Atmospheric nitrogen deposition has minor impacts on the abundance and diversity of arbuscular mycorrhizal fungi and their contribution to soil carbon stock in tropical forests

Knowledge about arbuscular mycorrhizal fungi (AMF) is crucial for understanding nutrient limitations on primary productivity and soil organic carbon (C) storage in terrestrial ecosystems. Both theoretical models and empirical evidence hold that nitrogen (N) addition in phosphorus-limited ecosystems can either increase or decrease AMF diversity and abundance. However, many of these studies involved high-level N additions, which do not reflect realistic levels of atmospheric N deposition, thus leading to biased estimations of AMF and their role in the soil C stock. Here, we assessed AMF diversity and abundance under N addition using data from five tropical forests, ranging from 88%, 31%, and 25% arbuscular mycorrhizal tree dominance to dual-mycorrhizal tree dominance, and combined it with a global synthesis of tropical/subtropical forests. Our field study showed that N addition based on realistic N deposition (≤50 kg N ha⁻¹ yr⁻¹, comparable to the actual rate of atmospheric N deposition in the studied sites) caused little change in AMF abundance and diversity, as confirmed by our meta-analysis. The responses of AMF abundance to N addition did not differ significantly across forests with varying mycorrhizal dominance. However, high-level N addition (>50 kg N ha⁻¹ yr⁻¹) from a global dataset reduced AMF abundance and diversity. AMF responses were correlated with plant C, soil nutrient availability, and/or pH. Our findings further indicate that current atmospheric N deposition is unlikely to enhance soil C content via AMF. Given that N deposition has been stable or even declined in major global economies, we propose that previous studies may have overestimated AMF responses to atmospheric N deposition, which neither increased nor reduced AMF abundance and diversity as previously thought.