Robert Jandl

Soil respiration under climate change: prolonged summer drought offsets soil warming effects

Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To assess the combined effects of warming and reduced precipitation on soil CO2 efflux, we conducted a two-way factorial manipulation experiment (4 °C soil warming + throughfall exclusion) in a temperate spruce forest from 2008 until 2010. Soil was warmed by heating cables throughout the growing seasons. Soil drought was simulated by throughfall exclusions with three 100 m2 roofs during 25 days in July/August 2008 and 2009. Soil warming permanently increased the CO2 efflux from soil, whereas throughfall exclusion led to a sharp decrease in soil CO2 efflux (45% and 50% reduction during roof installation in 2008 and 2009, respectively). In 2008, CO2 efflux did not recover after natural rewetting and remained lowered until autumn. In 2009, CO2 efflux recovered shortly after rewetting, but relapsed again for several weeks. Drought offset the increase in soil CO2 efflux by warming in 2008 (growing season CO2 efflux in t C ha−1: control: 7.1 ± 1.0; warmed: 9.5 ± 1.7; warmed + roof: 7.4 ± 0.3; roof: 5.9 ± 0.4) and in 2009 (control: 7.6 ± 0.8; warmed + roof: 8.3 ± 1.0). Throughfall exclusion mainly affected the organic layer and the top 5 cm of the mineral soil. Radiocarbon data suggest that heterotrophic and autotrophic respiration were affected to the same extent by soil warming and drying. Microbial biomass in the mineral soil (0–5 cm) was not affected by the treatments. Our results suggest that warming causes significant C losses from the soil as long as precipitation patterns remain steady at our site. If summer droughts become more severe in the future, warming induced C losses will likely be offset by reduced soil CO2 efflux during and after summer drought.

Effects of season and experimental warming on the bacterial community in a temperate mountain forest soil assessed by 16S rRNA gene pyrosequencing

Climate warming may induce shifts in soil microbial communities possibly altering the long-term carbon mineralization potential of soils. We assessed the response of the bacterial community in a forest soil to experimental soil warming (+4 °C) in the context of seasonal fluctuations. Three experimental plots were sampled in the fourth year of warming in summer and winter and compared to control plots by 16S rRNA gene pyrosequencing. We sequenced 17 308 amplicons per sample and analysed operational taxonomic units at genetic distances of 0.03, 0.10 and 0.25, with respective Goods coverages of 0.900, 0.977 and 0.998. Diversity indices did not differ between summer, winter, control or warmed samples. Summer and winter samples differed in community structure at a genetic distance of 0.25, corresponding approximately to phylum level. This was mainly because of an increase of Actinobacteria in winter. Abundance patterns of dominant taxa (> 0.06% of all reads) were analysed individually and revealed, that seasonal shifts were coherent among related phylogenetic groups. Seasonal community dynamics were subtle compared to the dynamics of soil respiration. Despite a pronounced respiration response to soil warming, we did not detect warming effects on community structure or composition. Fine-scale shifts may have been concealed by the considerable spatial variation.

Carbon sequestration and forest management

Forest management has the potential to increase the terrestrial C pool. According to the rules of the Kyoto Protocol and of the United Nations Framework Convention on Climate Change, forestry can generate a sink for greenhouse gases that can contribute to meeting the national commitment to emissions reductions. Afforestation is a common strategy that over the course of decades leads to the incorporation of carbon dioxide (CO2) in plant biomass. However, site types such as wetlands and peatlands may even be a source of greenhouse gases when they are afforested. Adapted management of existing forests may have a less obvious or slower effect on the terrestrial C pool. It is mainly relevant in countries that already have a large forest cover. We analysed the effects of harvesting, rotation length, thinning, fertilizer application and tree-species selection. All these treatments have an impact on the forest productivity and consequently on C sequestration in the ecosystem. Many forest treatments are already an integral part of sustainable forestry practice. In the context of C sequestration and its accounting in national greenhouse-gas budgets, ecosystem stability is highly rated. Forests that are robust against disturbances up to a certain degree of severity are better suited for political commitments than stands of maximum productivity with a high risk of damages.

