Populations of Sinorhizobium meliloti congregate in the 30–60 cm section of the soil profile in a stand of dryland lucerne (Medicago sativa): is this where lucerne fixes its nitrogen?

Lucerne (Medicago sativa L.) cv. Hunterfield was inoculated with Sinorhizobium meliloti and sown in a red clay loam at Condobolin, New South Wales. The soil had been limed at 6 different rates 8 years previously. In 2001, when the stands were 3 years old, the populations of S. meliloti were enumerated, with a plant-infection test, in 4 sections of the soil profile (i.e. 0–15 cm, 15–30 cm, 30–60 cm and >60 cm). The numbers of rhizobia in the soil were very high (120 000 rhizobia/g) in the 30–60 cm section. In sharp contrast, much lower numbers (0–15 cm, 75 rhizobia/g; 15–30 cm, 190 rhizobia/g; >60 cm, 287 rhizobia/g) were detected in the other sections of profile. Liming had no effect on size of S. meliloti population. It was concluded that, under the conditions of the experiment, it was reasonable to assume that lucerne nodulated most abundantly between 30–60 cm below ground. This zone may represent the location of maximum nitrogen fixation. Some implications of the conclusions are discussed.