Soil microbial biomass carbon and nitrogen in pure and mixed stands of Pinus massoniana and Cinnamomum camphora differing in stand age

• Soil microbial biomass concentrations were the highest in the mixed stands.
• Positive effects of admixing on microbes were more pronounced over time.
• Ratio of Cmic/Corg decreased with stand age.
• Ratio of Cmic/Corg increased with soil depth, inconsistent with conventional models.
• Microbial variables were correlated to organic carbon, nitrogen and soil properties.

Converting monocultures into mixed forests has become a common trend of forest management in recent decades owing to enhancement of ecosystem functioning and sustainability due to complementary resource use, environmental benefits and improved soil properties in mixed forests. These positive admixing effects are expected to be more pronounced over time since the above- and below-ground interactions become more intense as forest grows. Although soil microbes play pivotal roles herein, the interrelationship between tree species composition and soil microorganisms is not well understood, especially in the context of varying forest age. In this study, we compared soil microbial biomass carbon (Cmic), microbial biomass nitrogen (Nmic), soil organic carbon (Corg), and total soil nitrogen (Ntot) in Pinus massoniana and Cinnamomum camphora mixed plantations at 10-yr, 24-yr and 45-yr old stands with their monospecific counterparts in the soil depth of 0–10 cm, 10–20 cm and 20–30 cm. The results showed admixtures created age-dependent strong positive effects on soil microbial biomass and carbon sequestration. 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. We also found the Cmic/Corg and Nmic/Ntot ratios increased with increasing soil depth, likely indicating increasing efficiency of microbes in soil carbon decomposition along the profile, which has not yet been captured by conventional process models. The sensitivity of microbial parameters to tree species composition, stand age, and soil depth revealed in this study not only sheds light on the interpretation of the higher soil quality in mixed than in pure species plantations using the dynamic admixing effects but also highlights the need of further research to improve model representation of the soil processes in deep profile.