Soil Bacterial and Archaeal community largely reflects changes in soil properties induced by sheep grazing in an arid shrubland of Patagonia

Abstract

The aim of this study was to analyze the diversity of Bacteria and Archaea in arid soils from the Patagonian Monte under different sheep grazing pressures, and their relation with soil physicochemical properties. Upper soil samples associated to vegetated patches (P) and inter-patches (IP) were collected at a grazed site (G) and a nongrazed site (NG) within the field “San Guillermo” (42˚51’S, 64˚54’W). Aliquots of the soil samples were used to determine soil moisture, pH, texture, organic C, total N, C/N ratio, and the concentration of ammonium, nitrate and nitrite. The diversity and community structure of Bacteria and Archaea in DNA extracted from soil samples was analyzed by MiSeq sequencing of 16S rRNA genes. Bioinformatic analyses were performed in QIIME2 software and sequences were classified using the Greeengenes database. Soil moisture was low (< 3.6%), pH varied between 7.8 and 8.3, and texture was sandy loam. Soil organic C, total N, ammonium and nitrite concentrations were higher (p < 0.05) at NG (C: 5.79 ± 0.66 mg/g; N: 0.50 ± 0.03 mg/g; NH4 +: 2.25 ± 0.12 µg/g; NO2 - : 0.30 ± 0.03 µg/g) than at G (C: 3.29 ± 0.68 mg/g; N: 0.30 ± 0.05 mg/g; NH4 +: 1.80 ± 0.16 µg/g; NO2 - : 0.16 ± 0.03 µg/g), but nitrate concentration and the C/N ratio did not differ significantly among samples (p > 0.05). Two-way nested analysis of similarities based on the Bray Curtis index detected differences in microbial community composition at G and NG (global R: 0.59, significance 0.3%), and this result was also supported by cluster and MDS analyses. The families with higher contributions to these differences (> 1%) were Gaiellaceae and Rubrobacteraceae (Actinobacteria), Nitrososphaeraceae (Thaumarchaeota), and unclassified families of the phyla Euryarchaeota, Acidobacteria, Proteobacteria and Actinobacteria. Four soil physicochemical variables (moisture, clay percentage, organic C and nitrite concentration) showed the highest correlation (Rho: 0.71, significance 1%) with the Bray Curtis resemblance matrix based on microbial community composition. No differences were detected between P and IP in physicochemical properties, nor in microbial community composition (p > 0.05). In this study, grazing disturbance exerted a major effect shaping soil Bacterial and Archaeal community, through their response to changes in soil physicochemical properties, both in vegetated patches and in bare soil areas.