Effects of warming, wetting and nitrogen addition on substrate-induced respiration and temperature sensitivity of heterotrophic respiration in a temperate forest soil

Soil heterotrophic respiration and its temperature sensitivity are affected by various climatic and environmental factors. However, little is known about the combined effects of concurrent climatic and environmental changes, such as climatic warming, changing precipitation regimes, and increasing nitrogen (N) deposition. Therefore, in this study, we investigated the individual and combined effects of warming, wetting, and N addition on soil heterotrophic respiration and temperature sensitivity. We incubated soils collected from a temperate forest in South Korea for 60 d at two temperature levels (15 and 20 degrees C, representing the annual mean temperature of the study site and 5 degrees C warming, respectively), three moisture levels (10%, 28%, and 50% water-filled pore space (WFPS), representing dry, moist, and wet conditions, respectively), and two N levels (without N and with N addition equivalent to 50 kg N ha(-1) year(-1)). On day 30, soils were distributed across five different temperatures (10, 15, 20, 25, and 30 degrees C) for 24 h to determine short-term changes in temperature sensitivity (Q(10), change in respiration with 10 degrees C increase in temperature) of soil heterotrophic respiration. After completing the incubation on day 60, we measured substrate-induced respiration (SIR) by adding six labile substrates to the three types of treatments. Wetting treatment (increase from 28% to 50% WFPS) reduced SIR by 40.8% (3.77 to 2.23 mu g CO2-C g(-1) h(-1)), but warming (increase from 15 to 20 degrees C) and N addition increased SIR by 47.7% (3.77 to 5.57 mu g CO2-C g(-1) h(-1)) and 42.0% (3.77 to 5.35 mu g CO2-C g(-1) h(-1)), respectively. A combination of any two treatments did not affect SIR, but the combination of three treatments reduced SIR by 42.4% (3.70 to 2.20 mu g CO2-C g(-1) h(-1)). Wetting treatment increased Q(10) by 25.0% (2.4 to 3.0). However, warming and N addition reduced Q(10) by 37.5% (2.4 to 1.5) and 16.7% (2.4 to 2.0), respectively. Warming coupled with wetting did not significantly change Q10, while warming coupled with N addition reduced Q(10) by 33.3% (2.4 to 1.6). The combination of three treatments increased Q(10) by 12.5% (2.4 to 2.7). Our results demonstrated that among the three factors, soil moisture is the most important one controlling SIR and Q(10). The results suggest that the effect of warming on SIR and Q(10) can be modified significantly by rainfall variability and elevated N availability. Therefore, this study emphasizes that concurrent climatic and environmental changes, such as increasing rainfall variability and N deposition, should be considered when predicting changes induced by warming in soil respiration and its temperature sensitivity.