UPTAKE OF NITROGEN BY PINUS-RADIATA AND RETENTION WITHIN THE SOIL AFTER APPLYING N-15-LABELED UREA AT DIFFERENT FREQUENCIES .2. SEASONAL TRENDS AND PROCESSES

Applications of N-15-labelled urea enriched at 2.33 atom % excess were applied at different frequencies and in different seasons to 2-year-old Pinus radiata growing on coastal sand dunes at Christchurch, New Zealand. A single application of 90 g N per tree in spring was compared with either three by 30 or nine by 10 split applications. Applications of 30 g N per tree in either autumn, spring or summer were also compared. The uptake of fertiliser-nitrogen into the foliage was initially the same for all treatments. Uptake of N-15 ceased with the lower application rate after 100 days but continued at the higher application rate until about Day 200. The pattern of foliar uptake mirrored the availability of a fertiliser-nitrogen pool in the soil as determined by leachate and soil analysis. Losses of fertiliser-nitrogen as ammonia from volatilisation were minimal although small frequent applications resulted in greater losses. For applications of 30 g N per tree leaching losses were apparently minimal with immobilisation accounting for the majority of nitrogen not taken up by the trees. For the single application of 90 g N per tree up to 30% was lost by leaching. For the split applications a greater proportion of the nitrogen was retained within the soil. However the heterotrophic microflora apparently outcompeted the tree for this additional nitrogen. The twin processes of leaching and immobilisation act to rapidly limit the availability of fertiliser-nitrogen to the tree. Leaching is important for heavy single applications while immobilisation dominates for smaller and frequent applications. Consequently the tree's utilisation remains low at about 20% regardless of the frequency, season or rate of application. Fertiliser applications should aim to maximise the uptake of nitrogen into the tree in the first year. Single applications should be used but if groundwater contamination is a problem or nitrogen leaching may displace other nutrient ions in short supply then split applications may have some benefit.