The fate of labelled mineral nitrogen after addition to three pasture soils of different organic matter contents

Abstract

Ammonium and nitrate labelled with ¹⁵N were added separately to three soils representing different stages of organic matter accumulation under pasture. The fate of the labelled nitrogen was determined after 6 weeks, and again after a further 29 weeks in the presence and absence of ryegrass plants.

Recoveries of the labelled nitrogen within the soil–plant system were high (90–l00%), except where nitrate was added to the high-nitrogen soil (69%). The labelled ammonium was immobilized rapidly, particularly in the low-nitrogen soil, but nitrate reacted with the soils much more slowly. Thus recovery of labelled nitrate nitrogen by the plants was generally greater than that of ammonium. However, the total mineral nitrogen (labelled and indigenous) in unplanted soils and the total plant uptake of nitrogen were not appreciably affected by the chemical form of the added nitrogen. This suggested that an isotopic exchange reaction had occurred between the labelled mineral nitrogen and indigenous organic nitrogen. The greatest increase in total nitrogen uptake by the plants after addition of nitrogen (i.e. the apparent fertilizer efficiency of the added nitrogen) occurred in the low-nitrogen soil (85% and 81% recovery for ammonium and nitrate respectively). The highest recovery of labelled nitrogen by the plants occurred where nitrate was added to the low- and intermediate-nitrogen soils (both 80%).

The indigenous organic nitrogen of the three soils was distributed between four fractions in similar patterns. The labelled nitrogen 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.