B4WarmED forest warming experiment: Effects of soil moisture on soil organic matter decomposition

Background/Question/Methods

One of the largest potential positive feedbacks to climate warming is CO₂ arising from the enhanced decomposition of soil organic matter (SOM) in warmer soils. Drier soils in a warmer world, however, could alter the response of SOM decomposition to warming. In upland soils, where microbial decomposition of SOM can be limited by water availability, soil drying has the potential to decrease the response of SOM decomposition to warming. The goal of this study was to determine the effect of soil moisture on the temperature response of soil respiration at the B4WarmED (Boreal Forest Warming at an Ecotone in Danger) warming experiment at two upland southern boreal forest sites in northern Minnesota. This open-air warming experiment uses infrared heat lamps and soil heating cables to heat 7-m² plots continuously during the growing season by 1.7 or 3.4°C.  Experimental warming treatments were initiated in 2009. In June 2012, we established a rain removal manipulation factorially with the higher level of warming treatment using rainout shelters that were deployed during precipitation events for the rest of the growing season to achieve a reduction in growing season precipitation of ca. 40%. We measured soil CO₂flux every two weeks during this time period.

In conjunction with the field portion, a laboratory soil incubation used repeated measurements of the temperature response of microbial respiration with southern boreal forest soils incubated at three different moisture contents to determine the relative importance of soil moisture in controlling the release of CO₂ from warmer soils.

Results/Conclusions

Experimental warming stimulated soil CO₂ flux in the first three years of heat treatment (2009 – 2011). By the fourth year of experimental warming (2012), however, there was no effect of warming treatment on soil CO₂ flux. The rain removal manipulation removed the target level of 40% of precipitation from both of the sites during June to September 2012. Soil CO₂ flux decreased significantly in the rain removal treatments, but rain removal did not alter the response of soil CO₂ flux to warming. In contrast, evidence from the laboratory incubations demonstrated that soil respiration increased less with warming in drier soils, indicating that warming-induced soil drying could decrease the potential warming enhancement of soil organic matter decomposition. Taken together, these results suggest that the drying of upland soils in a warmer world may limit the response of SOM decomposition to warming.