FATE OF LABELLED MINERAL NITROGEN AFTER ADDITION TO 3 PASTURE SOILS OF DIFFERENT ORGANIC MATTER CONTENTS

Ammonium and nitrate labelled with 15n were added separately to 3 soils representing different stages of organic matter accumulation under pasture. The fate of the i5jj was determined after 6 weeks, and again after a further 29 weeks in the presence and absence of ryegrass plants. Recoveries of the 15N within the soilrplant system were high (90-100%), except where nitrate was added to the high-nitrogen soil (69%). The labelled ammonium was immobilized rapidly, particularly in the low-N soil, but nitrate reacted with the soils much more slowly. Thus recovery of labelled nitrate N by the plants was generally greater than that of ammonium. However, the total mineral N (labelled and indigenous) in implanted soils and the total plant uptake of N were not appreciably affected by the chemical form of the added N. This suggested that an isotopic exchange reaction had occurred between the labelled mineral N and indigenous organic N. The greatest increase in total N uptake by the plants after addition of N (i.e. the apparent fertilizer efficiency of the added N) occurred in the low-N soil (85% and 81% recovery for ammonium and nitrate respectively). The highest recovery of *% by the plants occurred where nitrate was added to the low- and intermediate-N soils (both 80%). The indigenous organic N of the 3 soils was distributed between 4 fractions in similar patterns. The 1% was incorporated into each of these organic fractions, but there was a tendency for greater amounts to remain in the non-distillable acid-soluble fraction.