Effects of solid oxygen fertilizers and biochars on nitrous oxide production from agricultural soils in Florida

Nitrous oxide (N₂O) is a potent, long-lived greenhouse gas with a global warming potential 298 times greater than CO₂ and has increased in the atmosphere from ~270 to 330 ppb. Agricultural soils are major sources of N₂O, especially under hypoxic/anaerobic conditions that promote heterotrophic denitrification. Factors such as soil aeration, moisture from heavy rainfall or flooding, available carbon and nitrogen, temperature, and pH regulate N₂O production. In Florida, extreme rains and hurricanes frequently cause waterlogging that elevates N₂O emissions and reduces nitrogen-use efficiency, while vegetable production commonly receives substantial N fertilization (e.g., ~224 kg N ha⁻¹). Biochar amendments have variably reduced N₂O in prior studies, depending on biochar and soil properties. Solid oxygen fertilizers (SOFs; calcium peroxide, magnesium peroxide) can alleviate hypoxia in flooded soils by releasing oxygen, potentially suppressing denitrification-driven N₂O formation. The objective was to evaluate how SOFs and biochars affect N₂O production and mineral N dynamics in two contrasting Florida agricultural soils (mineral, organic) with and without N fertilization under flooded, short-term laboratory incubation.

Prior work attributes soil N₂O emissions primarily to denitrification, with episodic increases after precipitation and under low redox potential. Biochar has been studied for soil improvement and C sequestration, showing variable effects on N₂O emissions across soils and biochar types, influenced by biochar chemistry (volatile matter, ash content), soil texture, C/N interactions, and fertilizer form. Reviews and meta-analyses (e.g., Cayuela et al.) propose both biotic and abiotic mechanisms for biochar’s mitigation of N₂O, but outcomes remain inconsistent. SOFs (e.g., MgO₂, CaO₂) have been used to alleviate plant hypoxic stress in flooded conditions by increasing redox potential via oxygen release; however, their effect on soil anoxia and N₂O suppression has been little studied.

Two Florida agricultural soils were used: a mineral soil (Kanapaha fine sand; 1.2% organic C) and an organic soil (Pahokee Muck; 33.9% organic C). Initial soil properties are reported (texture, pH, CEC, nutrients). Biochars were produced from corn residues (CB) and pine bark (PB) via slow pyrolysis: feedstocks chopped (~2 cm), dried, heated at 10 °C min⁻¹ to 550 °C, held 4 h, cooled, oven-dried (65 °C), crushed, sieved (2 mm). Biochar properties (pH 1:10, total C/N/P, proximate analysis) were characterized. SOFs were calcium peroxide (CaO₂; solubility 1.65 g L⁻¹, pH of 1% suspension 11.7) and magnesium peroxide (MgO₂; solubility 86 mg L⁻¹, pH 10.3). Experimental design: two simultaneous laboratory incubations (23–25 °C, March–April 2017). Treatments (per soil): S (control), S+N (224 kg N ha⁻¹ as ammonium nitrate, 33.5% N), S+CB, S+CB+N, S+PB, S+PB+N, S+N+CPO, S+N+MPO. Biochar and SOFs were applied at 0.5% w/w. Moisture was brought to 60% of maximum water holding capacity (MWHC), pre-incubated 3 d, then adjusted to 100% MWHC at experiment start; moisture was maintained throughout. Replication: 3 per treatment per soil (total 48 units; 2 soils × 8 treatments × 3 reps). Experiment 1 (N₂O): jars (473 cm³) with 100 g mineral or 50 g organic soil, loosely covered, dark incubation ~3 weeks. Gas sampling after 12 h closures at scheduled intervals: MS days 1,2,3,4,6,9,13,21; OS at 0.5,1,2,3,4,5,7,9,13,17,23. N₂O quantified by GC (Shimadzu GC-2014, ECD). Concentrations converted to rates via ideal gas law; cumulative N₂O by linear interpolation. Experiment 2 (mineral N): destructive sampling at days 1, 3, 13, and 21 (MS) or 23 (OS). Mineral N (NH₄⁺, NO₃⁻) extracted with 2 M KCl and measured on a discrete flow autoanalyzer (SEAL AQ2). Microbial biomass C and N measured at end via chloroform fumigation and 0.5 M K₂SO₄ extraction. Soil and biochar pH (soil:water 1:2; biochar:water 1:10), total C and N by dry combustion (Thermo Flash EA 1112), Mehlich-1 extractable nutrients by ICP-AES; soil texture, MWHC, and CEC by standard methods. Statistics: Completely randomized design; one-way ANOVA (SAS PROC ANOVA); mean separation by Duncan’s Multiple Range Test at α=0.05.

