Effect of climate/soil moisture content on aluminium toxicity and carbon stocks of two high country soils under glasshouse conditions

Aluminium (Al) toxicity is a key factor limiting pasture production in the high country of New Zealand. Many high country soils are acidic, with a pH less than 5.5. Low soil pH makes Al soluble, leading to high concentations of exchangeable Al in soils and plant Al toxicity. Aluminium toxicity reduces plant vigour and yields; resticting root growth and limiting nutrient and water uptake. The harsh climatic conditions experienced in the high country could also play a role in the exacerbation of Al toxicity. Literature on the influence of low moisture extremes on exchangeable Al concentrations is scarce. Nomad white clover was grown for 10 months under glasshouse conditions at Lincoln University in two acidic high country soils collected from Omarama and Glenmore Stations, Central Otago. The trial was a full factorial design of two soil types, three soil moisture levels and four fertiliser treatments. Lime (100% CaCO3) was applied at three rates (0,2 and 8 t lime/ha) with one treatment of P and lime (2 t lime/ha and 150 mg P/kg). Sulphur was applied to all treatments at 120 kg S/ha (Gypsum). Plants were harvested four times during the experiment and yields were determined. Herbage was analysed for macro and micro nutrient concentrations by ICP-OES and NIRS. Soils were analysed for pH and exchangeable Al, with bulk soil samples also analysed for P and S contents at completion of the experiment. A significant interaction between soil moisture and exchangeable soil aluminium was observed for the Glenmore soil. The medium moisture treatment (23% VWC) had lower exchangeable Al, 4 mg/kg compared to the low (17% VWC) and high (33% VWC) moisture treatments which contained 5 and 5.6 mg exchangeable Al/kg respectively, for the zero lime treatment. The low Al Omarama soil in comparison, only contained Al concentrations of 1.3, 1.1 and 0.8 mg Al/kg in the same treatments at the completion of the experiment. This experiment did show an interesting interaction and strongly suggests that soil moisture influences exchangeable Al, but only when Al concentrations are greater than 4 mg Al/kg. Similar experiments are required using high Al soils to confirm this result. The Glenmore soil contained higher initial concentrations of exchangeable Al (6.6 mg Al/kg), had greater carbon stocks (6.9 kg C/m3) and produced higher shoot yields (10.6 g DM/pot) than the Omarama soil (3.8 mg Al/kg , 3.5 kg C/m3 and 7.8 g DM/pot). The high P fertility of the Glenmore soil (Olsen P 18) probably supressed soil Al affects on yield. High soil carbon may have also masked the effects of soil moisture on exchangeable Al for the Glenmore soil. Moisture treatments caused a significant growth limitation on the low moisture treatment (17% VWC), with a yield of 8.7 g DM/pot compared to 9.5 and 9.4 g DM/pot for the medium (23% VWC) and high (33% VWC) moisture treatments. Herbage nutrient concentrations were effected predicatably by lime and P addition. Shoot concentrations of P and Mo increased with lime addition, while shoot concentrations of Zn, Mn, B and Al decreased. Shoot nutrient uptake followed the same trends as concentration