Response of soil respiration and its components to experimental warming and water addition in a temperate Sitka spruce forest ecosystem

Future climate change is expected to alter the terrestrial carbon cycle through its impact on soil respiration. In this study, we determined the responses of soil respiration and its components to experimental warming with or without water addition. A replicated in situ heating (∼2 °C above ambient soil temperatures) and water addition (170 mm in total each year) experiment was carried out for the first time in a temperate plantation forest of Sitka spruce over the period 2014–2016. R_h was measured inside deep collars (35 cm deep) that excluded root growth, while R_s was measured using the static chamber approach and near-surface collars (5 cm deep) and R_a calculated by subtracting R_h from total soil respiration (R_s). Experimental warming significantly increased R_s, R_h and R_h/R_s, but had no effect on R_a. In contrast, none of the respiration components were affected by water addition. Warming increased annual R_h by 62% but had no effect on R_a. Overall, warming did not significantly increase annual R_s. Warming showed a stronger impact on R_s in the non-growing season but had a smaller impact in the growing season. Warming increased R_a in the non-growing season but decreased it in the growing season. The effects of warming on R_h were similar for the two periods. Our results highlight the differential response of R_a and R_h to warming, which was mediated by water addition or season. For this and other similar forest sites that don't experience water limitation, global warming may have a positive feedback on atmospheric CO₂ concentrations through enhanced soil respiration.