Admixing effects on belowground processes in Pinus massoniana and Cinnamomum camphora forests over time
Converting monocultures into mixed forests has become a common trend of forest management in recent decades as tree species mixtures are generally believed not only to increase biodiversity but also enhance ecosystem sustainability and functioning. The positive diversity-productivity relationships have been proved to be true in many manipulated biodiversity ecosystems that productivity increased with species diversity based on aboveground growth characteristics in many manipulated biodiversity experiments owing to niche partitioning and complementary resource exploitation on particular growth stage. A debate, however, has followed regarding the underlying mechanism (complementarity and sampling effect) and time effect. In this study, we compared belowground fine root biomass, soil microbial biomass carbon (Cmic), microbial biomass nitrogen (Nmic) and soil microbial diversity in Pinus massoniana and Cinnamomum camphoramixed plantations at 10-yr, 24-yr and 45-yr old stands with their mono-specific counterparts in the soil depth of 0-10 cm, 10-20 cm and 20-30 cm.
The results showed admixtures created strong positive effects on fine root biomass, soil microbial biomass, diversity and carbon sequestration in all the development stages. Firstly, the concentrations of Corg, Ntot, Cmic and Nmic were highest in the Pinus-Cinnamomum mixed stands in the whole soil profile at all development stages. These positive admixing effects were non-additive in the 10 and 45 years old stands, while additive in 24 years old mixed stands. Secondly, although the fraction of microbial carbon or nitrogen (i.e., Cmic/Corg or Nmic/Ntot ratio) in soil decreased with stand age, the positive effects of admixture on soil microorganisms became more pronounced in terms of Cmic concentrations as forests grew. However, the admixing effects on fine root biomass over time showed otherwise, which was more closely related to the aboveground stand structure and standing fine root biomass.
Our results revealed that soil microbial parameters were sensitive to the changes of species composition in plantations and stand age, and they could be used as a useful indicator to interpret the effects of plantation management on the soil quality.