Effect of Different Synthetic Nitrogen Forms and Levels on Nitrification and Denitrification Key Genes Abundances: Implications for Oligotrophic Forest Soil Management

Climate change and anthropogenic nitrogen addition alter the soil physicochemical properties and microbial activity in oligotrophic forest soil. Unbalanced and non-selective nitrogen fertilizer application is lost as gas emissions (N2O, NO) and also contributed to eutrophication through NO3− leachate. Similarly, NO3− infiltrates and contaminated drinking water sources lead to human thyroid dysfunction. In order to protect depleting timber growth due to nitrogen deficiency and increasing ecological concerns from nitrogen misapplication, we reviewed the effects of different synthetic nitrogen forms and levels on the biogeochemical process. In this review, we focused on the most recent findings from research articles, review articles, and meta-analyses on forest soil and also followed the complementary insights from agricultural soil so that we may be able to highlight how these observations contribute to the understanding of the forest soil nitrogen cycle. Firstly, we elaborated the role of nitrification and denitrification in the nitrogen transformation process. Secondly, we discussed the effect of different nitrogen forms and levels on nitrification and denitrification functional gene abundances. Thirdly, we analyzed the possible effect of gene abundances on the nitrogen conversion process. Finally, we revealed that different forms and levels of synthetic nitrogen not only alter the nitrogen conversion pathways by increasing the gene abundances through substrate availability but also shift the gene dominance, thereby modifying soil physicochemical properties, such as pH. This collectively changes the conditions, which are critical for gene expression potential involved in the nitrogen conversion process. These findings may create a direction for sustainable and eco-friendly fertilizer application in nitrogen-deficient soil.