The natural inactivation of enteric pathogens in soil is a critical component of the
multi-barrier approach to prevent infectious disease in humans by enteric
microorganisms when biosolids are used as a fertiliser and soil conditioner on
agricultural land. The addition of biosolids to agricultural soil modifies the soil
microbial community and ecological interactions. Ecological processes, especially
the activities of predatory protozoa, may have a critical role in reducing the survival
of enteric pathogenic bacteria when biosolids are applied to agricultural soil. To test
this hypothesis a series of field experiments were established on two soils of
contrasting organic matter content and fertility status, amended with different sludge
types, to examine the interactions between the soil microbial biomass, total protozoa
numbers, environmental and soil factors and their effects on the decay of the enteric
indicator bacteria, Escherichia coli, in biosolids-amended soil. Soil microbial
biomass carbon (SMBC) concentrations were influenced by soil physico-chemical
properties and, in particular, larger background biomass concentrations were
measured in unamended control soil containing the largest amount of organic matter.
The microbiological content and substrate availability of the supplied materials also
influenced the extent of the increases in SMBC. Soil protozoa numbers consistently
increased in both experimental field soils from background values of 3-3.5 log10 g-1
ds to 4-4.5 log10 g-1 ds after sludge application. The extent of the increase was
consistent with the effect of the organic amendments on SMBC. Laboratory
investigations indicated the direct involvement of bacteriophagous protozoa activity
in the soil ecological processes responsible for E. coli inactivation in biosolidsamended
agricultural soil. This was linked to the addition of an active protozoa
population to the soil in sludge, as well as to the stimulation of protozoa indigenous
to the soil due to inputs of substrates and microbial biomass in sludge. Consequently,
the survival of enteric organisms is a self-limiting process, due to the stimulation of
microbial predatory activity in amended soil. Overall, the results provide assurance
that assumptions relating to soil decay during waiting periods stipulated for
agricultural use of sludge are highly conservative. They also confirm that the
cropping/harvesting restrictions prescribed in legislation and guidance controlling the
application of biosolids on farmland allow the natural attenuation of pathogens to
protect human health with a significant margin of safety.