Soil Microarthropod Contributions To Decomposition Dynamics: Tropical-Temperate Comparisons Of A Single Substrate - Statistical Data Included

Ecology, Sept, 1999 by L. Heneghan, D. C. Coleman, X. Zou, D. A. Crossley Jr., B. L. Haines

INTRODUCTION

The decomposition of organic matter and the enrichment of the soil with the labile nutrients necessary for plant growth are a biological process operating within the constraints imposed by a range of complex and interacting physical factors (Aber and Melillo 1980, Berg and Staaf 1980, Dyer et al. 1990). Soil fauna are contributors to this process and to the maintenance of soil fertility (Swift et al. 1979, Coleman and Crossley 1996). In the presence of microarthropods (e.g., free-living Acari and Collembola), which are prevalent components of this fauna, mass loss of newly senesced litter increases, amounts of inorganic nitrogen may be greater, and primary productivity can be enhanced (Seastedt 1984, Setala and Huhta 1991). Since few of these animals directly consume decaying organic matter, much of the regulation which they impose on decomposition is through their trophic interactions with the microbial community (Moore et al. 1988, Lussenhop 1992). As a consequence of these linkages of below-ground trophic cascades to ecosystem processes, alterations to the microarthropod assemblage structure have been shown to have significant effects on soil respiration and the leaching of a wide range of nutrients from laboratory microcosms (Heneghan and Bolger 1996).

Contrasts of decomposition between temperate and tropical forests are hampered by the lack of comparability in methodologies, the paucity of comprehensive studies, and the lack of a unifying conceptual framework (Swift and Anderson 1989). It is generally accepted, however, that for any given quality of substrate, decomposition in the humid tropics proceeds more rapidly than in the temperate region (Madge 1965, Ladd et al. 1985). The determining factors of litter decomposition rates, namely climate, edaphic structure, resource quality, fauna, and microbes, come into play in all terrestrial systems, though their relative importance may vary along a latitudinal gradient. Lavelle et al. (1993) speculated that in the humid tropics, biological systems of regulation, i.e., the mutualistic interactions of fauna and microbes, are the paramount determinants of decomposition dynamics for any one leaf type. This is because climatic variability is so greatly reduced in the humid tropics that it represents a constant and no longer acts as a constraint on biotic activity. This is in marked contrast to temperate forests, where seasonal climatic patterns strongly constrain the biota. In a similar vein, Couteaux et al. (1995) suggest that decomposition should be high at the transition between Mediterranean and Atlantic climates, where favorable moisture and temperature occur simultaneously. Although, in temperate regions, modifications of microarthropod assemblages can influence the availability of N (e.g., Seastedt and Crossley 1983, Heneghan and Bolger 1996), differences in assemblage structure have not been shown to influence mass loss of decomposing litter (Andren et al. 1995, Hoover and Crossley 1995).

We looked at the decomposition of a single substrate (Quercus prinus L.), in tropical and temperate sites under the influence of divergent microarthropod assemblages. Assemblages of microarthropods are known to diverge along a latitudinal gradient, with tropical sites having more diverse assemblages than do temperate ones (Stanton 1979). Puerto Rico was one of our tropical sites because it was assumed to harbor a different diversity of microarthropods compared with Central American forests (Pfeiffer 1996). Thus in this experiment we explored three related issues: First, what is the role of climate, substrate quality and biotic assemblages in regulating the decomposition of the chosen substrate? This study is unique in that we transplanted a single substrate, Q. prinus, collected at a single watershed, and followed its decomposition simultaneously at tropical and temperate locations. Second, are microarthropods more influential in decomposition dynamics in the tropics? To examine this we manipulated the abundance of microarthropod populations in litterbags containing the Q. prinus leaves by using naphthalene, an arthropod repellent. The warmer and moister soils of the tropics provide optimal conditions for microbial growth and for the development of strong interactions between the microbes and the fauna which feed on them. We expected, therefore, accelerated rates of decomposition at the tropical sites and an early and consistent faunal influence on the rate of mass loss. Third, is there a relationship between the structure of the microarthropod assemblages and their contribution to decomposition, an important ecosystem function? We speculated that, as a consequence of stronger fauna-microbial interactions, differences in the microarthropod assemblages at the tropical sites could result in site-specific differences in the rates of decomposition. If this contention is true, then when mass loss from litterbags from the tropical sites are compared, the differences between litterbags with microarthropods would be greater than the differences between the litterbags with reduced microarthropod populations.