Negative Phosphorus Budgets Challenge Sustainable Intensification of Grasslands

That's the conclusion of a study that suggests a fourfold rise in the amount of mineral and organic phosphorus needed on grasslands by 2050.

Grasslands provide grass and fodder to sustain the growing need for ruminant meat and milk.

Soil nutrients in grasslands are removed through withdrawal in these livestock products and through animal manure that originates from grasslands and is spread in croplands. This leads to loss of soil fertility, because globally most grasslands receive no mineral fertilizer. Here we show that phosphorus (P) inputs (mineral and organic) in global grasslands will have to increase more than fourfold in 2050 relative to 2005 to achieve an anticipated 80% increase in grass production (for milk and meat), while maintaining the soil P status.

Combined with requirements for cropland, we estimate that mineral P fertilizer use must double by 2050 to sustain future crop and grassland production. Our findings point to the need to better understand the role of grasslands and their soil P status and their importance for global food security.

Global food demand will rise rapidly in the coming decades. In particular, meat and milk consumption are projected to increase markedly due to rising incomes and decreasing prices especially in developing countries. The global area of permanent grassland (3.3 billion hectares) covers 26% of the Earth’s ice-free land, and provides an important contribution to global food security by supplying proteins and energy to ruminants. Between 1970 and 2005 the world’s grasslands expanded by about 4%. Grassland degradation is likely to accelerate given the expected growth of livestock production10, overgrazing and consequent soil erosion, nutrient deficiency, weed encroachment and desertification.

Sustainably meeting global food demand is one of humanity’s grand challenges, and P is increasingly considered to be a new global sustainability issue because of its finite reserves. Like nitrogen, P is a major limiting nutrient in agriculture and is removed from arable and grassland soils by crop and grass withdrawals and erosion. Soil P removed in the harvest needs to be replaced by weathering and inputs through organic (mostly manure) and mineral fertilizers to sustain crop and grass production.

Most global studies on the agricultural P cycle focus on arable land. However, we are not aware of any global study addressing the P cycle in grasslands and its connection with croplands. In this paper we assess grassland P budgets between 1970 and 2005, as well as P requirements to sustain projected future grass production. We consider soil P budgets to assess changes in soil fertility, and the budget for the grassland system to assess the P exchange between grasslands and croplands. The classification of livestock production systems (intensive and pastoral) and grassland systems is shown in Supplementary Fig.1.

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