Investigations of soil aluminium and legumes for pastoral farming in the Central Otago high country : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Soil acidity and associated high concentrations of exchangeable aluminium (Al) present significant challenges to the success of legume species and farm sustainability in the high country of Central Otago, Aotearoa New Zealand. Such grasslands rely heavily on legumes for soil nitrogen (N) and quality livestock feed. However, traditional legume species are limited by the district's edaphic and climatic conditions. To address these challenges, this PhD investigated soil chemistry, Al solubility and legumes to support sustainable pastoral farming in the high country. Understanding the mechanisms controlling Al solubility, soil acidity, and buffering capacity is imperative in addressing the challenges of farming on acid soils. The chemistry of acidic grassland soil was investigated to assess Al fractions and associated properties. The low base saturation (< 10%) and very acidic soil pH (pHKCl 2.9–4.1) indicated that the soil had likely shifted beyond the Al-base cation exchange buffering system into hydroxy Al dissolution or organic Al dissolution pH buffering systems. Statistical modelling showed Al solubility in the A horizon soil was controlled by organically bound Al. This finding underscored the importance of the saturation of Al to carbon (C), in addition to soil pH, in predictive models of exchangeable Al. The equation: logAlKCl = 0.74 – 0.49pHKCl + 0.57log(AlCu/C) effectively predicted exchangeable Al in the A horizon of the investigated soil. Moreover, the low pH coefficient indicated that the A horizon was weakly buffered by the exchange of Al3+ and H+ on soil organic matter binding sites. Conversely, no unequivocal explanation of Al solubility was determined in the B and C horizons. Nevertheless, the linear relationship between exchangeable Al and pH indicated pH can be used to predict exchangeable Al in the B and C horizons by the equation: logA Thirteen forage legume species were assessed across three field experiments in Central Otago. Plant survival varied among species, with local climate emerging as the pivotal determinant of legume survival and yields. Lotus (Lotus pedunculatus Cav.) exhibited 100% survival over 18 months at a high altitude, acidic, cool climate site, whereas other perennial legume species showed limited persistence (0–55% survival). At two hotter, drier sites, crimson clover (Trifolium incarnatum L.) had the greatest Year 1 establishment and yields of assessed annual legume species. Moreover, strong Year 1 seed set and subsequent regeneration in Year 2 were observed for crimson clover, striated clover (Trifolium striatum L.), and subterranean clover (Trifolium subterraneum L.) (cv.s Denmark and Narrikup). The differences in survival among these legume species underscore the detrimental effect of climate on many legumes and highlight lotus and crimson clover as key species of interest for farms in high country environments. Following strong survival rates in Year 1 at the high altitude, acidic, cool climate experimental site, lotus and Russell lupin (Lupinus polyphyllus Lindl.) were selected for a glasshouse experiment to investigate potential mechanisms of Al tolerance associated with root growth in acid soil. Shoot and root biomass and root morphology were quantified in response to lime treatments in 0–9 cm and 9–18 cm horizons of 20 cm deep pots. The soil pH was 4.4, 4.9 and 5.4; the exchangeable Al concentrations were 24, 2.5 and 1.5 mg kg-1 for 0, 4.5, and 6.7 g lime L-1 treatments. No response to lime was shown in the mean total shoot and root biomass of lotus (13.2 g DM plant-1) nor Russell lupin (2.9 g DM plant-1). Furthermore, lotus plants did not differ in root length or surface area between the 0–9 and 9–18 cm horizons. However, within 0–9 and 9–18 cm horizons, Russell lupin showed increased root biomass, length, and surface area when lime was not added or was added at a low rate. Greater root biomass in lotus and Russell lupin was associated with a 50% reduction in soil exchangeable Al in the 0–9 cm horizon (16 mg kg-1), compared with the 9–18 cm horizon (32 mg kg-1) and the control soil (27 mg kg-1) in the 0 g lime L-1 treatment, suggesting that this reduction in exchangeable Al concentration is a key mechanism for Al tolerance in these legume species.