Seasonal persistence of faecal indicator organisms in soil following dairy slurry application to land by surface broadcasting and shallow injection

Dairy farming generates large volumes of liquid manure (slurry), which is ultimately recycled to agricultural

land as a valuable source of plant nutrients. Different methods of slurry application to land exist;

some spread the slurry to the sward surface whereas others deliver the slurry under the sward and into

the soil, thus helping to reduce greenhouse gas (GHG) emissions from agriculture. The aim of this study

was to investigate the impact of two slurry application methods (surface broadcast versus shallow injection)

on the survival of faecal indicator organisms (FIOs) delivered via dairy slurry to replicated

grassland plots across contrasting seasons. A significant increase in FIO persistence (measured by the

half-life of E. coli and intestinal enterococci) was observed when slurry was applied to grassland via

shallow injection, and FIO decay rates were significantly higher for FIOs applied to grassland in spring

relative to summer and autumn. Significant differences in the behaviour of E. coli and intestinal

enterococci over time were also observed, with E. coli half-lives influenced more strongly by season of

application relative to the intestinal enterococci population. While shallow injection of slurry can reduce

agricultural GHG emissions to air it can also prolong the persistence of FIOs in soil, potentially increasing

the risk of their subsequent transfer to water. Awareness of (and evidence for) the potential for ‘pollution-swapping’

is critical in order to guard against unintended environmental impacts of agricultural

management decisions