Escherichia coli Export from Manured Fields Depends on the Time between the Start of Rainfall and Runoff Initiation

After rainfall or irrigation begins, surface‐applied chemicals and manure‐borne microorganisms typically enter the soil with infiltration until the soil saturates, after which time the chemicals and microbes are exported from the field in the overland flow. This process is viewed as a reason for the dependence of chemical export on the time between rainfall start and runoff initiation that has been documented for agricultural chemicals. The objective of this work was to observe and quantify such dependence for Escherichia coli released from solid farmyard dairy manure in field conditions. Experiments were performed for 6 yr and consisted of manure application followed by an immediate simulated rainfall event and a second event 1 wk later. The nonlinearity of the release seen in laboratory and plot studies did not manifest itself in the field. The number of exported E. coli cells in runoff was proportional to rainfall depth after runoff initiation in each trial. The proportionality coefficient, termed export rate, demonstrated a strong dependence on the runoff delay time that could be approximated with the exponential decrease. The export rate decreased by one order of magnitude when the rainfall depth at runoff initiation increased from 18 to 42 mm. The same dependence could approximate data from the simulated rainfall event 1 wk after the manure application, assuming that the initial E. coli content in manure after 1 wk of weathering was 10% of the initial content. Overall, accounting for the dependence of manure‐borne E. coli export on the runoff delay time should improve the accuracy of E. coli export predictions related to the assessment of agricultural practices on microbial water quality. Core Ideas The manure‐borne E. coli export rate strongly depended on the runoff delay time. Manure‐borne E. coli export was well fitted using a constant export rate. On average, a 10‐fold decrease in E. coli export was observed between weeks.