Effect of nitrogen fertiliser additions on nitrogen fluxes and plantation productivity in young eucalyptus cloeziana (F. Muell) plantations

Forest plantations with fast growing species have become a major landscape feature innmany countries because of an increasing worldwide demand for wood. Many of thesenplantations are established on land with relatively infertile soils or on land that has beennused for other purposes but has now been abandoned. Nutrition is, therefore, a commonnlimitation on plantation productivity and nutrient deficiencies often develop in earlynstages of plantation development. A better understanding of the dynamics of plantnnutrient demand and supply will help managers design management regimes that helpnovercome these difficulties and create sustainable forest plantations.n n Nitrogen deficiencies are common in many plantation areas and this study wasnundertaken to understand the early patterns of nitrogen uptake and cycling in a youngnEucalyptus cloeziana plantation in Southeast Queensland. It was undertaken becausenprevious studies have suggested nitrogen is a nutrient limiting the productivity of thisnspecies in many soils in the region.n n The study was based around several nitrogen fertiliser trials that examined the responsenof the plantation to increasing rates of nitrogen addition (28 kg ha-1 up to 320 kg ha-1).nThe first of these trials (the qOld Siteq) had been established in 1998 by the QueenslandnForest Research Institute (QFRI) and was one year old when the present studyncommenced. A second trial using a similar fertiliser schedule was established by thenQFRI in 1999 (the qYoung Siteq). Since the two trials were located only 10 km apart itnwas assumed that they could be regarded as forming a chronosequence allowing theninvestigation of nitrogen uptake and use over the first three years of the life of anplantation in this region.nn n Studies of soil nitrogen mineralisation over two years showed that there was more ornless continuous mineralisation throughout the year but that the greatest rates of nitrogennrelease were measured at the beginning of the wet season in the months of September tonNovember (up to 47.7 kg ha-1 month -1 of mineral nitrogen). The rate of release was mostnclosely related to soil temperature in preceding months. Most nitrogen was released innthe form of ammonium.n n Plantation biomass was measured in trees aged between one and three years.nRegressions were developed linking diameter at breast height (D130) and biomass.nBiomass increased rapidly with age reaching about 33 t ha-1 at three years. The greatestnvolume increase was measured between year one and two at the Young Site and whennthe volume increased by about 44 m3ha-1 during the year. Overall, the trees grew wellnand there was no significant increase in biomass with applications of fertilizer nitrogennsuggesting that the plantation productivity would not be, in fact, limited by nitrogennavailability. Distribution of biomass between foliage, branches, and wood changed overntime. The proportion of leaf mass to total aboveground biomass decreased sharply withnincreasing stand age and reached about 12 % of total biomass at age three in the OldnSite.n n Foliar nitrogen concentration decreased over time and there was a significant effect ofnfertiliser application on foliar nitrogen. However, the ratio of foliage biomass to totalnbiomass was similar in all fertiliser treatments after three years indicating that fertilisingndid not cause a large change in the way nitrogen was used. The total nitrogen innaboveground biomass increased over time and reached around 132 kg ha-1 at three years.nThe ratio of foliage nitrogen to total nitrogen content in biomass decreased over timenand reached about 40 % at age three in the Old Site.n n Although frequent mowing was undertaken to remove weeds a substantial weed biomassndeveloped in these young plantations between mowings. These weeds accumulated anlarge amount of nitrogen. At age 18 months, there was 135 kg ha-1 of nitrogennaccumulated in the weeds at the Young Site. There was much lower amount of nitrogennaccumulation in the weeds growing under older plantations because of thencommencement of canopy closure. Only 39 kg ha-1 of nitrogen was accumulated in thenweeds when trees reached thirty month at the Old Site. These results highlight thencritical role of weed management in young plantations.n n Various forms of nitrogen fertiliser are being used for agricultural purposes but urea isnbecoming widely used in Queensland. The efficiency of nitrogen uptake was explorednusing 15N-labelled tracer method. This was applied as 1% enriched 15N-labelled Urea.nThe fate of this fertiliser nitrogen was followed by excavating soils surrounding thenfertilised trees and the destructive sampling of trees. The maximum uptake of appliednnitrogen after six months was found 64 % of the applied 15N-labelled fertiliser. n Over time some of the nitrogen taken up from soil and from fertilisers is recycled and isnavailable for re-use. The amount of litterfall in plantations aged between two and threenyears was about 4.1 t ha-1 yr-1. The amount of nitrogen returned in this litter was aboutn38 kg ha-1 yr-1. The litter produced and the amount of nitrogen returned in litterfall wasnnot affected by earlier fertiliser applications although there was some evidence ofnincreasing levels of foliar nitrogen concentrations in trees subject to higher nitrogennfertiliser rates. Litter decomposition is relatively slow and there was little influence ofnfertiliser on the decomposition processes. The release of nitrogen, however, was slowernthan the release of other nutrients because much is temporarily immobilised. By the endnof twelve months some 8 kg N ha-1 was being released from liter in three year oldnplantations and was available for plant uptake.n n Some nitrogen was retranslocated from aging leaves before litterfall and this wasnmeasured over a 12-month period when the trees were between two and three years old.nThe amount was generally around 13 kg N ha-1 yr-1 in the first six months that covernspring and around 4 kg N ha-1 yr-1 in the second six months that cover summer andnautumn. Nitrogen retranslocation was significantly affected by fertiliser application ratesnwith more being translocated in trees subject to higher fertilisation rates.nn n The plantations used in this study had been established at a site that was assumed to bendeficient in nitrogen. In fact, it appears nitrogen was not limiting for plantation growthnover the period of study. The declining availability of soil nitrogen has been matched bynnitrogen from litterfall, weed decay and internal retranslocation. The plantations hadnreceived a basal dressing of phosphorus but there was some evidence that phosphorusncould have been limiting plantation growth. The trees are only now reaching the crucialnexponential growth phase when nutrient demands will be high so that further monitoringnshould be carried out to follow nitrogen and phosphorus dynamics in the next few years.n