Modified Biochars Reduce Leaching while Maintaining Bioavailability of Phosphate to Dragoon Lettuce (Lactuca sativa) in Potting Tests

The traditional use of soluble phosphate fertilizers in agriculture depletes finite global supplies and accounts for a major nonpoint source of phosphorus pollution. In potting experiments, we tested whether two modified softwood biochars that strongly bind phosphate could fertilize romaine "Dragoon" lettuce while retarding P leaching. Modifications included doping with MgO (MgO-BC), or binding, postpyrolysis, of a cationic polymer, poly-(diallyldimethylammonium) chloride (pDADMAC-BC). The former sorbs phosphate via coordination with MgO nanoparticles or coatings and the latter through enhanced anion exchange. The presterilized potting soil (SS) was a mixture of fine sand and peat moss. The test sets contained modified biochars (3 or 2% g-C/g-SS) either preadsorbed with phosphate or added along with the same amount of soluble phosphate, all at the rate of 180 or 120 mg-P/pot. We also tested sets containing the P-rich mineral dolomite. Control sets included SS and SS amended with unmodified biochar, with or without soluble phosphate, and SS amended with MgO-BC but without soluble phosphate. All pots were fertilized weekly with a phosphate-free Hoagland nutrient solution. Sets amended with the modified biochars, either preadsorbed with phosphate or added along with soluble phosphate, gave dramatically higher plant yields than the control or dolomite sets. Sets amended with modified biochars preadsorbed with phosphate leached a small fraction (0.1-23%) of P relative to the controls fertilized with soluble phosphate. Plant uptake of Mg was high in sets amended with MgO-doped biochars and induced a toxic response when those biochars were incorporated uniformly in a fine powdered form. Arbuscular mycorrhizal fungi added to some sets decreased the root:shoot ratio but otherwise had little impact. The results indicate that the tested modified biochars can appreciably reduce P leaching while providing a bioavailable source of P for crop growth.