Seasonal changes of principal anions contents and other soil properties in acidified forest soils

Acidification of forest soil is a natural degradation process enhanced by anthropogenic activities. The depositions of principal inorganic anions are the main external acidity inputs to forest ecosystems. The aim of the study was to describe seasonal changes of sulphate and nitrate behaviour in soils and influence of their depositions on the selected forest soil properties. The following soil properties were investigated: soil pH, DOC, selected elements contents and Al species content. The Jizera Mountains area (Czech Republic) was chosen as a representative soil mountainous ecosystem affected by acidification. Soil and precipitation samples were collected at monthly intervals from April to October during the years 2008-2010 under beech and spruce stands. Prevailing soil types were classified as Alumic Cambisols under beech and Entic Podzols under spruce stands (according to FAO classification). Soil samples were collected from surface fermentation (F) and humified (H) organic horizons and subsurface B horizons (cambic or spodic). The collected soil samples were analyzed immediately in a "fresh" state. Unsieved fresh samples were extracted by deionised water and content of anions (sulphate, nitrate, chloride and fluoride) in these extracts were determined by ion-exchange chromatography (IC); the Al speciation was performed by means of HPLC/IC. The extracts were also used for determination of main elements content (Al, Ca, Mg, Ca, Na and Fe) by means of ICP-OES. Content of anions and main elements content, pH and conductivity were determined also in the precipitation samples (throughfall, stemflow and bulk). Statistically significant differences in distributions of monitored anions between the tested soil horizons were observed. The highest content of sulphate was determined in F and B horizons. On the contrary, contents of nitrate were highest in F horizons and lowest in B horizons. Higher annual variability in the investigated characteristics was proven for organic horizons compared to mineral ones. More distinctive seasonal variation was observed in both organic horizons (F and H), because these horizons are more affected by actual soil conditions and environment. Higher contents of anions were determined in summer each year and decreasing contents were observed from summer sampling period to autumn sampling months. With one exception in autumn 2009, when the highest amounts of sulphate and nitrate were observed. Moreover, sulphate and nitrate affected mobility and behaviour of observed elements. Based on the results it can be concluded that contents of observed anions were changing not only within the soil profile (spatial variation), but also with time (seasonal variation).