Drying–rewetting events reduce C and N losses from a Norway spruce forest floor

Periods of prolonged summer drought are likely to be expected for this century, with possibly strong effects on carbon (C) and nitrogen (N) mineralization in soils. Drought generally reduces mineralization rates, but the possibility of excess mineralization pulses during rewetting raises the question about the net effect of drying–rewetting events. In this experiment, we measured C and N mineralization in undisturbed soil columns that were either kept under continuously moist conditions (control) or that were subjected to drying-rewetting. We had three treatments (D1–D3) with different drying intensity (increasing from D1 to D3) but uniform rewetting intensity (4 mm d−1). Soil columns were taken from a Norway spruce forest in Bavaria, Germany. The CO2 fluxes from control and treatment groups were identical before drying. Over the 80 d drought period, total CO2 emissions from D1, D2, and D3 were only 72, 52 and 43% of that from the control, respectively. Rewetting resulted in a fast increase of CO2 fluxes to approx. the same level as in the control. Rewetting could not restore soil moisture of the dry soil to the level of the control, presumably because of preferential flow and water repellency of soil organic matter. No significant excess C mineralization during the 40 d rewetting period was observed. Adding up total CO2 fluxes during drought and rewetting period, the treatments D1, D2, and D3 emitted only 88, 71 and 67% of the CO2 emitted by the control. Measurements of dissolved organic carbon (DOC) did only show minor differences between control and treatment columns, indicating that no significant accumulation of DOC took place during the drought period. Radiocarbon signature of emitted CO2 indicated that C mineralization was reduced with decreasing water availability and no new substrate became bioavailable. Net N mineralization over the course of the whole experiment was reduced by drought to 77, 65 or 52% of the control. Net nitrification was virtually zero during drought whereas net ammonification continued at reduced levels. In summary, we found that drying–rewetting generally reduced C and N mineralization in this soil and that the total reduction increased with drought intensity.