Environ. Sci. Technol. 2006, 40, 2552-2558
Partitioning CO2 Effluxes from an Atlantic Pine Forest Soil between Endogenous Soil Organic Matter and Recently Incorporated 13C-Enriched Plant Material IRENE FERNANDEZ,* ANA CABANEIRO, AND S E R A F IÄ N J . G O N Z AÄ L E Z - P R I E T O Departamento de Bioquı´mica del Suelo, Instituto de Investigaciones Agrobiolo´gicas de Galicia, Consejo Superior de Investigaciones Cientı´ficas (CSIC), Apartado 122, E-15780 Santiago de Compostela, Spain
Soil CO2 effluxes from recently added 13C-labeled phytomass versus endogenous soil organic matter (SOM) were studied in an acid soil from Atlantic pine forests (NW Spain). After several cultures to incorporate fresh 13Cenriched Lolium perenne to a Humic Cambisol with predominance of humus-Al over humus-Fe complexes, potential soil C mineralization was determined by laboratory aerobic incubation (84 days). Isotopic 13C analyses of SOM fractions were assessed to know in which organic compartments the 13C was preferentially incorporated. Although in the 13C-labeled soil the C mineralization coefficient totalized less than 3% of soil C, the 13C mineralization coefficient exceeded 14%, indicating a greater lability of the newly incorporated organic matter. Organic compounds coming from added phytomass showed a higher lability and contributed considerably to the total soil CO2 effluxes (52% of total soil CO2 evolved during the first decomposition stages and 27% at the end), even though added-C comprised less than 4% of total soil C. Good determination coefficients, when values of CO2-C released were fitted to a firstorder double exponential kinetic model, support the existence of two C pools of different lability. Kinetic parameters obtained with this model indicated that phytomass addition augmented the biodegradability of the labile pool (instantaneous mineralization rate k increased from 0.07 d-1 to 0.12 d-1) but diminished that of the recalcitrant pool (instantaneous mineralization rate h decreased from 2.7‚10-4 d-1 to 1.6‚10-4 d-1). Consequently, the differentiation between both SOM pools increased, showing the importance of SOM quality on CO2 emissions from this kind of soil to the atmosphere.
Introduction Soil organic matter (SOM) comprises several compartments of different biochemical composition, biological stability, and C turnover (1). Humic matter is an important component of the passive highly stable SOM pool, and it is the key factor in stabilization, accumulation, and dynamics of organic C in soils (2). SOM also strongly influence sustainability, fertility, * Corresponding author phone: 34-981590958; fax: 34-981592504; e-mail:
[email protected]. 2552
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 8, 2006
and productivity of forest ecosystems (3, 4), particularly in acidic and sandy soils where fertility is mainly determined by SOM content and quality. Therefore, understanding SOM composition and the contributions of different organic pools to the total CO2 effluxes from forest soils as well as the mechanisms involved in SOM mineralization is important and influence many forest and environmental processes from a climate change perspective. Formerly, C isotopic studies on SOM turnover were mainly carried out using 14C radioactive isotope (5-7) before environmental restrictions to use it, whereas at present most of C isotope research on soils to study the C cycle is done with the stable isotope 13C at natural abundance levels (812) and using artificially 13C-enriched (13-15) or 13C-depleted (16) soils. Publications where C4 plants were introduced as isotopic tracers into soils developed under C3 vegetation are quite common (17-19), but papers using 13C-enriched C3 plants to follow soil decomposition processes (15, 20) or to know from which SOM pools CO2 effluxes are coming from are less frequent. The use of highly 13C-enriched plant material can provide a method to track the fate of organic inputs into the diverse SOM fractions as well as to monitor CO2 effluxes from the different SOM pools and their changes during biodegradation processes. In this research Lolium perenne, an herbaceous C3 species commonly used in revegetation experiments, was chosen to obtain the 13C-enriched phytomass; its composition, with large proportion of N-containing compounds, such as proteins, amino acids, nucleic acids, etc. (21) and low fiber content (lignin, cellulose) ensures higher biodegradability and earlier SOM incorporation than other more lignified species. The main objective of this research was to estimate SOM turnover rates and to contribute to a better understanding of the soil CO2 emissions to the atmosphere, by quantifying the CO2 effluxes coming from recent or old soil organic compounds in Atlantic pine forests with predominance of humus-Al complexes in their SOM.
Experimental Section Soil Analyses. Methods described by Guitia´n Ojea and Carballas (22) were used to determine the following soil properties. Soil texture was determined (
