The influence of earthworms on carbon and nitrogen cycling processes in agroecosystems based on organic or inorganic nutrient inputs.

摘要

My research was based on the field and laboratory experiments designed to investigate the effects of earthworms on C and N cycling processes in agroecosystems that received organic or inorganic sources of nutrients. A field experiment was established, in 1991, in which earthworm populations were manipulated, using electroshocking to reduce natural populations in some field enclosures and the addition of field-collected worms to augment natural populations in others. The experiment was established in maize (Zea mays) agroecosystems in which nitrogen was provided in the form of NH{dollar}sb4{dollar}NO{dollar}sb3{dollar}, cow manure or legume cover crop. Electroshocking was effective at reducing earthworm population to about 30% of their natural abundance. The addition of field-collected worms was not as effective at increasing earthworm populations as electroshocking was at reducing populations, but increased the biomass of L. terrestris. The total earthworm biomass at the site declined throughout the study period which was due to: (i) the extreme climatic conditions that occurred throughout the study period and (ii) the conversion of the field from perennial alfalfa to cultivated maize. I designed a laboratory experiment in which I replicated the field experiment in soil microcosms with and without earthworms and with similar nutrients. Earthworms significantly increased the amounts of extractable NO{dollar}sb3sp-{dollar} and decreased the amounts of microbial biomass-N in the microcosms, especially in those that received no organic inputs. Earthworms increased soil respiration rates significantly throughout the study period. I investigated the effects of earthworms on litter decomposition in the field enclosures of the main field experiment. Earthworms increased the loss of C and N from surface maize residues and gathered the highest quality residues around the mouths of their burrows, accelerating the decomposition of the redistributed residues and creating concentrated areas of high microbial activity. The results of these experiments are summarized in a conceptual model that depicts the influence of earthworms on C and N cycling processes by comparing the fluxes of these two elements in the presence of earthworms to their fluxes when earthworms are absent.