Carbon and Nitrogen in Danish Forest Soils—Contents and Distribution Determined by Soil Order

the C/N ratio is used to characterize living and dead organic matter. The total C accumulated in biomass is Increasing atmospheric CO2 concentrations, and widespread deposition of N to terrestrial ecosystems has increased the focus on soil generally limited by the availability of N. Forest trees C and N pools. The aim of this study was to estimate the size and are often adapted to N limited conditions by mechadistribution of organic C and N pools in well-drained Danish forest nisms that exert tight controls on the N cycles (Melillo, soils. We examined 140 forest soil profiles from pedological surveys 1981; Aber et al., 1989). In these N limited ecosystems, of Danish forest soils. We calculated total C and N pools in organic increased supplies of plant available N from deposition layers and mineral soils to a depth of 1 m. The profiles represent may result in increased biomass production, and consevariations in texture (sandy to loamy), and soil order (USDA soil quently additional fixation of C (Mäkipää et al., 1999). taxonomy Spodosols, Alfisols, Entisols, and Inceptisols). The average total organic C and N contents were 12.5 and 0.61 kg m 2 respectively. Holland et al. (1997) estimated that N deposition alone There were large differences in total C and N among soil orders. might allow for the sequestration of an additional 1.4 Spodosols had the greatest C content (14.6 kg m 2), and Alfisols the 103 to 2 103 Mg C yr 1 worldwide. Therefore, N input least (8.8 kg m 2), while the N content was highest in Alfisols (0.75 was seldom considered harmful in terms of biomass kg m 2) and least in Spodosols (0.51 kg m 2). The main contributor production. However, since the early 1980s, the focus to the high C content in Spodosols is the spodic horizons containing has been on the negative impacts of deposition of atmoilluvial humus, and thick organic horizons. Carbon and N concentraspheric N in forest ecosystems (Gundersen and Rasmustions decreased with soil depth. Soil clay content was negatively correlated to C content and positively correlated to N content. Soil order sen, 1995), including acidification, induction of nutrient and horizon designations may be useful in predicting the total C and deficiencies, leaching of essential nutrients and harmful N content of Danish forest soils, and may also predict potential for elements, requiring attention and countermeasures C sequestration following afforestation of arable land. (Skeffington, 1990). Soil organic matter may diminish because of changes in climate, land use, and land management (Harrison T increasing concentration of atmospheric CO2 et al., 1995; Murillo, 1994; Schlesinger, 1995; Lal et al., has directed focus on global C cycles and pools. 1995). The size of the annual input of organic material Atmospheric and oceanic compartments of global C and decomposition rates are key factors regulating SOM budgets have attracted the most attention, while terpool sizes. Because these factors depend on forest prorestrial systems have been treated as a near neutral ductivity and soil fertility, soil properties are the primary compartment (Eswaran et al., 1995). Because terrestrial factor determining the C content of forest soils within ecosystems constitute major C sinks this has proven climatic regions (same moisture and temperature reinadequate in accounting for all global C sources and gimes as proposed by Soil Taxonomy) (Tate, 1992; Essinks. Several studies have suggested that soils represent waran et al., 1995). a substantial C compartment (Murillo, 1994; Westman The role of SOM as a C and N reservoir, the sensitivity et al., 1994). In ecosystems where net primary producof SOM to environmental changes, and the use of C/N tion exceeds decomposition, net C accumulation takes place, in living biomass above and belowground and in ratios as an indicator of ecosystem stability have necessiorganic matter in the forest floor (O horizons) and soil. tated precise estimations on the soil C and N pools worldSoils may act as significant C sinks because of the accrewide. tion of soil organic matter (SOM) (Eswaran et al., 1995; Our objective in this study was to estimate the size and Cannell and Milne, 1995). Soil C pools may exceed distribution of organic C and N pools in well-drained aboveground terrestrial biomass C pools by up to three Danish forest soils. We gathered all data available; soil times (Eswaran et al., 1993). profiles characteristics, and chemical and physical variBiomass in living plants is accumulated by the autoables that are related to C and N storage in soils, we trophic synthesis of organic compounds. This process then analyzed and discussed these data to establish relaincorporates both C and N in the living biomass making tionships with soil forming processes. the two elements intimately associated in both living We examined hypotheses concerning the relationship and dead organic material. Because of this relationship between accumulation of C and N and soil order; and soil order differences in C and N concentration and H. Vejre, Centre for Forest, Landscape and Planning, Royal VeteriC/N ratio changes with soil depth. Further, we examine nary and Agricultural University, DK-1871 Frederiksberg C, Denmark; I. Callesen, L. Vesterdal, and K. Raulund-Rasmussen, Centre whether the forest floor C and N storage is influenced for Forest, Landscape and Planning, Danish Forest and Landscape by soil order. Research Institute, Hørsholm Kongevej 11 DK-2970 Hørsholm, Denmark. Received 26 June 2001. *Corresponding author ([email protected]). Abbreviations: SOM, soil organic matter. Published in Soil Sci. Soc. Am. J. 67:335–343 (2003).