Spatial and temporal variation of soil solution chemistry and ion fluxes through the soil in a mature Norway Spruce (Picea abies (L.) Karst.) stand

In this study we investigated the spatial and temporal variation in soil solution chemistry and of water and ion fluxes through the soil in a forest ecosystem. Our aim was to evaluate the relevance of these variations for the accuracy of average areal soil solution concentrations and ion fluxes with seepage at 90 cm depth.Twenty spatially distinct subcompartments of approximately 1 m² were established within a mature stand of Norway spruce and ceramic suction lysimeters were installed at depths of 20, 35 and 90 cm. A tensiometer was placed close to each suction lysimeter, and one throughfall sampler was established for each subcompartment.Soil solution samples were analysed for major ions (H⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe³⁺, Al³⁺, Cl^-, NO₃^-, SO₄^2-. We calculated water fluxes for each subcompartment separately by a numeric simulation of the soil water flux close to the lysimeters. The ion fluxes at each lysimeter were calculated by multiplying the simulated water fluxes with the ion concentrations on a fortnightly base. Averaging these 20 independent ion fluxes gave the areal average flux and an estimate of its statistical accuracy. The spatial variation of ion concentrations in the soil solution was high with coefficients of variance ranging from 5% to 128%. Part of the spatial variation was related to stem distance. Temporal variation of the concentrations was less than spatial for most ions. The spatial variation of water and ion fluxes with seepage was also substantial; for example the fluxes of SO₄^2- -S calculated for each subcompartment ranged from 21 to 119 kg ha^-1 yr^-1, with an arithmetic average of 47 kg ha^-1 yr^-1. For H₂O, Mg²⁺, Cl^-, and SO₄^2- , the spatial heterogeneity of seepage fluxes was largely explained by the heterogeneity of throughfall fluxes. No such relationship was found for nitrogen.Despite using 20 replicates, the 95% confidence intervals of the average annual areal fluxes with seepage were found to be 20–30% for most ions.