The impact of changes in the timing of precipitation on soil respiration and soil-N in Mediterranean evergreen oak woodlands

Background/Question/Methods

Climate effects on photosynthetic carbon assimilation are the most important mediators of productivity, thereby providing the substrate for soil respiration, which constitutes a major determinant of carbon balance in terrestrial ecosystems. Particularly in seasonally dry climates, such as in the Mediterranean basin, climate change is expected to largely alter annual carbon balance by increasing both length and severity of droughts and the temporal unpredictability of rainfalls. Hence, knowledge about the behavior of Mediterranean ecosystems confronted with new environmental trials is of utmost importance for future ecosystem management. To assess the effects of precipitation variability on soil respiration and soil-N-cycling in a cork oak woodland (montado), large ´rain-out shelters´ were constructed. The water manipulation treatment was based on historical precipitation data with average annual precipitation of 680 mm. The median number of rain events during the growing season is 24, with the median length of the dry period being 7 days. During the growing season of October 2010 - June 2011, the two water manipulation treatments were: ´weekly watering treatment´, where natural conditions were simulated with a normal dry period of 7 days, and ´3-weekly watering treatment´, with the normal dry period increased three-fold to 21 days. The aim was to assess the effects of prolonged dry periods, without altering total annual precipitation inputs, on productivity of the herbaceous understorey and its respective feedbacks on soil respiration and nitrogen availability. 

Results/Conclusions

Increasing the dry period between precipitation events from 1 to 3 weeks did not affect productivity of the understory. Similarly, seasonal development of soil respiration and soil nitrogen were not significantly different between the two experimental treatments. Soil respiration increased continuously throughout the growing season, following increasing temperatures and productivity patterns. In contrast, soil N was lowest during the active growing season, increasing towards summer with reduced plant growth and increased litter accumulation from understory die-off. Surprisingly, soil water content was of no importance for either of the parameters during the study period. The observed patterns suggest that the drought adapted ´montado´ understorey community determines soil respiration and soil N patterns in the ecosystem. As productivity did not change with an increase in the dry period, soil respiration and N-cycling were also not affected. However, the effects of prolonged droughts within the growing period of the herbaceous understorey are currently being investigated by increasing the experimental dry period to 6 weeks.