Soil calcium content as the driving factor for vegetative structure and soil microbial function diverging across a fire chronosequence of the boreal forests in northeast China

The role of biophysical variables in constructing community structure changes with the time since fire. The major objective of this study is to verify the transition stage and its underlying variables for the postfire forest and soil microbial function in the boreal forested area of China. A 50-year fire chronosequence was presented, and biomass of forbs, shrubs and woody plants was separately weighted to assess their contribution to the whole community with the year since fire (YSF). Simultaneously, soil biophysical properties were measured for stands in different time periods after fire. Soil microbial functions, i.e. growth efficiency (GE) and carbon use efficiency (CUE), were calculated based on ecoenzymatic and soil nutrient stoichiometry. In terms of vegetative structure, forbs' proportion decreased from 75% to 1.5%, but the proportion of woody plants increased from 0.04% to 70% across this fire chronosequence. GE and CUE of soil microorganisms averaged 0.242 and 0.236 and were significantly higher in 9, 15 and 31 YSF than in 2 and 3 YSF. Soil metal content was significantly increased at the late stage of this fire chronosequence, and soil calcium content showed a positive correlation with woody plant biomass and a negative correlation with soil microbial function. Overall, the present work highlights that the time period of 15 and 31 YSF is a hallmark stage for aboveground vegetative structure and soil microbial function to change in different trends and that the calcium content may partly account for these two divergent trajectories.