Soil carbon dioxide fluxes and profile concentrations in two boreal forests

Because a large fraction of the world’s carbon exists in the soil of boreal forests, understanding how soil temperature and moisture affect soil respiration is vital for predicting soil response to climate change. We measured soil respiration and CO2 concentrations within soils of floodplain and upland forests in interior Alaska from 1996 to 1997. At each site, a 0.10-ha-area shelter was constructed that prevents summer precipitation from infiltrating into the soil. Measurements of soil profile CO2, soil respiration, soil temperature, and soil moisture were made inside (treatment) and outside (control) the sheltered areas through two growing seasons and the winter of 1996–1997. Sheltered soils had decreased profile concentrations and surface flux of CO2. At the upland control site, individual flux rates ranged from 0.10 to 0.95 g·m–2·h–1 in the summer and at sites under the shelter from 0.10 to 0.53 g·m–2·h–1. Rates at the floodplain control site ranged from 0.11 to 1.45 g·m–2·h–1 and under the shelter from 0.11 to 0.55 g·m–2·h–1. Fick’s Law could predict surface CO2 flux when the CO2 concentration gradient within the profile accurately represented the soil surface gradient and biological sources and sinks of the gas did not overwhelm flux calculations. Résumé : Étant donné qu’une fraction importante du carbone dans le monde se trouve dans le sol des forêts boréales, il est vital de comprendre de quelle façon la température et l’humidité du sol affectent la respiration du sol pour prédire la réaction du sol à un changement climatique. Nous avons mesuré la respiration du sol et les concentrations de CO2 dans le sol de forêts situées sur une plaine alluviale et sur un plateau de l’intérieur de l’Alaska de 1996 à 1997. À chaque endroit, un abri d’une superficie de 0,10 ha a été construit pour empêcher les précipitations estivales de s’infiltrer dans le sol. Les mesures du CO2 dans le profil de sol, de la respiration, de la température et de l’humidité du sol ont été prises à l’intérieur (traitement) et à l’extérieur (témoin) des sites couverts pendant deux saisons de croissance et pendant l’hiver 1996–1997. Les sols protégés avaient des concentrations de CO2 dans le profil et un flux de CO2 en surface plus faibles. Pendant l’été sur le plateau, les taux individuels de flux variaient de 0,10 à 0,95 g·m–2·h–1 dans les zones témoins et de 0,10 à 0,53 g·m–2·h–1 dans les zones sous abri. Sur la plaine alluviale, les taux variaient de 0,11 à 1,45 g·m–2·h–1 dans les zones témoins et de 0,11 à 0,55 g·m–2·h–1 dans les zones sous abri. La loi de Fick pouvait prédire le flux de CO2 en surface lorsque le gradient de concentration de CO2 dans le profil représentait fidèlement le gradient à la surface du sol et que les sources et les puits biologiques de CO2 ne venaient pas fausser les calculs de flux. [Traduit par la Rédaction] Billings et al. 1783 Between 16 and 24% of the world’s soil carbon is in the vast boreal forest (Gates 1993), which comprises 1.2 × 109 ha (Walter and Breckle 1986). This estimate is conservative because of significant, unquantified reservoirs of carbon immobilized in permafrost (Billings 1987). Any future changes in climate could increase depth of thaw in these soils, thus influencing soil water availability in this region. Increased depth of thaw and altered soil moisture will affect rooting depth, root respiration, and soil microbial community composition. Under such a scenario, soil respiration could be altered dramatically. This could have a significant effect on the global carbon cycle (Billings 1987, 1995). Because soil respiration is an indicator of the biological activity occurring in a soil, monitoring soil respiration in conjunction with related soil parameters can help us evaluate how microbial and root activity in active-layer soils may respond to a changed climate (Schlentner and Van Cleve 1985; Thierron and Laudelout 1996). Soil CO2 efflux has been studied in boreal (Gordon et al. 1987; Schlentner and Van Cleve 1985; Weber 1985), arctic (Billings et al. 1982, 1984), temperate (Wiant 1967; Witkamp 1969; Raich and Schlesinger 1992; Solomon and Cerling 1987; Landsberg 1986), and tropical systems (Trumbore et al. 1995). However, few of these studies have examined the effects of experimentally altering water availability on both soil profile CO2 dynamics and surface efflux. Measurements of this kind are especially important in the boreal zone, where depth of soil thaw affects water availability and both gas profile and surface fluxes. Johnson (1993) discussed the need for studying factors controlling forest soil CO2 fluxes such as soil porosity. This Can. J. For. Res. 28: 1773–1783 (1998) © 1998 NRC Canada 1773 Received December 22, 1997. Accepted August 15, 1998. S.A. Billings1 and D.D. Richter. Nicholas School of the Environment, Duke University, Durham, NC 27708, U.S.A. J.Yarie. School of Agriculture and Land Resources Management, University of Alaska Fairbanks, Fairbanks, AK 99775, U.S.A. 1Author to whom all correspondence should be addressed. e-mail: [email protected] I:\cjfr\cjfr28\cjfr-12\X98-145.vp Wednesday, January 20, 1999 1:23:48 PM Color profile: Disabled Composite Default screen