The effects of walnut shell and wood feedstock biochar amendments on greenhouse gas emissions from a fertile soil

Abstract

Land application of biochar, as a strategy to enhance soil fertility and reduce greenhouse gas (GHG) emissions is receiving widespread interest. Short-term soil incubations (29days) were used to investigate the effects of agriculturally relevant biochar applications from two contrasting feedstocks and temperatures on CO2 and N2O emissions from a fertile agricultural soil amended with different types of fertilizer (organic and synthetic). In addition, the effects of biochar on the denitrification process were examined using an acetylene based method to ascertain N2O and N2 emissions during denitrification. Complementary incubation experiments without soil (biochar and biochar with compost) examined the impact on natural or amended organic matter (compost) and biochar stability and surface chemistry were also investigated. Batch incubations (25°C) of biochar (softwood pyrolyzed at 410°C [WF410] and 510°C [WF510] and walnut shell pyrolyzed at 900°C [WA900]) amended soils were performed to determine emissions of CO2 and N2O due to complete (absence of acetylene [C2H2]) and incomplete denitrification (presence of C2H2). Similarly, GHG emissions from the complementary incubations were also measured. Concurrent biochar surface compositional changes were investigated with attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy. Biochar effects on CO2 emissions were not significantly different from controls. WA900 biochar (high pH) affects N cycling resulting in significantly higher emissions of N2O under conditions of complete denitrification and of N2 under conditions examining incomplete denitrification. WF410 (highest H/C ratio and lowest surface area) treatments with compost resulted in higher GHGs emissions which is attributed to a priming effect of the compost organic matter (COM). In addition, WF410 was most susceptible to degradation, evident from infrared spectroscopic analysis of the biochars. Although these results suggest that not all biochars provide substantial benefits as a soil amendment, the data do demonstrate potential for development of biochars with beneficial impacts on GHG emission mitigation and enhancement of soil C stocks.