Nutrient effects on nodulation and N2 fixation in actinorhizal symbioses

In Frankia-actinorhizal symbioses phosphorus and nitrogen greatly affect nodulation as well as plant growth. In this thesis I studied how phosphorus and nitrogen affect nodulation in two actinorhizal plants having different infection pathways, the root hair infected Alnus incana (L.) Moench and the intercellularly infected Hippophae rhamnoides L. Furthermore, I studied the possibility to use N2-fixing and non-N2-fixing plants for land reclamation of Italian marginal clay soils. The methods were performed in the laboratory as well as in the field, thus to combine studies of high precision (laboratory work) and high relevance (field work). In both species studied phosphorus stimulation and nitrogen inhibition of nodulation were systemic, indicating that the shoot takes active part in regulating nodulation. Further, phosphorus and nitrogen effects were specific on nodulation and not mediated via plant growth. In Alnus incana phosphorus stimulation and nitrogen inhibition was found to occur already at an early stage, cortical cell divisions, in nodule formation. In Hippophae rhamnoides phosphorus stimulated nodulation as well as N2 fixation and nitrogen uptake, measured by 15N techniques. The land reclamation experiment showed that N2-fixing and non-N2-fixing plants, in particular drought tolerant species, can be used to reclaim the soil studied. In conclusion, phosphorus stimulation and nitrogen inhibition of nodulation occurred in actinorhizal plants having different infection pathways indicating a generality of the observed nutrient effects. The phosphorus stimulation of nodulation could be explored in field to increase nodulation, N2 fixation and yield of N2-fixing plants, but the possible uptake of P by mycorrhiza in field should be considered. Land reclamation of marginal clay soils is a long-term process but my studies show the potential use of actinorhizal plants in this process.