Host plant height explains the effect of nitrogen enrichment on arbuscular mycorrhizal fungal communities

Nitrogen (N) enrichment is widely known to affect the root-associated arbuscular mycorrhizal fungal (AMF) community in different ways, for example, via altering soil properties and/or shifting host plant functional traits. However, empirical knowledge of their relative importance is still lacking. Using a long-term N addition experiment, we measured the AMF community taxonomic and phylogenetic diversity at the single plant species (roots of 15 plant species) and plant community (mixed roots) levels. We also measured four functional traits of 35 common plant species along the N addition gradient.

We found divergent responses of AMF diversity to N addition for host plants with different innate heights (i.e., plant natural height under unfertilized treatment). Furthermore, our data showed that species-specific responses of AMF diversity to N addition were negatively related to the change in maximum plant height. When scaling up to the community level, N addition affected AMF diversity mainly by increasing the maximum plant height, rather than altering soil properties.

Our results highlight that plant height drives the AMF community dynamics under nitrogen enrichment at both species and community levels, thus providing important implications for understanding the response of AMF diversity to anthropogenic nitrogen deposition.

Methods
Individual root sampling

At the end of August 2017 (7 years after the start of the experiment), we sampled roots from 15 common plant species occurring in all N addition treatments. For logistic reasons, we sampled only 4 plots per N addition treatment (16 plots in total).  We carefully washed the samples to separate the soil around the roots, and selected the fine roots (diameter < 2 mm) of 6 individuals or 1 g of each selected plant species in each plot and mixed as one root sample. This resulted in a total of 240 individual root samples collected for AMF DNA extraction, which were brought back to the laboratory within 24h and stored at −80°C.

Mixed-root sampling

At the end of August 2020, 6 plots of each N addition level (24 plots in total) were samples to investigate AMF community structure at the plant community level. Soil cores (3.8 cm diameter; 25 cm depth) were taken randomly at 5 locations within each plot and mixed into a single composite sample. Fine roots were carefully separated from each soil sample and washed. This resulted in a total of 24 mixed root samples, which were brought back to the laboratory within 24h and stored at −80°C for DNA extraction. The remaining soil samples were air-dried, sieved (2 mm) and stored at 4 °C to measure soil properties (see below).