Enhanced Foliar Litter Decomposition Rate of Pinus massoniana When Admixed with Broadleaf Species

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In the global ecosystem, the slow decomposition of coniferous forest litter has caused a number of ecological problems, among which is the decay of China’s Pinus massoniana litter. It has been pointed out that converting pure P. massoniana plantations into mixed forests with broadleaf species can improve ecosystem services. Therefore, the selection of mixed species is key for the success or failure of the conversion of near-natural forests. In this study, from the perspective of apoplastic decomposition, the leaf litter of P. massoniana was mixed with three common native broadleaf species, namely Choerospondias axillaries, Cinnamomum camphora, and Cyclobalanopsis glauca, using an indoor incubation method to systematically analyse the differences in the decomposition rates of apoplastic material in each mixture, and to provide a theoretical basis for the selection and mixing of tree species for the management of near-natural forests in P. massoniana forests. After 175 days of indoor incubation of the foliar litter under dark conditions at 25 °C, the residual dry matter of the mixed apoplastic litter of P. massoniana and the three broadleaf trees was lower than that of P. massoniana. It indicated that the incorporation of broadleaf apoplastic foliage promoted litter decomposition, with the most pronounced effect in the case of admixture with C. Camphora. Compared with the group of pure P. massoniana alone, the remaining mass and residual rate decreased by 0.56 g and 9.45%, respectively. The regression equation of Olson’s negative exponential decay model showed that the P. massoniana + C. Camphora mixture had the fastest decomposition rate (k) of 1.305, an increase of 0.237, a decrease in half-life of 0.11 years, and a decrease in turnover period of 0.49 year, compared to the P. massoniana alone group. Most of the measured values throughout the incubation period were significantly lower than the predicted values, suggesting that there was a non-additive and synergistic effect of litter mixing.