Effect of high levels of macronutrients on the trace element nutrition of Pinus radiata

Abstract

A number of experiments, using both field trial and pot trial techniques, were undertaken to investigate the effect of N addition at high rates on the growth and nutrition of radiata pine. Two field trials were designed to investigate the effect of increasing rates of N addition on young radiate pine plantation (Taheke) growing in a fertile ex-pasture soil north east of Rotorua and the effect of increasing rates of N and B addition on young radiata pine growing in the nutritionally poorer soils of Ashley Forest in Canterbury. One pot trial in sand culture investigated the effect of N source on the growth of radiata pine seedlings while a further two pot trials investigated the effect of increasing N and B addition at two rates of P.

In 1989 urea was added at 0, 250 or 500 kg N ha⁻¹ to enhance the mild stem deformities that had been observed in young radiata pine trees at the Taheke site. The soil had a high nutrient status, presumably because of its pastoral land-use history, a high potential to nitrify and the major form of inorganic N was nitrate. While the amount of radiata pine stem deformity did increase during the trial, this was not a result of the fertiliser addition. The measurements of tree height and diameter were also not significantly affected by N addition. Subsequent soil analysis indicated that the amount of native N in the top soil was already high, approximately 3.1 and 1.8t ha⁻¹ in the 0 to 10 and 10 to 20cm depths respectively, and therefore the fertiliser additions did not represent a large input to the system and the addition of N did not significantly affect the amount of total N% or mineral N in the soil.

At Taheke, the increasing addition of N significantly increased the foliar concentrations of N in the short-term and significantly decreased the concentrations of P, Ca and Mg during the trial presumably as a result of dilution and/or translocation. There appeared to be a relationship between Cu and N, in the foliage, and the number of kinks in the upper stem of the radiata pine trees. As Cu:N ratios in the foliage increased, the number of kinks decreased.

The characteristics of the trial site at Ashley Forest contrasted to those at Taheke because the soils had a low nutrient status, had a very low potential to nitrify and the major form of inorganic N was ammonium. The B status of the soil was also marginal for radiata pine growth. The fertiliser treatments consisted of N, added as urea, at 0, 200 or 400kg N ha⁻¹ by B, added as ulexite, at 0, 3.7 or 7.4 kg ha⁻¹. The foliage sampling technique followed the life of fascicles from just after initiation in November, 1989, to full extension in May, 1990. The results showed that the concentrations and of amounts per fascicle of N and B significantly increased in response to the addition of the respective fertilisers while P and Mg significantly declined with increasing N addition. Retranslocation occurred for both P and B particularly during times of low soil moisture. The amount of retranslocation was increased with increasing N addition while the increasing addition of B reduced the amount of B retranslocation.

The results indicated that without B addition the foliar concentrations of B were marginal (i.e. 8 to 12µg g⁻¹) and were further reduced by the addition of N. This also occurred for P where the foliar concentrations were low (i.e. <0.12%) and significantly declined with increasing N addition even though a basal dressing of 14kg of P ha⁻¹ had been added.

The addition of N increased fascicle weights, compared to the control, particularly at 200kg N ha⁻¹, but had no significant effect on tree heights at 6 months or at 3 years after fertiliser addition. However, when twelve trees were biomassed 1 year after fertiliser addition at Ashley, the total above-ground weight of trees that had received 7.4kg B ha⁻¹ and/or 400kg N ha⁻¹ was significantly greater then the control trees. The results from the biomass indicated that 3.2% of the added N and 0.5% of the added B was had been taken up into the above-ground portion of the tree. There was no evidence of residual fertiliser N in the soil, however the addition of B at 7.4kg ha⁻¹ had significantly increased the non-specifically adsorbed and specifically adsorbed soil B fractions in the 0 to 10cm and 10 to 20cm depths. The weed components at Ashley, mainly gorse and broom, may have been important sinks for the added fertiliser.

A sand culture pot trial further investigated the effect of the inorganic N-source, either ammonium or nitrate, on the growth and nutrition of radiata pine seedlings. The results indicated that nitrate-N nutrition increased the uptake of cations, particularly Ca, Mg and Fe, while reducing the uptake of P. The roots of the nitrate-fed seedlings were darker in colour and were more numerously and finely branched than the ammonium-fed seedlings.

Two sand culture pot trials, one at 4µg P m1 ⁻¹ and the other at 8µg P m1⁻¹ , further investigated the interaction between the addition of ammonium (at rates of 14, 35 and 70µg N ml⁻¹ ) and B (at rates 0.01,0.05 and 0.1 µg B m1⁻¹ ) on radiata pine seedlings. Increasing N addition increased: N uptake, shoot dry weights, the dilution of nutrient concentrations in the shoots, the translocation of nutrients to the shoots, the efficiency of nutrient uptake by the roots, the physiological use efficiency of nutrients and the seedling height difference. The root dry weights decreased with increasing N addition. As B:N ratios declined in the shoots, with increasing N addition, the amount of seedling height difference (a simple measure of stem deformity) increased. The correlation was significant. The increasing addition of B, in the sand culture pot trials, increased seedling B concentrations and B:N ratios and the amount of B in the roots.

Across the two ammonium by B pot trials, the higher addition of P affected radiata pine growth by increasing the amount of N in the shoots.