Joséphine Demay (INRAE Aquitaine-Bordeaux, ISPA) – 2020-2023

Previous studies1,2 have shown that converting 60% of arable land to or-ganic farming could meet global food demand, provided that food sys-tems are reorganized (food waste reduction, diet change, change in spa-tial distribution of livestock system, etc.). Beyond this threshold, nitrogen limitations would be so severe that production could not meet demand. This study complements previous research by examining the availability of phosphorus (P) in soils. Mineral phosphate fertilizers, which account for more than half of global phosphorus inputs into agricultural soils, are, like nitrogen fertilizers, prohibited in organic farming.

Simulations of a world with 100% organic farming agriculture revealed a phosphorus deficit in agricultural soils in the long term. This woudl be particularely severe in regions where current soil phosphorus fertility is low, crop yields are high, and leguminous crops are widespread. And it twill be added in every regions to a short term limitation of agricultural production due to nitrogen. After 100 years of worldwide organic far-ming, phosphorus limitation would lead to a 12% decrease in produc-tion on cultivated land compared to current levels. When combined with the impact of biotic and nitrogen stresses, the total decline in production would be 56%. In view of phasing out mineral phosphate fertilizers, this thesis discusses solutions for the sustainable management of phospho-rus reserves in agricultural soils, which can only be achieved by reducing losses, particularly through erosion, or by recycling phosphorus from human excreta.

Supervisors

Sylvain Pellerin (UMR ISPA)

Thomas Nesme (UMR ISPA)

Bruno Ringeval (UMR ISPA)

Pietro Barbieri (Bordeaux Sciences Agro)