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

Abstract

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.

Keywords:

• post-rice crop
• legumes
• Mn²⁺
• flooded soil
• toxicity
• anaerobic

Acknowledgements

The authors wish to acknowledge the Charles Sturt University Postgraduate Research Scholarship for the PhD scholarship granted to Shamsul Haque. In addition, we acknowledge the NSW Department of Primary Industries for providing access to the hydroponic plant growth facility. We also acknowledge Dr. Saliya Gurusinghe for reviewing the manuscript and Dr. Paul Weston for support provided with statistical analysis.

Disclosure statement

No potential conflict of interest was reported by the author(s)