Organic acid production by plants and microorganisms was quantified in sand media amended with biosolids in the presence and absence of com (Zca mays L.) in a sand-culture hydroponic medium, Tolal quantities of organic acids were greatest in ireatmenls containing both plants and biosolids. with lesser amounts in treatments wilh plants alone, biosolids-treated media alone, and a nutrient solution-irrigated blank medium, Biosolids enhanced organic acid production in the rhizosphere and influenced the composition ol' organic acid mixtures. Only lactic, acetic, butyric, and oxalic acids were detected in media without plants. When the medium was planted, additional organie acids were recovered including tartaric, maleic. suecinic, valerie, glutaric, pyruvic, and propionic. Lactic, acetic, and butyric acids were predominant in solutions recovered from Ihe planted media and collectively accounted for 0.65 lo 0,75 of the COO mole fraction. Oxalic, maleie, and tartaric acids were the second mosl abundant and varied from 0.05 to 0.1 of the mole fraction, followed by suecinic, valeric, glutarie. propionie, and pyruvic acids, comprising <0,05 ofthe mole fraeiion. Plant growth stage had no effect on relative proportions of organic acids but did intiuenee the total quantities of organic acids recovered. Biosoiids sources did nol have a signilicant elTect on either the quantity or composition of organic acids in any media. The predominance of organie aeids that are mierobial fermentation products suggests that the carbon contained in root exudates and biosoHd amendments was transformed into a mixture of various fermentation 871 872 B.-J. Koo et al.products that accumulated in the rhizosphere solution and sand medium ns a result of microbial growth and activity.
Red clover (Trifolium prateuse L.), sweet clover (Meliotus alba Medik.), alfalfa (Medicago sativa L.), and crown vetch (Vica villosa Roth) were grown in nitrogen‐free Hoagland's solution in a declining nutrient condition. Increases in acidity of the nutrient solutions were recorded over the experimental period. In general, the excess bases and the excess bases/nitrogen ratio varied considerably among harvests. Significant positive relationships between total N and proton efflux, excess bases and H+ excretion, and dry weight and H+ efflux were observed. The milligrams H+ produced per gram N fixed for the four legumes were 49 for red clover, 43 for crown vetch, 42 for alfalfa, and 37 for sweet clover. By extrapolating the protons produced per gram dry weight in this study to yields commonly observed in the field, annual hydrogen production per hectare would amount to 9.7 kg for red clover, 4.6 kg for sweet dover, 15.2 kg for alfalfa, and 4.5 kg for crown vetch. The amounts of acidity produced per hectare per year calculated from the values of N2 fixation were 5.2 to 14 kg by alfalfa, 4.2 to 9.4 kg by red clover, 3.2 to 7.1 kg by sweet clover, and 3.9 to 6.8 kg by crown vetch. The study demonstrates that production of H+ through symbiotic fixation by legumes is an important source of acidity in agricultural ecosystems. The acidifying effect of legumes could, in the long‐term, result in acidification of the bulk soil causing the downward leaching of exchangeable cations and a decrease in base saturation.
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