Epsilon Archive for Student Projects

Abstract

Nitrous oxide (N₂O) is a potent greenhouse gas and the dominant ozone-depleting substance of the 21st century. Agriculture has been responsible for approximately 74% of anthropogenic N₂O emissions in the 2010s. Large fractions of annual N₂O fluxes occur during short periods with high emissions ("hot moments”) such as wet-dry cycles (WDC) and freeze-thaw cycles (FTC), the latter remaining poorly represented in current IPCC emission factors. Biochar amendments have shown mitigation potential for soil N₂O emissions, yet the dependency of this effect on soil type, fertilizer type, biochar dose, and environmental conditions remains insufficiently understood.

This laboratory mesocosm study investigated the effects of woodchip biochar applied at four rates (NBC=0, BC1~6, BC2~15, and BC3~24 t ha-1) on N₂O emissions from a clayey soil (pH 5.7) and a sandy loam (pH 7.1). The soils were fertilized with either synthetic NPKS (Nitrogen, phosphorous, potassium & sulphur) fertilizer or dairy slurry manure. Emissions were measured for 58 days at high temporal resolution using a photoacoustic gas spectrometer (INNOVA 1312) under controlled temperature conditions (15°C). Samples were exposed to one WDC at 70% water-filled pore space (WFPS) followed by one FTC at 80% WFPS (−22°C for 7 days).

During the WDC, biochar significantly reduced cumulative N₂O emissions in the clayey soil, with BC2 reducing emissions under NPKS fertilization by 77% and BC3 reducing emissions under manure by 65%. No significant reductions were observed in the sandy loam, potentially linked to its higher pH, lower organic carbon content, and greater within-treatment moisture variability due to observed soil settling. During the FTC, emissions were substantially higher than during the WDC, accounting for 93.6% of total cumulative emissions in the clay soil and 74.7% in the sandy loam. No significant biochar-induced reductions were observed during the FTC alone, although consistent trends suggested potential effects obscured by high variability and limited replication. Over the full experimental duration, BC2 significantly reduced cumulative N₂O emissions for the manure in the clayey soil by 52.8%.

These findings highlight the importance of soil physicochemical properties (particularly pH, texture, and carbon content) in determining biochar’s N2O mitigation efficacy. The dominance of FTC-induced emissions underscores the need for dedicated studies on biochar effects during freeze-thaw events, incorporating mechanistic measurements to guide future mitigation strategies.