Mycorrhizal mediated feedbacks influence net carbon gain and nutrient uptake in Andropogon gerardii

•  The carbon sink strength of arbuscular mycorrhizal fungi (AMF) was investigated by comparing the growth dynamics of mycorrhizal and nonmycorrhizal Andropogon gerardii plants over a wide range of equivalent tissue phosphorus : nitrogen (P : N) ratios. •  Host growth, apparent photosynthesis (A_net ), net C gain (C_n ) and P and N uptake were evaluated in sequential harvests of mycorrhizal and nonmycorrhizal A. gerardii plants. Response curves were used to assess the effect of assimilate supply on the mycorrhizal symbiosis in relation to the association of C with N and P. •  Mycorrhizal plants had higher C_n than nonmycorrhizal plants at equivalent shoot P : N ratios even though colonization did not affect plant dry mass. The higher C_n in mycorrhizal plants was related to both an increase in specific leaf area and enhanced photosynthesis. The additional carbon gain associated with the mycorrhizal condition was not allocated to root biomass. The C_n in the mycorrhizal plants was positively related to the proportion of active colonization in the roots. •  The calculated difference between C_n values in mycorrhizal and nonmycorrhizal plants, C_diff , appeared to correspond to the sink strength of the AMF and was not an indirect result of enhanced nutrition in mycorrhizal plants.