The balance between arbuscular mycorrhizal fungal diversity and plant growth benefits from optimizing nitrogen inputs in agroecosystems

Excessive nitrogen (N) inputs exacerbate ecological problems and compromise multiple ecosystem services delivered by soil biodiversity. Arbuscular mycorrhizal fungi (AMF) facilitated nutrient acquisition by plants from soil and alleviated reliance on external inputs. However, how N inputs influence the relationships between AMF diversity and crop performance, especially whether there exists an optimal threshold to maximize crop reliance on AMF and minimize reliance on external N remains unclear. A field experiment was conducted to monitor the responses of AMF diversity and maize growth among N input levels (0, 120, 240, and 360 kg urea-N ha-1). Results showed that increasing N inputs decreased AMF colonization rate and spore density. Across the N input levels, the N inputs at 120 and 240 kg N ha-1 increased AMF richness and biomass, respectively. Moreover, N input level at 360 kg ha-1 (local doses) increased maize growth including yield and total biomass but decreased plant P content, indicating that the role of AMF was constrained under high N inputs. Notably, an optimal N input level that resulted in win-win support of AMF diversity and crop growth was obtained, which was ca. 52 % of local doses (i.e. 190 kg ha-1). Together, understanding the relationships between beneficial microbes and crop with nutrient input gradients was critical for advancing future sophisticated fertilizer management harnessing multiple ecological services and crop productivity at the lowest external economic and environmental cost.