Knowledge for better food systems

Reconsideration of the planetary boundary for phosphorus Environ

This paper argues that that the human use of phosphorus, primarily in the industrialized world, is causing the widespread eutrophication of fresh surface water. It is based on Rocktrom’s concept of ‘planetary boundaries. It also discusses the prospect that we have reached/are reaching ‘peak phosphorous’ and suggests that the evidence here is mixed, although a P shortage in coming years is possible.
This paper argues that that the human use of phosphorus, primarily in the industrialized world, is causing the widespread eutrophication of fresh surface water. It is based on Rocktrom’s concept of ‘planetary boundaries. It also discusses the prospect that we have reached/are reaching ‘peak phosphorous’ and suggests that the evidence here is mixed, although a P shortage in coming years is possible. Carpenter S R and Bennett E M (2011). "Reconsideration of the planetary boundary for phosphorus Environ". Res. Lett. 6, 014009

Abstract

Phosphorus (P) is a critical factor for food production, yet surface freshwaters and some coastal waters are highly sensitive to eutrophication by excess P. A planetary boundary, or upper tolerable limit, for P discharge to the oceans is thought to be ten times the pre-industrial rate, or more than three times the current rate. However this boundary does not take account of freshwater eutrophication. We analyzed the global P cycle to estimate planetary boundaries for freshwater eutrophication. Planetary boundaries were computed for the input of P to freshwaters, the input of P to terrestrial soil, and the mass of P in soil. Each boundary was computed for two water quality targets, 24 mg P m − 3, a typical target for lakes and reservoirs, and 160 mg m − 3, the approximate pre-industrial P concentration in the world's rivers. Planetary boundaries were also computed using three published estimates of current P flow to the sea. Current conditions exceed all planetary boundaries for P. Substantial differences between current conditions and planetary boundaries demonstrate the contrast between large amounts of P needed for food production and the high sensitivity of freshwaters to pollution by P runoff. At the same time, some regions of the world are P-deficient, and there are some indications that a global P shortage is possible in coming decades. More efficient recycling and retention of P within agricultural ecosystems could maintain or increase food production while reducing P pollution and improving water quality. Spatial heterogeneity in the global P cycle suggests that recycling of P in regions of excess and transfer of P to regions of deficiency could mitigate eutrophication, increase agricultural yield, and delay or avoid global P shortage.

Conclusions

Human release of P to the environment is causing widespread eutrophication of surface freshwaters. Yet the global distribution of P is uneven, and soils of many regions remain P-deficient even as soils of other regions are P-saturated (MacDonald et al 2011). Heterogeneity complicates the task of managing to provide both food and high quality freshwater, surely one of the key challenges of environmental management in the 21st century. The planetary boundary for freshwater eutrophication has been crossed while potential boundaries for ocean anoxic events and depletion of phosphate rock reserves loom in the future. The solution to this problem is widespread adoption of better practices for conserving P in agricultural ecosystems, so that P is cycled effectively among soil, crops, livestock and people without contributing to eutrophication of surface waters. At the same time, P-deficient regions of the world should be supplemented by P from P-rich regions. Such subsidies could come in the form of recycled P in fertilizer (e.g. solid P-rich material from manure digestors) or P in food. The paper is attached below.
 

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