Effects Of Thinning On Fungi And Understory And Litter Decomposition Under Chinese Pine Plantations

China has the largest area of artificial forest plantations in the world and most of them are covered by coniferous plantations. Because of slow decomposition rate in needle litter, the pure coniferous plantations are facing the problem of stand quality decreasing. Thinning is one of the effective measures to solve this problem, which promotes the decomposition of litter and speeds up the return of nutrients to soil. It is important to study its mechanism from the aspects of fungi and litter types. During 2011 to 2013, the community structures of macrofungi, litter layer fungi, soil fungi and understory (litter types) were examined by field investigations or laboratory experiments under four thinning intensities (control 2700 trees per hm2, light 1925 trees per hm2, moderate 1700 trees per hm2, heavy 1400 trees per hm2) in 34-year-old Pinus tabuliformis (Chinese pine) plantations. Subsequently, based on the change rules of fungi communities and understory compositions (litter types) after thinning, we designed the simulated decomposition experiment in laboratory to found out the impact of fungal compositions and litter types separately or interactively on litter decomposition rate and N, P, K cycling, where different litter composition inoculated with different fungi communities are put in artificial climate chambers with the same environmental gradient. The results showed that:(1) The macrofungi, litter and soil fungal communities were all changed after thinning, where litter layer fluctuated more than the others. The dominant species in macrofungi, litter and soil fungal communities were changed to more beneficial for litter decomposition. The richness and diversity of macrofungi, litter and soil fungal communities increased after thinning, especially in moderate or heavy thinning.(2) Thinning enhanced the diversity and biomass of understory under Chinese pine plantations, which influenced litter types (more deciduous litter was mixed in litter layer after thinning). Moderate thinning had the highest value of understory diversity and biomass, where the richness and diversity of shrub layer were 119.88% and 36.12% higher than that in control, the richness and diversity of herb layer were 100.01% and 24.03% larger than that in control, and the the biomass of branches and leaf of shrub and herb was 101.84%,152.32% and 94.78% higher than that in control, respectively.(3) After 80 days simulated decomposition experiment in laboratory, the decomposition rates of coniferous litter, deciduous litter and mixed litter (three litter types) were all increased with the increase of fungal species. Mixing litter developed positive non-additive effects. In the early period of decomposition(0-20 days), the decomposition rate of coniferous and deciduous litter in mixed litter increased by 24.43-31.93% and 14.72-43.38% respectively (the increment amplitude became larger with the increase of fungal species), while only that of deciduous litter in mixed litter increased in the middle and later periods of decomposition(40-80 days), and increased by 29.35% at the end of decomposition (80 days).(4) After 80 days simulated decomposition experiment in laboratory, with the increase of fungal species, N net releases in three type litter were all enhanced. Mixing litter developed positive non-additive effects on N net release, which resulted from the significant improvement of N net release from deciduous litter (108.81-356.59%). These positive non-additive effects enhanced with the increase of fungal species.With the increase of fungal species, P releases from coniferous and deciduous litter were both improved in the middle and late period of decomposition. Deciduous litter had the largest P net release followed by mixed litter, then coniferous litter. Mixing litter developed positive non-additive effects on P net release, which resulted from the significant improvement of P net release from coniferous litter (42.23%).With the increase of fungal species, K releases from three type litter were all improved in the late period of decomposition(60-80 days). Deciduous litter had the largest K net release followed by mixed litter, then coniferous litter. Mixing litter developed negative non-additive effects on K net release from coniferous litter (the negative effects were decreased by the increase of fungal species) while developed positive non-additive effects on K net release from deciduous litter (17.31%).Generally, moderate thinning (1700 trees per hm2) increases the fungal diversity, and accelerates the development of understory (changing litter type, more deciduous litter will be mixed into litter layer) under Chinese pine plantations. The increases of fungi species and the changes of litter type (mixtures of deciduous and coniferous litter) are beneficial for litter decomposition and nutrient cycling, which is one explanation for thinning accelerating litter decomposition in coniferous plantations.