Along forest succession stages, soil nutrient status always presents vertical stratification along soil horizons. However, the mechanism underlying such soil depth-dependence is still a universally open question.  

Researchers have suggested that the microbiomes play a pivotal role in the allocation of decomposed organic matter across the top organic matter layer and subsoil horizons in forest ecosystems.  

To test this idea, Zhen Bai of Institute of Applied Ecology, Chinese Academy of Sciences, and his colleagues investigated the carbon (C) and nitrogen (N) contents, extracellular enzyme activities and microbial dimensions (e.g., specialization, abundance, and diversity) in different soil horizons along a temperate forest succession gradient. 

They found that the early-stage organic matter layer was characterized by high substrate quality (i.e., high N content but low C/N ratio), extracellular enzyme (e.g., β-N-acetyl-glucosaminidase) activities, as well as highly diversified and exploitative traits (e.g., high richness and diversity, a powerful ability to mobilize easily available nutrients). However, the O-horizon lost such copiotrophic life strategies along with forest succession, i.e., presenting the age-related increase in C/N ratios but decreases in N contents, enzyme activities and microbial richness.  

In the subsoil horizons, however, the substrate quality, extracellular enzyme activities and microbial richness were either lowest at the middle succession stage or showed an increasing trend between the middle and later stages.  

Particularly, the N content showed an age-related decline at the top organic matter layer; however, the N content varied little at the upper subsoil horizon, and even significantly increased at the lower subsoil horizon across forest succession stages. 

The results reveal top-down sieving processes, i.e., depth-dependent nutrient availability is first dependent on the substrate preference of microbial communities at organic matter layer and is then determined by microbial adaptations to the corresponding translocation of decomposition products at subsoil horizons.  

Their work was published online in Catena with the title of “Soil depth-dependent C/N stoichiometry and fungal and bacterial communities along a temperate forest succession gradient”. 

The research was funded by the National Key Research and Development Program of China, the Hunan Key Laboratory for Structure and Ecosystem Service of Subtropical Forest and the National Natural Science Foundation of China.