Microtopographic differences in soil properties and microbial community composition at the field scale

Soil biogeochemical processes, such as soil-atmosphere greenhouse gas (GHG) fluxes, are highly variable in space and time, even in homogenously managed agricultural fields. Although soil properties which regulate biogeochemical processes are known to vary predictably with topographic position at the hillslope to watershed scale in landscapes with noticeable topographic relief, the role of microtopography in influencing field scale spatial variability in soil properties in relatively flat landscapes, such as the Midwest U.S., is poorly understood. Moreover, microtopographic effects on soil microbial community composition and its functional importance in mediating biogeochemical processes have previously received little attention. We, therefore, investigated the effect of microtopography on soil properties and microbial community composition which may regulate soil biogeochemical process rates. We sampled paired poorly-drained depressional and well-drained upslope areas in eight replicate maize fields in Champaign County, Illinois. We observed microtopographic differences in soil pH, HCl-extractable Fe(III) concentrations, and soil organic C concentrations (P < 0.05). After normalizing for site variance, we also observed microtopographic differences in microbial community composition based on bacterial 16S rRNA, fungal ITS2, and nitrogen-cycling functional genes (archaeal and bacterial amoA, nirK, nirS, and Clade 1 nosZ), with soil pH and HCl-extractable Fe(III) exhibiting strong correlations with community structure (P < 0.05). Taxa capable of anaerobic metabolisms, such as Geobacter and Anaeromyxobacter, were relatively more abundant in poorly-drained, depressional soils (P < 0.05), suggesting a possible functional difference in the distinct communities. Overall, our findings demonstrate microtopographic differences in soil physicochemical properties and microbial community composition which could contribute to spatial variation in biogeochemical process rates.