Experimental forest soil warming: response of autotrophic and heterotrophic soil respiration to a short-term 10°C temperature rise

We warmed the top soil of a mature coniferous forest stand by means of heating cables on control and trenched plots within 24 h by 10°C at 1 cm soil depth (9°C at 5 cm depth) and measured the effect on the autotrophic (RA) and heterotrophic (RH) component of total soil CO2 efflux (RS). The short time frame of warming enabled us to exclude confounding fluctuations in soil moisture and carbon (C) flow from the canopy. The results of the field study were backed up by a lab soil incubation experiment. During the first 12 h of warming, RA strongly responded to soil warming; The Q10 values were 5.61 and 6.29 for 1 and 5 cm soil depth temperature. The Q10 values for RA were almost twice as high as the Q10 values of RH (3.04 and 3.53). Q10 values above 5 are above reasonable plant physiological values for root respiration. We see interactions of roots, mycorrhizae and heterotrophic microbes, combined with fast substrate supply to the rhizosphere as an explanation for the high short-term temperature response of RA. When calculated over the whole duration (24 h) of the field soil-warming experiment, temperature sensitivities of RA and RH were similar (no significant difference at P < 0.05); Q10 values were 3.16 and 3.96 for RA and 2.94 and 3.35 for RH calculated with soil temperatures at 1 and 5 cm soil depth, respectively. Laboratory incubation showed that different soil moisture contents of trenched and control plots affected rates of RH, but did not affect the temperature sensitivity of RH. We conclude that a single parameter is sufficient to describe the temperature sensitivity of RS in soil C models which operate on larger temporal and spatial scales. The strong short-term response of RA may be of relevance in soils suspected to experience increasingly strong diurnal temperature variations.