Delayed recovery of soil respiration after wetting of dry soil further reduces C losses from a Norway spruce forest soil

This experiment investigated the effects of prolonged summer drought on soil respiration (SR) in a mountainous Norway spruce forest in south Germany. On three manipulation plots we excluded summer throughfall in the years of 2006/2007 and measured SR fluxes in comparison to three control plots. Using radiocarbon measurements we quantified the contribution of rhizosphere (RR) and heterotrophic respiration (HR) to total SR. In both manipulation years, mean CO2 emissions (+/- SE) from the throughfall exclusion (TE) plots were smaller than from the control plots with 5.7 t C ha(-1) (+/- 0.3) compared to 6.7 t C ha(-1) (+/- 0.2) in 2006 and 5.9 t C ha(-1) (+/- 0.3) compared to 7.0 t C ha(-1) (+/- 0.4) in 2007. Under control conditions, CO2 originated mainly from HR (60-95% of SR). Prolonged drought reduced HR, whereas RR was not affected or even increased slightly. Reduction of CO2 emissions on the TE plots was found up to 6 weeks after differences in matric potential conditions disappeared, possibly either because water repellency inhibited homogeneous rewetting of the organic horizons or because of severe damage to the microbial population. No evidence was found for the release of new, formerly protected substrates by preceding drought. Continuous measurements in 2008 (no manipulation) did not reveal increased CO2 emissions on the TE plots that could compensate for the reduction during the years 2006/2007. Based on our results, we postulate a negative feedback between increased frequency and magnitude of summer droughts and SR in Norway spruce stands.