Reconsideration of the planetary boundary for phosphorus

Abstract

Phosphorus (P) is a critical factor for food production, yet surface freshwaters and somecoastal waters are highly sensitive to eutrophication by excess P. A planetary boundary, orupper tolerable limit, for P discharge to the oceans is thought to be ten times thepre-industrial rate, or more than three times the current rate. However this boundary doesnot take account of freshwater eutrophication. We analyzed the global P cycle to estimateplanetary boundaries for freshwater eutrophication. Planetary boundaries werecomputed for the input of P to freshwaters, the input of P to terrestrial soil, and themass 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 boundarieswere also computed using three published estimates of current P flow to the sea. Currentconditions exceed all planetary boundaries for P. Substantial differences between currentconditions and planetary boundaries demonstrate the contrast between large amounts of Pneeded for food production and the high sensitivity of freshwaters to pollutionby P runoff. At the same time, some regions of the world are P-deficient, andthere are some indications that a global P shortage is possible in coming decades.More efficient recycling and retention of P within agricultural ecosystems couldmaintain or increase food production while reducing P pollution and improving waterquality. Spatial heterogeneity in the global P cycle suggests that recycling ofP in regions of excess and transfer of P to regions of deficiency could mitigateeutrophication, increase agricultural yield, and delay or avoid global P shortage.