Biochar and compost soil amendments affect soil carbon and greenhouse gas emissions

Greenhouse gases including nitrous oxide and carbon dioxide emitted from agricultural soil contribute to raising global temperatures. Organic soil amendments may potentially reduce these emissions and contribute to nutrient use efficiency and soil productivity. Poultry manure biochar, poultry manure/crop residue compost and a combination of the two were incorporated into mounded Ferralsol on a blueberry farm in northern NSW, Australia and planted with Southern Highbush seedlings. Carbon addition was standardised at 4 t C ha^-1, with biochar applied at 10 t ha^-1, compost at 23 t ha^-1 and a combination of the two at half those rates each. Soil was sampled at establishment, and nitrous oxide emissions and plant growth monitored for ten months. Significantly increased C and pH were observed in the 0-10 cm layer of the amended soils relative to the control soils. Average soil C in the control was 5.4%, with biochar 6.1%, compost 6.2% and combination 6.3%. Soil pH (in CaCl₂) in the control soil averaged 5.1, with all amendments being significantly (p‹0.05) higher at between 5.6-5.8. Nitrous oxide flux varied across the seasons (4-64 µg N₂O-N m^-2 h^-1), initially emitting at 32-49 µg N₂O-N m^-2 h^-1 at establishment, declining over winter and increasing during early summer and again in late summer. No clear association between emissions of N₂O and treatments was observed. Compost amended soil initially lowered N₂O emissions, which subsequently increased in summer, whereas biochar amended soil lowered N₂O in spring. There was no treatment effect detected in plant canopy volume after 10 months. Further soil sampling, gas and plant growth monitoring will provide additional information as this trial continues.