Herbicides Increase Emission of Ammonia by Pearl Millet and Congo Grass

Abstract

Core Ideas Post‐emergent herbicides application increase the emission of ammonia by cover crops. Post‐emergent herbicides application on congo grass decrease the total N in the residues. Glyphosate application on congo grass or pearl millet strongly reduces the total‐N content in the straw. In previous works, we demonstrated that some herbicides could increase N losses from the soil–plant system under greenhouse conditions, but the N‐output pathways were not investigated. In this context, the aim of this work was to evaluate ammonia (NH3) emission by soil and plants and the total‐N accumulation in the straw following herbicide application to pearl millet (Pennisetum glaucum) and congo grass (Urochloa ruziziensis). Two field experiments were conducted in a randomized block, with a factorial design and eight replicates per treatment. The treatments were: 1‐control: without herbicide application, 2‐ glyphosate, 3‐glufosinate‐ammonium, and 4‐paraquat. Periodic measurements of NH3 emission by plants or soil at 48‐h intervals were considered as repeated measurements. At the end of the experiment, the dry mass, ammonium (NH4+) in both plant tissues and soil, and total N in shoots were measured. All evaluated herbicides increased emission of NH3 by pearl millet and congo grass with net losses ranging from 1.5 to 7.0 kg ha−1 of NH3 after herbicide application, but soil NH3 emission was not affected. In congo grass, all herbicides reduced total N and dry mass content in plant tissues, but only the glyphosate herbicide reduced it in pearl millet. Herbicide usage increases emission of ammonia by plants and can decrease the total‐N accumulation in cover crop straw. Further research should be done to investigate how these ammonia losses and changes in total N from aboveground biomass affect crop production as well as the recommended N fertilizer application rates.