Abstract
Australian vineyard soils are naturally low in phosphorus, an element essential to the functioning of all life-forms. In most vineyards, occasional additions of mineral phosphorus fertilisers are used to enhance grapevine growth and production but a more sustainable approach can be taken through optimal management of leaves, pruning and grape marc, which can result in the majority of P being recycled backed into the vineyard. However, since phosphorus availability is controlled by the forms of phosphorus rather than total amount of P present, this project examined the P forms present and the changes in P composition in the main waste biomass forms over a full season. The amount of P potentially returned to the vineyard was considerable relative to fertilisation rates and was a combination of directly (immediately) plant available and more stable (slow released) forms, which differed between plant part and was also influenced by the phosphorus status of the vine.
Summary
Australian vineyard soils are naturally low in phosphorus, an element essential to the functioning of all life-forms. In most vineyards, occasional additions of mineral phosphorus fertilisers are used to enhance vine growth and production. These can be viewed as replacing phosphorus lost from vines in three main ways: pruning of canes in winter, loss of leaves in autumn and removal of fruit at harvest. These losses are in effect incidental to wine production, which itself contains very little phosphorus. This contrasts with the situation for most other agricultural products, especially grain crops, where substantial quantities of phosphorus are contained in the final product. Thus, in theory, it should be possible to “close the phosphorus loop” in wine production systems through careful management of the biomass waste streams that contain substantial quantities of phosphorus. This would provide a direct (though small) economic benefit by removing the need for phosphorus fertiliser. More importantly, closing the phosphorus loop would provide an important demonstration of commitment to sustainability principles and environmental and social stewardship, as the sources of fertiliser phosphorus are non-renewable and their exploitation (e.g. historical mining in Nauru and current mining in Moroccan-occupied Western Sahara) is politically sensitive.
It is well known that grapevines store significant quantities of phosphorus in their roots and trunk to overcome short term deficiencies of soil supply during early canopy development. During later development some phosphorus is lost in senesced leaves and pruned canes. Generally, senesced leaves and prunings are retained in the vineyard, although senesced leaves are often blown around and so phosphorus contained in these leaves is not necessarily recycled efficiently to the soil; management of prunings varies widely. In addition, phosphorus is removed from the vineyard at harvest, at a rate that has been estimated at approximately 0.3-0.6 kg of phosphorus per tonne of grapes. In some management systems, phosphorus removed at harvest is returned into the vineyard as grape marc. Although the total concentration of phosphorus in various parts of grapevines has previously been studied, very little information exists on the chemical forms of phosphorus present. Phosphorus is present in cells in a variety of forms including the simple orthophosphate anion and its protonated counterparts, and a range of organic forms including phospholipids, which make up cell membranes, nucleic acids (DNA and RNA), which store and carry genetic information, and inositol phosphates, which act as storage compounds for phosphorus, especially in seeds. The relative quantities of these different chemical forms of phosphorus affect how efficiently and quickly phosphorus in biomass returned to the soil is cycled and, in particular, on the timing of availability of this phosphorus for uptake by the grapevine. In this project, 31P NMR spectroscopy was used to determine the phosphorus speciation in grapevine biomass over a full season, focusing on phosphorus speciation in the major biomass losses of prunings, leaf fall and harvested fruiting bodies (berries and stalks)