The distinct responses of bacteria and fungi in different‐sized soil aggregates under different management practices

Abstract

Soil aggregates can be considered massive microbial incubators. Thus, the study of the bacteria and fungi colonizing different soil aggregate fractions helps to elucidate the factors influencing the associations between microhabitats in soil aggregates and microbial functioning. The main objective of this study was to evaluate the total bacterial and total fungal abundances and soil quality-related parameters of four aggregate size fractions (>2 mm, 2–0.25 mm, 0.25–0.053 mm and <0.053 mm) at two depths (0–5 cm and 7–12 cm) under three different management regimes (no cultivation, direct seeding [DS] and traditional tillage [TT]). We analysed the pH, electrical conductivity and organic carbon (OC) and nitrogen (N) concentrations in both bulk soil samples and aggregate fractions. For DNA extracted from each aggregate fraction,we performed qPCR analysis of 16S rRNA (an estimate of bacterial abundance) and internal transcribed spacer (an estimate of fungal abundance). The TT decreased the proportion of macroaggregates (>2 mm), which constituted only 16.7% (with 0.17 OC/100 g soil) under this treatment at 0–5-cm depth, compared with 50.7% (with 1.34 g OC/100 g soil) in uncultivated soil. The abundances of bacteria and fungi exhibited contrasting responses to soil management across aggregate fractions, which differed by depth. The abundance of bacteria, but not fungi, was closely related to OC and N concentrations, with higher values in macroaggregates than in microaggregates for TT and DS at 0–5 cm depth. Uncultivated soil showed no OC differences across aggregate fractions and the bacteria were distributed uniformly among them. Fungal abundance was higher in microaggregates, mainly in DS, concurrent with lower bacterial abundance. Therefore, bacteria and fungi responded differently to changes in properties at the aggregate scale induced by soil management within the soil surface horizon.