Uptake of nitrogen by Pinus radiata and retention within the soil after applying 15N-labelled urea at different frequencies. 1. Growth response and nitrogen budgets

Abstract

The uptake of fertiliser-nitrogen and retention within the soil was determined after one growing season in a 2-year-old Pinus radiata plantation on coastal sand dunes at Christchurch, New Zealand. Applications of 15N-labelled urea enriched at 2.33 atom % excess were applied at different frequencies and in different seasons to single-tree root-isolated plots (area, 7 m 2). 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 was about 20% of that applied for all treatments irrespective of frequency, rate or season of application. Uptake was positively correlated with the size of the tree at the time of application. Seventy-two percent of fertiliser-nitrogen was retained within the soil following the application of 30 g N in spring but only 39% for the single application of 90 g N per tree. Splitting the application increased the retention of fertiliser-nitrogen to 54% and 67% for the three-split and nine-split applications, respectively. Those trees receiving 90 g N showed a diameter response of 9% compared to controls. Height growth was not affected by fertiliser. Above-ground biomass was 19% higher than controls with the main increase being in foliage formed during the experiment (+26%). The largest growth responses were below ground with overall root biomass 55% higher than controls with fine roots (less than 2 mm) 80% higher. The only response to 30 g N per tree was below ground with fine-root biomass being 27% higher. Split applications of urea reduced losses of fertiliser, probably as leaching, but the increased nitrogen within the soil has not benefitted the trees in the first year. The relatively low losses from applying 30 g N per tree and the increase in fine-root biomass suggest that an optimal fertiliser strategy in young pines would be to apply a single small dressing followed by a heavier application a year later when root density and sink size have increased. There is no evidence for the superiority of applying nitrogen in any particular season.