Soil microbial biomass and mineralizable carbon of water-stable aggregates

Biophysical alterations of agricultural soils following adoption of zero tillage (ZT) deserve investigation in order to better understand the processes of soil organic C (SOC) sequestration and turnover. We determined the vertical distribution of soil microbial biomass C (SMBC), C mineralized in 24 d under standard conditions, and basal soil respiration (BSR) in five water-stable aggregate classes, Four soils (loam, silt loam, clay loam, and clay) from the Peace River region of northern Alberta and British Columbia were sampled following 4 to 16 yr under comparison of conventional shallow tillage (CT) and ZT, Macroaggregates (>0.25 mm) had greater SMBC, more C mineralized in 24 d? and higher BSR than microaggregates at a depth of 0 to 50 mm, Differences between macro- and microaggregates in these properties decreased with soil depth, Carbon mineralized in 24 d and SMBC were 9 +/- 9% greater (mean of four soils +/- standard deviation among soils) under ZT than under CT in macroaggregates, but were 6 +/- 11% Lower in whole soil due to lower amounts in microaggregates under ZT than under CT, Macroaggregate-protected SOC to a depth of 200 mm was 6.7 +/- 1.9 g m(-2) under CT and 9.8 +/- 2.6 g m(-2) under ZT, Soil organic C in macroaggregates, which had high concentrations of active pools of SOC, appeared to have been shunted into the more stable microaggregate fraction after disturbance with CT, Unlike in temperate, humid climates, decomposition of SOC during the passage from macro- to microaggregates may have been limited by the frigid, semiarid climate.