Exploring global changes in nitrogen and phosphorus cycles in agriculture induced by livestock production over the 1900-2050 period

Abstract

Crop-livestock production systems are the largest cause of human alteration of the global nitrogen (N) and phosphorus (P) cycles. Our comprehensive spatially explicit inventory of N and P budgets in livestock and crop production systems shows that in the beginning of the 20th century, nutrient budgets were either balanced or surpluses were small; between 1900 and 1950, global soil N surplus almost doubled to 36 trillion grams (Tg) · y(-1) and P surplus increased by a factor of 8 to 2 Tg · y(-1). Between 1950 and 2000, the global surplus increased to 138 Tg · y(-1) of N and 11 Tg · y(-1) of P. Most surplus N is an environmental loss; surplus P is lost by runoff or accumulates as residual soil P. The International Assessment of Agricultural Knowledge, Science, and Technology for Development scenario portrays a world with a further increasing global crop (+82% for 2000-2050) and livestock production (+115%); despite rapidly increasing recovery in crop (+35% N recovery and +6% P recovery) and livestock (+35% N and P recovery) production, global nutrient surpluses continue to increase (+23% N and +54% P), and in this period, surpluses also increase in Africa (+49% N and +236% P) and Latin America (+75% N and +120% P). Alternative management of livestock production systems shows that combinations of intensification, better integration of animal manure in crop production, and matching N and P supply to livestock requirements can effectively reduce nutrient flows. A shift in human diets, with poultry or pork replacing beef, can reduce nutrient flows in countries with intensive ruminant production.

Keywords: emissions; global nitrogen and phosphorus cycle; soil nutrient budget.

Fig. 1.

Global animal stocks for 1900, 1950, 2000, and 2050 for cattle (A), pigs (B), and sheep and goats (C). AFR, Africa; EUR, Europe; NA, North America (Canada, United States); NAS, North Asia (Russian Federation, Belarus, Ukraine, Republic of Moldova); OCE, Oceania (Australia and New Zealand); SAS, South Asia (rest of Asia); SCA, South and Central America.

Fig. 2.

Agricultural soil N budget for 1900, 1950, 2000, and 2050 (baseline scenario). The soil N budget is calculated using Eq. 1.

Fig. 3.

Global recovery of N and P in crop (A and B) and livestock (C and D) production for 1900, 1950, 1970, 2000, and 2050 (IAASTD baseline scenario). Recovery is calculated using Eq. S5 (crop) and Eq. S6 (livestock).