Mineralization and carbon turnover in subarctic heath soil as affected by warming and additional litter
Introduction
Soils of the arctic ecosystems contain approximately 14% of the global soil carbon (C) stock (Post et al., 1982). Even if the most recalcitrant fractions are excluded, the carbon amount per unit area is 50% higher than the global average (Jonasson et al., 2001). These soils also contain a large nitrogen (N) store, although the availability to plants is dependent on microbial mineralization and immobilization of nutrients (Jonasson et al., 2001).
The Arctic is experiencing rapidly advancing global warming, which can have a serious impact on the biogeochemical cycling of carbon and nutrients in the soil. If the predicted 3–5 °C increase in the annual mean temperature over the next 100 years (ACIA, 2004) leads to enhanced decomposition and mineralization of nutrients, higher availability of nutrients together with direct temperature effects can support higher plant biomass, but at the same time, the higher decomposition may lead to losses of carbon to the atmosphere (Mack et al., 2004).
Warmer temperatures are not only expected to increase plant biomass, but also to alter the plant community composition. It is already evident, that the abundance of deciduous shrubs is increasing (Sturm et al., 2001; Stow et al., 2004), and that the mountain birch tree-line has risen northwards and towards higher altitude (Kullman, 2003). These trends are predicted to further intensify in response to global warming (ACIA, 2004; van Wijk et al., 2004; Walker et al., 2006), which leads to higher amount of litter on the ground. It is not well known how the additional litter affects microbial processes in soil and whether it will increase or decrease the losses of soil C. Litter is the major source of soil organic matter, but an introduction of this additional substrate to soil may also induce a priming effect, that is, stimulated decomposition of the recalcitrant SOM fractions in soil (Kuzyakov et al., 2000). Evidence for a priming effect after litter addition has been observed in coniferous forests (Subke et al., 2004).
Warming itself, higher nutrient availability and the increased amount of litter can all potentially alter resource limitation of soil bacteria. Soil bacteria are in general considered to be C limited. However, recent evidence shows that this may not always be the case in the Arctic, where bacteria appear N or phosphorus (P) limited (Nordin et al., 2004; Rinnan et al., 2007). If the higher nutrient availability following warmer temperatures and more available litter induces a shift in resource limitation from N or P to C, increased consumption of the soil C stock and release of CO2 can be expected. This would be a serious feedback on climate change.
In this work, we assessed how increasing temperatures in the Arctic directly and indirectly, via the increased litter inputs, affect the mineralization of N, P and C. This was done by a field incubation of soil modified from the traditional buried bag method (Eno, 1960) in a long-term field experiment with factorial warming and litter addition treatments. In addition, we determined whether warming and litter addition affect the soil bacterial growth rate by measuring incorporation of radioactive precursors of bacterial DNA (thymidine) and proteins (leucine) into bacterial cells (Bååth, 1992, Bååth, 1994). The same technique was used to reveal how warming and litter addition affect the patterns of substrate limitation of bacterial growth.
We hypothesized that bacterial growth would be higher under elevated temperature, and that this would result in enhanced mineralization of nutrients. We also expected enhanced carbon losses from the warmed soil. The additional mountain birch litter was thought to serve as an extra source of energy and nutrients, and thus it was hypothesized to stimulate bacterial activity and to alter the resource limitation of the bacteria.
Section snippets
Experimental site
Soil processes were investigated at a field experiment located close to the Abisko Scientific Research Station in subarctic Sweden (68°21′N, 18°49′E). The experiment has been running since 1999 with warming and litter addition as factorially applied treatments arranged in six blocks. The warming treatment was achieved with dome-shaped open-top plastic tents, which increased the air temperature by 3–4 °C and the soil temperature by approximately 1 °C. Plots in the litter addition treatment
Soil characteristics
The soil characteristics after 6 years of warming manipulation during the growing season and yearly litter additions were mainly affected by litter addition (Table 1). Litter addition significantly increased the concentrations of DON (P<0.01), microbial biomass P (P<0.05) and total P (P<0.05).
There were no treatment effects on bulk density, water content, soil organic matter, root biomass, inorganic N and P, microbial biomass C and N, DOC, or total C and N (Table 1). The ratios of microbial
Discussion
Realistic warming of ecosystem plots over six growing seasons had no effects on soil chemical and microbial characteristics when these were measured in June, after a long winter season without temperature manipulations. The lack of significant warming effects on microbial biomass is largely in agreement with previous results from a slightly drier nearby heath exposed to a similar period of warming (Jonasson et al., 1999) as well as with results from wet sedge and mesic tussock tundra in Alaska (
Acknowledgments
This study was financially supported by the Academy of Finland and the European Commission Marie Curie Intra-European fellowship to R. Rinnan. The Danish Natural Science Research Council supported the set-up and maintenance of the site, the EU ATANS grant supported logistics and Abisko Scientific Research station provided facilities for the fieldwork. We are grateful to Fredrik Demoling for assistance in the limiting factor assay, and to Gosha Sylvester, Esben Vedel Nielsen and Karna Heinsen
References (41)
Thymidine incorporation into macromolecules of bacteria extracted from soil by homogenization-centrifugation
Soil Biology & Biochemistry
(1992)- et al.
Adaptation of a rapid and economical microcentrifugation method to measure thymidine and leucine incorporation by soil bacteria
Soil Biology & Biochemistry
(2001) - et al.
Can the extent of degradation of soil fungal mycelium during soil incubation be used to estimate ectomycorrhizal biomass in soil?
Soil Biology & Biochemistry
(2004) - et al.
Organic matter manipulations have little effect on gross and net nitrogen transformations in two temperate forest mineral soils in the USA and central Europe
Forest Ecology and Management
(2005) - et al.
The effect of biocidal treatments on metabolism in soil. V. A method of measuring soil biomass
Soil Biology & Biochemistry
(1976) - et al.
Biogeochemistry in the Arctic: patterns, processes and controls
- et al.
Litter, warming and plants affect respiration and allocation of soil microbial and plant C, N and P in arctic mesocosms
Soil Biology & Biochemistry
(2004) - et al.
Interactions between plants, litter and microbes in cycling of nitrogen and phosphorus in the arctic
Soil Biology & Biochemistry
(2006) Recent reversal of Neoglacial climate cooling trend in the Swedish Scandes as evidenced by mountain birch tree-limit rise
Global and Planetary Change
(2003)- et al.
Review of mechanisms and quantification of priming effects
Soil Biology & Biochemistry
(2000)
Comparative aspects of cycling of organic C, N, S and P through soil organic matter
Geoderma
Remote sensing of vegetation and land-cover change in Arctic Tundra Ecosystems
Remote Sensing of Environment
An extraction method for measuring soil microbial biomass C
Soil Biology & Biochemistry
Impacts of a Warming Arctic: Arctic Climate Impact Assessment
Rapid method of determining factors limiting bacterial growth in soil
Applied and Environmental Microbiology
Chemical Analysis of Ecological Material
Measurement of protein synthesis by soil bacterial assemblages with the leucine incorporation technique
Biology and Fertility of Soils
Temperature-dependent shift from labile to recalcitrant carbon sources of arctic heterotrophs
Rapid Communications in Mass Spectrometry
Plant functional types as predictors of transient responses of arctic vegetation to global change
Journal of Vegetation Science
Increased ectomycorrhizal fungal abundance after long-term fertilization and warming of two arctic tundra ecosystems
The New Phytologist
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