Humidity conditions and the development of bacterial communities in soils of contrasting texture

Abstract

The influence of different moisture conditions on the development of whole bacterial communities, as determined by the size of their biomass, was investigated in loamy sand and loose sandy soil. During the 4 months of the trial the soils were: (A) continuously stored at field-moist state (control); (B) constantly kept at about 59% of the maximum water holding capacity (WHO, corresponding to 100% of the capillary water capacity (CWC); (C) repeatedly air-dried and remoistened to 50% of the WHC at 2 week intervals ; (D) continuously stored at air-dry state. In untreated control loamy sand the mean bacterial biomass was found to be 0.19 mg dry mass g −1 dry soil, one third larger than the biomass found in loose sandy soil. This resulted both from the greater numbers as well as the larger mean size of the bacterial cells inhabiting the heavier soil. At persistent moistening of the samples to 100% of the CWC a doubling of the bacterial biomass was noted in both soils. The moisture level corresponding to the natural field-moist state thus limited the development potential of bacterial communities. After four drying-remoistening cycles the mean bacterial biomass was for air dry samples smaller than in the control: by one fourth and one third in the heavier and lighter soil, respectively. For remoistened samples the mean values were found to be higher than the control - by 16% in the heavier and by 18% in the lighter soil. The recurring increase in moisture level following a period of severe water deficit thus favoured bacterial biomass development, and that to a larger extent in the lighter soil than in the heavier one. In continuously air-dry samples the bacterial biomass in the soil decreased by one third and in the lighter soil by as much as four fifths in relation to the control. The diminishing of biomass size resulted primarily from the decrease in the mean size of bacterial cells. The reaction of bacterial communities to changes in moisture conditions was clearly dependent on soil type.