ABSTRACT Biological soil crusts (BSC) are commonly found in soils in the drylands regions, which can influence stabilization, water retention, nutrient cycling (particularly carbon (C) and nitrogen (N) dynamics), and several ecological processes. However, the composition of BSC in Brazilian soils undergoing the desertification process remains poorly understood. This study aimed to characterize the bacterial community in BSC formed in a Brazilian semiarid region under the desertification process. Thus, a highly desertified region was selected from which 34 BSC samples were collected. The total DNA of the BSC was extracted from 0.5 g samples, and the bacterial community was sequenced by a Next Generation Sequencing (NGS) platform (Miseq – Illumina®) using universal primers (515F and 806R). Bioinformatic analysis was carried out in QIIME (v.1.9), and the Operational Taxonomic Units (OTU) table was constructed following the Sumaclust methodology. The pH of BSC, C, N, and phosphorus contents was analyzed. Our study identified a diverse bacterial community in the BSCs. Cyanobacteria, Chloroflexi, and Proteobacteria phyla presented the greatest relative abundance (%) across the samples. Cyanobacteria were dominated by the orders Nostocales and Leptolyngbyales. The prediction of the putative functions found that mostf OTU were related to phototrophy, photosynthetic cyanobacteria, and photoautotrophy. The study found correlations between bacterial phyla and BSC properties, with Cyanobacteria positively related to C. Chloroflexi, Armatimonadetes, and WPS-2 were negatively correlated with C and N contents. These results suggest the critical roles bacteria communities play in BSCs from the Caatinga biome and highlight the potential impact of environmental factors on their diversity and functions. (BSC regions stabilization retention particularly (C (N dynamics, dynamics , dynamics) processes However understood Thus 3 collected 05 0 5 0. NGS (NGS Miseq Illumina® Illumina 515F F 806R. 806R R . 806R) v.1.9, v19 v v.1.9 1 9 (v.1.9) (OTU methodology analyzed Chloroflexi % (% Leptolyngbyales phototrophy cyanobacteria photoautotrophy properties Armatimonadetes WPS2 WPS 2 WPS- v1 v.1. (v.1.9 ( v.1 (v.1. v. (v.1 (v. (v

ABSTRACT: Soil enzymes play a fundamental role in nutrient cycling in forest systems. The stoichiometry of C, N, and P–acquiring enzymes has been used to indicate nutrient limitation in the soil. However, the enzymatic stoichiometry remains poorly understood in pure and mixed eucalypt plantations. Thus, this study aims to assess the activity of enzymes in the soil to address the hypothesis that the introduction of N2-fixing trees could influence the enzymatic stoichiometry on C, N, and P cycling. The activity of β-glucosidase (BG), urease (U), and acid phosphatase (AP) was assessed in soil (0-20 cm depth) of pure Eucalyptus grandis without (E) and with N fertilization (E+N), and a mixed system with E. grandis and Acacia mangium (E+A), and a pure A. mangium (A) plantation at 27 and 39 months after planting. The activities of BG/U, BG/AP, and U/AP were used to calculate the enzyme C/N, C/P, and N/P ratios, respectively. Rates of N–acquiring enzymes were higher in E and E+N, while soil microorganisms invested in P–acquiring enzymes in A and E+A. The vector length and angle demonstrated that C demand by microorganisms does not change in relation to N and P, regardless of the treatment. However, N demand decreased in relation to P in A and E+A (mainly at 27 months). Our results suggest that enzymes activity in pure eucalypt systems is limited for their soil-litter nutrient contents. At the same time while acacia and mixed plantation seem to invest in P–acquiring enzymes to improve biological N2 fixation promoted by diazotrophic bacteria associated to acacia.