Composition of Soil Bacterial and Nematode Communities within Soil Aggregates in a Kiwifruit Orchard under Cover Crop Treatment

Soil, which exhibits difference in nutrient contents and aggregate sizes, provides spatially distinct habitats for biota. Cover crops influence the compositions of soil organism communities, playing an indispensable role in regulation of underground food webs and ecosystem functions. However, the effect of cover crops on soil microbes and nematodes distribution within different aggregate sizes remains unknown. Thus, a field experiment in a kiwifruit orchard with cover crops was conducted to estimate the distribution of soil nematodes and bacteria with different soil aggregate sizes (mega-aggregate (>2 mm, LMA), macro-aggregate (0.25-2 mm, SMA), and micro-aggregate (<0.25 mm, MA)) and cover crop treatments (four cover crop species (CC) and no cover crop as control (CK)). The results showed that bacterial compositions varied with both aggregate sizes and cover crop treatments. The composition of bacterial community was significantly different between mega-aggregate and micro-aggregate, and bacterial community diversity was significantly higher in micro-aggregate compared with mega-aggregate. Moreover, cover crop treatment dramatically changed the compositions of bacterial communities. However, the nematode communities were mainly impacted by soil aggregate sizes. Larger aggregates (mega-and macro-aggregates) contained higher abundance of omnivores/predators and lower abundance of fungivores. In contrast to bacterial community, the richness of nematode community was lower in micro-aggregates compared with larger aggregates (mega-and macro-aggregates). Redundancy analysis (RDA) and structural equation model (SEM) showed soil organic carbon (SOC) was the main soil factor that directly and indirectly affected both bacterial and nematode communities. The investigations of both bacterial and nematode communities could provide a better understanding on carbon and nutrient cycling across aggregate size fractions.