Decomposition, the breakdown of dead plant and animal material, is a fundamental process that affects soil fertility and ecosystem carbon storage. Most of what is known about decomposition is from studies in high rainfall areas, but this knowledge does not translate well to dryland ecosystems. Some recent studies suggest solar ultra-violet radiation has a major influence on decomposition in drylands; however, other studies indicate the level of mixing of wind/water-transported soils with litter is the key factor. This project seeks to resolve these competing explanations through a series of laboratory studies and field experiments in Arizona designed to measure interactions among these factors. These linkages will be assessed in the context of woody plant encroachment into grasslands, a globally extensive vegetation change in drylands.
This investigation will yield new insights into processes that affect soil fertility and carbon storage in drylands by combining the disciplines of plant community ecology, ecosystem science and earth science in a novel framework. The findings will be relevant nationally and internationally, as dryland ecosystems characterize major portions of the US and global land area, and may be significant carbon sinks. In addition to dissemination in the scientific community, results will be communicated to land management personnel and organizations through outreach programs. The study will provide training opportunities for four graduate and numerous undergraduate students. The collaborating institutions (University of Arizona, New Mexico State University, University of Kentucky and Loyola University) have substantial minority enrollments, and efforts will be made to recruit students from underrepresented groups.
Death is an integral part of the cycle of life. In ecosystems, plants acquire inorganic compounds from their environment (carbon, nitrogen, phosphorus, etc.) and build them into complex organic compounds that nourish animals either directly (e,g,, herbivores that consume plants) or indirectly (carnivores that consume herbivores). Eventually, all plants and animals in the ecosystem die (Figure 1). What is the fate of the nutrients they have accumulated during their life-time? These nutrients are released back to the environment via the process of 'decomposition'. Decomposition, the breakdown of dead plant and animal material, is a fundamental ecosystem process that affects long-term soil fertility and carbon storage. Most of what is known about decomposition is from studies in high rainfall areas, but this knowledge does not translate well to dryland ecosystems. Some recent studies suggest solar ultra-violet radiation is a major driver of decomposition in drylands; however, other studies indicate the level of mixing of wind/water-transported soils with litter is the key factor.  Research supported by this NSF grant sought to resolve these competing explanations. A collaborative effort initiated in 2008 involved scientists in the School of Natural Resources and the Environment at the University of Arizona (Steve Archer and Dave Breshears), New Mexico State University (Heather Throop), the University of Kentucky (Rebecca McCulley) and Loyola University New Orleans (Paul Barnes). Field experiments in the Sonoran Desert of Arizona and complimentary controlled environment studies at Loyola University, New Mexico State University, and the University of Kentucky were used to ascertain how light energy-soil movement interact to affect decomposition rates in the context of woody plant encroachment into grasslands, a globally extensive vegetation change in drylands.  Soils and litter can be redistributed by wind and water. Several key insights related to wind (Figure 2) and water movement of soil particles have emerged from this research. Comparisons of wind- and water-related processes required focusing on estimates of material "transported" (or moved) rather than that which was "eroded" (or lost). It is well known that fluvial-driven sediment transport occurs in conjunction with large rainfall events, and that aeolian transport of sediment occurs during particularly windy periods. Surprisingly, however, the findings revealed that small amounts aeolian transport occurred on a sufficiently frequent basis, that when totaled, these small increments contributed substantially to annual totals. Also quantified were the effects of land use (burning or grazing) and climate (average vs. extreme precipitation or drought events), and the effects of vegetation patch structure on sediment flux rates. A new approach for estimating sediment deposition onto plant litter was developed to enable quantification of this important input term.  This research has also yielded new insights into processes that affect soil fertility and carbon storage in drylands by combining the disciplines of plant community ecology, ecosystem science and earth science in a novel framework. In particular, we have found, 1) that sunlight (UV and visible) can be an important driver of decomposition in drylands (Figure 3) but this can be ameliorated by soil coverage of litter (Figures 4), 2) soil litter mixing results in the formation of adhering "soil films" that consist of soil particles and microbial products, which in turn, 3) enhances microbial decomposition processes (Figure 5). Thus, decomposition in these drylands is driven by the interaction of these two important factors (Figure 6). Additionally, we have developed new approaches and techniques to characterize the UV environment of surface litter by adopting techniques that have been used to monitor UV exposure in humans. These findings, and others, have contributed new insights into the processes controlling decomposition in drylands (grasslands, savannas and deserts) and are relevant nationally and internationally, as these ecosystems characterize major portions of the US and global land area, and may be significant carbon sinks. Understanding fundamental processes governing carbon storage and nutrient recycling are critical to evaluating and predicting the effects and drivers of global climate change.  Numerous and diverse training opportunities were provided at the University of Arizona for undergraduates (14 student workers [5 male; 9 female; including Hispanic and Asian], 8 REU's from six different institutions [1 male; 7 females; 1 Hispanic], and 2 undergraduate independent studies [both females, 1 Asian, 1 Hispanic]), graduate students (2 PhDs [1 male, 1 female]; 3 MS [1 male, 2 female; 1 Black]), two post-docs [both male], four secondary school teachers [2 male; 2 female; 1 Hispanic], and two PhD-level Senior Research Scientists [1 male; 1 female]. Collaborators within the University of Arizona included faculty in Atmospheric Sciences, Chemical & Environmental Engineering, Plant Science, Soil, Water & Environmental Science, Education, Biosphere-2, iPlant, and the Arizona Science Teacher Advancement and Research Training program. External collaborators included 10 different scientists representing federal entities (USGS, Los Alamos National Labs), other universities (Oregon State, Oklahoma State, UCLA), and other countries (Univ. Alcala, Univ Alicante).
