Livestock husbandry, in particular large intensive feedlots, has become a major problem both for recycling of organic phosphorus and for emission of phosphorus pollutants. Worldwide, the structure of animal agriculture has changed as livestock are concentrated in fewer but larger operations. In the United States, in spite of losing nearly a fourth of the livestock operations between 1982 and 1997, the total number of animal units (an equivalent that converts various kinds of animals into cattle based on individual nutrient excretion) has remained fairly constant at c. 91-95 million. In China, a large number of intensive feedlots appeared in suburbs and rural areas during the last decade. According to a national investigation, the output of hogs, meat chicken (broilers), and egg chicken (layers) produced by intensive feedlots and farms accounted for 23%, 48%, and 44% of the national total in 1999, respectively.
As livestock operations have become fewer, larger, and more spatially concentrated in specific areas, animal wastes have also become more concentrated in those regions. This leads to a considerable phosphorus surplus in manure, as the amount of manure nutrients relative to the assimilative capacity of land available on farms for application has grown, especially in specific high-production areas. Consequently, off-farm manure export requirements are increasing.
But because of its bulk, uneven distribution, and prohibitive cost of transport beyond a limited radius, a large proportion of manure phosphorus is now subject to disposal instead of recycling. If construction of necessary infrastructures for appropriate disposal of manure lags behind, animal wastes become a major source of phosphorus loads in surface waters. Uncontrolled phosphorus emission from intensive feedlots and farms in China has escalated in parallel with the gradual growth in total animal feeding operations and the rapid shift in breeding structure. The emission of China's livestock was estimated at 36% of its national phosphorus load to aquatic environments in 2000. Thus, livestock husbandry is the most significant source of phosphorus flux to surface waters in China, similar to the situation in European countries in the early 1990s.
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