Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/1997
Publication Date: N/A
Interpretive Summary: Knowledge of soil organic carbon sequestration and turnover is important in understanding biogeochemical cycles, land management effects on soil quality, and the contribution of soils to greenhouse gas emissions. Macroaggregates (>0.25 mm diam.) had a greater concentration of soil microbial biomass carbon and mineralizable carbon than microaggregates near the soil surface, but more similar concentrations of these active carbon pools at lower depths. We observed a redistribution of active carbon pools within water-stable aggregate classes suggesting that active carbon pools became more associated with macroaggregates under zero tillage and more with microaggregates under conventional tillage. Soil under zero tillage contained more macroaggregate-protected organic carbon than under conventional tillage, indicating that reduced soil disturbance could lead to greater organic carbon sequestration and improved soil quality with improvements in aggregation.
Technical Abstract: 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 basal soil respiration (BSR), C mineralized in 24 d under standard conditions, and soil microbial biomass C (SMBC) in five water-stable aggregate classes of four soils (loam, silt loam, clay loam, and clay) managed for 4 to 16 yr under conventional shallow tillage (CT) and ZT in northern Alberta and British Columbia. Macroaggregates (>0.25 mm) had higher BSR, more C mineralized in 24 d, and greater SMBC than microaggregates at a depth of 0 to 50 mm, with differences between macro- and microaggregates decreasing with soil depth. Macroaggregate- protected SOC to a depth of 200 mm was 67 kg/ha under CT and 98 kg/ha under ZT. Soil organic C in macroaggregates, with high concentrations of active pools of SOC, appeared to have been shunted into the more stable microaggregate fraction upon disturbance with CT. Unlike in temperate, humid climates, decomposition of SOC during the passage from macro- to micro-aggregates may have been limited by the frigid, semiarid climate.