DTPA soil extractable and plant heavy metal concentrations with soil-added Cd treatments

Summary The numbers, weights, and ages of root nodules were determined in a homogeneous 6- to 7-year-old stand ofHippophaë rhamnoides in a coastal sand-dune area. Large variations in the number of nodules, which ranges from 30 to 281 per m² were found. Most of the nodules occurred between 10 and 60 cm below the soil surface. In the surface layer and lower than 60 cm only small numbers of nodules were present. It is concluded that relatively large samples have to be investigated to obtain an impression of the nodulation of a given stand and to permit a comparison of the contribution of dinitrogen fixation in different stands.Close correlation was found between nodule numbers and the age of the roots, young roots having the highest numbers. However, the wide variation in the number of nodules per square metre could not be explained solely by the age distribution of the roots. The possible role of some other factors is discussed. No relationship was found between the nodule weight and the age of the nodule. The mean dry weight of living nodule material was 5.3 g per m². Most nodules reached an age of up to 3 years. The significance of nodule decay for the distribution of the endophyte in the soil and in the process of nodule formation is discussed.