Aluminium toxicity and magnesium uptake in spruce forest

In response to the problems in quantifying the effects of aluminium in plant nutrient uptake and growth as mentioned in the previous post, De Wit et al. (2010) specifically measured the effects of dissolved al on mg uptake in Norway spruce forests, in a long term field manipulation experiment from 1996.

Table from De Wit et al. (2010) showing the changes to BS after the addition of dilute aluminum carbonate into soil. 

The sites chosen were homogeneous-sandy soils with low al and n concentrations and Norway spruce stand. Dilute aluminium carbonate was added to 12 adjoining plots during snow and frost free seasons. Frequent measurements of soil at O horizon, pine needles and bark samples, crown density and color were observed and taken for analysis.

BS at O horizons were found to have significantly decreased while exchangeable al increased after 3 years of treatment. Mg in needles were also found to be reduced as compared to the control.

However, De Wit et al. (2010) found no significant impact of al on tree volume, crown, density annual increment and height increment. Plant root growth was not affected as hypothesized by many researchers. Only reduced mg uptake was significant, and this is not due to fluctuations in mg in soils. This mg deficiency has been reported in Lehstenbach, Germany by Alewell et al. (2000) but in nutrient poor soils.

Despite reports of al toxicity, it seems that spruce trees are more tolerant to acid deposition than previously thought. Similar experiments should be carried out in other areas to verify al toxicity levels in plants. However, this is a challenge to tropical areas as the vegetation is much more heterogeneous in a given area. Field experiments will need to be aided with laboratory experiments to test al sensitivity for all types of vegetation in a given area in order better quantify the effects of al in tropical regions.

References:

De Wit, H.A., Eldhuset, T.D. and Mulder, J. (2010) Dissolved Al reduces Mg uptake in Norway spruce forest: Results from long-term field manipulation experiment in Norway. Forest Ecology and Management. Vol. 259, pp. 2072-2082.

Alewell, C., Manderscheid, B., Gerstberger, P., Matzner, E., 2000. Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE-Bavaria, Germany. Journal of Plant Nutrition and Soil Science—Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 163, 509-516.

Discussing dissolved aluminium in soils

Studies have shown that dissolved aluminium in soils are a threat to forest health. Aluminium in soils occur naturally as aluminium is one of the most abundant metals in the earth’s crust. Upon acid deposition, aluminium is dissolved and mobilised from clays as the negative clay sites are replaced by hydrogen ions. In the soil solution, aluminium ions hydrolyse and are taken up by plants, inhibiting nutrient uptakes and plant root growth. Nutrient deficiency in trees have been found to be present at sites with high aluminium in soil solutions (Alewell et al. 2000).

Picture showing controlled experiment of plant root growth in different aluminium concentrations. Taken from http://www.summitfertz.com.au/research-and-agronomy/soil-ph.html

However, there are also healthy trees found under similar aluminium concentrations (Huber et al. 2004). There is no direct relationship between forest/tree health with acid deposition or dissolved aluminium toxicity in soils. Some researchers have thus used bases such as calcium, magnesium and potassium to aluminium ratio as a predictor of forest vitality (De Vries et al. 2003). However, as different types of vegetation, soils and ecological environments have different sensitivities to dissolves aluminium in soils, it is difficult to determine a threshold of base to aluminium ratio. Laboratory experiments of aluminium toxicity have been disputed by whole-ecosystem experiments as well. Laboratory experiments have been criticised for using potentially toxic levels of aluminium in controls and the amount of nitrogen added will have influence on root growth. Whole-ecosystem experiments are inconclusive about the role of aluminium in reducing root growth and nutrient uptake due to the presence of multiple variables (De Wit et al. 2001).

There is a need to reconcile both methods, in order to prove a direct, unambiguous relationship of nutrient uptake and root growth with aluminium toxicity.

References

Alewell, C., Manderscheid, B., Gerstberger, P., Matzner, E., 2000. Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE-Bavaria, Germany. Journal of Plant Nutrition and Soil Science—Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 163, 509– 516.

De Vries, W., et al. (2003) Intensive monitoring of forest ecosystems in Europe2: atmospheric deposition and its impacts on soil solution chemistry. Forest Ecology and Management 174, 97–115.

De Wit, H.A. et al. (2001) Aluminium: the need for a re-evaluation of its toxicity and solubility in mature spruce stands. Water Air and Soil Pollution: Focus 1, 103–118.

Huber, C., et al. (2004) Response of artificial acid irrigation, liming, and N-fertilisation on elemental concentrations in needles, litter fluxes, volume increment, and crown transparency of a N saturated Norway spruce stand. Forest Ecology and Management 200, 3–21.

Soil Acidification and Heavy Metal Contamination in Soils

Soils are acid/base systems which have the ability to absorb and release metals which have been introduced into the soil by anthropogenic or natural means. Soils are naturally acidic or alkaline based on their bedrock and climatic conditions. This measure of acidity or alkalinity in soils is important as it directly affects the growth of plants and the leach of heavy metals into soils which will cause crop contaminations.

In alkaline soils, i.e. sodic soils, plant growth is poor due to competition of sodium ions with other important ions such as magnesium and potassium for uptake in plant roots. However, alkaline soils have excesses of base cations which can bond with oxidised heavy metals (metal oxides) to fix heavy metals in soil. Alkaline soils thus are less likely to cause health problems when contaminated with heavy metals, but this is dependent on the type of metal oxides present in soil solution. However, these soils are too salty for crop growth. 

Picture showing Sodic Soils. Taken on 2 October 2014 in Inner Mongolia, China. Sodium salt levels are very high, with salts crystallizing to form a white layer on top. Nearby lake is salty as well. 

