Micrometer-scale physical structure and microbial composition of soil macroaggregates

Soil macroaggregates are discrete, separable units of soil that we hypothesize contain smaller assemblages of microorganisms than bulk soil, and represent a scale potentially consistent with naturally occurring microbial communities. We posed two questions to explore microbial community composition in the context of the macroaggregate: 1) Is there a relationship between macroaggregate physical structure and microbial community composition in individual macroaggregates? And, 2) How similar are the bacterial communities in individual sub-millimeter soil macroaggregates sampled from the same 5-cm core? To address these questions, individual macroaggregates of three arbitrary size classes (250-425, 425-841, and 841-1000 mu m) were sampled from a grassland field. The physical structures of 14 individual macroaggregates were characterized using synchrotron-radiation based transmission X-ray tomography, revealing that a greater proportion of the pore space in the small- and medium-sized macroaggregates is as relatively smaller pores, resulting in greater overall porosity and pore mineral interface area in these smaller macroaggregates. Microbial community composition was characterized using 16S rRNA pyrosequencing data. Rarefaction analyses indicated that the membership of each macroaggregate was sufficiently sampled with only a few thousand sequences; in addition, the community membership varied widely between macroaggregates and the structure varied from those communities strongly dominated by a few phylotypes to communities that were evenly distributed among several phylotypes. We found no strong relationship of physical structure with community membership; this may be due to the low number of aggregates (10) for which we have both physical and biological data. Our results do support our initial expectation that individual macroaggregate communities were significantly less diverse than bulk soil from the same grassland field site. (C) 2013 Elsevier Ltd. All rights reserved.