ENHANCED SOIL STRUCTURING BENEATH WHITE CLOVER AND ITS IMPACT ON NUTRIENT TRANSPORT
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Previous work at IGER has revealed that soil structural differentiation under white clover is phenomenally rapid and enhanced when compared with ryegrass. White clover is one of the most nutritious and widely distributed forage legumes. Its use is advocated in sustainable systems of livestock production because of its ability to acquire atmospheric N through biological fixation in the root nodules. It thus provides an economically viable alternative to the N-demanding conventional system, and a possible solution to reduce the environmental impacts of nitrate leaching from agricultural land. There are, however, potentially negative impacts associated with improving soil aggregation through the use of clover that need further investigation. It appears that legume-based systems are not environmentally benign: similar amounts of N and P are leached from beneath grass-clover swards as those leached from beneath fertilised grass operating at the same level of production. In some circumstances, clover rich swards can give rise to very high levels of nitrate leaching. Thus, this observation of clover induced soil aggregation has important implications for the pollutant transport qualities of soils and for the organic/conventional agriculture debate. Re-packed soil columns of four soil series and 0.5 m intact monoliths of the Crediton series were planted with white clover, perennial ryegrass and a mixture of the two species, and managed according to an organic and conventional farming regime. Visual observations revealed a rapid enhancement in soil structure beneath white clover compared to ryegrass and unplanted soil. A novel technique to determine oxygen diffusion as an indicator of soil porosity, gave a diffusion rate that was nearly nine times greater than that of the grass treatments and fifteen times greater than the unplanted control soil, with intermediate values for the mixed treatment Thus enhanced structural differentiation beneath white clover was supported by greater permeability to air and freer drainage to water. Structural stability tests suggested that white clover improved the ability of the soil to maintain its structure under the action of water, and was estimated to be three times more stable than ryegrass. There was also evidence which implied improved shear strength and resistance to mechanical forces. Differences in soil structure were verified with water retention measurements, which showed a greater proportion of macropores. The void structure was simulated with the 30 Pore-Cor network model, which also suggested a number of larger pores and a saturated hydraulic conductivity which was four times greater than ryegrass. This also highlighted inadequacies in the current standard ISO protocol for water retention. The solute transport studies showed elevated levels of nitrate and phosphate leaching. Concomitant transport of bromide inferred structural differentiation and changes in leaching dynamics. In addition, white clover allowed the passage of greater volumes of water. Most importantly, this was manifested at the soil profile scale and therefore likely to be of consequence in the field. The implications of the research are that enhanced soil structure beneath white clover alters the transport of gases, water, nutrients and other dissolved substances. Further understanding of these soil processes are needed before advocating the use of forage rich legumes in sustainable systems, and for the development of management strategies.
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