Structural and functional succession in the nematode fauna of a.soil food web

Soil microplots were amended with organic materials of varying nature and complexity but providing similar amounts of carbon. Materials were either placed on the soil surface or incorporated. Unamended and mineral fertilizer control plots were established: Plots were maintained vegetation-free so that the food web activity was fueled by resident soil organic matter and the input material. Enrichment-opportunist bacterivore nematodes increased rapidly in response to low C/N plant materials and, to a lesser extent, to more complex materials. General-opportunist bacterivores increased in all plots, but at a slower rate. Fungivore nematodes also increased gradually in all plots but most rapidly in those amended with higher ON and more complex materials. Indices derived from nematode faunal analysis suggested a constant rate of succession from enrichment-opportunist to general-opportunist bacterivore guilds across all treatments, probably mediated by bacterial abundance and differences in life course characteristics of the respective taxa. The rate of succession from bacterivore to fungivore nematodes was greatest in plots receiving high ON materials. Succession to fungivory, presumably indicating a shift from bacterial to fungal decomposition channels, was slowest in those plots with a high level of organismal metabolic activity, as measured by soil respiration. The cumulative amounts of N mineralized in the plots were directly related to the enrichment index (EI), based on the abundance of opportunistic bacterial- and fungal-feeding nematodes. The amounts of mineralized N were inversely related to the slope of the channel index (CI), that is, the rate at which decomposition changed from bacterial to fungal. Maintenance of adequate soil fertility in systems driven by organic input may require maintenance of food web structure and function as indicated by high levels of enrichment-opportunist bacterivore nematodes. That will require frequent supply of labile organic sources. Allowing food web succession to guilds that indicate lower mineralization potential will result in lower levels of soil fertility. (C) 2003 Elsevier Science B.V. All rights reserved.