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Mechanisms of soil erosion as affected by climatic and management factors Helena Soinne 23.4.2009 Helsinki University Centre for Environment MTT Research Finland Mechanisms of soil erosion as affected by climatic and management factors Helena Soinne 23.4.2009 Helsinki University Centre for Environment MTT Agrifood Research Finland Presentation outline Erosion How and what? Erosion control Soil structure Aggregates Management Climate Erosion and climate change To sum up... Aims Helena Soinne 23.4.2009 Erosion Transport of soil constituents by natural forces primarily water and wind Natural process Sedimentation of eroded material forms the parent material of many present day agricultural soils Translocation of eroded colloidal material leads to development of illuvial sub surface horizons Human activity (agriculture) accelerates - Problems in water quality Helena Soinne 23.4.2009 Water erosion Erosion can be connected to radical changes in landscape Heavy rains, steep slope Massive loss of fertile soil, agricultural importance Rare in Finland Photo by UH, Environmental Soil Science Erosion of agricultural fields Removal of soil material is evidenced by turbid drain water, colloid-size particles Helena Soinne 23.4.2009 Colloidal material Continuum of particle size ranges, 1 nm - 1µm Fine clay, Fe- and Al-oxides, organic macromolecules, bacteria and viruses Small particles are chemically active Mobilised colloids act as carriers for Pesticides Phosphorus Heavy metals Environmental importance Non-polar organic compounds Helena Soinne 23.4.2009 Soil erosion Erosion is controlled by Rainfall intensity and runoff - Impact of raindrops can break down soil aggregates - Runoff water detaches and transports soil material Slope gradient and length - accumulation of runoff - velocity Vegetation Soil erodibility Helena Soinne 23.4.2009 Soil erosion Photo by UH, Environmental Soil Science Photo by Tommi Peltovuori Erodibility = the ease by which soil particles are detached by rain or by the running water Structurally unstable soils may disperse in water into basic particles i.e. sand, silt and clay Stability of soil aggregates – most important property governing soil erodibility Helena Soinne 23.4.2009 Erosion control Eroded material carrying contaminants and nutrients poses a threat to the receiving ecosystems (eutrophication) Suggested measures to decrease these threats: - Buffer strips Aim to protect the waterways - Constructed wetlands Erosion is a sign of structural imbalance in the soil Sealing and blocking of pores– infiltration and oxygen exchange becomes difficult Permanent loss of soil and nutrients Erosion control should aim to keep the soil and nutrients in the field Helena Soinne 23.4.2009 Erosion control on the field Effect of raindrops should be diminished vegetation cover minimal tillage Surface runoff should be diminished Enhancing infiltrability Increasing surface storage Aggregate stability should be increased Helena Soinne 23.4.2009 Soil Structure - aggregates Micro-aggregates <250 µm Clay-size particles bonded together by cementing agents (organic molecules, calcium carbonate, amorphous Al and Fe oxides, disordered aluminosilicates) Dense, high internal strength Macro-aggregates >250 µm Combinations of micro-aggregates Held together by roots and fungae Aggregates of different size classes have different stability Helena Soinne 23.4.2009 Soil structure - aggregate destabilisation Disintegration of macro-aggregates into microaggregates Wetting action: breakdown caused by compression of entrapped air during wetting or by differential swelling Water drop impact Dispersion of clay into the solution (micro-aggregates) Diffusion in wet soil High ionic strength – less dispersion Helena Soinne 23.4.2009 Soil structure - aggregates Factors affecting aggregate stability Internal: - Ionic strength + - Clay mineralogy and content +- - Organic matter + (-) - Fe and Al oxides + External: - Age + - Biological factors + - Agricultural management + - Climate Helena Soinne +23.4.2009 Structure and management Effect of management on soil structure: Tillage - Brake up natural aggregates, new surfaces for colloid detachment - Increase / decrease infiltration Vegetation cover - Dissipate raindrop energy, reduce wetting rate - Roots Organic matter amendments - increase in biological activity Fertilisers - Increase in ionic strength Helena Soinne 23.4.2009 Structure and climate Wetting and drying Drying is driving force for aggregation (micro-aggregates) Cycles decrease macro-aggregate stability ? - Effect is influenced by wetting rate and soil water content Freezing and thawing Freezing resembles drying – Traditionally frost has been considered to increase aggregate stability in clay soils Cycles: results showing decrease and increase in macroaggregate stability have been reported (water content) Spring: weak structure and high runoff - Soil saturated, snow melting, vegetative cover minimal Helena Soinne 23.4.2009 Erosion and climate change The erosion rates are expected to increase if precipitation becomes more intense and frequent (ClimSoil 2008) Frozen soil under snow cover stays intact However: Photo taken on March 18th in 2007 by Tommi Peltovuori Helena Soinne 23.4.2009 Erosion and climate change In the Boreal zone, increasing winter temperatures may further increase the potential for erosion Less snow cover Run-off volumes higher Duration of soil frost is shorter Freeze-thaw cycles Erosion throughout the year? Importance of erosion control Photo taken on March 18th in 2007 by Tommi Peltovuori Helena Soinne 23.4.2009 To sum up…. Climate change Heavier rains Winter rains Mobilisation of colloids Milder winters Transport of nutrients - Freeze-thaw cycles Soil structure is essential in erosion control Stabilisation of different aggregate size fractions is due to different mechanisms (micro- and macro-aggregates) - Management should have positive effect on aggregates in all size classes Climate change - changes in temporal distribution of precipitation - Challenge for soil management Helena Soinne 23.4.2009 Aims To characterise (quality) the eroded material Variation between land-use types, "no-erosion -field"? Seasonal variation To get knowledge that helps in predicting the environmental risk caused by erosion and to find management practises that reduce the potential for erosion To identify mechanisms behind the detachment of soil particles from the soil Environmental conditions that increase the instability of aggregates Conditions where high potential for erosion To identify problematic circumstances typical for certain management-type Helena Soinne 23.4.2009 Thank you! Helsinki University Centre for Environment MTT Agrifood Research Finland