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 (CO2, N2O, and CH4). In this paper we present results from the first 6 mo of this study. The daily average efflux of CO2 increased exponentially with increasing soil temperature and decreased linearly with increasing soil moisture. A linear regression of temperature and the natural logarithm of CO2 flux explained 92% of the variability. A linear regression of soil moisture and CO2 flux could explain only 44% of the variability. The relationship between soil temperature and CO2 flux is in good agreement with the Arrhenius equation. For these CO2 flux data, the activation energy was 63 kJ/mol and the Q10 was 2.5. The daily average uptake of CH4 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 CH4 uptake and 49% of the variability in the relationship between soil moisture and CH4 uptake. We predicted the annual CO2 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 CO2 flux. We estimate that the annual CO2—C flux in 1991 was 712 g/m2 from unheated soil and 1250 g/m2 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 CO2.

Soil solution and extractable soil nitrogen response to climate change in two boreal forest ecosystems

“…Neither in CO 2 T-T nor in T-T did soil solution NH 4 concentrations reflect patterns in soil extractable NH 4 ; in CO 2 T-T NH 4 concentrations decreased despite an increase in extractable NH 4 while in T-T soil solution NH 4 increased even though extractable NH 4 did not change. The patterns found in CO 2 T-T were similar to those observed in Harvard forest (Peterjohn et al 1994). Behavior of dissolved NH 4 is more complicated than that of NO 3 since NH 4 more easily adsorbs to the soil than NO 3 ; soil solution NO 3 concentrations were four to five times higher than NH 4 concentrations even though soil extractable NO 3 concentrations were ten times lower.…”

“…Despite these general patterns, effects of elevated temperature on soil nutrient pools are less clear. Few data are available addressing the effects of warming on soil extractable N. Peterjohn et al (1994) measured increases in soil inorganic N pools in response to warming. Shaw and Harte (2001) measured no increase despite increases in N mineralization rates, which was ascribed to increased plant uptake.…”

“…Shaw and Harte (2001) measured no increase despite increases in N mineralization rates, which was ascribed to increased plant uptake. Effects of warming on soil solution concentrations of dissolved inorganic N (DIN; NH 4 and NO 3 ) have been mixed as well with studies reporting increases (Joslin and Wolfe 1993;McHale and Mitchell 1996), decreases (Ineson et al 1998) or no change (Peterjohn et al 1994) in soil solution DIN concentrations upon soil heating. Rustad et al (2000) hypothesized that soil solution DIN concentrations will increase only in N-saturated ecosystems, whereas most N will be taken up by vegetation in N-limited ecosystems.…”

Elevated CO2and warming effects on CH4uptake in a semiarid grassland below optimum soil moisture

“…While CH 4 uptake often decreases in response to elevated CO 2 and increases in response to warming under wetter soil conditions [Peterjohn et al, 1994;Ineson et al, 1998;Ambus and Robertson, 1999;Phillips et al, 2001;McLain et al, 2002;Sjögersten and Wookey, 2002;McLain and Ahmann, 2008], we observed the opposite during times when soil moisture was below the optimum soil moisture content for CH 4 uptake ( Figure 6). Indeed, CH 4 uptake responses to elevated CO 2 and warming effects were more sensitive under dry soil conditions.…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…These N additions are within the critical load of N for sensitive ecosystems [8,9]. This N loading may be further exacerbated by increases in N availability from climate influences (i.e., warming and elevated CO 2 ) on internal N cycling, which can increase decomposition, mineralization, and biological nitrogen fixation (BNF) [10][11][12]. The consequences of N loading are well known and include changes in biodiversity [7,13,14], composition [15,16], productivity [17], leaching [18,19], and possibly nitrification [17].…”