Effects of Mechanical Stresses and Strains on Soil Respiration

Abstract

Mechanical stresses and strains are imposed on soils by natural processes, such as wetting and drying cycles, and also by anthropogenic actions such as tillage and traction (Dexter, 1988). If the imposed stress exceeds a critical value, the pore structure of the soil changes (Mitchell, 1993). In a general context applicable to agricultural activities, compressive stresses lead to a bulk reduction in porosity, whereas shear and tensile stresses under certain conditions increase porosity. Since the pore space is also the habitat for soil micro-organisms, changes in pore properties induced by mechanical stresses and strains can have a marked effect on microbial activity. Numerous field studies have been conducted to evaluate the change in microbial activity resulting from the application of mechanical energy in the form of different tillage systems (Carter, 1992; Chan et al., 1992; Beare et al., 1994; Franzluebbers et ai., 1994). An increase in tillage intensity usually results in an increase in microbial activity measured as evolved CO₂. It has been suggested in these studies that tillage opens up pores thus exposing previously physically protected organic matter to attack by organisms. Other changes likely to influence microbial activity following tillage include changes to soil climate, water status and aeration (Jenkinson et al., 1992). The depletion of the carbon pool in soil through tillage has significant implications for soil quality and greenhouse gas emission (Kern and Johnson, 1993).