Effects of sample storage on microbial Fe-reduction in tropical rainforest soils

Many biogeochemical investigations of soil processes require temporary and/or long-term sample storage, which may alter soil biogeochemistry and impact the outcome of laboratory experiments. This study examines the influence of storage conditions on soil iron (Fe) reduction. We subjected soil samples from the Luquillo Experimental Forest, Puerto Rico to: (a) 2-mm sieving/homogenization within an anoxic environment, and storage at 4 °C or (b) 22 °C; (c) air-drying at 30 °C followed by 2-mm sieving/homogenization and storage at 22 °C; and (d) storage at 22 °C with no sieving/homogenization (control). We assessed changes in Fe-reduction after one week, six months and 12 months of storage by incubating soils in a 95% N2:5% H2 headspace for 7–15 days. After one week of storage, FeII production was linear with time yielding similar Fe-reduction rates (ave. 2.8 mmol kg−1 d−1) in all treatments except the unhomogenized controls, which exhibited only slightly lower rates (2.2 mmol kg−1 d−1). Storage for a duration of 12 months at 4 °C decreased FeII production following 7 days of anoxic incubation, while air-drying soils maintained similar FeII production levels. Most probable number counts of Fe-reducing microbes were comparable in the air-dried and 4 °C stored soils, but bacterial isolates from the air-dried soils exhibited higher net FeII production over a one week incubation in pure culture media than strains isolated from soils stored at 4 °C. When microbial activity was not limited by low temperature (e.g., during the 22 °C storage treatment), homogenization slowed FeII production. The observed FeII production and cell counts in these tropical soils suggest that storage at cold temperatures, as well as homogenization when microbial activity is not suppressed during storage, reduced the resilience of the resident Fe reducing microbial communities, a finding consistent with studies on other microbial processes in tropical soils. Overall our results suggest air-drying tropical soils maintains their capacity for FeIII reduction during long-term sample storage more effectively than cold-storage. •In the 1st week, storage method doesn't impact iron reduction in tropical soils.•Iron reduction is best preserved long-term in air-dried soil or intact soil blocks.•Long-term refrigeration and homogenization lower FeIII reduction capacity.•Different soil storage methods select for different Fe-reducing microbes.