Lucerne dry matter and N-fixation, when sown with or without lime and inoculant

Abstract

This paper quantifies the effect of inoculation and lime on lucerne growth and nitrogen fixation over the establishment and following season. The field experiment at Ashley Dene, in Canterbury, had an initial soil pH of 5.2 and moderate exchangeable aluminium content (4.2 mg/kg). The 15N natural abundance (δ15N) method was used to quantify nitrogen fixation. The pH of the top-soil was increased to ca. 5.5 and the Al content was decreased to 2.0 and 1.3 mg/kg soil by the application of 1 and 2 t lime/ha, respectively. In Year 0 the dry matter yield increased from 3 to 4 t DM/ha with the addition of 2 t lime/ha. The nitrogen content of lucerne plants was 2.2% (w/w), and was unaffected by inoculant or lime in Year 0. Similarly, the δ15N was 0.81‰ and unaffected by inoculant or lime. In Year 1, the N percentage of inoculated lucerne plants was 4.2% (w/w) compared with 3.6% in uninoculated plants. The δ15N value was 0.81‰ in uninoculated plants compared with -0.23‰ in inoculated lucerne plants. The inoculated lucerne yielded 7.8 t DM/ha compared with 2.4 t DM/ha when uninoculated. The calculated proportion of legume N derived from atmospheric N2 (%Ndfa) estimated that 70% of the nitrogen content in the inoculated lucerne shoots was derived from nitrogen-fixation in Year 1. There was no effect of lime on lucerne DM, or δ15N values in Year 1. This suggests lucerne rhizobia tolerated moderate levels of Al in acidic soils. The application of lime and inoculant are therefore recommended for lucerne, particularly in areas where there is no history of lucerne. The available soil N was sufficient to meet crop demand in the establishing year. Lucerne was then reliant on biological nitrogen fixation for yield in Year 1 which suggests lucerne preferentially used soil available N in Year 0, before commencing N fixation. Keywords: Aluminium, lucerne, lime, Medicago sativa L., nitrogen fixation.