Soil depth-dependent C/N stoichiometry and fungal and bacterial communities along a temperate forest succession gradient

Abstract The vertical stratification of nutrient availability and related microbial adaptations in different soil horizons vary along forest succession gradients. However, the driving forces behind such age-related nutrient statuses and microbial dynamics and their depth-dependent distinctions remain unclear. To bridge these knowledge gaps, the current study investigated the carbon (C) and nitrogen (N) contents, C- and N-acquiring extracellular enzyme activities and fungal and bacterial communities in the organic layer (O), surface mineral soil (A) and mineral subsoil (B) in three temperate forest stands (35 years (y), 82 y and 200 y) in the Changbai Mountain region of northeastern China. The O-horizon presented age-related increasing C/N ratios as well as decreases in N%, enzyme (e.g., β-N-acetyl-glucosaminidase) activities and fungal and bacterial richness and diversity. In addition, the dominant early-stage (i.e., 35 y) O-horizon fungal and bacterial species presented positive relationships with the N content and copiotrophic characteristics, such as a powerful ability to mobilize available nutrients. However, the dominant late-stage (i.e., 200 y) O-horizon microbial species were negatively related to the N content but positively associated with the C content and/or C/N ratio and were characterized by an oligotrophic capacity for adapting to N deficiency and recalcitrant substrate decomposition. In the A- and B-horizons, however, the substrate availability (e.g., C and N contents) and nutrient cycling activities (e.g., enzyme activities and fungal and bacterial diversities) exhibited either a decreasing tendency from the early to the middle stage (i.e., 82 y) or an increasing tendency from the middle to the late stage. Such different or even opposing nutrient statuses and microbial adaptations between the decomposed organic matter and soil horizons suggest that the belowground biochemical characteristics are strongly determined by litter quality succession and microbial feedbacks to the depth-dependent allocation of available nutrients.