Natural variations in snow cover do not affect the annual soil CO2 efflux from a mid‐elevation temperate forest

Climate change might alter annual snowfall patterns and modify the duration and magnitude of snow cover in temperate regions with resultant impacts on soil microclimate and soil CO2 efflux (Fsoil). We used a 5-year time series of Fsoil measurements from a mid-elevation forest to assess the effects of naturally changing snow cover. Snow cover varied considerably in duration (105–154 days) and depth (mean snow depth 19–59 cm). Periodically shallow snow cover (<10 cm) caused soil freezing or increased variation in soil temperature. This was mostly not reflected in Fsoil which tended to decrease gradually throughout winter. Progressively decreasing C substrate availability (identified by substrate induced respiration) likely over-rid the effects of slowly changing soil temperatures and determined the overall course of Fsoil. Cumulative CO2 efflux from beneath snow cover varied between 0.46 and 0.95 t C ha−1 yr−1 and amounted to between 6 and 12% of the annual efflux. When compared over a fixed interval (the longest period of snow cover during the 5 years), the cumulative CO2 efflux ranged between 0.77 and 1.18 t C ha−1 or between 11 and 15% of the annual soil CO2 efflux. The relative contribution (15%) was highest during the year with the shortest winter. Variations in snow cover were not reflected in the annual CO2 efflux (7.44–8.41 t C ha−1) which did not differ significantly between years and did not correlate with any snow parameter. Regional climate at our site was characterized by relatively high amounts of precipitation. Therefore, snow did not play a role in terms of water supply during the warm season and primarily affected cold season processes. The role of changing snow cover therefore seems rather marginal when compared to potential climate change effects on Fsoil during the warm season.

Effect of a dense Allium ursinum (L.) ground cover on nutrient dynamics and mesofauna of a Fagus sylvatica (L.) woodland

The aboveground biomass built up annually by Allium ursinum (L.) contains similar amounts of nutrients as the foliage of mature Fagus sylvatica (L.) stands. The decomposition of the A. ursinum stand in early summer provides N rich forage for grazing mesofauna, especially favouring collembolans and accelerating mineralization of soil organic matter. Short term decreases of soil pH had no negative effect on populations of collembolans. Synergistic effects from soil fauna and microbes may accelerate nitrogen release from decomposing leaf litter. A positive feed back may have emerged. High animal abundance and diverse mesofauna populations are capable of high rates of litter fragmentation. Consequently, favourable conditions for microorganisms are created and allow high rates of mineralization and release of nutrients. Our data show that substantial amounts of nitrogen are lost from the system. Undisturbed forest ecosystems are considered to recycle mineralized nitrogen efficiently though. But temporal uncoupling of the N cycle due to microbial activity and delayed or decreased N uptake of higher plants can cause enhanced leaching even from undisturbed systems. The tendency to loose nutrients is apparent from high nitrate concentrations in the soil solution throughout the year. When nutrient losses from A. ursinum subsystems are considered, lateral nutrient imports from adjacent parts of the ecosystem have to be taken into account. If lateral import does not counterbalance losses, maintenance of the soil nutrient status must occur by weathering or a decline is to be expected.

Revitalization experiments in magnesium deficient Norway spruce stands in Austria

Amelioration of degraded forest ecosystems on acidic substrates showing the new type of forest decline is a major goal of forest management. A number of experiments show positive effects of Mg-application to systems suffering from Mg-deficiencies. The current paper compares experiments conducted in the Austrian part of the Bohemian Massif, where both effects on soil solution chemistry and effects on plant nutrition, vitality and growth were investigated. It turned out that any type of Mg-source is able to improve Mg-nutrition of trees; both a neutral salt like KIESERITE as well as alkaline reacting magnesite and dolomite derived materials. A positive reaction of vitality and growth could however only be induced with dolomitic lime or magnesite. Using mineral NPK fertilizers, even with high Mg-content, induced Mg-deficiencies and led to nutritional imbalances. In addition significant NO3 --leaching occured. On the other hand an organic slow release fertilizer (BACTOSOL*) amended with magnesite derived fertilizers (BIOMAG**) led to balanced nutrition and a fast recovery of tree health status, as judged by crown transparency, vitality index and growth rates. In both cases, when either magnesite derived compounds or combinations with the organic slow release fertilizer were applied, NO3 - -leaching occured only during the first three years after fertilization. The leaching rates declined afterwards to values comparable to unfertilized plots, while Mg-content of the soil solution could be elevated compared to the CONTROL, showing the sustainability of proper fertilization.

