Effects of the Combined Application of Nitrogen Fertilizer and 2,4-D on Nitrification Ammonia Oxidizers and Herbicide Bioavailability in a Volcanic Soil. A Microcosm Study

Purpose 2,4-D (2,4-dichlorophenoxy acetic acid) is an herbicide used in weed control worldwide; however, there is no information regarding its effects on the nitrification process in volcanic soils fertilized with urea and how its bioavailability can be modified during this process. Methods This study was carried out under microcosm incubation of an Andisol amended with an equivalent field dose of 200 kg N ha −1 and 0.672 kg 2,4-D ha −1 for 30 days. In this study were evaluated the changes produced on enzymatic activity, nitrogen kinetic mineralization, ammonia-oxidizing bacteria (AOB) abundance and herbicide persistence. Additionally, a 2,4-D adsorption study was carried out in batch with urea application. Results The main results on fertilized soils show that 2,4-D stimulated temporarily urease and dehydrogenase activity. Herbicide application increased N-NH 4 + concentration by approximately 23%, and N-NO 3 − significantly decreased by 18%, compared to fertilized soil. The abundance of AOB amo A gene decreased at all times. On the other hand, 2,4-D adsorption decreased by urea application, but its persistence was not affected, with a low half-life of approximately 4 days. Conclusions This study indicated that 2,4-D influences the urea mineralization process by increasing ammonium and decreasing nitrate levels as a consequence of increased urease activity and the inhibition of AOB populations. On the other hand, the bioavailability of 2,4-D decreased due to the pH modifications produced by the application of urea. In general, we conclude that the combined application of fertilizers and some herbicides should be studied to estimate the efficacy and environmental effect of both agrochemicals.