Heterotrophic bacterial production measured on soil microaggregates sampled using a Biological Laser Printer

Although routinely measured in aquatic systems over the last 30 years, heterotrophic bacterial production (3H-leucine method) has only more recently been measured in terrestrial ecosystems to elucidate the role of soil biogeochemical processes in global carbon cycles and climate change. Studying bacterial metabolism at the microaggregate scale (100–300 μm) may unmask important microscale biogeochemical relationships amongst bacterial processes and the surrounding soil. Biological laser printer technology was used to sample soil core sections (ca. one mm thick) into ca. 100–300 μm microaggregates. Bacterial production was measured on collections of five microaggregates each (180 samples over 5 cm depth profile) by suspending microaggregates into filtered rainwater with 3H-leucine and incubating for 4 h. Bacterial production varied by over two orders of magnitude amongst groups of five microaggregates collected over the same millimeter scale core slice. Range for all measurements along the core was below detection (0.005) to 897 fg C aggregate d−1. As expected, variability amongst adjacent (within 2 mm of each other) microaggregates groups generally decreased with depth along with the total magnitude of bacterial production rate. Variation in bacterial production at the microaggregate scale was greater than that seen at larger scale (ca. 50 mg; theoretical 3200 microaggregates) sampling of the same core. This is the first demonstration of using a biological laser printer for fine scale sampling of soil core slices and subsequent measurement of bacterial metabolism.