Litterfall quality modulates soil ammonium and nitrate supply through altering microbial function in bamboo encroachment of broadleaf forests

Plant invasion can change soil microbial function and nitrogen (N) supply through the input of litter with different qualities and quantities, among other factors. However, the underlying mechanism for litter quality to regulate soil N mineralization via microbial function with plant invasion remains unclear. We examined the effect of litterfall with contrasting chemical properties on soil net ammonification and nitrification rates, enzyme activity, and associated microbial function using reciprocal litterfall transplanting between a natural broadleaf forest and a Moso bamboo forest formed after the encroachment of a natural broadleaf forest. Our results showed that, on average, litterfall removal decreased ammonification rates by 33.6% but increased nitrification rates by 137.8%, while litterfall replacement increased nitrification rates by 55.9% in the natural broadleaf forest. Litterfall removal increased ammonification rates by 33.9%, while litterfall replacement increased ammonification rates by 42.1% at the end of the experiment in the bamboo forest. Structural equation modeling revealed that higher quality litterfall (higher cellulose content and lower lignin/N ratio) from the broadleaf forest enhanced net ammonification by altering fungal composition and increasing their ligninolytic capacity. However, the lower quality litterfall from the bamboo forest increased net nitrification rates through altering bacterial community and increasing urease activity. Our results indicate that the increased N supply resulting from litterfall replacement could counteract the low NH₄⁺ availability in the bamboo forest soil, but not in the broadleaf forest soil. The contrasting modulation effects of litterfall quality on soil mineral N production would have cascading effects on bamboo encroachment and forest succession with implications for ecosystem management.