SOIL-WATER CONTENT AS A KEY FACTOR DETERMINING THE SOURCE OF NITROGEN ((NH4)-H-+ OR (NO3)--) ABSORBED BY DOUGLAS-FIR (PSEUDOTSUGA-MENZIESII) AND THE PATTERN OF RHIZOSPHERE PH ALONG ITS ROOTS

Pseudotsuga menziesii (Mirb.) Franco was grown on an acid sandy soil (pH(H2O) = 3.87), fertilized with ammonium nitrate as the nitrogen source (50 and 150 kg N.ha-1), and maintained at two soil water levels. At both nitrogen levels, ammonium uptake was much higher in the wet soil than in the dry soil, whereas nitrate uptake was not affected by soil water content. From calculation of the nitrogen supply at the root surface, it was concluded that P. menziesii preferentially took up ammonium. Under dry soil conditions, movement of ammonium in the soil was impeded, so that the ammonium preference could not manifest itself; thus a large part of the nitrogen absorbed was in the nitrate form. Proton excretion per unit organic nitrogen formed was greater in the wet soil than in the dry soil owing to a larger ammonium contribution to total nitrogen uptake. In all treatments, the rhizosphere at the root tip was more alkaline than the bulk soil, whereas along the older root zones, acidification of the rhizosphere occurred. The strongest alkalization at the root tip occurred in the dry soil. Analysis of the rhizosphere pH pattern with a simulation model on proton transport in the soil showed that the results could only be explained by a gradual transition from OH-excretion at the root tip to H+ excretion along the older root zones. Soil water content was an important factor in determining rhizosphere pH. By extrapolating the results to forest conditions, it was determined that rhizosphere acidification may occur when the nitrification rate is low and the soil is not too dry.