Nutrient management in a Pinus radiata plantation after thinning: the effect of nitrogen fertilizer on soil nitrogen fluxes and tree growth

The influence of N fertilizer on soil mineral N fluxes, canopy development, and tree growth was studied in a thinned 11-year-old Pinus radiata D. Don plantation. Ammonium sulphate and single superphosphate were applied in an incomplete factorial design, but only the main effects of N application at 0 (control) or 200 kg N-ha-1 are considered here. Spring application of fertilizer increased the quantity of mineral N in the forest floor plus surface soil (0-0.30 m) from 1.2 to 194 kg.ha-1. Within 51 weeks this had fallen to 8.3 kg.ha-1, and after 89 weeks had returned to prefertilizer levels. In the unfertilized soil, rates of net mineralization were low with little seasonal variation. Nitrogen fertilizer increased N mineralization; over the 2 years of measurement fertilized and unfertilized soils mineralized 155 and 77 kg N.ha-1, respectively. There was no net immobilization of fertilizer N. There was no leaching of mineral N from the unfertilized soil whereas 149 kg N.ha-1 was leached below 0.30 m during the 2 years after fertilizer application. Nitrogen uptake increased from 71 kg ha-1 in the control to 203 kg ha-1 in the fertilized treatment. Fifty-one percent (103 kg ha-1) of N uptake by trees in the fertilized treatment occurred within 20 weeks of fertilizer application. Fertilized trees took up 58% of the available N (N added as fertilizer plus N mineralized), while 42% was leached. Ammonium dominated the soil mineral N pool and mineral N fluxes, with nitrate generally accounting for less than 10% of mineral N in both fertilized and unfertilized soils. Leaching of mineral N from the fertilized soil (Nleach, kg.ha-1.week-1) was highly correlated (r2 = 0.92) with soil mineral N content (Nstart, kg.ha-1) and effective rainfall (rainfall minus evaporation, Reff, mm.week-1) according to the relationship Nleach = aNstart + bReff, while N uptake (kg.ha-1 week-1) was highly correlated (r2 = 0.91) with soil mineral N content and N mineralization (Nmin, kg.ha-1.week-1) according to the relationship Nuptake = aNstart + bNmin. Fertilizer increased needle N concentrations and content by 52 and 87%, respectively, after 58 weeks, and resulted in a 17% increase in leaf area index after 71 weeks. These differences were reflected by an increase in basal area increment of 23% during the 2 years since fertilizer application. The rapid uptake of N fertilizer was associated with storage in existing biomass. Uptake of fertilizer N should, therefore, increase with plantation biomass. Consequently, it should be possible to increase the uptake of N fertilizer, and minimize leaching, by applying fertilizer before, rather than after, thinning. Such a strategy may be particularly appropriate for soils that have a low capacity to retain applied N.