Contrasting effects of straw and biochar on microscale heterogeneity of soil O2 and pH: Implication for N2O emissions

Little is known about the microscale heterogeneity of O2 and pH in the interfaces between soil and amendments. In this study, planar optodes were applied to continuously measure the micro-scale O2 and pH dynamics in soil amended with a patch of straw and its biochar. The recalcitrant biochar with high porosity had stronger capability to maintain the oxic zone around the patch area. Mainly through diffusion of alkali carbonates, the biochar increased the soil pH within a few hours but in constrained area (<4.5 mm from the surface of the biochar patch). Such high pH coupled with oxic conditions largely restricted N2O emissions in the biochar treatment. The sufficient labile carbon from straw induced fast O2 consumption with microoxic development in the straw-soil interfaces, while its porous structure could enhance O2 diffusive inputs in the core area, therefore, the microoxic area was formed as a concentric ring around the straw patch. Such enriched oxic-microoxic transient zones would induce nitrification coupled denitrification, which led to the high N2O emissions. Additionally, the microbial degradation of straw resulted in a pulse decline of soil pH, which possibly inhibited the N2O reductases, consequently enhanced N2O emissions. Those results demonstrate the contrasting effects of straw and straw derived biochar on microscale O2 and pH localization as well as the associated N2O emissions. It will contribute to a better understanding of the driving factors for N transformations on a microscale and has the potential to become valuable tool in environmental monitoring.