Impact of soil nitrogen concentration on Glomus spp.-Sinorhizobium interactions as affecting growth, nitrate reductase activity and protein content of Medicago sativa

Our objective was to evaluate how increasing levels of N in the medium (0, 4, 8 and 16 mmol N added kg(-1) soil) affect the interaction between Sinorh izobium and arbuscular mycorrhiza (AM) fungi in the tripartite symbiosis wi th Medicago sativa. Growth response, nutrient acquisition, protein content, and nitrate reductase (NR) activity were measured both in plant shoots and roots. Results showed that N levels in soil did not affect mycorrhizal col onization but they strongly influenced nodulation, particularly of mycorrhi zal plants. Mycorrhizal colonization was required for a proper nodulation w hen no N was applied to soil. In contrast, the addition of 4 mmol N kg(-1) soil reduced nodulation only in mycorrhizal plants and 8 mmol N added kg(-1 ) soil allowed nodule formation only in non-mycorrhizal plants. Nodulation was totally inhibited in all treatments with the addition of 16 mmol N adde d kg(-1) soil. N addition enhanced NR activity in all the treatments, while AM colonization increased the proportion of NR allocated to roots. This ef fect was more pronounced under the lowest N levels in the medium. The two A M fungal species showed different distribution pattern of enzymatic activit ies in plant tissues indicating specific physiological traits. Protein cont ent as well as the relative proportion of protein in roots were greatly inc reased after mycorrhizal colonization. Glomus intraradices-colonized plants had the highest protein content in shoot and root. Mycorrhizal effects on growth, N acquisition and biochemical variables cannot be interpreted as an indirect P-mediated effect since P content was lower in mycorrhizal plants than in those which were P fertilized. Mycorrhizal colonization increased the N content in plants irrespective of the N level, but the effectiveness of AM fungi on plant N acquisition depended on the AM fungus involved, G. i ntraradices being the most effective, particularly at the highest N rate. N -2 fixation, enhanced by AM colonization, contributed to N acquisition when a moderate N quantity was available in the soil: Nevertheless, under a hig h N amount the nodulating process and/or fixing capacity by Sinorhizobium w as reduced in AM plants. In contrast, the AM fungal mycelium from a particu lar mycorrhizal fungus may continue to contribute efficiently to the N upta ke from the soil even at high N levels. These results demonstrate the parti cular sensitivity of AM fungal species in terms of their growth and/or func tion to increasing N amounts in the medium. A selection of AM fungi used to address specific environmental conditions, such as N fertilization regimes comparable to those used in agronomic practices, is required for a better use of N applied to soil.