不同水平硝态氮对蚕豆根系质子释放量的影响

为了解豆科植物在加速土壤酸化中的作用及其机制,通过水培试验和恒pH自动电位滴定法研究了4个硝态氮供应水平对蚕豆根系质子释放量和介质pH的影响,分析了蚕豆根系释放质子的机制。结果表明,当不供应外源硝态氮时,蚕豆根系释放的质子数量最多,以干根计算培养10次的累积释放量可达37.5mmol·g-1;此条件下质子释放导致培养液pH下降幅度最大,可达1.93。随着外源硝态氮供应量的增加,蚕豆根系释放的质子数量和介质pH的下降幅度均减小。当培养液NO3-浓度为1.0mmol·L-1时,培养试验前期根系有少量质子释放,培养试验后期有少量OH-释放。研究表明,蚕豆生长介质中的质子主要来源于蚕豆根系质子的直接释放,有机酸对介质质子的贡献可以忽略。蚕豆根系对阴、阳离子的不平衡吸收是根系释放质子的主要原因,添加硝态氮可以减少蚕豆根系质子释放量。

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.