The effects of nitrogen and phosphorus additions on the mutualisms between plants, arbuscular mycorrhizal fungi, and rhizobia bacteria

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) and rhizobia bacteria are two microbial symbionts that commonly interact with leguminous plants.  AMF provision plants with soil resources, especially phosphorus, in exchange for host plant photosynthate.  Similarly, rhizobia provide useable forms of nitrogen to plants in exchange for plant carbon resources and other plant exudates.  Given that each plant-microbe interaction is nutritionally based, the availability of nutrients, especially phosphorus and nitrogen, is expected to alter the costs and benefits of each interaction.  AMF and rhizobia often simultaneously infect a common plant.  The presence and identity of coinfecting microbial symbionts can alter the dynamics of each interaction.  We conducted a full-factorial greenhouse study manipulating the presence and identities of AMF and rhizobia on the late-successional prairie legume Amorpha canescens.  In order to evaluate how soil resource availabilities influenced each pairwise interaction, and coinfection combination, we applied phosphorus and nitrogen in a full factorial design.  Along with fitness responses of all three partners, we measured carbon and nitrogen contents of the aboveground and belowground plant tissues and belowground tissues, as well as the carbon, nitrogen, and phosphorus content of the soil. 

Results/Conclusions

The nitrogen content of plant material was not influenced by nitrogen addition.  AMF (F_1,98=37, P<0.001) and rhizobia (F_1,98=106, P<0.001) infection increased the nitrogen content of plant shoots, but only rhizobia increased nitrogen in plant roots (F_1,105=130, P<0.001).  Overall plants grew better with nitrogen addition, but this effect was driven by variation in AMF-infected plants with nitrogen addition compared to the no-nitrogen treatments (F_1,507=4.56, P=0.03).  We also saw an effect of nitrogen addition and AMF inoculation on the C:N ratio in soils (F_{1, 79}=4.04, P=0.05).  With nitrogen addition, non-AMF plants had a higher C:N ratio, and soil with AMF inoculum had a lower C:N ratio.  Nitrogen addition influenced rhizobia responses by decreasing the number of nodules formed (F_1,339=6.03, P=0.01).  However, AMF ameliorated this decrease in nodulation, as there was no effect of nitrogen addition on nodulation in AMF treatments.  Surprisingly, we found that AMF hyphal colonization density increased with phosphorus addition (F_1,162= 6.4, P=0.01).  These results suggest that A. canescens depends primarily on belowground interactions for nutrient acquisition.  Additionally, we demonstrate that the effects of increased nitrogen availability on plant growth, soil properties, and the mutualistic interactions between plants and rhizobia are mediated by the presence of AMF.