BayCEER Workshop 2017

Drying and rewetting mobilizes phosphorus and nitrogen from undisturbed forest floors.

Drying and rewetting (D/W) of soils often resulted in the release of soluble phosphorus (P) and nitrogen (N) thereby changing the availability of both nutrients. Most experiments on D/W have been conducted with disturbed mineral soil samples and with rewetting of the soil samples by water extraction. Here we studied the effect of D/W on the leaching of P and N from undisturbed forest floors of a European beech and a Norway spruce site under near field conditions of desiccation and rewetting. We hypothesized that even under realistic rewetting of undisturbed forest floors, the leaching of P and N is increased after D/W and that the effects are less pronounced for spruce than for beech because of the larger hydrophobicity of the spruce forest floor. Undisturbed forest floor columns were subjected to desiccation at 20°C until a matrix potential of -100 MPa (pF 6.0) was reached, while controls were kept at moist conditions. Columns were irrigated by 22 mm day^-1 from day 1-3 and by 10 mm day^-1 from day 4-14 given in automated short pulses. Leachates from the soil columns were analyzed for orthophosphate, total P, NH₄, NO₃ and total N. In the spruce forest floor the concentrations of total P in leachates and the leachate fluxes were strongly increased after D/W. Less increase of solute P was observed for beech coinciding with less rewetting of the forest floor. Leaching of total N from the spruce forest floor was not affected by D/W, however, concentrations and leaching of NH₄ increased while leaching of NO₃ decreased. For beech the leaching of total N and NH₄ increased after D/W while NO₃ leaching decreased. The results indicate that also under realistic conditions D/W of forest floors increases solute P and leads to changes in the ratio of NH₄/NO₃ in solution, thereby altering the availability of the nutrients.