Erosion effects on soil microbial carbon use efficiency in the mollisol cropland in northeast China

●Soil erosion decreased soil microbial CUE and increased microbial uptake of carbon.●Soil erosion decreased microbial CUE by decreasing substrate C,N and MBC and increasing soil pH.●Soil microbes had to increase their uptake rate to cope with the loss of substrates with increasing erosion rate.●Soil microbial respiration increased with increasing degree of erosion.●Soil microbial growth rate remained relative stable under different degrees of soil erosion.●Microbial CUE in soil surface was less responsive to erosion than that in deeper soil.Soil microbial carbon use efficiency(CUE)is an important synthetic parameter of microbial community metabolism and is commonly used to quantify the partitioning of carbon(C)between microbial growth and respiration.However,it remains unclear how microbial CUE responds to different degrees of soil erosion in mollisol cropland.Therefore,we investigated the responses of soil erosion on microbial CUE,growth and respiration to different soil erosion rates in a mollisol cropland in northeast China based on a substrate independent method(18O-H2O labeling).Soils were sampled at four positions along a down-slope transect:summit,shoulder,back and foot.We found microbial CUE decreased significantly with increasing soil erosion rate in 5−20 cm soil,but did not change in 0−5 cm.The decrease of microbial CUE in subsoil was because microbes increased C uptake and allocated higher uptake C to microbial basal respiration with increasing soil erosion rate.Microbial respiration increased significantly with soil erosion rate,probably due to the more disturbance and unbalanced stoichiometry.Furthermore,soil microbes in surface soil were able to maintain their growth rates with increasing degree of erosion.Altogether,our results indicated that soil erosion could decrease microbial CUE by affecting soil physical and chemical properties,resulting in more decomposition of soil organic matter and more soil respiration,which had negative feedbacks to soil C sequestration and climate changes in cropland soil.

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.