Impacts of dicamba and 2,4-D drift on ‘Ponkan’ mandarin seedlings, soil microbiota and Amaranthus retroflexus

The auxinic herbicides 2,4-D and dicamba are very prone to volatilization and can be carried by wind hundreds of meters to non-target environments. Effects of auxinic herbicide drift on susceptible broadleaf crops from adjacent fields are well documented; however, such effects on the soil microbiota and/or weeds also from adjacent fields have rarely been studied. In this research, the impact of simulated drift of 2,4-D and dicamba subdoses (0D, 1/256D, 1/64D, 1/4D, 1/16D and 1D) were evaluated on ‘Ponkan’ mandarin (Citrus reticulata Blanco) seedlings, mandarin rhizosphere microbiota and Amaranthus retroflexus. The full field dose (1D) of dicamba and 2,4-D was 720 and 2345 g ae ha−1, respectively. Dicamba promoted 15 and 18% more foliar abscission and chlorophyll degradation than 2,4-D at 28 days after treatment (DAA). Dicamba also reduced the photosynthesis rate by 60% compared to the control at 28 DAA. Both herbicides at 1D increased intercellular carbon concentration by 15–20%, while reducing transpiration rates by up to 70–90% at 28 DAA. However, no herbicide affected chlorophyll fluorescence in mandarin seedlings. Regarding the soil microbiota, dicamba at 1/16D increased the soil respiration rate by 37% compared to the control. In contrast, 2,4-D produced a greater increase in CCO2 than dicamba at 1D. Both herbicides at 1D reduced carbon in microbial biomass by 75 and 80%. Doses of 1/4D or more of both herbicides were able to control and reduce the dry matter of A. retroflexus by ≤80% at 21 DAA. Drift of dicamba was more toxic than 2,4-D for mandarin seedlings; however, both herbicides affected several physiological processes of the plants. Furthermore, dicamba and 2,4-D also caused alterations in the soil microbiota and weeds of adjacent fields.