Spatial and temporal variation in soil Mn2+ concentrations and the impact of manganese toxicity on lucerne and subterranean clover seedlings

Abstract. Spatial and temporal variation in soilMn2+ was observed over a 12-month period at two field sites near Gerogeryand Binalong in southern New South Wales (NSW), Australia. Three pot experiments were then conducted to emulate therange of soil Mn2+ concentrations observed in the field and to determine the effect of different concentrations on lucerne andsubterranean clover seedling growth, as well as to determine the effect of heating a soil on pH and Mn2+ concentrations.Concentrations of soil Mn2+ in the surface 0.20mvaried at a given sampling date by up to 288% (2.5'9.7 mg/mL) and 183%(8.7'24.6 mg/mL) across the Gerogery and Binalong field sites, respectively. At both sites, the concentration of soilMn2+ in agiven plot also varied by up to 175% between sampling times. There was little consistency between sites for seasonalfluctuations of soil Mn2+, although in both instances, peaks occurred during months in which newly sown lucerne plantsmight be emerging in southern NSW. Pot experiments revealed that high concentrations of soil Mn2+ reduced lucerneseedling survival by 35%, and on seedlings that did survive, reduced shoot growth by 19% and taproot length by 39%.Elevated concentrations of soil Mn2+ also reduced subterranean clover seedling survival by up to 55% and taproot length by25%, although there were few effects on subterranean clover in treatments other than those imposing the highest soil Mn2+concentrations. The third pot experiment demonstrated that elevated soil temperatures led to increased soil pH and increasedsoilMn2+ concentrations, attributable to a decrease in biological oxidation of soilMn2+. This was in contrast to the commonlyanticipated response of a decline in soil Mn2+concentrations as soil pH increased.