- Organic soil (OS) consistently exhibited higher N₂O production rates and cumulative emissions than mineral soil (MS), except for the MS corn biochar + N (S+CB+N) treatment which produced very high emissions. - Effect of N fertilizer: N addition increased N₂O production by ~74× in MS and ~2× in OS compared to unfertilized controls. - Solid oxygen fertilizers (SOFs): In MS, both CaO₂ and MgO₂ with N reduced N₂O production to levels comparable to unfertilized soils, achieving 98–99% reductions relative to N-only controls. The lowest cumulative N₂O in MS was with CaO₂ + N (S+N+CPO): 51 µg N₂O kg⁻¹ soil over 21 days. In OS, CaO₂ + N reduced N₂O by ~25% relative to N-only, while MgO₂ + N did not significantly reduce emissions (S+N+MPO: 22,073 µg kg⁻¹ vs S+N: 23,757 µg kg⁻¹). - Biochars: Corn residue biochar with N (S+CB+N) greatly increased N₂O in MS, yielding the highest cumulative emission among MS treatments (23,185 µg kg⁻¹). In OS, CB + N reduced N₂O relative to N-only. Pine bark biochar with N (S+PB+N) did not significantly change N₂O compared to N-only in either soil. - Temporal dynamics: In OS, most treatments peaked in N₂O production within 24 h, while SOF treatments delayed peak timing and sustained lower rates. In MS, N-only and CB+N peaked around day 4; MgO₂ + N peaked around day 10; CaO₂ + N maintained low, stable rates. - Mineral N dynamics: In MS, N-fertilized soils without SOFs had higher NH₄⁺ (50–73 mg kg⁻¹) and declining NO₃⁻ (to 19–27 mg kg⁻¹). With SOFs, NH₄⁺ was lower (6–41 mg kg⁻¹) and NO₃⁻ was maintained at higher levels (52–57 mg kg⁻¹). In OS, most treatments showed initial high NO₃⁻ (~300 mg kg⁻¹) declining to <20 mg kg⁻¹, with NH₄⁺ rising to ~100 mg kg⁻¹; CaO₂ + N uniquely maintained higher NO₃⁻ (from 501 to 225 mg kg⁻¹) across incubation. - Microbial biomass: SOF + N treatments reduced microbial biomass C and N in both soils relative to controls; CB effects on biomass differed by soil. End-of-incubation pH increases (MS pH ~9.8–10.0 with SOFs; OS pH ~8.25–8.35) accompanied biomass reductions. - Mechanistic insights: SOFs likely reduced N₂O by supplying O₂ via H₂O₂ decomposition, increasing redox potential and suppressing denitrification; CaO₂ outperformed MgO₂ in OS due to much higher solubility (1.65 g L⁻¹ vs 86 mg L⁻¹). CB’s volatile matter may have supplied labile C driving denitrification in MS, whereas in OS with abundant C, CB reduced N₂O likely via different soil–biochar interactions.

The findings confirm that N fertilizer is a key driver of N₂O emissions, yet soil properties, especially organic C content and microbial biomass, determine the magnitude and temporal pattern of emissions under hypoxia. OS, rich in organic C, supported rapid and high N₂O production under saturation, peaking within a day, consistent with C-fueled heterotrophic denitrification. MS required added N and labile C (as with CB) to reach high emissions. SOFs mitigated N₂O by supplying oxygen, increasing redox potential and limiting denitrification. CaO₂’s higher solubility enabled effective O₂ release in both soils; MgO₂ was sufficient in MS but not in OS where microbial O₂ demand was greater. Maintenance of higher NO₃⁻ with SOFs indicates inhibited denitrification and potential for improved plant-available N under flooded conditions. Biochar effects were soil- and feedstock-dependent: CB + N markedly enhanced N₂O in MS, likely via added labile C from volatile matter, but reduced N₂O in OS where mechanisms may involve changes in electron flow, microbial community, or sorption; PB showed neutral effects with N in both soils. Reductions in microbial biomass with SOFs likely reflect elevated pH and exothermic dissolution. Overall, the study demonstrates that appropriately selected SOFs can suppress N₂O under hypoxia while conserving nitrate, but biochar outcomes are context-dependent.

This study demonstrates that under hypoxic (flooded) conditions, N-fertilized mineral soils are particularly prone to large increases in N₂O emissions. Solid oxygen fertilizers effectively mitigated N₂O: both CaO₂ and MgO₂ were highly effective in mineral soil, and CaO₂ reduced N₂O in organic soil while maintaining higher nitrate levels. Biochar effects depended on soil and feedstock: corn residue biochar with N increased N₂O in mineral soil but decreased it in organic soil; pine bark biochar with N had minimal impact in both soils. Results suggest SOFs can be used to alleviate hypoxic stress, reduce N₂O emissions, and sustain plant-available NO₃⁻ in flooded agriculture. Future work should include long-term incubations and field trials across soil types and SOF rates to optimize an SOF-based strategy for grower adoption.

- Short-term laboratory incubation; field conditions and longer-term dynamics (seasonal variability, plant uptake) were not tested. - Only two soils (one mineral, one organic) from Florida were studied, limiting generalizability. - Single application rate for biochars and SOFs (0.5% w/w); dose–response not evaluated. - Potential nitrate leaching under elevated NO₃⁻ with SOFs was not assessed. - Microbial community composition and mechanistic pathways were not directly measured; biochar mechanisms remain inferential. - Elevated pH from SOFs and possible heat effects reduced microbial biomass; plant responses under such pH changes were not evaluated in this study.