Trophic Interactions and Nitrogen Cycling in a Semi-Arid Grassland Soil. II. System Responses to Removal of Different Groups of Soil Microbes or Fauna

Abstract

(1) Perturbations were performed on organisms in a below-ground trophic food web in a semi-arid grassland, using five separate biocide treatments to observe changes in trophic structures, interactions, and nutrient cycling. Changes in N mineralization and trophic interactions as predicted on the basis of simple predator-prey microcosm studies were observed following removal of particular groups. (2) Five biocides: streptomycin (bactericide), captan and PCNB (fungicides), carbofuran (insecticide-nematicide), and cygon (acaricide) were applied in situ to soil in cylinders containing predominantly blue grama grass. The response of microbes, fungal grazers, soil inorganic N and plants were followed monthly between April and October 1982. (3) Grazing of bacteria or fungi by predators resulted in one or more of the following occurrences: (i) increased soil inorganic N, decreased predator populations following reduction of prey or increased plant growth after reduction of nematodes; (ii) reduction of one group of decomposers, e.g. bacteria, allowed a second decomposer group, e.g., fungi, to increase in numbers; (iii) compensatory responses of microbial feeders, e.g. decreases in bacterial feeders (protozoa and bacterivorous nematodes) were followed by compensatory increases in fungal feeders, which increased following the increase in their fungal food supply. (4) Continuing changes in nitrogen cycling were not observed, presumably because the function of the reduced group was compensated by increased numbers of the second group performing a similar function. Nematode-VAM interactions must be considered in food web and nutrient flows, as the percentage of VAM colonization of plant roots increased markedly when nematode populations were decreased by nematicide treatment. Production of predator biomass was as important as microbial and plant production in determining nutrient flow in this system.