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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.

China's pollution census triggers green five-year plan

China is poised to launch a green five-year plan as a national census finds wastewater runoff from farms to be a far greater source of pollution than industry.

Mae-Wan Ho

Census reveals major pollution from farms

CHINA’s first nationwide pollution census just completed finds 30.3 million tonnes of pollutants (chemical oxygen demand, COD) discharged into water in 2007, more than double the 13.8 million tonnes reported that year. The census included, for the first time, measurements of wastewater runoff from farms using chemical fertilisers and pesticides, which accounted for 13.2 million tonnes of COD. The report, which mapped data from 5.9 million sources, showed that China discharged about 209 billion tonnes of wastewater and 63.7 trillion m3 of waste gases in 2007.

The main water pollutants were 1.73 million tonnes of ammonia nitrogen, 900 tonnes heavy metals, 4.73 million tonnes nitrogen, and 423,000 tonnes phosphorus.

Other pollutants included:

 Sulphur dioxide emissions, 23.2 million tonnes

 Nitrogen oxides, 17.98 million tonnes

 Dusts, 19.21 million tonnes

 Soot, 11.7 million tonnes

 Solid waste, 3.8 billion tonnes (of which 45.7 million tonnes hazardous)

 Livestock faeces, 243 million tonnes

 Livestock urine, 163 million tonnes

 Plastic film on cropfields, 121,000 tonnes (80.3% recycled).

Wang Yanliang of the ministry of agriculture acknowledged the high contributions from intensive livestock farming and excessive use of fertilisers and pesticides in the fields.

That is due to the immense size of China's agricultural sector and the country's massive dependency on artificial fertilisers.

China uses only 7% of the world's land to feed 22% of its population. (According to a recent Greenpeace report, the country consumes 35% of the world's nitrogen fertiliser.) Wang said the ministry would improve the efficiency of pesticide and fertiliser use, expand biogas generation from animal waste, which it has already supported in successive five-year plans,1 and change agricultural lifestyles to protect the environment.

The extent of agricultural waste could prove a larger problem than the many factories dumping pollution into China's rivers and lakes because it is easier to control factories than millions of farms.

Reliance on chemical farming not necessary

Wen Tiejun, dean of the school of agriculture and rural development at Renmin University, said his research suggested that Chinese farmers used nearly twice as much fertilisers as they need, and the census should be used as a turning point. He said: 'For almost all of China's 5,000-year history, agriculture had given our country a carbon-absorbing economy but in the past 40 years, agriculture has become one of the top pollution sources. Experience shows that we don't have to rely on chemical farming to resolve the food security issue. The government needs to foster low-pollution agriculture.'

Indeed, Chinese peasants have farmed sustainably according to the circular economy of nature, as, for example, in the circular economy of the dyke-pond system2 that, in its heyday, supported 17 people per hectare without using fertilisers and pesticides.

The pollution report comes at an opportune time as the country's 11th five-year plan (2006-2010) is drawing to a close, and officials are preparing the groundwork for the 12th five-year plan. Environmental protection is given the top priority.

The new five-year plan aims to reduce carbon intensity - carbon emissions per unit of GDP - by up to 45% by 2020, when 15% of its energy use will be non-fossil fuel. It also aims to reverse deforestation by increasing the total forest cover by 40 hectares and increasing the total forest stock volume by 1.3 billion m3.

The plan will set specific targets for different economic regions of China and will become domestic law, so firms will be legally required to meet the emission reduction targets. Some experts estimate that environmental degradation currently costs the Chinese economy up to 8% of its GDP, so the plan, though painful to firms, is undoubtedly beneficial for China's long-term economic growth.

The plan is likely to call for more than $450 billion in investment to protect the environment, more than double the $219 billion of the 11th five-year plan. Wastewater treatment companies will be hoping to reap the benefits. China's environmental protection industry is reported to be growing at between 15% and 20% a year. The ministry of environmental protection has estimated that the production value of the industry will reach $161 billion in 2010.

Opportunity to restore 'circular economy agriculture'

What appears to be missing from the 12th five-year plan is a systems-approach that combines environmental protection with food and energy production. This is particularly important as new findings also reveal widespread acidification of the soil in China's major croplands since the intensification of agriculture began in the early 1980s, which is reducing soil productivity.

One approach based on the anaerobic digestion of livestock and other wastes, already supported by the Chinese government, is envisaged in a 'Dream Farm 2', which recreates the circular economy that has served China so well in the past.3 It uses anaerobic digestion to prevent pollution by retaining and recycling the wastes into food and energy resources, while incorporating renewable energies at the microscale such as wind, solar and hydroelectric, making use of locally available resources as much as possible.

In our most recent report on truly renewable and sustainable energies,4 we show it is both possible and profitable to phase out fossil fuel use altogether by 2050 for all nations of the world, rich and poor, provided there is sufficient political will, wisdom, and international cooperation. - Science in Society (No. 46, Summer 2010)                                                           

References

1.   Li KM and Ho MW. 'Biogas China'. Science in Society 32,  2006.

2.   Ho MW. 'Circular economy of the dyke-pond system'. Science in Society 32,  2006.

3.   Ho MW. 'Sustainable agriculture, green energies and the circular economy'. Science in Society 46, 2010; and Ho MW, Burcher S,  Lim LC et al. Food Futures Now. ISIS and TWN, 2008.

4.   Ho MW, Cherry B, Burcher S and Saunders P. Green Energies: 100% Renewables by 2050. ISIS and TWN, 2009.

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


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