Respiration Rates of Soil Aggregates in Relation to Tillage and Straw-management Practices in the Field

Soil aggregates were collected from a long-term field experiment on a clay soil having two tillage treatments (ploughing and ploughless) and two straw-management treatments (straw removed and straw incorporated). Aggregates were collected from the 0-12 cm and 12-24 cm depth layers. The respiration rates of the aggregates were measured in the laboratory using conductimetric respirometers. Respiration rates were measured with zero mechanical energy input and with mechanical energy inputs typical of those induced by tillage. Shallow (ploughless) tillage resulted in higher organic carbon (C-org) content in the surface layer and lower C-org content in the deeper layer than the ploughed treatment. Basal respiration rates were positively correlated with the C-org contents of the aggregates. It is concluded that some of the C-org is "physically protected" against microbial activity and it is hypothesized that soils with higher clay content have the potential to retain higher contents of physically protected C-org. There was no effect of straw-management treatment on either C-org or respiration rate. Similarly, there was no statistically significant effect on respiration rate of mechanical energies typical of those used in tillage. However, the application of energies higher than those used in tillage resulted in increased respiration rates. This leads to the hypothesis of energy thresholds for the removal of physical protection from soil C-org.