Effects of moso bamboo expansion on soil nitrification and mineral nitrogen content in broad-leaved forests in Tianmu Mountain, China.

The regulation of soil nitrogen (N) cycling by moso bamboo expansion is one of the key reasons underpinning its expansion into adjacent natural forests. To elucidate the underlying mechanism of how moso bamboo expansion affects soil nitrification and mineral N content, we selected three forest stands along the gradient of moso bamboo expansion in Tianmu Mountain, including secondary evergreen broad-leaved forest, mixed bamboo-broadleaf forest and moso bamboo forest to measure soil net N mineralization rate, net N nitrification rate, potential ammonia oxidation, ammonia-oxidizing archaea (AOA) and bacteria (AOB) gene abundance, soil mineral nitrogen (NH₄⁺ and NO₃^-) content by laboratory incubation. We further examined the relative contributions of AOA and AOB to both potential ammonia oxidation and N₂O emission with selective inhibitor methods. The results showed that moso bamboo expansion increased soil pH and NH₄⁺/NO₃^- ratio. Compared to the broad-leaved forest, the mixed moso bamboo-broadleaf forest and moso bamboo forest increased in soil pH of 0.23 and 0.59, and NH₄⁺/NO₃^- ratio of 56.7% and 164%, respectively. Moso bamboo expansion decreased soil C/N ratio, net N mineralization rate, and net nitrification rate, while the mixed moso bamboo-broadleaf forest and moso bamboo forest showed decrease in soil C/N ratio of 8.7% and 23.4%, in net N mineralization rate of 24.4% and 40.0%, and in net nitrification rate of 24.2% and 45.9%, respectively. Changes in soil potential ammonia oxidation rate and N₂O emission rate mirrored the trend of soil net N nitrification rate, with moso bamboo expansion significantly inhibiting soil nitrification and mitigated N₂O emission. Moso bamboo expansion significantly decreased AOA gene abundance but increased AOB gene abundance. Selective inhibition experiment confirmed a reduced proportional contribution of AOA-driven potential ammonia oxidation alongside an increased AOB-driven potential ammonia oxidation component. Pearson correlation analysis and structural equation modeling revealed that moso bamboo expansion elevated soil pH and decreased C/N ratio, inhibited AOA-driven potential ammonia oxidation and net nitrification rate, consequently amplified NH₄⁺/NO₃^- ratio. Therefore, AOA-driven potential ammonia oxidation is an important driver for moso bamboo expansion, as it creates an NH₄⁺-enriched condition favorable for moso bamboo expansion.