Leaching losses of inorganic N and DOC following repeated drying and wetting of a spruce forest soil

Abstract

Forest soils are frequently subjected to dry–wet cycles, but little is known about the effects of repeated drying and wetting and wetting intensity on fluxes of ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$, ${\text{NO}}^{ - }_{3}$ and DOC. Here, undisturbed soil columns consisting of organic horizons (O columns) and organic horizons plus mineral soil (O + M columns) from a mature Norway spruce stand at the Fichtelgebirge; Germany, were repeatedly desiccated and subsequently wetted by applying different amounts of water (8, 20 and 50 mm day⁻¹) during the initial wetting phase. The constantly moist controls were not desiccated and received 4 mm day⁻¹ during the entire wetting periods. Cumulative inorganic N fluxes of the control were 12.4 g N m⁻² (O columns) and 11.4 g N m⁻² (O + M columns) over 225 days. Repeated drying and wetting reduced cumulative ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$ and ${\text{NO}}^{ - }_{3}$ fluxes of the O columns by 47–60 and 76–85%, respectively. Increasing ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$ (0.6–1.1 g N m⁻²) and decreasing ${\text{NO}}^{ - }_{3}$ fluxes (7.6–9.6 g N m⁻²) indicate a reduction in net nitrification in the O + M columns. The negative effect of dry–wet cycles was attributed to reduced net N mineralisation during both the desiccation and wetting periods. The soils subjected to dry–wet cycles were considerably drier at the final wetting period, suggesting that hydrophobicity of soil organic matter may persist for weeks or even months. Based on results from this study and from the literature we hypothesise that N mineralisation is mostly constrained by hydrophobicity in spruce forests during the growing season. Wetting intensity did mostly not alter N and DOC concentrations and fluxes. Mean DOC concentrations increased by the treatment from 45 mg l⁻¹ to 61–77 mg l⁻¹ in the O tlsbba columns and from 12 mg l⁻¹ to 21–25 mg l⁻¹ in the O + M columns. Spectroscopic properties of DOC from the O columns markedly differed within each wetting period, pointing to enhanced release of rather easily decomposable substrates in the initial wetting phases and the release of more hardly decomposable substrates in the final wetting phases. Our results suggest a small additional DOC input from organic horizons to the mineral soil owing to drying and wetting.