China's soils ruined by overuse of chemical fertilisers

Cropland soils in China are turning acid from the overuse of nitrogen fertilisers, decreasing productivity, polluting the environment, and contributing huge amounts of greenhouse gas emissions. Researchers recommend reducing fertiliser use, but have not considered phasing it out altogether by adopting organic agriculture.

Mae-Wan Ho

Intensive chemical agriculture turns soils acid

THERE has been a significant decline in soil pH since the 1980s in China's major croplands, mainly from the overuse of nitrogen fertilisers. This was revealed in a study carried out by Chinese, UK and US researchers led by Zhang Fu Suo at the China Agricultural University in Beijing.1

'Serious soil acidification will threaten food security and environmental safety worldwide,' Zhang said.2 'Our work has shown that soil quality or soil health should be paid more attention in intensive agricultural production systems receiving high nitrogen and other resource inputs.'

The researchers recommend optimal nutrient-management strategies that can significantly reduce nitrogen fertiliser rates without compromising crop yield, but have not considered adopting organic agriculture and phasing out nitrogen fertilisers altogether.

Soils are strongly buffered by inorganic ions, by the weathering of soil mineral, and in the acidic range, by interactions with aluminium and iron, so that its pH remains relatively constant. (pH is a measure of acidity and alkalinity on a scale of 0 to 14, 7 being neutral. A pH of less than 7 indicates acidity; the lower the pH, the greater the acidity. pH is inversely related to the concentration of hydrogen ions (H+), as a higher concentration indicates higher acidity.)

Soils become acid very slowly under natural conditions, over hundreds to millions of years. Old soils and soils in high rainfall regions tend to be more acid. Naturally acid soils occupy approximately 30% of the world's ice-free land and are commonly associated with phosphorus deficiency, aluminium toxicity, and reduced biodiversity and productivity.

Chinese agriculture has intensified greatly since the early 1980s on a limited land area with large inputs of chemical fertilisers. Grain production and fertiliser nitrogen (N) consumption reached 502 megatonnes (Mt) and 32.6 Mt respectively in 2007, increasing 54% and 191% since 1981. High levels of N fertiliser can acidify soils both directly and indirectly, and the rates of N applied in some regions are very high compared with those of North America and Europe. This has degraded soils and environmental quality in the North China Plain and the Taihu Lake region in south China, traditionally famous for its scenic beauty but now infamously putrid and polluted.3,4

A national soil survey had been conducted during the early 1980s, and pH was determined in all top soils sampled. For comparison, the team collected all published data on top soil pH from 2000 to 2008 and compiled two (unpaired) datasets on the basis of six soil groups according to geography, and two subgroups of cereal crops and cash crops. Both cropping systems receive very high fertiliser inputs compared with other agricultural systems worldwide, especially cash crops like greenhouse vegetables that have expanded rapidly since the 1980s.

The results showed significant drops in pH of 0.13 to 0.8 except in the highest pH soils, which represent only a small percentage of Chinese cultivated soils. In all other soil groups, acidification has been greater in cash crops (pH decreased by 0.3 to 0.8) than cereals (0.13 to 0.76) (see Table 1 on p.5).

Soils in group I (see Table 1) are the most acidic in south China and have acidified further since the 1980s.  Athough the net pH decreases for group I soils were small compared to the other groups, the impact may be more pronounced because these soils are approaching acidity at which potentially toxic metals such as aluminium and manganese could be mobilised.

Overuse of nitrogen fertilisers largely to blame

The results are backed up by data from 154 agricultural fields in which they measured the same site in the 1980s and in the 2000s. The average drop in pH in these sites is well over 0.5.

Still more data from 10 field sites in which soil pH was measured regularly over a period of 8 to 25 years also showed decreases in pH ranging from 0.45 to 2.20, and only in chemically fertilised plots, not in unfertilised soils, or soils with no crop planted.

