Arbuscular mycorrhizal fungi enhance nitrogen acquisition from, but not carbon loss of, organic matter in soil.

The effect of arbuscular mycorrhizal fungi (AMF) on decomposition can be regulated by their role in plant nitrogen acquisition due to their obligate biotrophic lifestyle. However, few studies have addressed the relationship between these two processes. We conducted an experiment using mycorrhizal-defective mutants and wild-types of two plant species with C and N dual-labelled litter as tracers. A meta-analysis of related studies was also performed to test the generality of the experimental results. Both our experiment and meta-analysis found that AMF enhanced plant N acquisition from organic substrates, while substrate N and C remaining in the soil were not significantly reduced. We propose that AMF may reduce N loss from the system, which retains substrate N for plant uptake. Under N limitation, AMF may stimulate the deamination of organic substrates or selective mining of N-rich soil organic matter. In addition, our meta-analysis found significant influences of experimental designs on the observed outcomes. We conclude that AMF may facilitate the decoupling between plant N acquisition from, and C loss of, organic materials. However, more studies that simultaneously trace C and N allocation from organic substrates are needed to elucidate the underlying mechanisms.