What levels of soil Mn2+ can pulse and legume crops tolerate when grown in rotation with paddy rice?

Elevated concentrations of manganese (Mn2+) can persist after drainage in soil following wetland rice production, leading to toxicity in subsequent legume crops. To evaluate Mn2+ concentration causing adverse impacts on crop establishment and growth, a controlled environment experiment using hydroponics was designed. Germination and growth of field pea, faba bean and mung bean were evaluated over 21 d with an increase in Mn2+ (0 - 50 mg Mn2+L-1). Germination of all crops was impacted significantly with an increase in concentration of Mn2+. Leaf chlorophyll content as well as all plant growth parameters assessed (dry weight, height) were negatively impacted (p < 0.05) by increasing concentrations of Mn2+. Field peas and faba beans proved more sensitive to Mn2+ than did mung beans at concentrations of 5 - 10 mg L-1. Field pea and faba bean specifically exhibited reduced height (30 - 60% and 4 - 41%, respectively), root length (12 - 30% and 20 - 25%, respectively), and shoot weight (41 - 69% and 47 - 81%, respectively). Mung bean was less impacted in terms of plant height with 23% reduction at 25 mg Mn2+ L-1 and shoot dry weight reduced by 48% at 50 mg Mn2+ L-1. Total chlorophyll content of mung bean was severely affected (86% reduction at 50 mg Mn2+ L-1), with field pea least impacted. Our findings suggest that significant Mn2+ toxicity could potentially occur in terms of legume germination and overall growth if legume crops are sown into post-rice soils with >= 5 mg Mn2+L-1. Soil Mn2+ level must be considered when developing cropping strategies following flooded rice.