Acidification and Nitrogen Eutrophication of Austrian Forest Soils

We evaluated the effect of acidic deposition and nitrogen on Austrian forests soils. Until thirty years ago air pollution had led to soil acidification, and concerns on the future productivity of forests were raised. Elevated rates of nitrogen deposition were believed to cause nitrate leaching and imbalanced forest nutrition. We used data from a soil monitoring network to evaluate the trends and current status of the pH and the C : N ratio of Austrian forest soils. Deposition measurements and nitrogen contents of Norway spruce needles and mosses were used to assess the nitrogen supply. The pH values of soils have increased because of decreasing proton depositions caused by reduction of emissions. The C : N ratio of Austrian forest soils is widening. Despite high nitrogen deposition rates the increase in forest stand density and productivity has increased the nitrogen demand. The Austrian Bioindicator Grid shows that forest ecosystems are still deficient in nitrogen. Soils retain nitrogen efficiently, and nitrate leaching into the groundwater is presently not a large-scale problem. The decline of soil acidity and the deposition of nitrogen together with climate change effects will further increase the productivity of the forests until a limiting factor such as water scarcity becomes effective.

Amelioration of Magnesium Deficiency in a Norway Spruce Stand (Picea abies) with Calcined Magnesite

Norway spruce has shown needle yellowing and defoliation symptoms on manysites of the Bohemian Forest in north central Austria since theearly 1980s. A forest amelioration experiment was set up toevaluate the response of spruce to magnesite based fertilizerswith different solubilities. Response variables were a crownvigor index, diameter growth, nutrient contents in needles,soil solution chemistry, and soil chemistry. Soil solution atfertilized plots showed an immediate response in base cationconcentrations, nitrate concentrations and pH-values.Fertilization increased the pool of exchangeable cations in theupper 15 cm of the soil and reduced levels of exchangeable Al. Themagnesium and calcium content of the needles of dominant trees wasgreatly improved. Fertilization has increased the mineralizationrate of soil organic matter. Diameter growth, as a measure of treevigor, showed a pronounced positive response with a time delay of5 years. Our results demonstratethat magnesite based fertilizers are a viable option to improvethe nutritional status of spruce forests on granitic bedrock.

Investigations of nitrogen fluxes and pools on a limestone site in the Alps

In the North Tyrolean Limestone Alps a site was investigated over a four-year period (1998–2001) in order to assess the nitrogen saturation status, the nitrogen budget (quantification of the net uptake of nitrogen by the canopy and of the nitrogen mineralization, nitrogen uptake from roots and N2O emission rates, proof of the origin of nitrate in the soil water with stable isotope analyses), and the effects of the actual nitrogen input on ground water quality. The main goals were to quantify the nitrogen input rate, the nitrogen pools in above-ground and below-ground compartments, nitrogen turnover processes in the soil as well as the output into the groundwater and into the atmosphere. The findings are based on continuous and discontinuous field measurements as well as on model results.While nitrogen input exceeded the Critical Loads of the WHO (1995), nitrogen deficiency and nutrient imbalances were verified by needle analyses. The atmospheric input of inorganic nitrogen was higher than the nitrogen output in 50 cm soil depth. A tracer experiment with15N helped to prove that not more than half of the applied nitrate could be discharged. This allows the conclusion that nitrogen is stored in the system and that the site cannot yet be said to be saturated with nitrogen. The same result was also obtained by modelling. In addition, it was proved that the nitrogen discharge did not stem from deposition but from processes within the system.

A holistic assessment of greenhouse gas dynamics from forests to the effects of wood products use in Austria

ABSTRACT Anthropogenic GHG emissions add a fast reinforcing feedback cycle to global carbon dynamics which continues to influence GHG concentrations in the Earths atmosphere. When looking at forest carbon cycles there is potential in utilizing another feedback cycle, namely the carbon cycle involving harvested wood products. To assess the potential of the mitigation options arising from these carbon flows, the forest-based sector in Austria was modelled to assess causal links, dependencies and dynamics involved in GHG-relevant processes. Carbon dynamics were investigated in forests and forest soil carbon, the forest product chain and life-cycle analyses for substitution of conventional products with wood products in a cascade of different modelling approaches and paradigms, and the results synthesized. It was found that material use of products from domestic timber sources has the highest climate change mitigation efficiency when originating from sustainably managed forests regarding biomass stocks. The emissions saved through building up a carbon stock from harvested wood products and through emissions substitution can be as high as ∼20 years of total annual Austrian emissions in 90 years. Additional conservation measures while sustaining sawnwood production and the related GHG benefits at a high level had the highest contribution to an overall carbon sink.