Forest plantations with fast growing species have become a major landscape feature in many countries because of an increasing worldwide demand for wood. Many of these plantations are established on land with relatively infertile soils or on land that has been used for other purposes but has now been abandoned. Nutrition is, therefore, a common limitation on plantation productivity and nutrient deficiencies often develop in early stages of plantation development. A better understanding of the dynamics of plant nutrient demand and supply will help managers design management regimes that help overcome these difficulties and create sustainable forest plantations.
Nitrogen deficiencies are common in many plantation areas and this study was undertaken to understand the early patterns of nitrogen uptake and cycling in a young Eucalyptus cloeziana plantation in Southeast Queensland. It was undertaken because previous studies have suggested nitrogen is a nutrient limiting the productivity of this species in many soils in the region.
The study was based around several nitrogen fertiliser trials that examined the response of the plantation to increasing rates of nitrogen addition (28 kg ha-1 up to 320 kg ha-1). The first of these trials (the "Old Site") had been established in 1998 by the Queensland Forest Research Institute (QFRI) and was one year old when the present study commenced. A second trial using a similar fertiliser schedule was established by the QFRI in 1999 (the "Young Site"). Since the two trials were located only 10 km apart it was assumed that they could be regarded as forming a chronosequence allowing the investigation of nitrogen uptake and use over the first three years of the life of a plantation in this region.
Studies of soil nitrogen mineralisation over two years showed that there was more or less continuous mineralisation throughout the year but that the greatest rates of nitrogen release were measured at the beginning of the wet season in the months of September to November (up to 47.7 kg ha-1 month -1 of mineral nitrogen). The rate of release was most closely related to soil temperature in preceding months. Most nitrogen was released in the form of ammonium.
Plantation biomass was measured in trees aged between one and three years. Regressions were developed linking diameter at breast height (D130) and biomass. Biomass increased rapidly with age reaching about 33 t ha-1 at three years. The greatest volume increase was measured between year one and two at the Young Site and when the volume increased by about 44 m3ha-1 during the year. Overall, the trees grew well and there was no significant increase in biomass with applications of fertilizer nitrogen suggesting that the plantation productivity would not be, in fact, limited by nitrogen availability. Distribution of biomass between foliage, branches, and wood changed over time. The proportion of leaf mass to total aboveground biomass decreased sharply with increasing stand age and reached about 12 % of total biomass at age three in the Old Site.
Foliar nitrogen concentration decreased over time and there was a significant effect of fertiliser application on foliar nitrogen. However, the ratio of foliage biomass to total biomass was similar in all fertiliser treatments after three years indicating that fertilising did not cause a large change in the way nitrogen was used. The total nitrogen in aboveground biomass increased over time and reached around 132 kg ha-1 at three years. The ratio of foliage nitrogen to total nitrogen content in biomass decreased over time and reached about 40 % at age three in the Old Site.
Although frequent mowing was undertaken to remove weeds a substantial weed biomass developed in these young plantations between mowings. These weeds accumulated a large amount of nitrogen. At age 18 months, there was 135 kg ha-1 of nitrogen accumulated in the weeds at the Young Site. There was much lower amount of nitrogen accumulation in the weeds growing under older plantations because of the commencement of canopy closure. Only 39 kg ha-1 of nitrogen was accumulated in the weeds when trees reached thirty month at the Old Site. These results highlight the critical role of weed management in young plantations.
Various forms of nitrogen fertiliser are being used for agricultural purposes but urea is becoming widely used in Queensland. The efficiency of nitrogen uptake was explored using 15N-labelled tracer method. This was applied as 1% enriched 15N-labelled Urea. The fate of this fertiliser nitrogen was followed by excavating soils surrounding the fertilised trees and the destructive sampling of trees. The maximum uptake of applied nitrogen after six months was found 64 % of the applied 15N-labelled fertiliser.
Over time some of the nitrogen taken up from soil and from fertilisers is recycled and is available for re-use. The amount of litterfall in plantations aged between two and three years was about 4.1 t ha-1 yr-1. The amount of nitrogen returned in this litter was about 38 kg ha-1 yr-1. The litter produced and the amount of nitrogen returned in litterfall was not affected by earlier fertiliser applications although there was some evidence of increasing levels of foliar nitrogen concentrations in trees subject to higher nitrogen fertiliser rates. Litter decomposition is relatively slow and there was little influence of fertiliser on the decomposition processes. The release of nitrogen, however, was slower than the release of other nutrients because much is temporarily immobilised. By the end of twelve months some 8 kg N ha-1 was being released from liter in three year old plantations and was available for plant uptake.
Some nitrogen was retranslocated from aging leaves before litterfall and this was measured over a 12-month period when the trees were between two and three years old. The amount was generally around 13 kg N ha-1 yr-1 in the first six months that cover spring and around 4 kg N ha-1 yr-1 in the second six months that cover summer and autumn. Nitrogen retranslocation was significantly affected by fertiliser application rates with more being translocated in trees subject to higher fertilisation rates.
The plantations used in this study had been established at a site that was assumed to be deficient in nitrogen. In fact, it appears nitrogen was not limiting for plantation growth over the period of study. The declining availability of soil nitrogen has been matched by nitrogen from litterfall, weed decay and internal retranslocation. The plantations had received a basal dressing of phosphorus but there was some evidence that phosphorus could have been limiting plantation growth. The trees are only now reaching the crucial exponential growth phase when nutrient demands will be high so that further monitoring should be carried out to follow nitrogen and phosphorus dynamics in the next few years.