BayCEER Workshop 2015

Drying-rewetting cycles release dissolved organic and inorganic phosphorus from forest soils

Introduction

Drying-rewetting cycles (D/W) frequently occur in topsoils and may mobilize soluble forms of P.

Material and Methods

Here we investigated the effect of repeated D/W on the release of dissolved inorganic (Pi) and organic P (DOP) from different layers of forest floors and from A-horizons. Samples were taken from 3 European beech sites and from 3 Norway spruce sites. Samples were subjected to three D/W cycles in the laboratory. Soil desiccation was to about pF 6 (-100 MPa) and samples were subsequently extracted with water. Controls were kept permanently moist at 50% water holding capacity.

Results

The release of Pi from forest floor layers by D/W cycles was larger than of DOP. Short term repeated cycles did not increase the effect. The release of Pi after D/W from the Oe horizons was largest (50-60 mg P kg^-1), exceeding the release from Oi and Oa layers and from A-horizons. In A-horizons, D/W induced a larger release of DOP than of Pi. There was no general difference in response to D/W between samples from beech or spruce sites. The release of Pi correlated to the microbial biomass in Oe + Oa layers but not in Oi layers. In Oi layers the release of Pi after D/W was positively related to the P mineralization in the controls. DOP release correlated to microbial biomass for all forest floor layers. When extrapolating the laboratory findings to the field scale, the release of total P following D/W might be equal to about 2 (beech) to 3 (spruce) kg P ha^-1, the differences being caused by larger forest floor depth under spruce.

Conclusions

In forest soils, D/W might thus have a significant influence on the dynamics of dissolved P.