Mineralization and carbon turnover in subarctic heath soil as affected by warming and additional litter

Abstract

Arctic soil carbon (C) stocks are threatened by the rapidly advancing global warming. In addition to temperature, increasing amounts of leaf litter fall following from the expansion of deciduous shrubs and trees in northern ecosystems may alter biogeochemical cycling of C and nutrients. Our aim was to assess how factorial warming and litter addition in a long-term field experiment on a subarctic heath affect resource limitation of soil microbial communities (measured by thymidine and leucine incorporation techniques), net growing-season mineralization of nitrogen (N) and phosphorus (P), and carbon turnover (measured as changes in the pools during a growing-season-long field incubation of soil cores in situ). The mainly N limited bacterial communities had shifted slightly towards limitation by C and P in response to seven growing seasons of warming. This and the significantly increased bacterial growth rate under warming may partly explain the observed higher C loss from the warmed soil. This is furthermore consistent with the less dramatic increase in the contents of dissolved organic carbon (DOC) and dissolved organic N (DON) in the warmed soil than in the soil from ambient temperature during the field incubation. The added litter did not affect the carbon content, but it was a source of nutrients to the soil, and it also tended to increase bacterial growth rate and net mineralization of P. The inorganic N pool decreased during the field incubation of soil cores, especially in the separate warming and litter addition treatments, while gross mineralized N was immobilized in the biomass of microbes and plants transplanted into the incubates soil cores, but without any significant effect of the treatments. The effects of warming plus litter addition on bacterial growth rates and of warming on C and N transformations during field incubation suggest that microbial activity is an important control on the carbon balance of arctic soils under climate change.

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.