In acidic soils, the problem of heavy metal contamination is more serious than in alkaline soils due to the release of metals bonded on soil surfaces into the soil solution. In places where acid rain from nearby polluting factories acidifies soils, aluminium is leached into soils when pH<4. Aluminium ions are toxic to plants and retards plant root growth (Krstic et al. 2012). Besides, as aluminium has 3 free electrons for bonding, after being leached out, negative soil surface adsorbs other metal ions available and causes nutrient deficiency in plants. However, addition of hydrogen ions also replaces metal ions in soils and releases metals, but the overall effect of adsorption is higher as aluminium has 3 free electrons for bonding as compared to a hydrogen ion which only has one. 

Thus, places in the south - the southern part of China where acidic soils and acid rain are both present, aluminium toxification levels are high. For some crops, they will fail to grow, but crops with higher tolerance levels can bioaccumulate aluminium and pass on aluminium in the food chain. 

Krstic, D. et al. (2012) Aluminium in Acid Soils: Chemistry, Toxicity and Impact on Maize Plants. Collected in Food Production - Approaches, Challenges and Tasks. Aladjadjiyan, A. eds. InTech,  Croatia. 

Acid Rain - Soil Interactions (n.d.) Available at: http://www.elmhurst.edu/~chm/vchembook/196soil.html

Soil Pollution in China

According to BBC, ‘Almost a fifth of China's soil is contaminated, an official study released by the government has shown’. Decades of poor environmental management problems and rapid industrialization has repercussions in the environment. However, even in the capital Beijing, these problems have not been given critical importance.

Photo showing extreme eutrophication problems in Beijing, China. Photo taken on 26 October 2014.

Soil quality standards have been implemented in China, but these standards are outdated (1996) and have been criticized by environmentalists for being irrelevant. There has been attempts to establish new soil quality standards, especially in farmlands, but "almost no major country has set a unified national standard” (Chen, quoted in Caixin, Chinese news). China has to set up and implement her own standards instead of relying on foreign help, as no other developed western countries had such critical levels of pollution in farmland. This has proved to be difficult, given the diversity of soil types in China. Critics of the new soil standards have mentioned the need to match the standards of soils to the soil type present in order for standards to be met.

This poses a serious problem to food security in China. Pollution from the soils contaminates the food produces and accumulates in the food chain. Reports have suspected that the outbreak of cancer deaths is linked to the pollution (Wong 2013). Lead in children’s blood may be due to heavy metal soil pollution as well. These are dire consequences resulting from poor pollution control and land use planning. Many farms in China operate in the vicinity of heavy metal polluting industries, resulting in undesirable effects on human health.

There is a need to spread the awareness of soil pollution. Soil is not dirt, it supports lives.

BBC (2014) Report: One fifth of China's soil contaminated. Online news, 18 April, Available at: http://www.bbc.com/news/world-asia-china-27076645

Wong, E. (2013) Pollution Rising, Chinese Fear for Soil and Food. Online news. The New York Times, 30 December. Available at: http://www.nytimes.com/2013/12/31/world/asia/good-earth-no-more-soil-pollution-plagues-chinese-countryside.html?pagewanted=all&_r=0 

Zheng, C. (2015) Gov't Digs Into Soil Pollution Problem with Proposal for New Standards. Online, Available at: http://english.caixin.com/2015-01-27/100778811.html

The different types of soils

Soils differ in nature mainly due to the type of bedrock (lithology) and the climatic conditions. The nature of soils must be understood in order to determine the influence of pollutants in the soil. For example, the impact of acid rain on acidic soils with heavy metal pollution in southern China will be different from the impact on sodic soils – salty/alkaline soils with heavy metal pollution in northern China.

Soils can be classified in many different ways, according to their colour, texture, chemical properties, percentage of clay, etc. One of the easiest and common way of classifying soils are according to their colour. A soil Munsell colour system is usually used to determine the redness/yellowness of the soil, and this categorisation gives us a basic idea on the acidity/alkalinity of the soil, and thus its climatic or paleoclimatic conditions.

The percentage of clay is also usually examined in order understand the extent of sorption which may take place. In soils, clay colloids are very important as they are where the chemical process of sorption takes place, which usually refers to the exchange of metal ions in the soil solution with clay colloids (usually negatively charged, variable). This affects the toxicity of metals in soils as it is the free metal concentration in soil solutions which is polluting and affects plants and humans.

Other factors such as soil structure and detailed chemical composition also affects the impact of pollutants on soils and their toxicity to the biosphere, hydrosphere and atmosphere. 

The Importance of Soils

Soils are complex, living and dynamic systems that are at the interface of four 'environments' - lithosphere, biosphere, atmosphere and hydrosphere. Any changes to these 4 environments will affect soils. The biosphere, atmosphere and hydrosphere are the main contributors to the pollution of soils. However, the climatic conditions of the situated environment and lithosphere controls the main characteristics of soils. The characteristic of soils will in turn determine the extent of impact of the pollutants. 

The heterogeneity of soils makes it difficult for the impact of pollutants on soils be quantified. As a result, soils often treated as a black box. Besides, since soils are good absorbents, they have been treated as stores for pollutants with no considerations for its sustainability. Usually, when soils start to leach their pollutants into the groundwater, soils have been too contaminated for effective treatment and they will have to be disposed at landfills. This will then greatly disrupt the carbon budget as soils are important carbon stores. Therefore, there is a need to promote the awareness for soil sustainability. 

Soils are important as they are 

1. Mediums for Plant Growth
2. Regulators of Water Supply
3. Assistants in the Recycling of Nutrients and Waste (N and C cycles)
4. Habitats of Organisms
5. Engineering Mediums