Soil ecosystems are essentially invisible. Despite their unobtrusiveness, it is in the underground portion of the biosphere that important cycling processes occur: nutrients are recycled, organic and inorganic contaminants are detoxified, and radiatively active trace gases are produced and consumed. These and other important ecosystem functions are brought about by an eclectic mix of organisms that vary in size from a few micrometers (e.g., bacteria) to a few centimeters (e.g., earthworms). It is the soil biota that comminutes, decomposes, and releases nutrients from plant and animal detritus deposited in the soil and transforms anthropogenic toxins to innocuous products. Much as in above-ground ecosystems, members of the soil ecosystem interact with each other through predatory, symbiotic, and parasitic relationships. These ecological interactions (e.g., predation) will cycle nutrients, detoxify organic pollutants, and produce gases like methane.
     Biological activities in the soil are exquisitely sensitive to moisture. As a porous medium with relatively small pores, soil has the capacity to hold water, much like a sponge. In soil, water acts as a medium for the movement and diffusion of substrates used by soil microflora and fauna. Because oxygen diffuses approximately 10,000 times faster in air than water, water in soil pores also serves as a barrier for the movement of oxygen. Soil moisture thus exerts a significant control on the function of soil ecosystems.
     One of the possible consequences of global climate change is a shift in the temporal and spatial distribution of rainfall. This will, in turn, change the seasonal and regional patterns of soil moisture dynamics. Since water plays such an important role in soil processes, these changes will alter the way soil ecosystems will cycle nutrients, detoxify, and produce trace gases. Over the years, one of the research questions explored in our laboratory has been how soil biological processes, such as nutrient cycling and methane production, respond to different soil moisture regimes. Will increased rainfall hinder or accelerate processes in the soil? What about lack of moisture?
     Nutrient conservation is important to sustainable agriculture. Promoting efficient nutrient cycling in agricultural ecosystems reduces the requirement for inorganic fertilizers and pesticides, and thus reduces the impact of nutrients on surface and ground water quality. Nutrient cycling is tightly coupled to soil moisture. Water, or lack of it, controls the diffusion of organic nutrients and the movement of soil fauna that help speed up nutrient cycling. Working with agricultural soils, Josef Gorres, visiting assistant research professor, Mary Savin, doctoral candidate and URI graduate fellow, (both of the Department of Natural Resources Science), and I conducted a series of experiments in which we carefully controlled the level of moisture in the soil and measured the rate at which nitrogen and carbon are released in mineral form at different soil moisture contents (see Fig. 1). Our results indicate that the mineralization of carbon and nitrogen respond differently to soil moisture. While nitrogen peaks in activity at relatively high soil moisture, carbon mineralization shows two peaks, one at high and one at low soil moisture. We believe that high moisture allows access of soil fauna to their microbial prey, releasing the carbon and nitrogen tied up in these microorganisms. As the water level drops, soil fauna have less access to their prey, there is less cycling of nitrogen through the fauna, and less nitrogen is released in plant-available form.
     Methane is an important greenhouse gas that is both produced and consumed by soil microorganisms, especially in wetland ecosystems. The water level in wetlands can fluctuate naturally with the season, or as a result of flood control management, ground water recharge, or nutrient retention. Methane production by soil micro-organisms requires the absence of oxygen (methanogenic bacteria are anaerobic), and thus is intimately tied to the presence of water, a barrier to oxygen diffusion. Working in collaboration with Ronald Jones, professor of microbial ecology, Florida International University in Miami, we studied methane production in soils of the Florida Everglades, one of the largest wetland ecosystems in the United States. The data show that methane evolution is controlled by both moisture and nutrient availability. The addition of phosphorus enhances the effects of soil moisture, allowing methane evolution at soil moisture levels that would not support methane production. It appears that phosphorus, found in extremely low concentrations in Everglades soils, increases microbial activity and thus oxygen consumption, hastening the onset of anaerobic conditions. Agricultural and residential land uses have the potential to cause nutrient pollution in watersheds. If runoff containing high levels of nutrients ends up in freshwater wetlands, it may increase the net flux of methane from the wetland to the atmosphere.
     Soil moisture is an important factor in nutrient cycling. As such, we can expect soil ecosystem functions to be deeply affected by temporal and spatial shifts in rainfall patterns and nutrient inputs resulting from global environmental change.

Recommended Reading
Amador, J.A., 1997. Soil form follows function. American Nurseryman 186: 64-67.
Kilham, K. 1994. Soil Ecology. Cambridge University Press, New York, 242 pages.
Sylvia, D.M., J.J. Fuhrmann, P.G. Hartel, and D.A. Zuberer. 1998. Principles and Applications of Soil Microbiology. Prentice-Hall, Upper Saddle River, New Jersey, 550 pages.
Amador, J.A., and R.D. Jones, 1997. Response of carbon mineralization to combined changes in soil moisture and carbon-phosphorus ratio in a low phosphorus histosol. Soil Science 162: 275-282.