Effect of microbial activity on penetrometer resistance and elastic modulus of soil at different temperatures

We explore the effect of microbial activity stimulated by root exudates on the penetrometer resistance of soil and its elastic modulus. This is important because it is a measure of the mechanical strength of soil and it correlates closely with the rate of elongation of roots. A sandy soil was incubated with a synthetic root exudate at different temperatures, for different lengths of time and with selective suppression of either fungi or bacteria. The shape of the temperature response of penetrometer resistance in soil incubated with synthetic exudate was typical of a poikilothermic temperature response. Both penetrometer resistance and small strain shear modulus had maximum values between 25 and 30 degrees C. At temperatures of 20 degrees C and less, there was little effect of incubation with synthetic root exudate on the small strain shear modulus, although penetrometer resistance did increase with temperature over this range (4-20 degrees C). This suggests that in this temperature range the increase in penetrometer resistance was related to a greater resistance to plastic deformation. At higher temperatures (> 25 degrees C) penetrometer resistance decreased. Analysis of the DNA sequence data showed that at 25 degrees C the number of Streptomyces (Gram-positive bacteria) increased, but selective suppression of either fungi or bacteria suggested that fungi have the greater role with respect to penetrometer resistance.