Long-term persistence of aluminium-tolerant and sensitive Phalaris lines on acidic soils and associated changes in soil acidity

Abstract

Persistence of an aluminium (Al)-tolerant phalaris F₁ hybrid (Siro 1146 Phalaris aquatica × P. arundinacea) was compared with that of 3 more Al-sensitive phalaris (P. aquatica L.) lines (cv. Australian and the progenitors of cvv. Sirosa and Sirolan) at 2 sites (Strathbogie and Baddaginnie) in north-eastern Victoria with strongly acidic soils 20 years after sowing. Soil pH_Ca and extractable aluminium (Al_Ca) were also measured to 1 m depth under Siro 1146 and annual grass pasture at each site. All grass treatments contained volunteer subclover. Siro 1146 persisted better than the other lines (P<0.05) at Strathbogie where the soil contained high Al_Ca concentrations down to 50 cm depth. Soils should be sampled and tested to at least this depth to determine their suitability for phalaris. At this site Australian phalaris persisted better than the progenitors of Sirosa and Sirolan (P<0.01), probably because Australian has more spreading ability and tolerance of set stocking than the winter active lines. At Baddaginnie, the soil contained lower concentrations of Al_Ca below 20 cm depth than at Strathbogie and persistence of the 3 Al sensitive phalaris lines was good despite the lower rainfall. The less drought-tolerant Siro 1146 persisted poorly at Baddaginnie, but had high ground cover due to high lateral spread of the survivors. At Strathbogie, the 10–20 cm layer of soil under Siro 1146 had a higher pH_Ca and lower Al_Ca than that under the other 4 phalaris and the annual grass treatments. Soil under Siro 1146 also had higher pH_Ca and lower Al_Ca down to 40 cm depth compared with the annual grass treatment, the differences being significant in the 20–30 cm layer. These differences were reversed at 50–100 cm, but only the effect on Al was significant. Although soils were not sampled at the beginning of the experiment, adequate replication and randomisation of the grass treatments showed that it was statistically improbable for observed final differences to be due to initial soil differences. The final differences may be due to greater amounts of nitrate being taken up from the upper layers by Siro 1146 over its long growing season compared with the annual grasses, leading to greater nitrate leaching from the upper layers and greater nitrate uptake from lower layers under the annuals (i.e. spatial separation of acid generation and consumption processes within the profile). Another possible reason for the greater acidification below 50 cm by the perennial is that its roots may have taken up more cations in this zone. Because of rising concern in some sectors of the public about the off-site, environmental effects of agriculture, the future role of more acid-tolerant phalaris cultivars growing near native vegetation in the high rainfall zone is discussed.