Effect of nitrate concentration on proton release by faba-bean roots

In the effort to understand the mechanisms of enhanced soil acidification by legume plants, the effect of nitrate dose on proton release by the roots of faba-bean under medium pH conditions was analyzed in solution culture experiment and by automatic potentiometric titration at a constant pH. The results show that the roots of faba-bean release the largest amount of protons when no external nitrate source is available, inducing a drastic decline (1.93 units) in medium pH. The cumulative amount of released proton after ten incubations reaches 37.5 mmol per gram dry-root. With increasing nitrate concentration, the amount of released protons and change in medium pH decrease. When nitrate concentration increases to 1.0 mmol·L^-1, only a small amount of proton is released at the early stage of the culture experiment (in fact, some amount of OH^- is released at the later stage of the experiment). When there is no external nitrate or it is available at low levels, legume plants derive N from biological nitrogen fixation, leading to higher cation than anion uptake. Under this condition, plant roots release protons to ensure that the charges are well in balance. Results also indicate that protons in the culture solutions come mainly from direct releases by faba-bean roots. The effect of organic acid on the amounts of proton in the medium is generally negligible. Addition of nitrate can reduce H+ release by faba-bean roots.