The effect of afforestation with Scots pine (Pinus silvestris L.) of sandy post-arable soils on their selected properties. II. Reaction, carbon, nitrogen and phosphorus

Despite the extensive literature on the effect on soil properties of afforestation of former arable land, we still lack full understanding of whether the changes proceed in the same direction and at the same rate, and of how long is required to achieve a state of soil equilibrium typical of a natural forest ecosystem. Therefore, as part of a study comparing post-arable sandy soils (Dystric Arenosols) afforested with Scots pine (Pinus silvestris L.) with arable soils and soils of continuous coniferous forests, the range and direction of changes in pH, organic carbon (Cₒᵣg), total nitrogen (Nₜₒₜ), ammonium (N-NH₄) and nitrates (N-NO₃) in soil solution, total (Pₜₒₜ) and available (Pₐᵥ) phosphorus were determined. The studies were carried out in south-east Poland (51°30′-51°37′N, 22°20′-22°35′E). Ten paired sites of afforested soils (five with 14- to 17-year-old stands and five with 32- to 36-year-old stands) with adjacent cultivated fields, and five sites of continuous forest with present stands of ca. 130–150 years old were selected. Soil samples were taken from the whole thickness of master horizons and, in the case of the A horizon of the afforested soils, from three layers: 0–5 (A₀–₅), 5–10 (A₅–₁₀) and 10–20 cm (A₁₀–₂₀). The cultivated soils in the Ap horizon showed higher pH (by ca. 1.0 unit), lower Cₒᵣg and C:N, similar Nₜₒₜ, lower N-NH₄, higher N-NO₃, higher Pₜₒₜ and Pₐᵥ contents compared with the Ah horizon of continuous forest soils. The results indicated decreased soil pH in the former plough layer of the afforested soils, with the greatest decrease observed in the 0–5 cm layer. In these soils, the Cₒᵣg content was considerably higher in the A₀–₅ layer, but lower in the two deeper layers and in the whole A horizon (0–20 cm) compared with the Ap horizon of the arable soils. The results indicate that the Cₒᵣg content, after an initial phase of decline, again achieved a level characteristic of arable soils. The Nₜₒₜ content in all layers of the A horizon of the afforested soils was lower than in the Ap horizon of the arable soils, and showed a reduction with stand age, especially in deeper layers. The C:N ratios in the mineral topsoil increased with stand age. N-NH₄ content increased and N-NO₃ decreased after afforestation. The Pₜₒₜ and Pₐᵥ contents in all layers and in the whole A horizon of the afforested soils, on stands of both ages, was lower than in the Ap of the cultivated soils. From the results, it could be concluded that, after more than 30 years of tree growth, the soils of the A horizon were still more similar to arable than to continuous forest soils with respect to Cₒᵣg, Pₜₒₜ and Pₐᵥ. With respect to pH, N-NH₄ and N-NO₃, especially in the 0–5 cm layer, they were more similar to continuous forest soils than to cultivated soils, but with respect to Nₜₒₜ and C:N ratio they were somewhere in between.