Soil and Environmental Impacts of Excessive Nitrogen Use in China.

China’s economy has made great strides since 1949, and especially since China initiated economic reforms and the open-door policy in the 1980’s. The growth in agricultural production has been one of the main national accomplishments. This has created the so-called ‘Miracle in China’ with 9% of the world’s arable land feeding 22% of the world’s population(Fan et al., 2011). The use of fertilizers has played a crucial role, accounting for about 50 % of the yield increase. The consumption of fertilizer in China has increased linearly since 1961. Total consumption of chemical fertilizers exceeded 64 Mt in 2009, and this was nearly 35% of total global fertilizer consumption. Use of nitrogen (N) fertilizer has increased from 0.5 Mt in 1961 to 46.6 Mt in 2009 (Consumption = production + import - export, Revised from National Bureau of Statistics of China, 1950-2010).

Unfortunately, since about 1990 the increase in grain production has been associated with a major decline in fertilizer nutrient use efficiency, especially N, and with widespread environmental damage. According to yearly data for grain yield and synthetic N consumption (National Bureau of Statistics of China, 1950-2010), the partial factor productivity of applied N (PFP, the ratio of yield to the amount of applied N) has been halved over the last 30 years. Recovery efficiency of N (% fertilizer N recovered in aboveground crop biomass, REN) for cereal crops was 35% on average in the 1990’s. However, this value has gradually reduced since then and the current REN is 28.3 % for rice, 28.2 % for wheat and 26.1 % for maize (Zhang et al., 2008), all of which are lower than the world values (40-60%). The low nutrient use efficiency may be attributed to fertilizer overuse and high nutrient loss resulting from inappropriate timing and methods of fertilizer application, especially in high yielding fields.

Decreasing N use efficiency indicated that more fertilizer N was being lost to the environment, causing further negative environmental impacts. For example, losses of N through leaching and run-off have led to drinking water pollution which affects 30 % of the population and results in eutrophication of 61% of lakes in the country. Annual synthetic fertilizer N-induced N₂O emission from Chinese croplands has increased from 120 Gg N₂O-N yr^-1 in the 1980s to 210 Gg N₂O-N yr^-1 in the 1990s (Zou et al., 2010). Another case study showed that soil pH in the major Chinese crop-production areas has declined significantly from the 1980s to the 2000s because of excessive N fertilizer inputs (Guo et al., 2010).

Therefore, rationalisation of N application to deliver greater N use efficiency and reduced environmental risks is urgently required in China. There is now overwhelming evidence that quantities of N fertilizer applied could be reduced with no detrimental effect on yield. Crop yields might even be increased by reduced use of fertilizer (Wilkinson et al., 2007; Fan et al., 2008). The great challenge ahead is to determine how crop productivity can be further increased to feed a growing population while minimizing N loss and its subsequent environmental damages for China. In reality, achieving such a target represents one of the greatest scientific challenges facing humankind (Tilman et al., 2002).