Effects of mixed plantation of alder and poplar on population of Frankia nodulation in soil

Nowadays, nitrogen-fixing tree species are often planted in combination with poplar to provide nutrients, though this can reduce the number of host plants and have a negative impact on soil Frankia population. In order to study the effects of mixed plantation of Caucasian alder (Alnus subcordata) and poplar (Populus deltoides) on Frankia nodulation potential, an experiment was carried out within a randomized block design. Five different mixtures of 17–years old P. deltoides (P) and A. subcordata (A) (100A, 70A30p, 50A50P, 30A70 and 100P) were established in Safrabasteh Poplar Research Station in Guilan province of Iran. The results showed that the presence of alder in mixture with poplar positively affected the diameter at breast height (DBH) and height of poplar trees. Mixed plantations with alder can improve Soil pH and the amount of soil nutrients and organic carbon. Furthermore, foliar nutrient concentrations of poplar trees were higher in mixed plantations than in pure plantations. The results of Frankia population analysis showed significant difference between the soils across different species mixture classes. The highest and lowest levels of Frankia population were observed in the rhizosphere soil of pure alder (49.17 g-1 soil) and pure poplar (0.89 g-1 soil), respectively. The Frankia population was negatively correlated with amounts of available P and K, yet it was positively correlated with pH. In addition, Frankia population showed positive and negative correlations with alder DBH and height, respectively. These results confirmed the effects of plant-host density on the viability and population of Frankia.