In the three major Chinese double-cropping systems - wheat-maize, rice-wheat, and rice-rice - annual N fertiliser application rates are usually above 500 kg N/ha. These systems contribute to increasing hydrogen ion concentration (i.e., acidity) by 20 to 33 kilomols (kmol)/ha/year. Greenhouse vegetable systems, the major cash crops, receive even greater N fertiliser inputs. In Shandong province, N fertiliser rates above 4,000 kg N/ha/year are common. Under this management, about 220 kmol H+/ha/year accumulates in the soil. The proton (H+) generation related to N in China is extremely high compared with other countries with lower N fertiliser rates.

Plant uptake of base cations (positively charged metal ions), which are then removed as harvests from fields, also leads to acid soils because the cations are replaced by hydrogen ions. Currently, about 25 tonnes of dry biomass are harvested annually in the three double-cropping systems, resulting in an estimated release to the soil of 15 to 20 kmol H+/ha/year that compensates for the base cations removed. In the greenhouse vegetable systems, the importance of base cations uptake varies greatly with plant species and yield but overall appears similar to the cereal systems.

Thus, the total H+ added to the soil due to nitrogen fertilisers and base cation removal is 30 to 50 kmol H+/ha/year for cereal systems, and 230 kmol/ha/year for greenhouse vegetable systems. In comparison, acid deposition due to acid rain is negligible, at 0.4 to 2.0 kmol/ha/year.

Nitrogen fertilisers pollute the environment and increase greenhouse gas emissions

Soil acidification occurs not just in China, but wherever and whenever intensive chemical fertilisation agriculture is practised in response to pressures to produce more food and, recently, bioenergy crops for biofuels,5 which means even less land for growing food in developing countries.

The overuse of chemical fertilisers is a major source of environmental pollution from agriculture, which China’s recent national pollution census identified to be a greater pollution source than industry (see article next page).

Collaborating scientists Keith Goulding and David Powlson of the UK’s Rothamsted Research Institute highlight another important aspect of chemical fertilisation:6 ‘The impact of N fertiliser overuse on greenhouse gas emissions is often overlooked. It arises through the carbon dioxide emitted when manufacturing fertiliser, and nitrous oxide, a powerful greenhouse gas, emitted when N fertiliser is applied to soil. Our work with Chinese collaborators shows that reductions in N use of 30% and, in some cases, much more are possible without any threat to China’s food security, and would make a significant contribution to reducing total greenhouse gas emissions from China. Avoiding N fertiliser over-use is a "multiple win": farmers save money, there is less water pollution, smaller greenhouse gas emissions, and a smaller acidification burden on soil and water.’

The real solution is to phase out chemical fertilisers altogether in favour of organic fertilisers.7

Dr Mae-Wan Ho is Director and co-founder of the UK-based Institute of Science in Society <www.i-sis.org.uk>, Editor of Science in Society magazine and scientific adviser to the Third World Network. The above is an edited version of an article published in Science in Society (No. 46, Summer 2010).

References

1.   Guo JH, Liu XJ, Zhang Y, Shen JL, Han WX, Zhang WF, Christie P, Goulding KWT, Vitousek PM and Zhang FS. ‘Significant acidification in major Chinese croplands’. Science 2010, 327, 1008-10.

2.   'Nitrogen fertiliser acidifies soils in China', environmentalresearchweb, 17 February 2010, http://environmentalresearchweb.org/cws/article/research/41738

3.   Ma J. ‘Disaster in Taihu Lake’, chinadialogue, 8 June 2007, http://www.chinadialogue.net/article/show/single/en/1082-Disaster-in-Taihu-Lake\

4.   Yang S-Q and Liu P-W. 'Strategy of water pollution prevention in Taihu Lake and its effects analysis'. Journal of Great Lakes Research 2010, 36, 150-8.

5.   Ho MW. ‘Biofuels: biodevastation, hunger & false carbon credits’. Science in Society 33, 2007.

6.   Rothamsted Research. ‘A 30% cut in fertiliser use in China would not threaten food security’, 17 February 2010, http://www.bbsrc.ac.uk/media/releases/2010/100217-cut-in-fertiliser-use-in-china.aspx

7.   Ho MW.  ‘Sustainable agriculture, green energies & the circular economy'. Science in Society 46, 2010.

*Third World Resurgence No. 237, May 2010, pp 2-5