot moments,” or brief periods that contribute disproportionately to N2O budgets. Future precipitation patterns may further complicate mitigation efforts by causing more favorable soil conditions that drive N2O emissions. The objectives of this thesis, therefore, were to: 1) Assess the sensitivity of N2O emissions to changes in precipitation; 2) Devise an approach to objectively identify hot moments; and 3) Identify the conditions that drive these hot moments. Six growing season simulations with nearly continuous N2O measurements from an indoor mesocosm facility were used to address each of these objectives.Four seasons comparing historical normal (1984-2014) versus end-of-century (2071-2099) precipitation patterns demonstrated that, for non-limiting soil nitrogen, the greatest N2O emissions occurred when soil water-filled pore space (WFPS) was between 40 and 80% and that cumulative emissions increased with the number of days above ~60% WFPS. Consequently, any future changes in precipitation that contribute to these conditions will likely increase N2O emissions. An assessment of 1350 rain events revealed that N2O emissions consistently increased within 24 hours of rainfall and when soil moisture was near ~60% WFPS and NH4+ was greater than 10 mg N kg-1 soil. However, emissions were suppressed when WFPS was ~60% and soil NH4+ was below 5 mg N kg-1 soil, demonstrating that soil NH4+ availability is an important determinant of the N2O emission response to rain. Finally, a new approach to categorize hot moments from background N2O emissions identified greater soil nitrate (NO3-), air temperature, and 10 cm WFPS among hot moments. Further, short-term pulses (up to 38 hours) of hot moments were sustained when nitrate (NO3-) was greater than 50 mg N kg-1 soil and WFPS was above 50%, and were controlled by short-term changes in WFPS and air temperature. These findings have important implications for the development of N2O emission models, agricultural management and mitigation strategies, and demonstrate the efficacy of conducting mesocosm experiments under controlled conditions.
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    Mitigation of Nitrate, Nitrous Oxide, and Ammonia Loss with Time And Source of Nitrogen Application in Corn
    (2022-08) Menegaz, Sonia
    The large need of nitrogen (N) for crop production and the negative impacts of fertilizer on the environment result in a compelling need to identify new or advanced N management practices to reduce N losses while maintaining productivity and profitability. Nitrate (NO3) leaching, nitrous oxide (N2O) denitrification, and ammonia (NH3) volatilization are the most common pathways of N loss when synthetic N fertilizer is applied in agricultural lands. The objectives of this 7-year study (2014-2020) were to evaluate the use of traditional management (urea applied at pre-plant) and advanced [enhanced efficiency fertilizers and split applications] N management practices on i) N losses (NO3, N2O, and NH3); ii) corn yield and profitability; and iii) cropping system N balance including plant N removal and soil N status. The field experiment was conducted at the University of Minnesota Southwest Research and Outreach Center, near Lamberton MN. Four treatments were applied and replicated four times in a randomized complete block design: a pre-plant application of 202 kg N ha-1 of either urea (U) or polymer coated urea (PCU; ESN) (E) and 135 kg N ha-1 as urea with urease inhibitor (Agrotain) sidedressed at development stage V4-6 with 67 kg N ha-1 applied before planting as either urea (U/U+) or ESN (E/U+). Across the 7 years, the advanced management practice E/U+ increased corn grain yield (1 Mg ha-1 or 10%) and total N uptake (TNU, plant + grain N; 18 kg N ha-1) compared to the traditional management practice (U), while E and U/U+ had similar grain yields and TNU to the other treatments. Furthermore, while net economic returns (NER) were not statistically different between treatments, E/U+ generated numerically greater NER per hectare than U ($63), E ($52), and U/U+ ($46). Nitrate-N leaching (measured from 2015 to 2020) was highly influenced by weather. Excess precipitation, especially after fertilizer application, increased NO3-N loads, which was lower for E compared to the other treatments; however, flow-weighted NO3-N concentration was not different between treatments, with only a trend for lower NO3-N concentrations for E. Nitrous oxide emissions (measured from 2018 to 2020) also increased with excess precipitation, especially after fertilizer application. Split application treatments and U did not reduce N2O-N loss, but in wet years N2O-N loss was lower for E compared to the other treatments. This, however, was not observed in drier years or when precipitation was evenly distributed. Unlike NO3-N and N2O-N losses, ammonia volatilization (measured from 2019 to 2020), decreased as the result of excess precipitation that helped incorporate the fertilizer deeper into the soil. Overall, NH3-N loss was lower for E compared to other treatments. Integration of all the variables measured into a N balance calculation showed to be a poor approach to estimate N efficiency or impact on environmental quality because plant N uptake overshadow treatment influence on N loss measurements. Our findings indicate that pre-plant ESN can be considered a strategy to reduce N losses while maintaining crop yield. While split treatments increased corn yield, they did not reduce N losses, which contrasts the common assumption held by many that split applications are better for the environment. Nonetheless, the traditional management practice of pre-plant urea was the least efficient, producing lower crop yields and increasing N losses compared to pre-plant ESN in wet years. This demonstrates that there are N management practices that can improve production and environmental protection and their merit should be explored and refined further.
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    Small Size, Huge Impact: Disproportionate Effects of Ponds on Aquatic Carbon Cycling and Atmospheric Greenhouse Gases
    (2023-05) Rabaey, Joseph
    The carbon cycle is essential for all life and is a major driver of Earth’s climate. Freshwater ecosystems (such as lakes, ponds, rivers, wetlands, etc.) play an outsized role in the global carbon cycle, acting in the transport, storage, and emission of carbon to the atmosphere. Freshwaters are significant sources of the greenhouse gases carbon dioxide (CO2) and methane (CH4) to the atmosphere despite only covering 1% of Earth’s surface area, and they have become critical a piece in global greenhouse gas models. Surprisingly, the freshwater ecosystems that may contribute the most to global emissions are small ponds, though causes of emissions and variation across ponds are not well understood. My dissertation aims to more fully understand carbon cycling in ponds and identify key factors that influence greenhouse gas emissions. Through measurements of ecosystem metabolism (i.e., the balance of photosynthesis and aerobic respiration), I found that production and respiration rates in ponds are some of the highest across all freshwater ecosystems, with shallow depths influencing many factors that lead to high rates. By measuring greenhouse gas emissions from a wide range of ponds, I found that the absence of oxygen and high phosphorus concentrations can combine to lead to elevated CH4 emissions. In addition, duckweed on the surface of ponds can exacerbate these conditions and is potentially a target for management strategies. Finally, I monitored greenhouse gas emissions in four ponds throughout an entire year and found that water column mixing and stratification greatly impacts the seasonal timing and magnitude of greenhouse gas emissions. Overall, this work emphasizes that ponds are dynamic ecosystems with high rates of carbon cycling and greenhouse gas emissions, with implications for management and global greenhouse gas dynamics.