Effects of 20 Years of Contrasting Tillage on Distribution of Ammonia Oxidisers and Denitrifiers Within Soil Aggregates

Microbial functional groups are heterogeneously distributed between soil aggregate fractions, which is strongly influenced by soil microenvironmental conditions. Intensive tillage practices disrupt soil aggregates, altering microbial niches and potentially affecting nitrogen (N) transformations, including processes leading to nitrous oxide (N₂O) emissions. This study aimed to identify linkages between soil aggregation and microbial community functional composition using field samples after 20 years of differential tillage management: conventional mouldboard tillage (CT) and noninversion minimum tillage (MT). Soil properties, including soil organic carbon (SOC) and the abundance of total bacterial, archaeal and fungal communities and N-functional guilds, were examined in two types of samples: (i) bulk soil and (ii) soil aggregates within three soil fractions: large macroaggregates (4–8 mm), mid-sized macroaggregates (2–4 mm) and small macroaggregates (< 2 mm) in relation to tillage system and soil depth. Our results revealed that MT led to an accumulation of SOC in the upper 0–10 cm of bulk soil. At the same depth, the mid-sized and small macroaggregates exhibited significantly higher SOC content compared to the large macroaggregates. While the mean diameter of aggregates did not significantly change under MT compared to CT, the stability of aggregates improved significantly compared to CT in both observed fractions (1–2 and 2–4 mm) at both depths (0–10 and 10–20 cm). Total bacterial, archaeal and fungal communities' abundance was significantly higher under MT than in CT. Among aggregate size fractions, the highest abundance of total bacteria and fungi was observed in the smallest macroaggregate fraction in the topsoil of MT. Nitrifier and denitrifier communities were more abundant under MT in the 0–10 cm soil layer than in CT and decreased with increasing sampling depth. Among the N-functional genes examined, our results indicated a trend towards higher abundances of bacterial amoA and nosZI genes in small macroaggregates within the MT 0–10 cm layer. AOA/AOB and nosZI/nosZII ratios increased with depth within MT, indicating tillage-specific niche differentiation as a result of changed environmental conditions. Overall, our findings suggest that MT influences aggregate stability and the abundance of N-cycling guilds but does not significantly alter their distribution across different soil macroaggregate size fractions.