Reducing herbicide leaching by split-dose applications in organomineral and mineral soils

Herbicides are widely used in agriculture to control weeds and maximise crop yields, yet up to 85% of applied compounds can be lost to surface and groundwater, contributing to contamination of drinking water sources. While mitigation strategies such as user training and equipment testing exist, herbicide exceedances persist and limited research has examined alternative application approaches, including split-dose strategies. This study aimed to (1) quantify the effect of split-dose applications of 4-chloro-2-methylphenoxyacetic acid (MCPA) and 3,6-dichloropyridine-2-carboxylic acid (clopyralid) on herbicide leaching to groundwater, and (2) assess their impact on soil microbial communities. A 16-week soil column experiment was conducted using mineral and organomineral soils. Herbicides were applied either as a single full dose (13.5 kg MCPA ha− 1; 2 kg clopyralid ha− 1) or as two half doses applied six weeks apart. Leachate concentrations were measured weekly. Peak concentrations of MCPA (69,620 µg L− 1) and clopyralid (10,630 µg L− 1) occurred in the first week following initial application. Following the second split dose, concentrations declined substantially, with less than 180 µg L− 1 detected, and all herbicides fell below detection limits by week 10. Analysis of 16 S rRNA gene sequences indicated that soil organic matter content and temporal variation exerted a stronger influence on microbial community structure than herbicide type or application strategy. Overall, split-dose application significantly reduced herbicide leaching without adversely affecting soil microbial communities or their role in biogeochemical cycling. These findings suggest that split herbicide dosing represents an effective management strategy, particularly in hydrologically sensitive environments.