Contrasting effects of indigenous arbuscular mycorrhizal fungi on nitrogen absorption of C3 and C4 grasses: Evidence from microcosm and 15N labeling experiments

<p><strong><em>Background and aims</em></strong></p> <p>Nitrogen (N) captured by arbuscular mycorrhizal (AM) symbiosis is a major pathway in the N uptake of host plants.  However, the relative contribution of arbuscular mycorrhizal fungi (AMF) to N uptake in different plant functional types has not been well assessed.</p> <p><strong><em>Methods</em></strong></p> <p>Two dominant plant species in semiarid steppe ecosystems on the Mongolian plateau, i.e. <em>Leymus chinensis </em>(C₃ grass) and <em>Cleistogenes squarrosa</em> (C₄ grass), were selected in this study.  We conducted a greenhouse manipulation experiment using novel microcosms combined with ¹⁵N labeling techniques and investigated the effect of indigenous AMF on plant growth and quantified their relative contribution to N uptake under high and low levels of available soil N. </p> <p><strong><em>Results</em></strong></p> <p>Indigenous AMF contribute more to N uptake in C₃ grass than that in C₄ grass, and mycorrhizal partners act as parasites for C₄ plant growth.  For <em>L. chinensis</em>, indigenous AM symbiosis suppressed plant growth under low soil N but improved plant growth under high soil N conditions.  AMF contributed to <em>c.</em> 23% and 20% of the total plant N uptake under low and high soil N conditions, respectively.  For <em>C. squarrosa</em>, indigenous AM symbiosis consistently inhibited plant growth under both low and high soil N conditions, and the percent contributions of AMF to N uptake were only <em>c.</em> 9% and 7%, respectively.</p> <p><strong><em>Conclusions</em></strong></p> <p>Our results demonstrate that indigenous AM symbiosis plays a vital role in N uptake by host plants, even in the absence of a positive growth response.  AMF can modify the fitness of C₃ and C₄ grasses and thereby alter plant community composition and ecosystem N cycling, particularly under high N conditions.  Our study has important implications for improving global N cycling models in the face of increasing global N deposition.</p>