Effects of Irrigation Regime and Seasonal Dynamics on the Rhizosphere Microbiome on Calcaric Cambisols in Western Iberian Vineyards: An Observational Study Across Seasons

Understanding how management and seasonality are associated with the soil microbiome is relevant for the sustainability of Mediterranean viticulture. This observational study compared the rhizosphere microbiome of rainfed and irrigated ‘Pardina’ vineyards on Calcaric Cambisols (Tierra de Barros, Extremadura, Spain) in five sampling campaigns (spring 2024–spring 2026) using 16S rRNA gene and ITS1 amplicon metabarcoding. The 30 soil samples were aggregated into ten community profiles per marker (five campaigns × two management), so inference is exploratory and limited by the absence of plot-level replication. Fungal communities in irrigated vineyards showed lower observed genus richness (median 180 vs. 297), Shannon diversity (3.05 vs. 3.84) and evenness than rainfed ones, with the same direction in all five campaigns, and a tendency toward higher dominance of a few genera (Berger–Parker 19.3% vs. 11.9%; not significant after correction), whereas prokaryotic richness and Shannon diversity did not differ consistently. Bray–Curtis PERMANOVA indicated a modest association between management and prokaryotic composition (R2 = 0.237, p = 0.048), which was not robust to permutations restricted within campaigns, and no detectable association for fungi (R2 = 0.151, p = 0.269); both results were sensitive to the inclusion of the spring 2026 campaign. The season and its interaction with management were not significant. Soil pH, available phosphorus and organic carbon differed between managements and planting density differed by design, so the patterns cannot be attributed to irrigation itself. The bacteria-to-fungi ratio, a sequencing-derived index, is reported descriptively only. These results identify candidate taxa and hypotheses to be tested in replicated experiments that include soil moisture and functional measurements.