Agricultural management in the Tai Lake region

Following the conversation from the eutrophication of Lake Tai (Taihu), probably the most publicized Chinese eutrophication event in Western media,  many will probably want to know more about the sources of the excess nutirents leading to massive algal blooms and deteriorating water quality.  In February, the New York Times reported that previous information on the extent of the deteriorating surface water quality in China severely underestimated the scale and influence of agricultural runoff, focusing mostly on point sources such as industrial effluent. Ma Jun, director of the Institute of Public and Environmental Affairs, a nonprofit research group in Beijing, rightly pointed out in the article that pollution emissions stemming from millions of rural farmers will prove an even bigger challenge than slapping fines on factories and industrial sources. But,  China’s Ministry of Environmental Protection is now beginning to realize that the agricultural contribution to poor surface water quality is just as prominent as the industrial contribution. Here I will give a brief overview of the issues at hand, from the perspective of nitrogen as an excess nutrient pollutant stemming from agricultural sources. In a previous post, we identified the main source of total nitrogen content in Lake Tai as having an agricultural origin.

In contrast to Europe and the United States, which could both be argued to have undergone agricultural intensification over a period of centuries, in China, the shift from traditional, complex, labor-intensive farming systems based on nutrient recycling happened over about four decades. Now, China’s agricultural system as a whole is heavily dependent on introduced inorganic nitorgen fertilizers and  is utilizing less and less organic and green manures to replenish nutrients that are removed from agricultural soils when crops are harvested. The agricultural scheme utilized in the Tai Lake region is one of the two most intensive double cropping systems in China: summer waterlogged rice followed by winter upland wheat.

Generally, inorganic nitrogen, which, in the case of China, is usually introduced to fields in the form of urea-based fertilizers, is quickly converted into ammonium (NH4+) and later into nitrate (NO3-)  by microorganisms in the soil according to the normal function of the nitrogen cycle. A portion of the nitrogen is also lost by NH3 volatization to the atmosphere, which contributes to air pollution and acid rain. Most plants, including rice and wheat. more readily take up the nitrate form of nitrogen than they do the ammonuium form. However, due to the negative repelling charge of nitrite with the surrounding soil (especially if the soil’s content is high in clay), the nitrate form is also highly mobile in soil and is easily carried by percolating water into surface water or groundwater.  Ammonium, on the other hand, sorbs to clay and organic matter, and although is less readily absorbed by plants, is held in soil for a prolonged period of time if not further converted to the nitrate form. Organic nitrogen supplements, including proteins or amino acids found in animal waste, are a slow release source of nitrogen for crops, as organic nitrogen  must first be converted into its inorganic forms in order to be taken up by plants.

Nitrogen that is quickly converted from urea fertizlier into nitrate is lost by leaching through soil into water.  Studies on the intensive wheat/rice double crop cycle followed in the Taihu region show that alternating wheat and waterlogged rice leads to an accumulation of nitrate after the wheat season. However, flooded rice fields in the following season results in much of the nitrate being lost by leaching. (Ju et al 2009) Farmers in the Tai Lake region also tend to use more fertilizer during the wheat season than wheat farmers in northern China, but result in lower grain yields. A possible explanation is that Tai Lake region farmers try to use increased amounts of fertilizer to compensate for poorer wheat cultivation conditions.

Much of the problem of nitrogen leaching lies in agricultural practices . One- poor wheat-growing conditions in the Lake Tai region lead farmers to overuse fertilizers (with little to no effectiveness achieved in crop gain) during the wheat season, and two- the traditional use of flooded rice paddies promotes nitrogen leaching.  We can see here that one aspect of the problem is psychological: with farmers holding to the belief that increasing inorganic fertilizer use will lead to increased crop yields. The other aspect is technical.

The System of Rice Intensification (SRI), developed in 1983 by Henri de Laulanie, a French agricultural practioner living in Madagascar, is one method that has been utilized worldwide to increase rice yields with fewer water and fertilizer inputs.  One key aspect to SRI is that, unlike traditional rice cultivation methods, rice plants are not kept under waterlogged conditions, but instead, rotated between wet and dry conditions. Alternating between wet and dry causes cracking of the earth, promoting aeration of the soil, increased root growth, and slower leaching of nitrogen from the topsoil. The wider root systems of each individual rice plant is then able to take up more of the applied nutrients (nitrogen) from the soil, resulting in healthier plants with more tillers, and better yields. In Sichuan Province, farmers have achieved 40% greater yields using SRI, with less water and nutrient inputs.

However, as China has a rice cultivation history that spans thousands of years, urging farmers to change their flooding methods will continue to be a difficult task. This, coupled with the relatively recent introduction of inorganic fertilizers and their overuse will probably be the biggest challenge in reducing the contribution of agricultural sources of nitrogen to water deterioration in the Tai Lake region. Experts (Want et al 2006, Richter et al 2000) have recognized the need to the economic “internalization of environmental externality”, calling for measures such as the removal of government subsidies on agricultural fertilizers (reducing over application of nitrogen to fields) and improvements on water pricing and water rights legislation (reducing unneeded water usage by adjusting prices to reflect the costs of using the resource and reflecting the needs of water quality improvements).

Studies cited:

Richter, et al. 2000.  “The N-Cycle as determined by intensive agriculture– examples from central Europe and China” Nutrient Cycling in Agroecosystems.

Wang, et al. 2006. “Toward Integrated Environmental Management for Challenges in Water Environmental Protection of Lake Taihu Basin in China”. Environmental Management.

Tags: Lake Tai, Nitrogen Cycle, SRI, System of Rice Intensification, Taihu
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