IMPLICATIONS OF N-ACQUISITION FROM ATMOSPHERIC NH3 FOR ACID-BASE AND CATION-ANION BALANCE OF LOLIUM-PERENNE

In the atmosphere, ammonia (NH3) is the third most abundant N species which, due to various natural and anthropogenic sources, can locally r each high concentrations. The acquisition of atmospheric NH3 by plant shoots will lead to two opposing effects on acid-base balance. Absorpt ion and dissolution of NH3 will cause an alkalinisation, while the ass imilation of NH3 results in an acidification. Different rates of these processes would lead to an acid-base imbalance with consequences for the ionic balance of the plant. As there is only a limited capacity fo r biochemical disposal of excess H+ in shoots, pH regulation may invol ve a pattern of (in)organic ion flow between shoots and roots followed by H+/OH- extrusion into the media via roots. The acquisition of NH3 as additional N source should lead to a reduction in the ratio of mol H+/OH- gained per mol N assimilated. We have recently investigated the NH3 acquisition by Lolium perenne L. cv. Centurion and studied the ef fects of gas phase NH3 on growth, acid-base balance and water-use effi ciency. The experiments, therefore, included the application of a rang e of (NH)-N-14, to the shoots and of N-15 as NO3- NH4+ or NH4NO3 to th e roots. After a summary of the main conclusions from those experiment s, we discuss the implications of the use of atmospheric NH3 for the m ineral composition of the plants. Over the range of NH3 supplied, plan ts from all treatments could utilize gas-phase NH3. Plants receiving N O3- via their roots had a higher capacity to use gaseous NH3 than thos e growing with NH4+. NH3 assimilation in shoots reduced both the acid load with NH4+ nutrition and the alkaline load with NOT supply to the roots. The most significant effect of fumigation on the ion balance wa s an increase in K+ within all treatments, and this effect was highest in the NH4+-fed plants. The results of the experiments support predic tions of a combination of neutralizing biochemical reactions as well a s transport of organic anion salts between shoots and roots as possibl e acid-base regulation mechanisms of the whole plant.