Responses of Trace Gas Fluxes and N Availability to Experimentally Elevated Soil Temperatures

Abstract

We are conducting a field study to determine the long—term response of belowground processes to elevated soil temperatures in a mixed deciduous forest. We established 18 experimental plots and randomly assigned them to one of three treatments in six blocks. The treatments are: (1) heated plots in which the soil temperature is raised 5°C above ambient using buried heating cables; (2) disturbance control plots (cables but no heat); and (3) undisturbed control plots (no cables and no heat). In each plot we measured indexes of N availability, the concentration of N in soil solutions leaching below the rooting zone, and trace gas emissions (CO 2, N 2 O, and CH 4). In this paper we present results from the first 6 mo of this study. The daily average efflux of CO 2 increased exponentially with increasing soil temperature and decreased linearly with increasing soil moisture. A linear regression of temperature and the natural logarithm of CO 2 flux explained 92% of the variability. A linear regression of soil moisture and CO 2 flux could explain only 44% of the variability. The relationship between soil temperature and CO 2 flux is in good agreement with the Arrhenius equation. For these CO 2 flux data, the activation energy was 63 kJ/mol and the Q 10 was 2.5. The daily average uptake of CH 4 increased linearly with increasing soil temperatures and decreased linearly with increasing soil moisture. Linear regression could explain 46% of the variability in the relationship between temperature and CH 4 uptake and 49% of the variability in the relationship between soil moisture and CH 4 uptake. We predicted the annual CO 2 flux from our study site in 1991 using two empirical relationships: the relationship between air temperature and soil temperature, and the relationship between soil temperature and CO 2 flux. We estimate that the annual CO 2—C flux in 1991 was 712 g/m 2 from unheated soil and 1250 g/m 2 from heated soil. By elevating the soil temperature 5°C above ambient, we estimate that an additional carbon flux of 538 g°m—2°yr—1 was released from the soil as CO 2.