Long-term nutrient trade-offs in mixed Pinus massoniana plantations with N-fixers: Implication for stage-specific productivity

Introducing nitrogen-fixing tree species (N-fixers) into Pinus massoniana plantations, a key industrial tree species for timber and resin production, is a common practice to enhance nitrogen (N) availability and boost biomass yield. However, the prolonged effects of this practice on ecosystem phosphorus (P) limitation and plantation sustainability remain uncertain. In this study, we identified a stage-dependent shift in P limitation in pure and N-fixers mixed P. massoniana plantations, based on comparisons between early (10-year) and later (18-year) plantation stages. In particular, when compared to pure plantations, N-fixers introduction led to a significant 42.2% increase of soil available P content in the early stage. However, this was followed by a 15.6% decrease in the later stage, which coincided with elevated ratios of soil available N:P, microbial biomass N:P and leaf N:P in P. massoniana , indicating a progressively intensifying P limitation. In the early stage, N-fixers introduction facilitated P mobilization by enhancing acid phosphatase activity and organic acid content, primarily due to stimulated microbial activity. Conversely, in the later stage, P. massoniana appears to adopt carbon-intensive strategies to cope with the heightened ecosystem P limitation, such as promoting the proliferation of ectomycorrhizal fungi and exuding organic acids. Overall, our findings reveal that while N-fixers introduction initially enhances soil P availability, its prolonged effects may exacerbate P limitation. These results highlight the necessity for stage-specific P management strategies to sustain nutrient balance and secure the long-term productivity of P. massoniana plantations, providing a critical scientific basis for the sustainable management of industrial forest resources. • N-fixer introduction enhanced soil P supply short-term via microbial stimulation. • Long-term N-fixer introduction suppressed P supply and disrupted N:P balance. • Stage-specific nutrient management is crucial for long-term yield stability plantations.