Effects of experimental warming on soil respiration in the semi-arid steppe to taiga forest transition zone in northern Mongolia

Background/Question/Methods

Recent monitoring in the Lake Hövsgöl region of northern Mongolia has shown that mean annual air temperature has increased by 1.8 °C over the last 40 years, greater than global average temperature increases. Although it is hypothesized that warmer temperatures will cause greater soil CO₂ efflux, responses of soil efflux to climate change may differ among ecosystems due to response variations of controlling factors. The objectives of our study were to (1) examine the environmental responses (soil temperature and moisture) to experimental warming, and (2) study the responses of soil CO₂ efflux to experimental warming, in three different ecosystems. Replicate plots with ITEX-style open-top passive warming chambers and non-warmed control areas were installed in three different ecosystems within a single valley: (1) semi-arid grassland on the south-facing slope not underlain by permafrost, (2) riparian zone and (3) larch forest on the north-facing slope underlain by permafrost. Aboveground air temperature was recorded for each treatment (control and OTC) by HOBO dataloggers, and belowground soil temperature and moisture (10 and 20 cm) by EC-TM sensors. Soil CO₂ efflux was measured periodically using a portable infra-red gas analyzer with an attached soil respiration chamber.

Results/Conclusions

Passive warming chambers increased air temperatures in all three ecosystems, with greater warming with greater openness of the canopy. Soil temperature at depths of 10 cm and 20 cm was significantly elevated by chambers in the steppe, less so in the forest, and not in the riparian area. OTCs were wetter than controls in forest and drier in the steppe and riparian plots. The highest soil respiration rates were observed in the riparian plots (0.71 ± 0.01 – 1.13 ± 0.06 g CO₂ m^-2 h^-1), followed by the forest (0.61 ± 0.02 – 0.91 ± 0.02 g CO₂ m^-2 h^-1), and the steppe plots (0.43 ± 0.01 – 0.60 ± 0.01 g CO₂ m^-2 h^-1).  Soil efflux measurements suggest that the warming by chambers had little effect on soil respiration, but that alteration of soil moisture by the chambers had a stronger effect. However, the relative effects of soil temperature and moisture on soil respiration varied across ecosystems. Overall, our results suggest that respiration responses to climate change in the steppe-taiga transition zone in northern Mongolia will likely differ depending on microclimate and habitat. Predicted precipitation change may have a stronger effect than the predicted temperature change in Mongolia.