Constraints and opportunities for lucerne (Medicago sativa L.), Caucasian clover (Trifolium ambiguum M. Bieb), and Russell lupin (Lupinus polyphyllus L.) in the high country of New Zealand

Abstract

This research focused on perennial legumes for the low fertility regions of the New Zealand high country. The aim was to improve production of legume species in these regions. A six-year field experiment at the Lees Valley evaluated the production of lucerne (Medicago sativa L.), Caucasian clover (Trifolium ambiguum M. Bieb.), white clover (T. repens L.), and red clover (T. pratense L.). Low soil pH and phosphorus deficiency were addressed by lime and P fertiliser application, but lucerne persistence was low at this site. Caucasian clover was the most persistent legume with the highest sown species component (93%) and yield (7.8 t DM/ha) in last season. Lucerne yielded 4.2 t DM/ha in Year 1, but this decreased to 2.7 t DM/ha in Year 3, and 1.0 t DM/ha in Year 6. Causes of these extreme responses were examined in a series of pot experiments. In the Lees Valley soil, 85% of lucerne seedlings died, when no lime was applied (pH 5.4, Al> 6.0 mg/kg) compared with 40% of Caucasian clover seedlings. The 2 t/ha lime application, increased lucerne seedling survival to 80% (pH 6.1, Al< 0.03 mg/kg). A similar survival rate was achieved for Caucasian clover with 1.0 t lime/ha. Results showed low pH and high Al content of the Lees Valley soil suppressed the growth of fine roots and nodulation of lucerne, more than Caucasian clover. However, the level of difference in Al tolerance between the two species could not fully explain the field experiment results at the Lees Valley. The main limitation for lucerne persistence was nitrogen deficiency, as a consequence of nodulation failure due to the Al toxicity. Three field experiments were conducted at Glenmore Station and Ashley Dene, to further confirm results. Russell lupin (Lupinus polyphyllus L.) was included in these later experiments as a potential Al tolerant legume. The percentage of nodulated plants for Russell lupin was constantly over 75%, over two years of the field experiments at Glenmore station and Ashley Dene. In contrast, for lucerne and Caucasian clover, nodulation decreased to zero and 25%, respectively over time in the high Al soil > 5 mg/kg at Glenmore station, but not under moderate levels of Al (ca. 3-4 mg/kg soil) at Ashley Dene. Nodule persistence of lupin was a key factor for plant survival in the high Al soil. At Glenmore station, Russell lupin was successfully established into resident vegetation and yielded over 10 t DM/ha in Year 2. Its vigorous growth was supported by consistent nodulation (75%), in the high aluminium content soil. This highlighted its potential to provide a nitrogen fixing perennial legume for environment that is unsuitable for lucerne. Caucasian clover establishment was low in both field experiments, but yielded 4 t DM/ha in Year 2 at Glenmore station. At Ashley Dene the nodulated lucerne plants produced 8.0 t DM/ha, compared with 2.0 t DM/ha in un-nodulated plants. The ¹⁵N natural abundance method showed ca. 70% of the nitrogen in nodulated plants was derived from BNF. This meant 122 kg N/ha of inoculated lucerne was fixed within six months. Thus lucerne was highly reliant on nitrogen fixation in an N deficient soil. Surface applied lime rates >4 t/ha, increased the top-soil pH from 5.2 to 5.8 and decreased exchangeable Al levels to less than 3 mg/kg in 0-75 mm of soil depth, in the Lees Valley and at Glenmore Station. However, toxic Al levels in deeper soil horizons and its variability compromised the efficiency of surface lime application to enable lucerne persistence at both sites. The measurable effects of lime were less obvious in 75-150 mm of soil depth. Similar changes in soil pH and Al levels were measured at Ashley Dene but only 2 t lime/ha was required. Combined results from the field and pot experiments suggested the main limiting for lucerne and Caucasian clover was nitrogen deficiency that should be provided from BNF in these low N and high Al soils. Therefore the fate of applied rhizobia inoculants, and the possibility of competition with any naturalized rhizobia strains inhabiting lucerne and Caucasian clover nodules was assessed. Eight naturalized strains of Sinorhizobium meliloti were identified from lucerne nodules grown in different high country regions. This was the first identification report of any naturalized S. meliloti occupying lucerne nodules in New Zealand. Soil pH and related Al levels affected the contribution of rhizobia strains to occupy lucerne nodules. At soil pH of 5.5 (2.8 mg Al/kg), 50% of the recovered isolates were the commercial strain. This proportion decreased to 6%, as the commercial genotype had been replaced by naturalized S. meliloti strains when the soil pH increased to 6.7 (Al< 0.3 mg/kg). Further studies are therefore required to evaluate competitive ability, nodulation and nitrogen fixation rate of these identified naturalized strains, compared with the commercial strain. In contrast for Caucasian clover, the applied commercial strain was the only identified Rhizobium leguminosarum from nodules, irrespective of sites. This indicated high specificity of Caucasian clover for rhizobia symbiont to nodulate the roots.