Epsilon Archive for Student Projects

Abstract

In order to study how the pool of sulphate and base cations in the soil have changed during the last 10-15 years, time trends on calculated sulphate and base cation budgets for eight small Swedish catchments, were investigated. In addition, a comparison between two methods for dry deposition estimation was performed.
Despite a decline in acid sulphur deposition over Sweden, only small signs of recovery can be seen in nature. This might be due to an acidificating net loss of sulphur from the soil pool, where an accumulation could have taken places during periods with high deposition. The pool of base cations in soil may also have been reduced and it might take time for the pool to recover. Such accumulations and reductions can be studied with budget calculations. However existing budgets are uncertain mainly because of difficulties to estimate dry deposition correctly. In this study, dry deposition of sulphur and base cations were calculated by their chloride ratio. Dry deposition of chloride was calculated from the difference in chloride in bulk deposition and stream water. In addition, another method using vegetation maps together with the ratio between throughfall and bulk deposition at sites near the studied areas, was utilized to get an alternative estimation.
A decrease in the soil pool can be seen and this, along with the fact that the decrease of sulphate in stream water does not follow the decrease in total deposition, indicates that the catchments are leaking sulphate. In the southern parts of Sweden the losses from soil were bigger than in the northern parts. Changes in the soil pool of base cations were in general small, apart from a great negative trend in the pool of sodium in the most southern site. All changes, however, can be explained by changed marine impact, since no trends for non­marine base cations could be found except in one northern site.
The two methods for dry deposition estimation gave different results. The method using vegetation maps indicated less dry deposition of sulphate than the method using chloride ratios. The map method gave negative dry deposition of non-marine base cations. This is incorrect and is a result of that the data used showed less throughfall than bulk deposition for many areas. However, the vegetation correction included in the map method can be useful for other monitoring sites, since it can be utilized for correcting throughfall measurements made in vegetation types not representative for the whole monitoring area.