Plants have developed diverse strategies to effectively uptake the scarce nitrogen (N) resource in soils with low pH and low N availability, typical for most forests. Some plant species can directly affect nitrification in soil through a process known as biological nitrification inhibition (BNI). Plants with BNI capacity release specific compounds to the soil via decomposition of plant litter or root exudation that inhibit nitrification pathways. BNI compound(s) and nitrifier group(s) targeted have been identified only for a few ecosystems and a few perennial grass or cultivated species. However, our knowledge on BNI in forest ecosystems is still very limited. Indeed, the questions about the nature of BNI compounds produced by trees and their effects on nitrifier community remains unanswered. Therefore, FORBES addresses this challenge by digging deeper than ever done before into the BNI capacity of forest tree species. More specifically, our general aim is to uncover the BNI capacity from forest tree species by identifying the compounds responsible for BNI and by testing their effect on target and off-target microbial strains and complex communities. To do so, we will use complementary approaches at the interface between microbiology, soil microbial ecology, forest science and chemical ecology, and we will address the following questions: (i) what is the relative contribution of root exudates vs litter to the production of BNI compounds by tree species?, (ii) which compounds produced by tree species explain their BNI capacity and their effect on multiple nitrifier strains? and (iii) what are the effects of BNI compounds on complex target (i.e. Nitrobacter) and off-target (i.e. other nitrifiers, denitrifiers) microbial communities in soil?