Retention of N-15 labelled urea in a radiata pine ecosystem : effect of split applications

Abstract

The efficiency of fertilizer nitrogen uptake and retention within the ecosystem were studied in a 2-year-old radiate pine stand. This was a third rotation site on coastal sand dunes. Application of 90g N/tree (150 kg N/ha), as either Single, 3-split or 9-split dressings, were applied as urea solution, labelled with 2.69 atom % N-15. A control and three seasonal treatments (30 g N/tree) were also included. The trees were in the centre of 7 m² root isolated plots. The experiment ran for a period of 17 months during which time N-15 uptake was monitored by foliar analysis. Steady levels were reached 6 months after the single application. The indicated that soil processes acted on the N-15 pool to achieve an equilibrium with soil-N. The final distribution of fertilizer nitrogen within the ecosystem was determined by a complete tree harvest (including roots) and soil sampling to 90cm. There was a 25% increase in above ground biomass formed in the year after fertilization with 90 g N/tree. The below ground response was more marked, with an 80% increase in fine (<2 mm) roots. Irrespective of the rate, the season of the splitting of the application, the tree’s uptake of fertilizer nitrogen was the same (mean = 21.3%). However, the retention within the soil varied from 40% with a Single application to 70% for the 9-split application. The total ecosystem retention varied from 60% (Single) to 90% (9-Split) excluding volatilization (<2%) and uptake by surrounding trees (<3%). The retention for the seasonal treatments was similar to the split treatments. The was a positive relationship between tree uptake of fertilizer nitrogen and initial sink size. Other major factors determining the range of tree uptakes (15-32%) were leaching and immobilization. The high soil retention of fertilizer nitrogen at the lower application rate, and for split applications, suggests immobilization is a dominant process. Leaching was most prominent for the Single 90 g N application. The extent of this loss may have precluded further uptake by the tree. Although total retention was increased with split applications, there was no immediate benefit to the tree. Given that this additional nitrogen is a very minor proportion of the total; in the soil, the probability of additional; responses seems unlikely. Split applications did not increase tree uptake of nitrogen. Indeed, in the absence of leaching a single application may be utilised more efficiently.