FOOD SECURITY - Science for Agriculture and Rural Development in Low-income Countries - page 46

about 12 Tg is lost to the environment, i.e., exceeding the total quantity of about 10
Tg currently applied annually in the US (Smil 2002, 2005).

Asia
Past productivity gains in East and South-east Asia would not have been possible
without the use of fertilizers during the last four decades (Khush 2005). NPK
fertilizer consumption in developed countries, such as the US has stabilized or
declined since the early 1980s, while it still shows a distinct upward trend in China
(Figure 7). The extensive use of fertilizers increasingly results in severe local
pollution problems, and may even show impact at global level (Li et al. 2005). It is
estimated that 25% of the lakes in China show signs of eutrophication, while a
recent study on water quality in northern China showed that nitrate concentrations in
groundwater exceeded the permissible limit of 50 mg l -1 in half of the investigated
locations (Fang et al. 2005).
A combination of high nitrogen (N) fertilizer inputs and a low proportion of
applied N taken up by the crop (= N recovery) is at the basis of these pollution
problems. In contrast to crop yields, N recoveries in East and South-east Asia do not
show significant improvements over time. However, during the 1990s, a major effort
to improve nutrient management in rice-based systems in Asia was launched,
applying a network mode (composed of 12 National Agricultural Research Systems
and the International Rice Research Institute), in which researchers worked with
farmers in multi-annual experiments (on-station and on-farm in the major rice
environments of the humid and subhumid tropics of Asia) to determine the causes of
low nutrient use efficiencies, and jointly develop more efficient, site-specific man-
agement packages (Dobermann et al. 2004). In more than 200 seasonal data sets, on
average, N recovery in rice was below 30% (Cassman et al. 1995, 1996), while

Mln t

45

China
USA

40

35

30

25

20

15

10

5

0

1960 1970 1980 1990 2000 2010

Ye a r

Figure 7. NPK fertilizer consumption (10 6 t) in China and the USA