Interannual and interseasonal soil CO2 efflux and VOC exchange rates in a Mediterranean holm oak forest in response to experimental drought

Asensio D., Peñuelas J., Llusià J., Ogaya R., Filella I. (2007) Interannual and interseasonal soil CO2 efflux and VOC exchange rates in a Mediterranean holm oak forest in response to experimental drought. Soil Biology and Biochemistry. 39: 2471-2484.
Enllaç
Doi: 10.1016/j.soilbio.2007.04.019

Resum:

Climate models predict drier conditions in the next decades in the Mediterranean basin. Given the importance of soil CO2 efflux in the global carbon balance and the important role of soil monoterpene and volatile organic compounds (VOCs) in soil ecology, we aimed to study the effects of the predicted drought on soil CO2, monoterpenes and other VOC exchange rates and their seasonal and interannual variations. We decreased soil water availability in a Mediterranean holm oak forest soil by means of an experimental drought system performed since 1999 to the present. Measurements of soil gas exchange were carried out with IRGA, GC and PTR-MS techniques during two annual campaigns of contrasting precipitation. Soil respiration was twice higher the wet year than the dry year (2.27±0.26 and 1.05±0.15, respectively), and varied seasonally from 3.76±0.85 μmol m-2 s-1 in spring, to 0.13±0.01 μmol m-2 s-1 in summer. These results highlight the strong interannual and interseasonal variation in CO2 efflux in Mediterranean ecosystems. The drought treatment produced a significant soil respiration reduction in drought plots in the wet sampling period. This reduction was even higher in wet springs (43% average reduction). These results show (1) that soil moisture is the main factor driving seasonal and interannual variations in soil respiration and (2) that the response of soil respiration to increased temperature is constrained by soil moisture. The results also show an additional control of soil CO2 efflux by physiology and phenology of trees and animals. Soil monoterpene exchange rates ranged from -0.01 to 0.004 nmol m-2 s-1, thus the contribution of this Mediterranean holm oak forest soil to the total monoterpenes atmospheric budget seems to be very low. Responses of individual monoterpenes and VOCs to the drought treatment were different depending on the compound. This suggests that the effect of soil moisture reduction in the monoterpenes and VOC exchange rates seems to be dependent on monoterpene and VOC type. In general, soil monoterpene and other VOC exchange rates were not correlated with soil CO2 efflux. In all cases, only a low proportion of variance was explained by the soil moisture changes, since almost all VOCs increased their emission rates in summer 2005, probably due to the effect of high soil temperature. Results indicate thus that physical and biological processes in soil are controlling soil VOC exchange but further research is needed on how these factors interact to produce the observed VOCs exchange responses. © 2007 Elsevier Ltd. All rights reserved.

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Seasonal soil VOC exchange rates in a Mediterranean holm oak forest and their responses to drought conditions

Asensio D., Peñuelas J., Ogaya R., Llusià J. (2007) Seasonal soil VOC exchange rates in a Mediterranean holm oak forest and their responses to drought conditions. Atmospheric Environment. 41: 2456-2466.
Enllaç
Doi: 10.1016/j.atmosenv.2006.05.007

Resum:

Available information on soil volatile organic compound (VOC) exchange, emissions and uptake, is very scarce. We here describe the amounts and seasonality of soil VOC exchange during a year in a natural Mediterranean holm oak forest growing in Southern Catalonia. We investigated changes in soil VOC dynamics in drought conditions by decreasing the soil moisture to 30% of ambient conditions by artificially excluding rainfall and water runoff, and predicted the response of VOC exchange to the drought forecasted in the Mediterranean region for the next decades by GCM and ecophysiological models. The annual average of the total (detected) soil VOC and total monoterpene exchange rates were 3.2±3.2 and -0.4±0.3 μg m-2 h-1, respectively, in control plots. These values represent 0.003% of the total C emitted by soil at the study site as CO2 whereas the annual mean of soil monoterpene exchange represents 0.0004% of total C. Total soil VOC exchange rates in control plots showed seasonal variations following changes in soil moisture and phenology. Maximum values were found in spring (17±8 μg m-2 h-1). Although there was no significant global effect of drought treatment on the total soil VOC exchange rates, annual average of total VOC exchange rates in drought plots resulted in an uptake rate (-0.5±1.8 μg m-2 h-1) instead of positive net emission rates. Larger soil VOC and monoterpene exchanges were measured in drought plots than in control plots in summer, which might be mostly attributable to autotrophic (roots) metabolism. The results show that the diversity and magnitude of monoterpene and VOC soil emissions are low compared with plant emissions, that they are driven by soil moisture, that they represent a very small part of the soil-released carbon and that they may be strongly reduced or even reversed into net uptakes by the predicted decreases of soil water availability in the next decades. In all cases, it seems that VOC fluxes in soil might have greater impact on soil ecology than on atmospheric chemistry. © 2006 Elsevier Ltd. All rights reserved.

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