Response of microbial communities to long-term fertilization depends on their microhabitat.

The objective of this study was to characterize the microbial communities attached to clay (< 2 μm), fine silt (2-20 μm), coarse silt (20-63 μm) and sand-sized fractions [> 63 μm; including particulate organic matter ( POM)] of an arable soil and analyse their response to more than 100 years of two different fertilization regimes. Mild ultrasonic dispersal, wet-sieving and centrifugation allowed the separation of soil particles with the majority of bacterial cells and DNA still attached. Fertilizations increased soil organic carbon ( SOC), total DNA and the abundance of bacterial, archaeal and fungal r RNA genes more strongly in the larger-sized fractions than in fine silt, and no effect was seen with clay, the latter representing above 70% of the total microbial populations. A highly positive correlation was found between microbial r RNA genes and the surface area provided by the particles, while the correlation with SOC was lower, indicating a particle-size-specific heterogeneous effect of SOC. The prokaryotic diversity responded more strongly to fertilization in the larger particles but not with clay. Overall, these results demonstrate that microbial responsiveness to long-term fertilization declined with smaller particle sizes and that especially clay fractions exhibit a high buffering capacity protecting microbial cells against changes even after 100 years under different agricultural management. [ABSTRACT FROM AUTHOR]