Glufosinate treatment of weeds results in ammonia emission by plants

The herbicide glufosinate, which is also called phosphinothricin (PPT), is known to inhibit glutamine synthetase and thus causes a blockage of ammonium (re)assimilation in plants. The objective of the present study was to test whether application of this herbicide results in an ammonia volatilization from the plants and to quantify nitrogen loss via ammonia emission. Four different weed species (Chenopodium album, Echinocloa crus-galli, Solanum nigrum, Tripleurospermum inodorum) were grown as monocultures in the greenhouse and treated with PPT when their canopies covered the soil. In the first experiment, whole shoot samples were taken during the following days and analysed for ammonium, pH and total nitrogen content. In the second experiment, apoplastic pH and ammonium concentration of the leaves were measured after herbicide application and used for the calculation of gamma-values (ratio between NH4+ and H+ concentration), the stomatal NH3 compensation point and the canopy net NH3 flux with a soil vegetation atmosphere transport (SVAT) model. Herbicide treatment caused a rapid increase in shoot ammonium concentration and the ammonium portion of the plant total nitrogen ranged from 0.6 to 0.9% and from 17 to 44% before and after PPT application, respectively. S. nigrum showed a strong increase in ammonium portion (35%) followed by a decrease (20%), which may have resulted from ammonia volatilization. The difference in total shoot nitrogen content per ground area at the start and 2 weeks after PPT application averaged for the three C3 weed species to a nitrogen loss of ca. 0.4 g N m-2 or approximately 13% of the total nitrogen in the weed canopy. Analysis of the apoplastic fluid yielded an increase in ammonium concentration and a pH decrease after an initial increase on day 1 after the PPT treatment. In order to evaluate the potential for ammonia loss, the Γ-value was calculated for both apoplastic and tissue water. S. nigrum showed the most dramatic increases in both apoplastic and tissue-water gamma-values 4 days after PPT treatment. The calculated stomatal NH3 compensation point was strongly elevated after PPT treatment. However, temporal changes of apoplastic pH and ammonium concentration varied between the species and the modelled ammonia emission ranged from 0.03 to 0.09 g N m-2. It is concluded that PPT application results in an ammonia emission of ca. <10% of the canopy nitrogen content. This is negligible with respect to the nitrogen balance of an agroecosystem; however, the ammonia pollution of the atmosphere has to be taken into account.