Forest conversion effect on soil nitrogen transformation and nitrous oxide emissions worldwide

• Forest conversion decreased soil total N storage and ammonium content. • Forest conversion increased soil autotrophic nitrification and N 2 O emissions. • Forest conversion decreased gross N mineralization and microbial biomass. • Soil (BD and carbon) and climatic (MAT and MAP) properties were indirectly altering soil N cycling. Understanding soil nitrogen (N) storage and loss following forest conversion is crucial to evaluate land resource utilization and environmental impacts. However, forest conversion altering soil N dynamics and nitrous oxide (N 2 O) emissions and their driving factors remain largely unknown. Here, we conducted a global meta -analysis of 97 published papers to evaluate the effect of forest conversion (e.g., conversion of primary forests to secondary forests, managed plantations, grasslands, or croplands) on soil N transformations and N 2 O emissions. We found that forest conversion decreased gross N mineralization and ammonium concentration due to reduced soil microbial biomass, total carbon, and total N. Conversely, forest conversion overall increased autotrophic nitrification and N 2 O emissions due to the increased ammonia-oxidizing bacteria and archaea abundances. Primary forest converting to croplands and plantations notably increased soil N 2 O emissions in temperate and tropical zones. Structural equation model showed that increased ammonium and nitrate levels, and decreased total carbon were the main factors inducing soil N 2 O emissions in unfertilized ecosystems, while the increased nitrate was a key factor inducing soil N 2 O emissions in fertilized ecosystems. Altogether, our results suggest that forest conversion overall limits soil N storage while stimulating N 2 O emissions, highlighting the importance of conserving primary forests to reduce N losses and the negative impacts on the environment.