Studies of aspects of biogeochemical cycling, with particular reference to soil sustainability and to the sea-salt effect
The concept of soil sustainability has been concisely reviewed, to show the central role of biogeochemical cycling in sustaining fertility of both natural and managed soils. The role of sea salt inputs from the atmosphere within the context of biogeochemical cycling of base cations, concentrating upon the ameliorative effects that base cation inputs from the atmosphere may have against soil acidification in acidification-sensitive upland soils in Scotland, is then discussed more fully. Three approaches were used to try to confirm the importance of the sea salt effect to Scottish upland soils. Studies under controlled laboratory conditions provided strong evidence that maintaining soils under field moist conditions caused considerable increases in exchangeable Ca2+:Na + ratios, which could be attributed to internal soil action redistributions in the absence of a sea salt input. The changes in soil pH over time were not as simple as had been initially expected, partly as a consequence of the complications caused by organic matter oxidation. This study clearly showed it is unwise to store field moist soils in a cooled room for a few weeks prior to analysis. Samples stored for 20 months were re-analysed to look for effects that could be attributed to the elimination of sea salt inputs, but the results did not provide evidence for a sea salt effect. Interpretation was difficult because of the possibility of errors occurring as a result of soil storage in a cold room for up to a few weeks prior to soil analysis, and the high degree of organic matter oxidation. The latter results in release of SO42-, which was removed by precipitation of aluminium sulfate. Repacked soil columns containing individual soil horizons were subjected to simulated high sea salt and low sea salt rainfall treatments over three months. One set of soil horizons was pre-treated to remove organic matter. The results of this experiment were much more unequivocal, with large increases in exchangeable Na+ and Mg2+ concentrations and corresponding soil pH shifts. Finally, it was hypothesised that, if soils with low mineral weathering rates in north east Scotland were susceptible to strong sea salt effects, this should be reflected directly in the associated river water composition. This was confirmed experimentally, and there was a strong link between susceptibility to sea salt effects and susceptibility to acidification.