The rhizosphere effects of <i>Fagus sylvatica</i> L. and <i>Fraxinus excelsior</i> L. saplings on greenhouse gas fluxes between soil and atmosphere

Abstract

Tree species influence the soil through stemflow and throughfall water, leaf litter and the root system. Little is known about the effects of tree roots on the C and N dynamics of the soil and the gas exchange with the atmosphere. In the present study, the effects of European beech (<i>Fagus sylvatica</i>) and Common ash (<i>Fraxinus excelsior</i> L.) saplings, as important European broad-leaved tree species, on C and N fluxes in the soil of a species-rich temperate forest were investigated under constant climatic conditions. The main objective was to identify root-induced changes in the greenhouse gas fluxes of CO₂, CH₄, and N₂O between soil and atmosphere. A stepwise experimental approach was used to extend the knowledge about rhizosphere effects on soil biogeochemistry. In the first step, the effects of simple C and N alteration by KNO₃ (equivalent to 200 kg N ha^-1 yr^-1) and glucose addition (equivalent to 9419 kg C kg ha^-1 yr^-1) on the fluxes of CO₂, CH₄, and N₂O were investigated for a basic understanding of the C and N dynamics in the incubated forest soil (Chapters 2 and 3). In the next step, the changes due to C and N alteration were compared with the putatively complex effects of ash roots on CO₂ and N₂O emissions in soil columns (Chapter 4). Finally, species-specific effects of the roots of beech and ash saplings on the C and N cycling of the soil were analysed in soil columns and novel double-split-root rhizotrons (Chapters 4, 5, and 6). The experimental investigation of the effects of NO₃^- and glucose addition on the greenhouse gas exchange (Chapter 2) revealed a large reduction in net CH₄ uptake due to increased N availability and saturating doses of C (reductions up to 86% and 83%, respectively). Moreover, addition of NO₃^- and glucose increased the N₂O emissions by factors of 8 and 39, respectively, whereas the CO₂ efflux remained constant after N addition and increased dramatically up to 11-fold after C addition (Chapter 3). A synergistic effect of C and N addition on all three investigated gas fluxes could be shown. The results of the simple C and N addition experiments suggest that the effect of the large C addition on all three investigated greenhouse gases, including the measured N emissions, was larger than the effect of elevated N availability, which might be important under a variable climate. The comparison of the effects of N addition and the presence of ash roots on CO₂ and N₂O emissions showed that the ash roots greatly reduced the N₂O emissions by up to 98%, whereas N addition increased the N₂O emissions just by 54% (Chapter 4). These results indicate that the effect of ash saplings on N₂O might not be exclusively explained by the N uptake of the roots, and that plant species effects of the rhizosphere changes should achieve a higher attention in future studies on the greenhouse gas balance of forest soils. As in the soil columns, the rhizotron experiment showed a large reduction of N₂O emissions by ash roots (Chapter 5). In contrast, the reduction of N₂O release in presence of beech saplings was only slight or not visible in the rhizotrons and the soil columns (Chapters 4 and 5). The CO₂ emissions from soil planted with ash tended to be higher than, or were similar to, the emissions from soil planted with beech (Chapters 4 and 5). Due to the higher relative contribution of root respiration to total soil respiration in ash rhizotrons (35.5 ± 8.5 vs. 9.0 ± 2.7 %, Chapter 5), we assume that a higher activity of saprotrophic fungi and a larger microbial-specific respiration was responsible for the similar CO₂ effluxes from soil under beech and ash (Chapter 6). In the rhizotron approach, the CH₄ uptake was significantly increased under ash compared to the control soil (Chapter 5), while beech saplings did not significantly affect the CH₄ uptake. In contrast to the observed changes in greenhouse gas fluxes, the C and N stocks of soil under beech and ash were only slightly different. In conclusion, the gas fluxes from the soil to the atmosphere can be used as sensitive indicators of even small changes in the biogeochemical processes of forests. Despite the higher CO₂ efflux from soil under ash, the greenhouse gas balance calculated as the sum of CO₂, CH₄, and N₂O fluxes tended to be more favourable for soil under ash than for soil under beech saplings in all experiments, which indicates a mitigating influence of European ash on the greenhouse gas balance of temperate forest soils. Further field and laboratory research on the relation between root systems and greenhouse gas fluxes from the soil are needed for realistic predictions of the future greenhouse gas balance under changing climatic conditions.