Microbial abundance and C and N stocks in tropical degraded Planosols from semiarid northeastern Brazil

•C stocks in degraded Planosol at semiarid region were halved.•Free-living diazotrophs increase N stocks in degraded Planosol by NBF.•Archaea population was higher at the Bt pedogenetic horizon.•Alkaline phosphatase producers did not differ due to soil degradation. Improper management and exploitation of the natural resources of the semiarid Planosols have triggered irreparable degradation processes in which vegetation can no longer reestablish itself naturally. We investigated preserved and degraded Planosols from Pernambuco State, Brazil, to determine total C and N stocks and quantify the functional and total microorganisms. We hypothesized that the degraded Planosol would have: i-significant reductions in C and N stocks, microbial biomass and activity in relation to the soil with preserved vegetation; ii-significant decreases in the proportion of total diazotrophic bacteria and archaea, ammonia-oxidizing bacteria, iron-reducing and -oxidizing bacteria and in alkaline phosphatase producers along the pedogenetic horizons. Thus, microbial biomass, soil basal respiration and metabolic and microbial quotients were obtained. Furthermore, total and functional gene copies were quantified by quantitative polymerase chain reaction (qPCR), and the functionality of biological nitrogen fixation (BNF) was determined using an acetylene reduction assay. In degraded Planosols, the activity from the present diazotrophs has been responsible for maintaining soil N stocks (i.e. soil N stocks without significant differences between the evaluated Planosols), but the soil C stocks were halved. Biomass and microbial activity did not show significant differences between two Planosols and there was no apparent metabolic stress from the identified microbiota. We found a significant proportion of structural and functional microorganisms in both Planosols. There were many archaea predominating in the Bt pedogenetic horizon in both Planosols, and a high proportion of microorganismal alkaline phosphatase was also found. We highlight the need for studies on BNF efficiency, especially focusing on archaea and free-living bacteria, as they are performing this important biological function, even in totally degraded semiarid soils.