Growth and nutrient cycling in cultivated protea neriifolia R.Br.
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The family Proteaceae is distributed predominantly in the south-western Cape Province of South Africa and south-western and south-eastern Australia, areas which fall within the climate term mediterranean ecosystems. A major characteristic of these areas is the low level of total nutrients in the soil, particularly nitrogen and phosphorus. In their natural habitat, therefore, Proteaceae occur on well-drained and highly leached soils of low nutrient status. Efficient nutrient cycling processes, combined with morphological adaptations designed to facilitate maximum absorption of available nutrients (for example, proteoid roots in the Proteaceae), conserve the limited nutrients available, allowing for the continued growth of these shrubs under conditions of low nutrient availability. In recent years, flowers of certain species of Proteaceae have become popular as cut-flowers. As a result, many species of Proteaceae are currently cultivated worldwide, under conditions that match as closely as possible those found in mediterranean ecosystems. Traditionally, the shrubs are cultivated on nutrient poor soils and of concern is the loss of nutrients through the removal of flowers for commercial sale. Therefore, the aim of the present study was to evaluate growth and mineral cycling in the proteaceaous shrub, Protea neriifolia R.Br., cultivated in a summer rainfall area in South Africa. Nutrient loss through flower removal and its effect on nutrient cycling was quantified. Optimum levels of ammonium nitrogen, phosphorus and potassium for the growth of P. neriifolia seedlings was determined and this formed the basis for the fertilization of mature P. neriijolia shrubs. The effects of inorganic fertilizers on growth and mineral cycling in mature P. neriijolia shrubs was monitored and the effectiveness of inorganic fertilizers, applied to redress nutrient loss, assessed. The primary response of seedlings of P. neriifolia to applied ammonium nitrogen, phosphorus and potassium was to ammonium nitrogen, with increased growth with increasing levels of applied ammonium nitrogen, to a maximum of 7 mM applied as 60 ml per week. Seedling response to applied phosphorus and potassium became noticeable only at higher levels of ammonium nitrogen supply, and at these levels seedlings were observed to respond favourably to relatively high phosphorus (0.65 mM) and potassium (1.25 mM) levels, also applied as 60 ml per week. Since nitrate nitrogen has been shown to be toxic to certain Proteaceae it was not tested in this investigation. However, results from the nursery trial suggested favourable P. neriijolia growth with a non-nitrate inorganic fertilizer with an NPK ratio of approximately 5: 1:3 (mass basis) and this was used as a basis for testing the effects of inorganic fertilizers on growth and nutrient cycling in mature P. neriijolia shrubs. Growth and nutrient cycling was monitored in mature P. neriijolia shrubs for four years: two years prior to the application of inorganic fertilizers and two years with the application of inorganic fertilizers, including unfertilized control shrubs. Two inorganic fertilizer preparations were tested. Both had as their base the commercial slow-release urea based fertilizer, Plantosan, which has an NPK ratio of 5: 1:3. This was supplemented with either ammonium sulphate or urea at a rate of 80 g per running metre every three months. Whole shrub dimensions showed similar growth of P. neriijolia shrubs cultivated in a summer rainfall area to the growth of the species in its natural habitat. Applied fertilizers did appear to increase growth, although these results became apparent only after 18 months. As recorded in other Proteaceae, the stem length of all shrubs decreased with increasing age of the shrubs although this decrease was less in shrubs receiving inorganic fertilizers. Branching did not appear to be affected by shrub age or the application of inorganic fertilizers. However, shrub reproductive productivity did increase with age, with greater increases in fertilized shrubs. Furthermore, flowers from fertilized shrubs were larger than those from unfertilized shrubs, although this phenomenon also only became apparent after 18 months. There was also a change in nutrient allocation patterns with those shrubs growing on soils of lowest nutrient availability directing more resources to root growth. This appeared to occur at the expense of stem material which, in each case, accounted for more than 50% of the total shrub biomass.Although shrub age and the application of fertilizers did influence total shrub growth, the timing of growth events were not affected. They were, however, not synchronous to growth events in P. neriijolia growing in its natural habitat. Vegetative growth showed a peak in early spring (September) and the peak reproductive period was in autumn (March, April and May). Maximum litter production (comprising more than 90% leaf litter) occurred in late autumn to early summer (May to December) and this, too, was not affected by shrub age or the application of inorganic fertilizers. Proteoid root occurrence was greatest in late winter/spring (August to November), co-incident with peak above-ground vegetative growth. There are two models that have been developed to describe the growth of overstorey shrubs in mediterranean ecosystems. In the first, the availability of nutrients is described as being of over-riding importance in determining growth events, while in the second model, soil moisture and temperature are regarded as primary growth detenninants. Neither model could adequately explain the shift in phenophase observed in P. neriifolia cultivated in a summer rainfall area. However, soil moisture and temperature do appear more important in determining phenophase events, particularly since the application of inorganic fertilizers did not appear to alter the timing of these growth events. Nevertheless, the importance of nutrients cannot be ignored as growth can occur only provided sufficient nutrients are available. Seasonal variations in nutrient concentrations of leaf, stem, floret and bract material were observed both prior to and after the application of inorganic fertilizers. However, of importance is that only shrubs receiving Plantosan plus ammonium sulphate retained comparable nutrient levels in tissue types compared with nutrient levels in the corresponding tissue types prior to the application of inorganic fertilizers. Shrubs receiving Plantosan plus urea and unfertilized shrubs had lower nutrient levels suggesting growth at the expense of previously absorbed nutrients. This was supported by a change in nutrient allocation patterns, particularly more nutrients in below-ground biomass in shrubs of the latter two treatments. The cycling of nitrogen, phosphorus and potassium was measured prior to and after the application of inorganic fertilizers. During nutrient cycling, the amount of nitrogen circulated was larger than the amount of potassium which, in turn, was larger than the amount of phosphorus. Three nutrient pools, in the above- and below-ground biomass and the soil, were measured and these comprise the plant/soil system. Inputs into the plant/soil system measured were from rainfall and inorganic fertilizers. Nutrient flows within the plant/soil system measured were leaching, due to rainfall, from the shrub onto the soil, litter production and decomposition, and nutrient uptake by the shrubs, into above- and below-ground nutrient pools. Losses from the plant/soil system recorded in this study were losses from the soil through stream-water, and the largest loss, loss through the removal of flowers for commercial sale. In the absence of flower harvesting the flow of nutrients in the plant/soil system, combined with inputs from rainfall, appeared adequate for the continued growth of the shrubs. However, in the presence of flower harvesting there appeared to be a nutrient budget deficit. This deficit appeared to worsen with increasing shrub age and increasing reproductive productivity in the absence of inorganic fertilizer applications. This was confirmed by nutrient depletion from the soil available nutrient pool. Although inorganic fertilizers did not dramatically alter soil total nutrient pools, soil levels of soluble nitrogen and available phosphorus did show slight increases but not in accordance with the levels of fertilizers applied. It is likely that a high proportion of the applied fertilizers was lost to the plant/soil system before becoming available to the system. This could have occurred through leaching through the soil profile although this was not measured in this study. Nevertheless, inorganic fertilizers did appear to compensate for nutrient loss through the removal of flowers for commercial sale, and the ammonium sulphate supplemented preparation resulted in the most favourable response. Due to a number of cultivation practices which affected the growth of the P. neriifolia shrubs, results are not always strictly comparable with natural systems. However, a number of results obtained in this invegstigation do have horticultural implications and these are briefly discussed with regard to the cultivation of the Proteaceae, particularly P. neriifolia cultivation.