Several previous field studies in temperate regions have shown decreased soil respiration after conventional tillage compared with reduced or no-tillage treatments. Whether this decrease is due to differences in plant residue distribution or changes in soil structure following tillage remains an open question. This study investigated (1) the effects of residue management and incorporation depth on soil respiration and (2) biological activity in different post-tillage aggregates representing the actual size and distribution of aggregates observed in the tilled layer. The study was conducted within a long-term tillage experiment on a clay soil (Eutric Cambisol) in Uppsala, Sweden. After 38 y, four replicate plots in two long-term treatments (moldboard plowing (MP) and shallow tillage (ST)) were split into three subplots. These were then used for a short-term trial in which crop residues were either removed, left on the surface or incorporated to about 6 cm depth (ST) or at 20 cm depth (MP). Soil respiration, soil temperature, and water content were monitored during a 10-d period after tillage treatment. Respiration from aggregates of different sizes produced by ST and MP was also measured at constant water potential and temperature in the laboratory. The results showed that MP decreased short-term soil respiration compared with ST or no tillage. Small aggregates (< 16 mm) were biologically most active, irrespective of tillage method, but due to their low proportion of total soil mass they contributed < 1.5% to total respiration from the tilled layer. Differences in respiration between tillage treatments were found to be attributable to indirect effects on soil moisture and temperature profiles and the depth distribution of crop residues, rather than to physical disturbance of the soil.

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Previous field studies in N Europe have shown that the impact of soil tillage on soil respiration is mostly indirect, caused by altered distribution of plant residues in soil affecting decomposition of residues. Tillage operations alter soil moisture and temperature conditions in soil, which control decomposition dynamics. Experiments under laboratory conditions allow indirect effects of altered residue decomposition to be distinguished from direct effects of mechanical disruption, i.e., the increased exposure of substrates within aggregates and micropores upon tillage. This study examined the effects of physical disruption of soils with different soil texture, land‐use history, and soil organic C content on soil respiration under controlled abiotic conditions. Undisturbed soil samples from 7 sites (arable land and grassland) were incubated at 20°C and three different water potentials (–1, –10, and –30 kPa). Soil respiration was measured before and after physical disruption with laboratory homogenizer, using an automated respiration apparatus. Soil organic C, water content, and bulk density explained 67% of the variation in base respiration. In half of the disrupted samples, bulk density was re‐adjusted by re‐compaction to conditions prevailing before disruption. Disruption and re‐compaction generally resulted in higher respiration flushes than disruption alone. Respiration peaks increased with water content. However, total C losses were small and corresponded to < 0.1 Mg C ha−1. Overall, physical soil disruption increased decomposition of soil organic matter only marginally and temporarily. It would be difficult to detect an effect of tillage on soil organic matter decomposition under field conditions.