N addition undermines N supplied by arbuscular mycorrhizal fungi to native perennial grasses

- AMF abundance and function were measured under different soil N conditions.
- N addition significantly decreased AMF function of nutrient transfer with host.
- Plant N was positively related to AMF allocation to nutrient transfer structures.
- AMF benefit perennial grasses by increasing N uptake.
- AMF may greatly reduce the need for N addition to improve perennial grass yields.

Arbuscular mycorrhizal fungi (AMF) form associations with plants and are ubiquitous in grassland and agriculture ecosystems. AMF are known to contribute to plant nitrogen (N) uptake, but the importance of AMF to ecosystem N cycling and overall plant N nutrition remains unclear, particularly in the context of agroecosystems. AMF abundance typically declines under N addition, but how this affects AMF function and subsequent N transfer to plants is unknown. We measured plant yield and plant N content in relation to AMF abundance and function under different soil N conditions, using both an N-addition experiment and a survey across perennial grassland sites with varying soil N levels. We used AMF root colonization to assess AMF abundance, but the presence of AMF does not necessarily relate to function (i.e. nutrient transfer with host plant), so we also used an allometric ratio of AMF structures and AMF fatty acid biomarkers as an index of AMF function. N addition significantly decreased AMF abundance by an average of 27%, and decreased function by an average of 42%, as measured by the allometric ratio. This pattern was supported by our survey study where soil N was negatively correlated with AMF abundance and function. In addition, plant N was positively related to higher levels of AMF allocation to nutrient transfer structures within host roots. Demonstrating these relationships across varying soil N levels at eight sites supported the hypothesis that AMF benefit perennial grasses by increasing N uptake. This is particularly important for perennial grasses grown for bioenergy because managing for higher AMF abundance and function may reduce or eliminate incentives for costly and environmentally problematic N addition.