The role played by microorganisms in the biogenesis of soil cracks: importance of substrate quantity and quality

The development of cracks in soils has a significant effect on important soil processes such as gaseous diffusion, water flow and root development. It is the heterogeneity and tortuosity of the resulting cracks which are important as they influence and control such processes. The microbial contribution to crack formation was therefore assessed by quantifying the heterogeneity and connectivity of cracks which developed following the addition of substrates differing in quantity and quality to a sandy loam soil. The heterogeneity and connectivity of the cracks was assessed using probability and Monte Carlo techniques respectively, which provided an estimate of how the occurrence of cracks varied with sample area and the nature of the route taken by a random walker in such a space. Increasing the amount of glucose added resulted in significantly (P<0.05) less heterogeneous and less tortuous cracks on subsequent drying of the soil, after the soil had been incubated for 2 weeks at 15°C in a slurried state. The addition of glucose or different forms of cellulose, such as a native pulverised cellulose and a highly crystalline cellulose, generally resulted in significantly (P<0.05) less heterogeneous and less tortuous cracks than those formed in the absence of a substrate addition. However, the addition of carboxymethylcellulose to soil initially prevented crack formation and after a 5 or 15 day incubation period resulted in the formation of cracks which were significantly (P<0.05) more heterogeneous than those generated by unamended soil after equivalent incubation periods. Soil amended with glucose, a water soluble source of carbon, resulted in the formation of cracks which were significantly (P<0.05) less heterogeneous and less tortuous than those generated in soil amended with either pulverised native cellulose or highly crystalline cellulose, both of which are water insoluble sources of carbon. The extent of the heterogeneity or connectivity of the cracks was shown to depend on the recalcitrance of the substrate and the incubation period before the soil slurries were allowed to dry and crack. Microorganisms were thus shown to contribute to the formation of cracks, as well as hindering crack formation, highlighting a dual role as potential degraders, as well as producers, of soil binding agents.