Background The replanting of broadleaved trees in pure coniferous plantations is widely implemented, as mixed plantations are generally more stable and functional. However, the effect of interspecific interactions between broadleaved and coniferous trees on internal nutrient cycles of conifers remains unclear.
Methods We selected pure coniferous plantations of a native (Pinus massoniana) and an exotic (P. elliottii) pine species and their corresponding mixed plantations with broadleaved trees (Schima superba) in subtropical China, and measured the nitrogen (N) and phosphorus (P) contents in the rhizosphere soils, fine roots, twigs, needles and needle litter of pines. We calculated the root capture, needle resorption and translocation of N and P by pines to determine the mobility of nutrients in trees.
Results Although the N and P in the rhizosphere soils increased due to the replanting of broadleaved trees, the N and P contents in the aboveground tissues of the two pine species did not increase in mixed plantations. Mixed planting had a negative effect on the N and P capture of native pine and a positive effect on that of exotic pine. The N and P resorption efficiencies increased in native pine but were unchanged in exotic pine after the replanting of S. superba. Native pine preferentially employed an aboveground nutrient resorption strategy, whereas exotic pine tended to adopt a belowground nutrient capture strategy after replanting. Translocation of N and P in trees was detected, which reflected the trade-offs between root nutrient capture and needle nutrient resorption.
Conclusions The effect of mixed planting varied between the species of native and exotic pines, and the internal nutrient cycles of both pine species might be dominated by interspecific interaction effects on nutrients rather than soil nutrients. Our study highlights the importance of selecting suitable broadleaved species for replanting in coniferous plantations.
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