Microbial community and functional diversity associated with different aggregate fractions of a paddy soil fertilized with organic manure and/or NPK fertilizer for 20 years

Microbial communities are sensitive to fertilizations, and the response of microbial communities to different fertilization regimes in bulk soils has been studied. However, it is unclear how microbial communities performance in soil aggregates. We determined the composition and functional diversity of the microbial community within different aggregate size fractions of a paddy soil fertilized with organic manure (OM) or/and mineral NPK fertilizer (NPK/NPKO) for 20 years, along with a control (no fertilizer, NoF). Soils were sampled in late December 2010 from a long-term experimental site established from an uncultivated wasteland in 1990. Five aggregate fractions, i.e., large macroaggregates (> 2 mm), macroaggregates (1-2 mm), small macroaggregates (0.25-1 mm), microaggregates (0.053-0.25 mm), and non-aggregated silt and clay particles (< 0.053 mm), respectively, were obtained by wet sieving. Microbial community composition was estimated based on phospholipid fatty acids (PLFAs) analysis. Microbial functional diversity was evaluated using the community level physiological profile method by Biolog Eco-microplate. The PLFA signature analysis revealed that microbial biomass tended to decrease with decreasing aggregate size. The application of fertilizer significantly increased the microbial biomass of large macroaggregates relative to the NoF treatment, but there was no significant difference among aggregates of the three fertilization treatments. Average well color development (AWCD) as well as all the functional diversity indices was almost highest in large macroaggregates and microaggregates of NoF, OM, and NPK treatments and in microaggregates of NPKO treatment, while all of them were lowest in silt and clay particles. NPKO treatment led to the greatest microbial functional diversity. AWCD values and McIntosh index (U) of most aggregates under the NPK treatment were significantly increased. The OM treatment did not significantly affect microbial functional diversity associated with different aggregate fractions. Aggregate fractions and fertilization significantly affected microbial community composition and functional diversity. Soil organic C and carbon to nitrogen (C/N) ratio dominated microbial community composition and metabolic profile associated with different aggregates, respectively.