Bacterivorous protists inhibit nitrification and N₂O emissions in cadmium polluted soils via negative feedback loops

Understanding the soil nitrogen (N) process under increasing anthropogenic activities, i.e., heavy metal pollution and N fertilization is essential for optimizing soil N management and tackling environmental problems. However, few studies assess how ubiquitous soil protists influence N process from a multitrophic perspective. Here, we conducted microcosm experiments to investigate how phagotrophic protists (Colpoda steinii) influence the autochthonous bacterial flora proxy for N process to drive the N transformation processes under different Cd pollution levels (0-3 mg kg^-1) with or without N fertilization. Because of hormesis, Cd stimulated the net nitrification rate and N₂O emissions by up to 65 % and 100 %, respectively, and this stimulation was stronger after N addition. However, protists attenuated and even reversed the stimulation of Cd on the net nitrification rate and N₂O flux, especially after N addition by correspondingly reducing N fertilization-enhanced nitrifiers and denitrifiers, which were also metabolically active under Cd pollution. With this negative feedback loop, protists reduced the net nitrification rate and N₂O emissions by up to 91 % and 36 %, respectively. This study offers novel insights to assess the effects of heavy metal pollution on soil nutrient cycling regarding soil predation, providing strategies for increasing N-use efficiency in agricultural ecosystems.