Metabolism of 13C-labeled glucose in aggregates from soils with manure application

Soil microbial biomass and microbial products play an important role in the stabilization of soil structure and, in turn, as a feedback, structure is believed to be a significant control of C dynamics in soils. We investigated the microbial mineralization and assimilation of added 13C-[U]-glucose within macro- and microaggregates from surface soils (Humic Gleysol) obtained from long-term plots amended or not with cattle manure (20 Mg ha−1 yr−1 for 18 yr). Slaking-resistant macroaggregates (250–1000 μm) and microaggregates (53–250 μm) were separated by wet sieving and incubated with 13C-labeled glucose (1000 μg C g−1 soil) and (NH4)SO4 (67 μg N g−1 soil) for 14 d at 25°C following a 7-d period of conditioning at 25°C. The production of 13C-labeled CO2 was measured periodically and the chloroform-labile C (microbial biomass) derived from glucose was determined at the end of the 14-d incubation. The added glucose was mineralized less but assimilated more in the microbial biomass of macroaggregates than in microaggregates, and this effect was generally greater in the manure-amended soil. Overall, the percentage of 13C-labeled glucose assimilated was inversely correlated (r=0.59) with that mineralized during the 14-d incubation. The size of the native biomass 14 d after glucose addition followed the same trend as that of the glucose-derived biomass. Our results support the hypothesis that stable macroaggregates, especially those from manured-soil, support a greater microbial biomass than microaggregates and constitute ‘hot-spots’ for the metabolism of readily-available substrates.