Many chemical reactions at the Earth's surface are influenced by biota. Ecosystem ecologists study the flow of matter and energy in ecosystems com-posed of organisms and their abiotic environments. The study of ecosystem ecology can blur distinc-tions between 'basic' research and environmental management. How does our behavior (including ur-ban development and land-use, water consumption, pollution) influence the movement of energy, water, and elements at local, regional, national or global scales? Will perturbations to chemical and energy cycles alter existing controls on ecosystem pro-cesses, and can we learn enough about them for effective regulation? Plants are critical in regulating biogeochemical cycles. Their growth controls the exchange of gases that support life in our current biosphere, and af-fects soil development. As primary producers, they influence the distribution of energy for higher tro-phic levels. Understanding how plants influence ecosystem processes requires a multidisciplinary approach drawing on plant physiology and bio-chemistry, community ecology, and biogeochem-istry. Due to their unique physiology and ecology, bryophytes differ from vascular plants in influenc-ing cycles of elements, energy, and water. For ex-ample, bryophytes have evolved an effective water relation system. Poikilohydry and desiccation tol-erance allow bryophytes to tolerate longer periods of water stress than vascular plants, and to recover quickly with rehydration. With poorly developed conduction systems, water and solutes are taken up over the entire plant surface. Lack of both game-tophyte stomata and effective cuticles in many spe-cies allows free exchange of solutions and gases across cell surfaces. Thus bryophytes often serve as effective traps for water and nutrients. This also makes them more sensitive to atmospheric chemi-cal deposition than vascular plants. Bryophytes also can tolerate a wide range of temperatures and are found in almost all terrestrial and aquatic environments, including harsh Antarc-tic environments where vascular plant cover is low (cf. Fogg 1998; Seppelt 1995). Without roots, bryo-phytes can colonize hard substrates like rock and wood that are poor habitat for vascular species. Bryophytes stabilize soils and prevent the loss of soil and nutrients via erosion, particularly on sand dunes (Martinez & Maun 1999) and in cryptogamic soil crusts (Eldridge 1999; Evans & Johansen 1999). Cation exchange on Sphagnum cell walls re-leases protons, generating acidity that may inhibit plant and microbial growth (Clymo 1963; Craigie & Maass 1966; Spearing 1972). Finally, bryophytes influence ecosystem succession (Brock & Bregman 1989) through terrestrialization of water bodies, de-position of benthic organic matter or paludification of upland systems. Bryophyte colonization often precedes the establishment of tree surfaces by other canopy-dwelling plants (Nadkarni et al. 2000). Due to their physiology and life history traits, bryophytes influence ecosystem functions by pro-ducing organic matter, stabilizing soils or debris, trapping sediments and water, and providing food and habitat for algae, fungi, invertebrates, and am-phibians. In this review, my objectives are to high-light several mechanisms by which bryophytes in-fluence carbon (C) and nitrogen (N) cycles within and fluxes from ecosystems. As such, I will focus on how bryophytes fix, intercept, transform, and/or release C and N. My goals are to 1) introduce im-portant processes controlling inputs and outputs of C and N in both terrestrial and aquatic ecosystems, 2) review work on the growth, decomposition, and leaching of bryophyte material, as well as biotic and abiotic controls on these mechanisms, and 3) suggest areas for future research that would ad-vance our understanding of bryophytes in biogeo-chemical cycling.