Antioxidants in soils

Background The organic matter content of soils, also called humus, varies from about 5% in many mineral soils up to 100% in organic soils, such as peats. 50% of the humus is made of carbon; and so, on a global scale, soils store large amounts of carbon. In fact it is estimated that they hold more carbon (1600 billion tonnes) than there is in all living plants on land (600 billion tonnes). The humus is subject to breakdown by micro-organisms and carbon is released into the atmosphere as CO2. The recent rise in atmospheric CO2 concentrations is now linked to global climate change. Understanding the breakdown process of humus is therefore an important part of research into the global carbon cycle and climate change. We know that the rate of breakdown of humus depends on the climate, especially temperature and rainfall, and on how the land is used (e.g. agriculture vs. forestry). But we still lack any detailed knowledge of the chemistry of the breakdown processes. This proposal seeks to find a possible general chemical mechanism involved. This novel approach takes as its starting point the fact that all organic materials in our oxygen-rich environment are susceptible to attack by highly reactive free radicals. For example, when butter goes rancid, it is because of oxidation by free radicals. Vegetable oil is less susceptible because it contains vitamin E, which is an antioxidant. Antioxidants stop the destructive oxidation by free radicals by being transformed into unreactive stable free radicals. It has recently been shown that breakdown of plant residues, such as lignin, involves free radicals. This oxidation by free radicals is likely to operative in soils when plant residues are degraded; but we do not know if soils contain antioxidants. The evidence suggests that they probably do. First, plant and animal remains added to soils will contain antioxidants. These are likely to persist for a time, particularly tannins, which have antioxidant properties and which are relatively resistant to microbial breakdown. Secondly studies have shown that soil humus contains unreactive stable free radicals. These are most likely to be derived from the reaction of the humus with reactive radicals. If this is the case, compounds within the humus are acting as antioxidants, by scavenging the reactive free radicals and terminating the oxidation process. Aims and objectives 1.To test the idea that soils have antioxidant properties, 2.If soils have antioxidant properties, to test whether the antioxidant properties vary from soil to soil and why. The first aim requires that we develop a method of measuring the antioxidant capacity (AOC) of soils. The second aim will be met by measuring the AOC of a variety of soils and by chemically analysing the soil extracts used for the AOC measurements. Potential applications and benefits If the ideas to be tested in this research are shown to be true they will benefit not only those interested in soil science, but also for example scientists interested in freshwater and marine environments, in which naturally occurring organic matter also undergoes degradation processes. There may be practical applications arising from the mechanisms that are to be investigated in this proposal. If it can be shown soils have antioxidant capacity, which controls the rate of breakdown of humus, then manipulation of that capacity could be used to control the breakdown process. During the breakdown, the release of atmospheric CO2 is accompanied by the release of soluble plant nutrients, such as nitrogen, into the soil. This could be important for our future use of fertilisers, and also for the protection of the water environment from excess nutrients (eutrophication).