Changes in aggregation do not correspond with changes in labile organic C fractions in soil amended with 14C-glucose

Dry-sieved aggregates (> 2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm and < 0.25 mm) were amended with 14C glucose at 615 ng C g−1 soil (low glucose, LG) or 2457 ng C g−1 soil (high glucose, HG). We followed the changes in water- and acid-extractable carbohydrate C, 14C microbial biomass and 14C microbial products and water-stable aggregation (MWD) in each aggregate size class over 56 days. CO2-efflux was measured to determine mineralisation of native and glucose-derived C in each aggregate size class. Glucose amendments did not affect the MWD of the > 2 mm aggregates. Both rates of amendment increased the MWD of the < 0.25 mm aggregate size class, but only the HG treatments generally increased the MWD of the 0.5–2 mm aggregate size classes. Increases in acid- and water-extractable carbohydrate C in the amended soils, relative to non-amended soils, were generally evident only up to 7 days after amendment. After this time water-extractable carbohydrate C in the amended aggregates was generally less than that in the non-amended aggregates. The proportions of 14C accumulated in the microbial biomass increased with declining macro-aggregate size, whereas the accumulations of 14C as microbial products were generally greater in the larger macro-aggregate size classes. These differences in C turnover were poorly correlated with clay content and pore size distribution in each aggregate size class. Glucose amendment initially resulted in inconsistent effects on the mineralisation of native C, however, after 21 days the HG treatments consistently increased the mineralisation of all but the 0.25–0.5 mm aggregate size class. This delayed priming effect was attributed to increased exposure of native organic C by enzyme activity in the amended soils and indicated that the amount of semi-protected mineralisable organic C increased with increasing macro-aggregate size. Despite the differences in water-extractable carbohydrate C, 14C microbial biomass and 14C microbial products between amendment treatments, generally all were poorly correlated with MWD in each treatment. This indicated that gross changes in the amounts of freshly deposited organic C did not reflect the labile organic C fractions involved in the stabilisation of aggregates in this sandy soil.