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Nitrate ammonification is important for soil nitrogen retention. However, the ecology of ammonifiers and their prevalence compared with denitrifiers, being competitors for nitrate, are overlooked. Here, we screen 1 million genomes for nrfA and onr, encoding ammonifier nitrite reductases. About 40% of ammonifier assemblies carry at least one denitrification gene and show higher potential for nitrous oxide production than consumption. We then use a phylogeny-based approach to recruit gene fragments of nrfA, onr and denitrification nitrite reductase genes (nirK, nirS) in 1861 global terrestrial metagenomes. nrfA outnumbers the nearly negligible onr counts in all biomes, but denitrification genes dominate, except in tundra. Random forest modelling teases apart the influence of the soil C/N on nrfA-ammonifier vs denitrifier abundance, showing an effect of nitrate rather than carbon content. This study demonstrates the multiple roles nitrate ammonifiers play in nitrogen cycling and identifies factors ultimately controlling the fate of soil nitrate. Nitrate ammonifiers are poorly known despite their importance for soil nitrogen retention. This study shows that they are phylogenetically diverse and globally distributed across terrestrial biomes and that the outcome of the competition with denitrifiers is controlled by soil nitrate. [ABSTRACT FROM AUTHOR]

Phosphorus (P) acquisition is key for plant growth. Arbuscular mycorrhizal fungi (AMF) help plants acquire P from soil. Understanding which factors drive AMF-supported nutrient uptake is essential to develop more sustainable agroecosystems. Here we collected soils from 150 cereal fields and 60 non-cropped grassland sites across a 3,000 km trans-European gradient. In a greenhouse experiment, we tested the ability of AMF in these soils to forage for the radioisotope 33 P from a hyphal compartment. AMF communities in grassland soils were much more efficient in acquiring 33 P and transferred 64% more 33 P to plants compared with AMF in cropland soils. Fungicide application best explained hyphal 33 P transfer in cropland soils. The use of fungicides and subsequent decline in AMF richness in croplands reduced 33 P uptake by 43%. Our results suggest that land-use intensity and fungicide use are major deterrents to the functioning and natural nutrient uptake capacity of AMF in agroecosystems., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)