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ItemModelling multi-year phosphorus flow at the regional scale: the case of Gippsland, Australia( 2015)Phosphorus (P) is an essential element for global food production, but it is geographically limited, non-substitutable, and non-renewable. In the traditional P management system, there exist a number of challenges to the sustainability of this vital resource, which, if not properly tackled, may lead to global P scarcity and hinder global food security. In order to provide effective policy and management response to overcome these challenges, and to safeguard global P sustainability, there is need for a sound understanding of the nature and magnitude of P flow through different systems at various geographical and temporal scales. An in-depth review of the available P flow analyses at different geographical and temporal scales has revealed that the regional scale which is significant in terms of the magnitude of P flow, has received limited attention in the multi-year analysis of P flow. Thus, there is a knowledge gap regarding the nature and magnitude of P flow over several years at the regional scale, but this understanding is essential for providing long-term and effective P management decisions. Therefore, utilizing the Substance Flow Analysis (SFA) method that relies on the mass balance principle, this study has performed a quantitative modelling of P flow over multiple years at the regional scale. In this regard, this study has developed SFA model of P in MATLAB/Simulink® software platform that can be utilized for analysing the nature and magnitude of multi-year P flow at the regional scale. This model takes into account both structurally and operationally, all the relevant P flows and storage associated with all key systems, subsystems, processes or components, and associated interactions of P flow to represent a typical P flow system at the regional scale. The main advantage of this model over available regional scale SFA models is that it is capable of analysing the trends or dynamic changes in P flow and storage over many years at an annual time step, whereas the available P flow models are static and can analyse P flow only for a particular year at a time. The unique capability of the model to comprehensively analyse various P flows and storage in a system, subsystems, or/and different components within subsystems and sub-subsystems while taking into account all interactions of P flow render it as a robust and powerful tool for the regional scale P flow analysis. This study has utilized this model in the case of Gippsland region in Australia to analyse the nature and magnitude of P flow and storage over a six-year period (2008-2013). This analysis has revealed that approximately 29% (4,445 tonnes) of the mean annual total inflow (15,349 tonnes) of P in this region eventually exited the system, indicating a substantial amount (10,904 tonnes) of P storage. The inflow of P mainly occurred as commercial fertilizer (10,263 tonnes) and livestock feed (4,443 tonnes), and the outflow mainly occurred as livestock products (4,181 tonnes); whereas the majority (66% or 7,218 tonnes) of P storage occurred in soils of the livestock farming system. The analysis has also revealed that the majority (approximately 90%) of the P flow and storage in this region was associated with the livestock farming subsystem. A significant annual variation in the magnitude of nearly all P flow and storage has been observed in the case of the main system (Gippsland region) and all subsystems. These variations in annual P flow and storage implies that making judgement based on a single year analysis may not represent the true picture of the magnitude of P flow, and therefore, emphasises the significance for multi-year analysis. This analysis also indicates that over the study period, a total of about 3,241 tonnes P were lost as soil erosion and runoff from different subsystems to water bodies in this region, eventually causing a substantial environmental and economic damage. Over the study period, a total of approximately 65,424 tonnes P storage (mainly in soils of the livestock farming subsystem) occurred in this region, which is more than the total quantity of P imported as commercial fertilizer into this region in that period. The accumulation of P in this manner over several years may lead to a massive stock of P in soils, which may ultimately intensify the risk of P loss as soil erosion and runoff. The findings of this analysis could be effectively utilized for making better P management decisions towards achieving P sustainability in this region. However, this study suggests that future research should investigate the reasons for the variations and trends in multi-year P flow as identified in the case of Gippsland region.