DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY Use of Carbon-13 and Carbon-14 to Measure the Effects of Carbon Dioxide and Nitrogen Fertilization on Carbon Dynamics in Ponderosa Pine

elevated CO2 (Allen et al., 2000) but no statistically significant differences in fine root production, microbial Soil C sequestration in predicted, future elevated CO2 environbiomass, or plant chemistry. Growth in elevated CO2 ments will be important to atmospheric CO2 levels, soil tilth, and fertility. An elevated CO2 study with ponderosa pines (Pinus poncan lead to changed plant species composition and litter derosa Laws) grown in chambers produced above ground vegetation quality and quantity (Oene et al., 1999). This together with a d13C of 244‰ and roots with 242‰. This together with carbon with changes in other constituents can alter decomposidating made it possible to follow soil C dynamics. Fifty percent of the tion of litter and C cycling back to the atmosphere California upland soil C, resistant to acid hydrolysis, was designated as (Strain and Thomas, 1992). Increases in N can act the resistant fraction. Carbon dating showed the mean residence times through enhanced plant growth, changes in litter quality, of this fraction to be 400 to 1500 yr greater than the total soil C for and either positive or negative alteration of in situ dethe horizons sampled. Young ponderosa pines grown in CO2 chambers composition rates (Fogg, 1988). The response of trees to produced negligible leaf litter. There were 32% more roots in the elevated atmospheric C and N fertilization is especially presence of either added N or double CO2 but 77% more in the presence of both. Root-derived soil C was equivalent to 10% of the important in that forests are considered to be major root C after the 6-yr growth period. Analysis of laboratory CO2 evolupotential C sinks in global change scenarios (Schimel, tion during extended incubation showed the active soil C pool repre1995; Fan et al., 1998). Atmospheric N may enhance sented 1 to 2% of the soil C with a field-equivalent mean residence both tree growth and soil C sequestration; it can also time (MRT) of 24 to 53 d. The slow pool represented 46 to 52% of contribute to forest decline (Nadelhoffer et al., 1999). the C with MRT of 24 to 67 yr depending on treatment and soil depth. Elevated CO2 concentration in the soil atmosphere has Analysis of the CO2 label during incubation from the elevated CO2 resulted in increased ecosystem C uptake but also has treatments, showed the root-derived 13C of the active fraction to have increased the rate of C cycling (Hungate et al., 1997). residence times similar to those of the total soil non labeled C at Soils may also act as C sinks under elevated CO2 partial ≈35 d. Root-derived C of the slow pool at 10 yr MRT turned over three to four time as fast as the general soil C. The 13C of the light pressures (Leavitt et al., 1994; Dixon et al., 1994). The fraction (LF), showed it to be most closely associated with the active changes in soil organic carbon (SOC) concentrations can pool. The particulate organic matter (POM) was part of the slow pool best be measured with experiments that measure differas determined with incubation. ences in SOC pool sizes and fluxes in response to management. Acid hydrolysis, together with C dating of the resistant fraction, can be used to determine the old resistant T increase in concentration of CO2 in the earth’s (Cr) fraction of SOC (Campbell et al., 1967; Martel and atmosphere by 1.5 mL L21 annually together with Paul, 1974; Trumbore, 1993). Carbon dating applied to increases in atmospheric N deposition in many parts a range of North American grassland soils (Paul et al., of the world has the potential for altering numerous 1997) has shown the MRT of SOC to be dependent on ecosystem processes (Hungate et al., 1997; Watson et al., climate, landscape position, soil management, and soil 1990; Trabalka, 1985). Elevated CO2 and N fertilization forming processes. The SOC pool resistant to acid hydrotreatments should increase below ground production lysis is generally 1200 yr older than the total SOC; MRT and root and microbial respiration, if other resources also increases rapidly with depth. Curve fitting of the (moisture, nutrients, etc.) are not limiting (Johnson et CO2 respiration data is used to estimate the pool sizes al., 1994; Strain and Thomas, 1992). Growth of loblolly and decomposition rate constants of the active (Ca) and pine (Pinus taeda L.) in 560 mL L21 CO2 in the Duke slow (Cs) pools (Collins et al., 2000; Paul et al., 2000). FACE experiment resulted in a 25% increase in producLong-term mineralization of soil C by microorganisms in tion (DeLucia et al., 1999). This was reflected in a 15% extended laboratory incubation allows the soil biota and increase in litter fall in the second year of exposure to its enzymes to act on the soil constituents, resulting in CO2 evolution. Plotting the CO2 efflux data on the basis of per unit time helps to determine the in situ availability S. Haile-Mariam and E.A. Paul, Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824-1325; W. Cheng, Enviof C to the soil microbiota and provides statistical paramronmental Studies Dep., Univ. of California, Santa Cruz, CA 95064; eters (Hess and Schmidt, 1995). D.W. Johnson, Earth and Ecosystem Science, Desert Research InstiAnthropogenic CO2 used in elevated atmospheric tute, 2215 Raggio Parkway, Reno, NV 89512; J.T. Ball, Biological Science Center, Desert Research Institute, Reno, NV 89512. Received Abbreviations: FACE, free-air CO2 enrichment; LF, light fraction; 20 Sept. 1999. *Corresponding author ([email protected]). MRT, mean residence time; POM, particulate organic matter; SOC, soil organic carbon. Published in Soil Sci. Soc. Am. J. 64:1984–1993 (2000).