Metabolism of C-13-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 investigate d the microbial mineralization and assimilation of added C-13-[U]-glucose w ithin macro- and microaggregates from surface soils (Humic Gleysol) obtaine d from long-term plots amended or not with cattle manure (20 Mg ha(-1) yr(- 1) for 18 yr). Slaking-resistant macroaggregates (250-1000 mu m) and microa ggregates (53-250 mu m) were separated by wet sieving and incubated with C- 13-labeled glucose (1000 mu g C g(-1) soil) and (NH4)SO4 (67 mu g N g(-1) s oil) for 14 d at 25 degrees C following a 7-d period of conditioning at 25 degrees C. The production of C-13- labeled CO2 was measured periodically an d the chloroform-labile C (microbial biomass) derived from glucose was dete rmined 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-am ended soil. Overall,the percentage of C-13-labeled glucose assimilated was inversely correlated (r = 0.59) with that mineralized during the 14-d incub ation, 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-s oil, support a greater microbial biomass than microaggregates and constitut e 'hot-spots' for the metabolism of readily-available substrates, (C) 2000 Elsevier Science Ltd. All rights reserved.