Pinus sylvestris root exudates indirectly facilitate Amorpha fruticosa growth performance by altering the nitrogen cycle

Facilitation is common among plants, but our understanding of indirect belowground facilitative mechanisms is limited. We followed up on previous research showing that Pinus sylvestris had strong facilitative effects on co-occurring Amorpha fruticosa shrubs by testing the effects of root exudates of the Pinus on various components of the N cycle in the soil around Amorpha roots.

We applied different concentrations of Pinus root exudates (0, 1x, and 4x) to soils, and measured the responses of the soil metabolome, microbial functional groups and the biomass of Amorpha.

A 1x concentration of Pinus root exudates promoted microbial metabolism and plant secondary metabolite biosynthesis. The same concentration increased the abundance of specific microbial genera in the rhizosphere involved in nitrification and nitrogen (N) fixation. These root exudates also increased the abundance of microbial taxa and genes involved in ammonification and nitrification, but inhibited microbial denitrification. In addition, pathway analysis indicated that Pinus root exudates indirectly facilitate Amorpha biomass by regulating its rhizosphere N-cycle. Together, these results indicate that 1x concentrations of Pinus root exudates promote the efficient conversion of ammonium to nitrate and the absorption and utilisation of organic N, while reducing gaseous N losses, thereby promoting the overall N uptake, and potentially growth, of Amorpha.

Our results point to the importance of root exudates as an indirect facilitative mechanism, operating through effects on microbes involved in the N cycle, but their ecological relevance should be further validated in the field.