Detrital foodweb interactions in North American grassland ecosystems

Abstract

Unique characteristics of grasslands which are important for understanding interactions of soil biota are: (1) the high proportion of primary production which occurs belowground; (2) the small amount of above-ground plant structural material; (3) the formation of Mollisols. These soils are usually quite fertile and contain large amounts of soil organic matter. Grassland soil biota live in a comparatively rich, structurally diverse habitat. Although the proportion of fungi and bacteria in grassland soils may vary from site to site, as a general rule they vary around equal proportions. The main consumers of the bacterial production are protozoa (mostly small naked amoebae) and free-living bacterial-feeding nematodes. Many taxonomic groups of microarthropods, previously thought to feed exclusively or predominantly on fungi, recently have been found to feed heavily on nematodes, some being obligate nematophages. Fungal production is consumed mostly by microarthropods and free-living, stylet-bearing nematodes. Microarthropods are the top predators in semi-arid grassland soils with different functional groups feeding on nematodes, microarthropods or both. Bacteria and, to a lesser degree, fungi, are responsible for most of the N mineralization in grasslands, but the protozoans, nematodes and microarthropods may contribute collectively as much as 40%. Although the biomass of the top predator microarthropods is small, this group could have a disproportionately large impact on the detrital community structure and function similarly to keystone predators in other communities. The dynamics of the different groups are closely interrelated. Therefore, it is necessary to monitor the dynamics of all major functional groups to obtain a complete understanding of the system. Pulses of decomposition, and hence saprophagic production, are extremely important, especially in semi-arid grasslands, where rainfall triggers high soil-organism activity. Relative growth rates of organisms, active after rainfall events in semi-arid ecosystems, may determine the lag times for population development, strength of interactions of the different groups and overall production of microbial biomass, its turnover rates and the resultant mineralization.