Native arbuscular mycorrhizal fungi increase the abundance of ammonia-oxidizing bacteria, but suppress nitrous oxide emissions shortly after urea application

The potential of the symbiosis between plants and arbuscular mycorrhizal fungi (AMF) to reduce emissions of the greenhouse gas N2O has gained scientific attention in the last years. Given the high nitrogen (N) requirements of AMF and their role in plant N uptake, they may reduce the availability of mineral N that could be subject to N2O emissions and leaching losses. We investigated the impact of AMF on the growth of tropical grass Brachiaria decumbens Stapf. and on N2O released after fertilization with urea in a mesocosm study. To evaluate the role of nitrification in N2O emissions, we used nitrification inhibitor dicyandiamide (DCD). The study included a full-factorial design (n = 6) with two AMF treatments (with and without AMF inoculation) and three fertilization treatments (control, urea and urea + DCD), applied after 92 days of growth. Plant growth, soil properties and N2O emissions were measured during the following 2 weeks and the abundance of nitrifiers was quantified one and two weeks after fertilization. The production of N2O increased after urea application but only without DCD, indicating the importance of nitrification in N2O emissions. The emissions of N2O after urea application were reduced by 46% due to the presence of AMP. Nevertheless, the abundance of ammonia-oxidizing bacteria (AOB) was increased by urea and AMF, while plant growth was reduced by the AMF. The increased root:shoot ratio of the biomass in AMF pots suggests competition between AMP and plants. This study demonstrated that immobilization of N by AMF can reduce N2O emissions after fertilization, even when plant growth is reduced. The inverse relationship between (higher) AOB abundance and (lower) nitrification rates suggests that changes in the activity of AOB, rather than abundance, may be indicative of the impact of the AMF-Brachiaria symbiosis on N cycling in tropical grasslands. Alternatively, the difference between N2O emissions from AMF and non-AMF pots may be explained by increased reduction of N2O in the presence of AMP. Longer-term studies are needed to verify whether the effects of AMF on N2O emissions and/or plant growth persist over time or are limited to initial immobilization of N by AMF in N-limited systems.