The influence of washings of living roots on nitrification

Abstract

The effects of washings of plant roots on the rate of nitrification in soil was investigated. The root washings were obtained by leaching columns of quartz chips in which plants were growing, with their roots bathed in nutrient solution. The root washings and ammonium solution were then added to columns of clay loam soil in which ammonium-ion was being converted to nitrate-ion at the rate of c. 370 mg N/kg soil/day. Rates of nitrate formation and ammonium disappearance were estimated by analysis of leachates from the soil columns. The root washings of all plants investigated (ryegrass, wheat, salad rape, lettuce and onion) reduced the rate of nitrification but the effects of rape and lettuce were temporary. Ryegrass root washings had the most pronounced and persistent effects and reduced the rate of nitrification (up to 84 per cent) in proportion to concentration. The rate of nitrification was not limited by supplies of ammonium-ion or other nutrients and there was no reduction of nitrate in the soil columns. As neither microbial immobilization of inorganic-nitrogen nor denitrification appeared to be taking place in the presence of root washings, it was concluded that root washings can in some way retard nitrification although no mechanism was demonstrated.