The Effects of the N-Fixing Tree Pentaclethra macroloba on the Above and Below Ground Communities Within a Primary Forest in the Northern Zone of Costa Rica

The soil microbial community is not only critical to nutrient recycling and mineralization of organic material, but also plays a fundamental role in influencing plant community composition (Kent & Triplett 2002, Buckley & Schmidt 2003, Leininger et al. 2006, Wardle et al. 2004, Ibekwe et al. 2007, Fierer et al. 2007, Litton & Giardina 2008). Conversely, differences in vegetation community also affect the soil fungal and bacterial composition, which then drives the decomposition and nutrient cycle processes. For example, a complex relationship exists between vegetation-derived carbohydrates, lignin, soil inorganic N, increased soil complexity and microbial community development that affects the soil organic C levels and productivity (Guggenberger et al. 1994, Zech and Kogel-Knaber 1994, Brookes 1995, Anderson 2003, He et al. 2003, Moscatelli et al. 2005, Bradford et al. 2008). Furthermore, increases in soil inorganic N stimulate increased production of more labile root-derived carbohydrates by plants which are used by the bacterial community, while preliminary plant decomposition selects for fungi which degrade the lignin, cellulose, hemicelluloses, and other complex materials (Guggenberger et al. 1994, 1995; Padmanabhan et al. 2003; de Boer et al. 2005; Fierer et al. 2007; Bradford et al. 2008; Talbot et al. 2008). Thus, it is the fungi that decompose more complex organic substrates more efficiently than bacteria leaving behind more recalcitrant residues and enhancing the organic carbon matter in the soil, and are more important as decomposers in older or restored soils (Holland and Coleman, 1987, Bardgett et al. 1993, Cambardella and Elliot 1994, Guggenberger et al. 1994, 1995, 1999; Beare 1997, Bardgett and McAlister 1999; Stahl et al. 1999, Griffith and Bardgett 2000, Zeller et al. 2001; Bailey et al. 2002, Talbot et al 2008). The soil bacteria, on the other hand, are more critical in decomposition and N nutrient cycling in managed, young, or recovering ecosystems (Moore and de Ruiter 1991, Lovell et al. 1995).