Terrestrial vegetation is often limited by nutrients, particularly nitrogen and/or phosphorus, and in some cases potassium, calcium, sulfur, magnesium, silica, and other micronutrients or trace minerals (Vitousek and Howarth, 1991). On a global scale, Fisher et al. (2012) found the average reduction in terrestrial plant productivity due to nutrient limitation to be between 16% and 28% (Figure 3.1). One major factor altering patterns of nutrient cycling is land use change due to deforestation. Deforestation alters nitrogen and phosphorus cycling, both of which can feed back to affect the carbon cycle and atmospheric CO2 concentrations. In this chapter, we examine carbon, nitrogen, and phosphorus cycling beneath forests and the effect of deforestation on these cycles.
In this section, we briefly describe the carbon cycle in undisturbed forests to provide the basis for understanding how deforestation alters this cycle. Next, we examine global pools of carbon stored in forests and the soils beneath these forests as well as the rate at which forests across different latitudinal belts take up carbon. Finally, we consider how deforestation might alter the carbon balance of forests and forest soils.
Carbon Cycle in Undisturbed Forests
Gross primary production (GPP) of an ecosystem represents the gross uptake of atmospheric CO2 that is used for photosynthesis. Plants use energy in the synthesis of new plant tissue and the maintenance of living tissues (Luyssaert et al., 2007). Because of the costs associated with growth and maintenance of leaves, wood, and roots, some photoassimilated compounds are lost from the ecosystem as autotrophic respiration (Ra) (Figure 3.2; Luyssaert et al., 2007). The fraction that is used for maintenance respiration can vary widely (i.e., 0.23–0.83 for different forest types as determined from a literature review of 60 different studies), yet in many forest studies it is generally assumed to be a constant value at 0.5 (DeLucia et al., 2007). The energy that is not used for respiration is the net primary production (NPP) and is equal to GPP – Ra. A large fraction of NPP is used in the production of leaves, wood, and roots and a portion of the standing biomass is transferred annually to litter (Luyssaert et al., 2007). This carbon enters the soil when C contained in leaf and woody litter, dead roots, mycorrhizal turnover, and carbon exudates from roots are transferred to the forest soil (Figure 3.2).