Influence of soil pH, phosphorus and sulphur on the frequency of rhizobia genotypes found in four pasture legume species grown in an acid high country soil

Abstract

South Island hill and high country soils typically have deficiencies of nitrogen, phosphorus, sulphur and molybdenum. The soils often have low pH and as such, high levels of exchangeable aluminium and manganese, causing plant toxicity and decreased growth potential. These factors, along with the harsh high country climate, provide significant challenges for the establishment and persistence of pasture legume species. Legumes form a symbiosis with rhizobia, which can fix atmospheric nitrogen, increasing crop quality and quantity while decreasing the nitrogen fertiliser requirements.

Four legume species were inoculated with their respective commercial rhizobia strain and grown under glasshouse conditions at Lincoln University for 18 months. The species examined were; Russell lupin (Lupinus polyphyllus), Caucasian clover (Trifolium ambiguum), Lotus (L. pedunculatus) and Lucerne (Medicago sativa). The plants were grown in an acid high country soil (Pukerangi moderately deep silt loam) from Armidale Station, Central Otago. The experiment had a factorial design with lime applied at three rates (0, 4000 and 8000 kg ha⁻¹ equivalent), phosphorus (P) applied at two rates (0 and 30 kg ha⁻¹) and sulphur (S) applied to most treatments at 120 kg S ha⁻¹ (Gypsum) with a complete ‘optimum’ treatment, and a zero S, P and lime treatment also included. Per treatment, 24 nodules were harvested (eight per pot from three pots), and the bacteria inhabiting the nodule were recovered into culture. DNA was extracted from these bacteria and genotyped using ERIC-PCR. A portion of the 16S rRNA gene from the most frequent genotypes per treatment was sequenced to identify the colonising bacterial species and determine the effects of soil pH and fertility on nodule occupancy.

Caucasian clover was predominantly colonised by the commercial strain, CC283b, however the plants did not survive in the ‘optimum’ treatment (L3P1). An isolate (genotype L) was sequenced and found to be a naturalized strain of R. leguminosarum that could be developed for use in New Zealand systems. The Russell lupin samples showed no presence of the commercial inoculant. Liming resulted in considerable change in the population diversity of species colonising the Lupin nodules. Lotus was mainly nodulated by the commercial strain, confirming the high specificity between the legume and its rhizobial symbiont. Lucerne showed a strong treatment effect, with the commercial strain dominant in all treatments but L0P0-S (no sulphur addition). A naturalized strain of Sinorhizobium meliloti was most common in the no sulphur treatment, and this could be tested for suitability as a commercial inoculant in high country soils that are extremely low in sulphur.

This experiment provided valuable insight with regard to the potential for successful nodulation under the range of conditions studied. Further field studies are necessary to confirm these results and discover whether the same results can be recreated under field conditions, particularly under the harsh climatic conditions, and grazing regime of the high country.