Showing papers by "Heikki Setälä published in 1996"

Influence of body size of soil fauna on litter decomposition and 15N uptake by poplar in a pot trial

Abstract: We combined the advantages of litter-basket and microcosm techniques to study the decomposition of 13 N tagged Douglas-fir litter and uptake of labelled N by poplar plants. This was done in microcosms in the greenhouse under artificial summer and winter conditions. The microcosms contained litter baskets that excluded certain soil fauna based on body size. Litter baskets of varying meshsize containing 15 N-enriched defaunated litter were embedded in a simulated coniferous forest floor of unlabelled organic and mineral soil layers. Four litter-basket treatments were established: 1, 5 mm mesh allowing entry of all fauna, but macropredators were excluded; 2, 5 mm mesh, but macropredators were included; 3, 0.5 mm mesh with entrance for mesofauna only; and 4, 48 μm mesh allowing only the microfauna to enter the baskets. After 40 weeks, the development of faunal populations outside the litter baskets was similar in each treatment, except for the absence of macropredators and a greater numbers of millipedes in treatment 1. Fauna colonizing the litter baskets clearly differed among treatments. No treatment differences were detected in plant biomass and in the uptake of total-N by poplar. However, the 15 N content in treatment 4 plants was significantly lower than in others. On a simple net loss basis, significantly larger amounts of the N and litter remained in the litter baskets with the 5 mm mesh than with smaller mesh (treatments 3 and 4). However, the dilution of 15 N in litter baskets indicated translocation of external organic matter into litter baskets, confirming the limitations of conventional litter bag methods that use N content and gravimetric measurements only. After including translocation and adjustment of 15 N content in litter baskets and surrounding pot soil, actual losses of N and mass were reversed: treatment 3>1>2>4, with treatment 4 being significantly less than others. Top predators had no apparent influence on any of the functional variables measured, suggesting that nutrient dynamics are mainly regulated by interactions occurring near the base of the detrital foodweb.

Food Webs and Nutrient Cycling in Soils: Interactions and Positive Feedbacks

Abstract: Traditionally, food web studies have been part of what may be called the population-community approach to ecology (O’Neill et al., 1986). Ecosystem ecologists using the process-functional approach have usually neglected population interactions in food webs, despite the fact that a mechanistic understanding of ecosystem processes such as decomposition of organic matter and nutrient cycling requires studies of the organisms performing these processes (Moore et al., 1988; Verhoef and Brussaard, 1990). Until recently, there have been few published theoretical and empirical studies relating population dynamics and food webs to ecosystem processes (and vice versa) (DeAngelis, 1992; Jones and Lawton, 1995). A combination of the two approaches can be fruitful for solving many problems in basic and applied ecology, and studies of food webs are likely to become a substantial part of this growing industry.