How drought affects tree roots as a major source of SOM

Ivano Brunner1, Claude Herzog1
1 Forest Soils and Biogeochemistry, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, CH-8903 Birmensdorf, Switzerland

O 4.2 in Below ground turnover of C and nutrients in forest soils

14.07.2014, 11:35-11:55, H17

Drought stress is one of the main environmental factors causing a reduction of plant growth. The decrease of the water potential in the soil induces in root tissues several signals such as hydraulic signals transmitted along the root axis or the production of the hormone abscisic acid. As a consequence, and since water always follows a water potential gradient, roots lower their hydraulic conductivity and their osmotic potential in order to avoid dehydration. Abscisic acid triggers a cascade of physiological responses, including the closure of stomata. Thus, dehydration avoidance during drought stress is the result of a delicate balance between root water uptake and stomatal movements (Aroca and Ruiz-Lozano 2012).

The closure of stomata has the negative effect that C assimilation rates are reduced and, as a consequence, that trees might be forced into a negative C balance. Stored C may then be mobilized to fulfill the metabolic needs – until reserves are depleted and trees die from C starvation (McDowell et al. 2008).

Roots may adapt to drought conditions with increased root growth into deeper (and moist) soil horizons, however, if severe drought is faced, there is an overall decrease in root biomass due to decreased water and carbon availability. Upon drought stress some morphological and nutritional traits of roots might become modified, e.g. length, diameter, tissue density, tip frequency, N concentration, or C/N ratio. Less is known about biochemical traits, e.g. starch, phenolic compounds (e.g. lignin), or aliphatic compounds (e.g. suberin) (Davies and Bacon 2003). For example, it has been shown that the total concentration of aliphatic monomers in oak roots from a dry environment is much higher than from a moist environment (Andreetta et al. 2013).

Root-derived SOM plays a major role for C cycling and C storage in soils, as a considerable proportion becomes incorporated into the soil as belowground input. In particular, the accumulation of aliphatic root-derived compounds might be important for the hydrophobic protection of SOM (Dignac and Rumpel 2013).

In this presentation we focus on how drought affects the quantity and quality of fine roots of trees. And we combine literature data with data from an own irrigation experiment in a Scots pine forest growing in a climatic dry area in Switzerland. After nine years of irrigation we have observed that biomass of fine roots has increased due to irrigation, whereas longevity of the fine roots has been distinctly reduced. However, C and N concentrations of the fine roots are not strongly influenced by the irrigation treatment, as well as the concentrations of phenolics and lignin (Herzog et al. 2014).

Andreetta A. et al. 2013. Biogeochemistry 112:41-58
Aroca R, Ruiz-Lozano JM. 2012. Springer, Berlin, pp. 113-127
Davies WJ, Bacon MA. 2003. Springer, Berlin, pp. 173-192
Dignac MF, Rumpel C. 2013. Biogeochemistry 112:1-6
Herzog C. et al. 2014 (in review)
McDowell NG. et al. 2008. New Phytologist 178:719-739

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last modified 2014-06-19