Soil microbial biomass and carbon dioxide flux under wheat as influenced by tillage and crop rotation

Soil organic matter is important both from an agronomic and an environmenta l perspective because it affects the capacity of the soil to sustain crop g rowth, and it is a source and sink of atmospheric CO2-C. Soil microbial bio mass comprises a small proportion of total soil organic matter, but it is m ore dynamic than total soil organic matter. Therefore, measurements of soil microbial biomass may show the effects of soil management on potential cha nges in soil organic matter before such effects can be detected by measurin g total soil organic matter. The effects of tillage and crop rotation on so il microbial biomass and activity were studied in 1995-1997 in the wheat ph ase of different cropping rotations that had been established in 1992 under zero tillage or conventional tillage in northern Alberta. Soil microbial b iomass was often significantly (P < 0.05) higher, but never significantly l ower, under zero tillage than under conventional tillage. However, CO2 evol ution (basal respiration) was usually higher under conventional tillage tha n under zero tillage, resulting in higher specific respiration (qCO(2)) und er conventional tillage than under zero tillage. The higher additions but l ower losses of labile C under zero tillage mean that more C is sequestered in the soil in the zero-tillage system. Thus, this system contributes less to atmospheric CO2 than conventional tillage, and that soil organic matter accumulates more under zero tillage. Plots preceded by summerfallow, especi ally under conventional tillage, usually had the lowest microbial biomass a nd CO2 evolution, and plots preceded by legume crops had higher microbial b iomass and lower qCO(2) than other treatments. Tillage and rotation had lit tle effect on total soil organic matter 5 yr after the treatments had been imposed, probably because of the cold climate of northern Alberta, but the results confirm that the labile forms of soil C are more sensitive indicato rs of soil organic C trends than total soil organic C. These effects of til lage and rotation on soil microbial biomass were similar to those on microb ial diversity reported previously. These results confirm that zero tillage and legume-based crop rotations are more sustainable crop management system s than conventional tillage and fallowing in the Gray Luvisolic soils of no rthern Alberta.