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Acidity Produced by Leguminous Plants through Symbiotic Dinitrogen Fixation

Dep. of Soil and Environ. Sci., Univ. of California, Riverside, CA 92521.

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ABSTRACT

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.