Biochar Increasing Internal Tolerance to Manganese Toxicity in a Manganese-Rich Acid Soil

Marquez, Josiah M. K.
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University of Hawaii at Manoa
Manganese (Mn) toxicity is a serious constraint on crop productivity in prime agricultural land in central Oahu. Liming alleviates Mn toxicity by raising soil pH and supplying calcium (Ca), reducing plant available Mn. Biochar shows promise as a sustainable alternative to lime in remediating Mn toxicity. We implemented a series of soybean greenhouse experiments to test the potential of five biochars with differing physico-chemical properties to remediate Mn toxicity. Additionally, to compare the effects of calcium nutrition and pH, we conducted another experiment with 4 increasing rates of two different Ca sources, lime (CaCO3) and a neutral Ca salt (CaSO4). Our results showed that pH increase was effective in alleviating Mn toxicity from the soil; however, Ca nutrition independent of pH also contributed to alleviation. Biochar treatments maintained soybean growth similar to the limed control, despite having toxic levels of Mn in tissue and soil, suggesting alleviation by increasing plant Mn tolerance. A second planting in the same biochar treatments resulted in Mn toxicity in almost all biochars, suggesting that biochars’ alleviating properties lack persistence in the soil, except for the anaerobic digest biochar, which continued to detoxify Mn. A follow-up bioassay grow- out was conducted, which gave results suggesting that alleviation from anaerobic digest biochar was from biochar-derived compounds absorbed into the plant. We propose that alleviation of Mn toxicity from biochar involves Mn tolerance through organic chelates, specifically phenolic compounds. Increasing internal tolerance with biochar can have implications not only in agriculture, but also in phytoremediation of heavy metals.
Biochar, Mn toxicity, soybean growth
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