Priming Effects Induced By C And N Additions In Relation To Microbial Biomass Turnover In Japanese Forest Soils

An understanding of the priming effect (PE), i.e. the effects of carbon (C) and nitrogen (N) inputs on the priming of pre-existing soil organic C mineralization, is fundamental to predicting future soil C cycling and climate feedback. Since the direction and magnitude of the PE depend on the turnover of microbial biomass C and N, we evaluated the PE in relation to pre-existing biomass C and N using dual labeling of glucose C-13 and mineral N-15 for Japanese moderately acidic (pH of 5.0) cedar and strongly acidic (pH of 4.1) cypress soils. The addition of C only to the cedar soil caused no PE with no change in biomass turnover relative to the control without C or N additions. The addition of N with C caused a positive PE, leading to a net loss of soil C stocks, and decreased pre-existing biomass N due to the higher turnover of biomass N relative to the control. The result for the cedar soil was in line with the stoichiometric decomposition theory but not with microbial N mining theory. By contrast, the addition of C only and both C and N to the cypress soil caused a negative PE. Our study revealed that increasing the pre-existing biomass N turnover can lead to a positive PE, accelerating net soil C loss, when the soil pH does not limit microbial activities.