Influence of Altered Temperature and Precipitation on Desert Microfauna and Their Role in Mediating Soil Nutrient Availability

摘要

Arid-land deserts comprise up to 30% of global land surface area and experience degradation by anthropogenic land use demands. Sixteen of the 19 climate models used by the International Panel on Climate Change predict increased temperature and prolonged periods of drought for much of the arid southwest US. Soil microfauna consume organic-bound nitrogen and excrete soluble inorganic and dissolved organic nitrogen that is available for plant uptake or as substrate for microbial saprotrophs. However, the growth and activity of nematodes and protozoa is restricted to periods of adequate temperature and moisture. The objective of this dissertation research project is to determine the possible effects of predicted climate changes on cool desert soil nematodes and protozoa and their role in nitrogen cycling. Climate changes are predicted to affect nematodes more adversely than protozoa. Nematodes are thought to contribute more to nitrogen cycling through dissolved organics while protozoa are thought to contribute relatively more to nitrogen cycling through inorganic nitrogen. A shift in community composition favoring amoebae over nematodes could shift the relative balance of organic and inorganic pools of soil nitrogen. Soil microfauna are not readily observed in their soil habitat, so extraction and enumeration techniques are adapted and tested for the desert microfaunal communitites. Desert soil nematodes and protozoa participate in energy channels deriving from vascular and non-vascular primary producers as well as bacterial and fungal saprotrophs. Environmental conditions influence the prey that are available to nematodes and protozoa and, thus, indirectly affect the relative composition of microfaunal feeding groups. Environmental conditions also directly affect microfauna. Not only are nematodes affected more negatively by adverse abiotic stress than amoebae, many species of bacterivorous nematodes appear to be susceptible to unique combination of the major abiotic stresses experienced in the desert. Findings from a two-year field experiment treated with elevated temperature and summer precipitation are consistent with lab and field mesocosm experiments, but highlight the uncertainly inherent in predicting long-term trends with brief experiments. In comparison with temperature and precipitation, long-term elevated carbon-dioxide enrichment was not shown to affect the abundance of desert soil microfauna directly but did affect the distribution of protozoa and the composition of nematodes indirectly through altered plant water use patterns. Continued work is needed to devise experimental systems that quantify the relative role of microfaunal functional groups in nutrient cycling.