Rhizosphere soil microbial communities and nitrogen transformation response to forest fire smoke

Abstract

Nitrogen (N) is a limiting element in terrestrial ecosystems, and soil microorganisms play a crucial role in N nutrient cycling. Forest fires, as significant drivers of global change, release large amounts of smoke pollutants that deposit nitrogen-containing compounds, such as nitrate (NO₃-N) and ammonium (NH₄-N), into the soil. These compounds enhance the availability of bioavailable N, influencing the geochemical cycling of N in forest ecosystems. However, our understanding of how forest fire smoke deposition alters soil microorganisms and influences soil N transformation processes remains limited. To address this, we employed metagenomic techniques to analyze differences in microbial communities and N transformation functional genes in the rhizosphere soil of Cunninghamia lanceolata (Lamb.) Hook under varying concentrations of smoke deposition. Our results indicated that, low-concentration smoke deposition significantly (P < 0.05) increased N concentration and N transformation enzyme activity in the rhizosphere soil compared to the control group. After 7 days of low concentration smoke deposition, the net ammonification rate and net nitrification rate were 2.51 and 3.02 times higher, respectively, than in the control. The abundance of functional genes related to soil N loss mediated by microorganisms, such as those involved in nitrification and denitrification processes, increased while functional genes associated with N fixation and transport exhibited less pronounced positive effects. This suggests that N input from forest fires may not persist in soil over time, as evidenced by decreased soil N concentration. Furthermore, Partial Least Squares-Path Modelling analysis demonstrated that soil N conversion enzyme activity had a significant positive effect on N functional microorganisms under low-concentration forest fire smoke deposition. Overall, these findings highlight that smoke deposition affects soil N transformation by altering soil enzyme activity, N content, and microbial communities, and lower smoke concentration appears to have a more beneficial impact on soil N transformation processes.