Biochar addition indirectly affects N2O emissions via soil moisture and plant N uptake

Studies in tropical and temperate regions and in the laboratory have shown that the addition of biochar into agricultural soils has potential to mitigate climate change by increasing crop yield per area, decreasing nitrous oxide (N2O) emissions and increasing soil carbon (C) storage. The impacts of biochar on plant productivity and soil processes are, however, highly variable depending on the properties of the biochar and the soil, plant species and environmental conditions. We studied the effects of biochar addition on soil moisture, yield of Phleum pratense (timothy), respiration and N2O emissions in mesocosms with a bare mineral soil or P. pratense stand. Biochar was made from spruce chips under rather low temperatures (400–450 °C) and was mixed into the whole soil layer of 45 cm during the preparation of the mesocosms. The mesocosms were fertilized with ammonium nitrate (NH4NO3;100 kg N ha−1) at the beginning of the experiment and after each harvest. Air temperature was maintained at 20 °C during the daytime and at 15 °C at night. Soil temperature was kept at a constant 15 °C. Biochar increased soil moisture increasing soil respiration and N2O emissions in the bare soil mesocosms, and yield, nitrogen (N) content and N uptake in P. pratense decreasing N2O efflux in the vegetated mesocosms under dry conditions (surface soil moisture 20–30%). Under wet conditions (surface soil moisture 40–50%), N2O emissions increased in the vegetated mesocosms simultaneously with the decreased N uptake in P. pratense harvest. Biochar could thus benefit agriculture, especially during the dry periods of the growing season, but might also increase N2O emissions. Biochar affected N2O efflux indirectly via soil moisture and plant N uptake.

► Biochar increased N2O & CO2 efflux and soil moisture in bare soil in dry conditions. ► Biochar increased yield and N content of Pleum pratense under dry conditions. ► Biochar decreased N2O efflux in vegetated soil in dry conditions. ► Biochar increased N2O efflux from wet vegetated soil. ► Biochar controlled N2O flux indirectly via soil moisture and plant N uptake.