Soil, Land Use And Environment Example

Soil, Land Use And Environment Example Soil, Land Use And Environment â€" Essay Example > To what extent can it be argued that the acidification of soils in humid temperate regions is the product of natural soil development as opposed to land use practice? IntroductionAcidification of soils is caused by the leaching and otherwise removal of cations (e. g. Ca and Mg) at a rate faster than can be supplied by weathering of the parent material. Leaching most often occurs where precipitation exceeds evapotranspiration, and includes the temperate regions of northwestern Europe, where soils have been subject to leaching since the end of the last ice age (Ellis and Mellor, 1995). Natural soils facing the same conditions for an extended time reach an equilibrium pH, but a wide variety of causes can change pH. Very low pH can cause a variety of problems, reducing the availability of nutrients for plant growth, and mobilising heavy metals such as aluminium, which can be very toxic when washed into fresh water systems. Natural causesPure rainwater in equilibrium with the atmospher e has a pH of 5.6. On meeting soil with a lower pH, the water is a sink of protons, as HCO3- ? H2CO3. However, the effect is a relatively small increase in pH, since the concentration of bicarbonate ions is low (Rowell and Wild, 1985). When meeting soil of pH above 5.6, rain has an acidifying effect, as carbonic acid dissociates, releasing protons. These exchange with the exchangeable cations such as Ca and Mg, which are then leached out of the system. In calcareous soils with pH 6.5, CaCO3 dissolves to Ca2+ and HCO3- which are also lost by leaching. The effect of rainwater as an acidifying agent is strongest in this last soil type (Rowell and Wild, 1985). Microbial respiration causes acidification by releasing CO2 which dissolves in soil water to form carbonic acid. Plants also contain many organic acids that are released to the soil. Nitrifying bacteria, also living in the soil, cause acidification by contributing to organic decomposition. In this process, NH4+ ions are oxidised to NO3- ions, with H+ ions as a by-product. Plant growth itself causes acidification, as base cations taken up by the roots are exchanged for H+ ions. The type of vegetation has a great effect on the contribution to acidity. Much research has been done into the effects of different tree species and woodland types (e. g. Hornung, 1985; Ovington, 1953), since the development of forests generally causes soil acidification. The processes in a new forest that contribute to acidification are the formation of an acid litter and mor humus, throughfall and stemflow (altering the chemistry of rainwater), production of organic acids, increased base cation uptake from the soil and increased precipitation interception and evapotranspiration. Not all of these processes act under all forests, and the effects are most marked under coniferous forests. However, there evidence that acidification of soils beneath conifers is largely reversed as the trees reach maturity. During the early years humus build-up is rapid, but later, as the canopy closes, the microclimate becomes less favourable for decomposition of litter, and thus nitrification by bacteria (Miles, 1985). The high soil pH created by young forests also affects the soil macrofauna, particularly earthworms. Earthworms are efficient decomposers of organic material, but can’t tolerate low pH. So, the decomposition of litter under a mature forest is further hindered by the loss of earthworm populations, and pH rises. Rising of pH during forest maturity is also due to changes in the ground vegetation as the forest closes in, for example the loss of relict moorland vegetation that has a strong acidifying effect.