Linking Key Properties of Biochar to Nitrous Oxide Emission in Soil: Chemical and Molecular Mechanisms

Abstract

Biochar is a product of biomass pyrolysis, by which bioenergy is produced. It has been suggested that biochar can be used as a soil amendment agent to improve soil physical (e.g. particle size distribution, bulk density and water retention), chemical (e.g. pH and cation exchange capacity) and biological (e.g. microbial community and enzyme activities) properties. Biochar can also increase soil carbon (C) inventory as a means of carbon storage and nitrogen (N) retention, improve plant growth, and reduce greenhouse gas emissions. Meanwhile, nitrous oxide (N2O) is a potent greenhouse gas with ca. 310 times that of carbon dioxide (CO2) and also a serious ozone depleting compound, contributing greatly to the global warming. To meet the challenges associated with climate change, greenhouse gas (particularly, N2O) emissions have to be reduced. Hence, studies on how biochar incorporation into soil affects N2O emission are of great significance. An increasing number of studies have shown that biochar amendment decreased the soil N2O emissions, while some other studies indicated no change or an increase in the N2O emission in biochar-amended soils. Various physical, chemical and molecular mechanisms have been proposed to describe the processes responsible for soil N2O emission, such as reduced N availability, decreased abundance of some denitrifying genes, accelerated the electron transfer to facilitate the last step of denitrification (N2O to N2), and increased adsorption of N2O. However, little direct evidence has been presented to support these mechanisms as biochar effect varies with soil type, biochar type (feedstock, pyrolysis temperature), plant species as well as environmental conditions.