Biochar application and summer temperatures reduce NO and enhance CH emissions in a Mediterranean agroecosystem: Role of biologically-induced anoxic microsites.

Biochar applications have been proposed for mitigating some soil greenhouse gas (GHG) emissions. However, results can range from mitigation to no effects. To explain these differences, mechanisms have been proposed but their reliability depends on biochar type, soil and climatic conditions. Furthermore, it is found that the mitigation capacity is dependent on how the biochar is aging under field conditions. The effects on NO, CH and CO emission rates of a gasification pine biochar (applied as 0, 5, and 30 t ha) were studied between 8 and 21 months of the application in an alkaline soil cropped to barley under Mediterranean climate. Together with GHG, soil chemical and biological properties were assessed, namely, changes in labile organic matter content and nutrient status, and pH, as well as microbial abundance, activity, and functional composition. During the 2 years of the application, significant changes were observed at the highest rate of biochar application such as higher contents of water, K, Mg, SO, higher basal respiration, and with non-significant changes in microbial community, though with some temporal effects. Regarding GHG, NO decreases coupled with CH increases in the summer sampling were measured, although only for the highest application rate scenario. Such effects were unrelated to pH, bioavailable nitrogen status, or bulk soil microbial community shifts. We hypothesized that the key is the porous structure of our wood biochar, which is able to provide more and diversified microbial microhabitats in comparison to bulk soil. At higher temperatures in summer, biologically-induced anoxic conditions in biochar pores acting as microsites may be promoted, where total denitrification to N occurs which leads to NO uptake, while CH production is promoted.