Lucerne (Medicago sativa L.) establishment after inoculation with different carriers of Ensifer meliloti sown on five dates at Lincoln University

Abstract

Lucerne (Medicago sativa L.) is the highest yielding temperate forage legume, and fixes as much as 360 kg N/ha/year. Its establishment in dryland Canterbury depends on understanding critical factors such as sowing time and inoculation techniques that increase chances of seedling survival and stand persistence. Measurements occurred for a complete growing season after sowing ‘Stamina 5’ lucerne inoculated with peat, lime coating, ALOSCA ® granules and a bare seed control on 4 October, 4 November, 2 December 2010 and 10 January and 7 February 2011 at Lincoln University Canterbury. No emergence differences were observed from the different inoculants. Coated seed produced a higher initial plant population of 331 plants/m² which was 68% of the 490 seeds/m² sown. It also had the highest population 12 months later although seedling numbers had declined by 27%.

Regardless of differences in plant population per treatment, dry matter (DM) yields were not different. Accumulated crop DM yields were however different among sowing dates and decreased with each successive sowing. SD 1 (4 October) gave the highest yield of 15 t/ha whilst SD 5 (10 January) yielded only 2.5 t/ha. Lower DM yields in the later sowing dates were explained by less intercepted light and reduced radiation use efficiency (RUE) probably due to changes in DM partitioning from shoots to roots. Specifically, SD 5 had a longer phyllochron (65 °Cd per node) than SD 1 (44.6 °Cd/node) at a base temperature of 5 oC. No differences were observed in canopy architecture for the different sowing dates as shown by a common extinction coefficient of 0.75. RUE for shoot production (RUEshoot) in regrowth crops of all sowing dates was 1.1±0.065 g DM MJ of intercepted photosynthetically active radiation (PARᵢ). This was 18% higher than the 0.9±0.033 g DM/MJ in seedling crops. Crops from the latest sowing grew 1.84±0.26 kg/ °Cd compared with 3.84±0.22 kg/ °Cd for the earliest sowing. Thermal time accumulation to 50% buds visible (Tt0-bv) decreased from 686 °Cd to 651 °Cd as the photoperiod lengthened in summer from 13 to 15 hours for SD 1 and SD 2.

Genotypic characterisation of bacteria isolated from inoculated root nodules and its identification using PCR and 16S methods generated bands for each isolate. These were sequenced and identified to genus level revealing the presence of seven common genotypes. Amongst these, Rhizobium tibeticum was the most dominant species with an occurrence of 14.5%. Ensifer meliloti, the rhizobia responsible for nodulation and subsequently N₂ fixation in lucerne was the second most dominant with an occurrence of 14%. It was also found present in all inoculant carriers characterised but not the bare seed control. These results suggest dominance and effectiveness of resident soil bacteria over introduced Ensifer meliloti in nodulating lucerne. The study emphasises the importance of sowing early in spring or summer to take advantage of the longer growing season and the use of the summer fallow to supplement soil moisture for successful lucerne establishment.