Nitrous oxide (N₂O), a potent greenhouse gas, is significantly emitted from agricultural soils, particularly under flooded (hypoxic) conditions caused by over-irrigation or heavy rainfall. In Florida, frequent heavy rains and hurricanes create waterlogged soils, leading to substantial N₂O emissions and crop damage. The recommended nitrogen (N) fertilization for Florida vegetable crops further exacerbates this issue. This research addresses this problem by investigating the potential of solid oxygen fertilizers (SOFs) and biochars to mitigate N₂O emissions from flooded agricultural soils. SOFs, like calcium peroxide (CaO₂) and magnesium peroxide (MgO₂), release oxygen, improving soil aeration and potentially reducing N₂O production by inhibiting heterotrophic denitrification. Biochar, a carbonaceous material produced from pyrolysis, has shown variable effects on N₂O emissions, potentially influencing soil microbial activity and nutrient availability. This study aims to evaluate the combined effects of SOFs and biochars on N₂O production and mineral nitrogen dynamics in two contrasting Florida soils – mineral soil and organic soil – with and without nitrogen fertilizer.

Extensive research highlights N₂O as a significant greenhouse gas with a long atmospheric lifespan and high global warming potential. Agricultural practices, particularly nitrogenous fertilizer application, are major contributors to N₂O emissions. Several factors influence soil N₂O production, including soil aeration, moisture content, organic matter, pH, carbon-to-nitrogen ratio, and available nitrogen. Flooding, often caused by heavy rainfall events, creates hypoxic conditions favoring heterotrophic denitrification, a primary pathway for N₂O production. Biochar, with its high carbon content and varied properties, has been studied as a potential soil amendment for carbon sequestration and greenhouse gas mitigation. However, its effects on N₂O emissions are variable and depend on factors such as biochar type, soil characteristics, and nitrogen fertilization practices. Previous research has explored the use of SOFs to alleviate hypoxic stress in plants by increasing oxygen availability. However, their role in mitigating N₂O emissions from flooded soils remains under-investigated. This study will build upon existing knowledge by evaluating the synergistic effects of both SOFs and biochars on N₂O production in diverse soil conditions.

Two laboratory incubation experiments were conducted using mineral soil (low organic carbon) and organic soil (high organic carbon) collected from Florida. Two biochars (corn residue and pine bark) were produced by slow pyrolysis. SOFs, CaO₂ and MgO₂, were applied at 0.5% (w/w). Treatments included control (S), N fertilizer (S+N), biochar with and without N, and SOFs with N. Soil samples were incubated for three weeks, with frequent N₂O measurements. Mineral N (NH₄⁺ and NO₃⁻) was measured separately to avoid interference with N₂O measurements. Soil microbial biomass carbon and nitrogen were also determined. Experiments were conducted in a completely randomized design, and data were analyzed using one-way ANOVA with Duncan's Multiple Range Test for mean separation.

Organic soil consistently exhibited greater N₂O production than mineral soil. Nitrogen fertilizer significantly increased N₂O production (74-fold in mineral soil, 2-fold in organic soil). In mineral soil, both SOFs (CaO₂ and MgO₂) significantly reduced N₂O production by 98-99% compared to the N-fertilized control. In organic soil, CaO₂ reduced N₂O production by 25%. The effects of biochars on N₂O production varied depending on soil type. Corn residue biochar increased N₂O production in mineral soil but decreased it in organic soil. Pine bark biochar had no significant effect on N₂O production. SOFs maintained higher levels of available nitrate-N (NO₃⁻) in both soils (52-57 mg kg⁻¹ in mineral soil and 225 mg kg⁻¹ in organic soil with CaO₂). Soil microbial biomass carbon and nitrogen were generally reduced by SOF application.

The results confirm that nitrogen fertilization greatly enhances N₂O production, particularly in mineral soils, highlighting the importance of considering soil properties in addition to fertilizer management. The substantial difference in organic carbon content between the two soil types explains the varying responses to N fertilization. The significant reduction in N₂O production by SOFs, particularly in mineral soil, indicates that the addition of oxygen successfully inhibited heterotrophic denitrification. The contrasting effects of biochar on N₂O production in the two soil types suggest complex interactions between biochar properties, soil conditions, and microbial processes. The maintenance of high NO₃⁻ levels with SOF application suggests that enhanced oxygen availability facilitates nitrification. The reduction in microbial biomass by SOFs could be attributed to increased pH or heat generation from the exothermic decomposition of peroxides. However, this effect may be less significant in organic soil due to the higher buffering capacity.

This study demonstrates the potential of SOFs to effectively mitigate N₂O emissions from flooded agricultural soils. The effectiveness of SOFs varies with soil type, with CaO₂ being more effective in organic soils due to its higher water solubility. Biochar effects are soil-specific. Further research is needed to optimize SOF application rates and explore long-term effects on soil health and crop production. Field studies are crucial to validate these laboratory findings and assess the practical applicability of this approach for sustainable agriculture.

This study was conducted under controlled laboratory conditions, which may not fully reflect the complexities of field settings. The short-term nature of the incubation may not capture long-term effects of SOFs and biochars on soil microbial communities and N cycling. The single application rate of SOFs and biochars may not represent optimal management practices for diverse soil conditions. Future research should address these limitations.