Intellectual Merit: This proposal intends to examine the manner in which the terrestrial S cycle varies with rainfall from the driest to the wettest parts of the planet. Sulfur, one of the most important macronutrients for life, is far less understood that its ?companion? element N. However, the magnitude of the human perturbation of the global S cycle, the complex ways in which ecosystems respond to S, and the basic question of how life drives the behavior of major elemental cycles makes a study of S an interesting scientific and societally important challenge. The project brings atmospheric, geochemical, and ecological expertise together to quantify the sources and rates of S inputs to ecosystems, to explore important avenues of internal S cycling within soils and plants, and quantify S removal pathways along a rainfall gradient from ~1 to ~4000 mm MAP y-1. The work plan also includes the measurement of total soil and plant S and S isotopes from multiple sets of climate gradients to learn about the patterns that climate imparts on terrestrial S storage and cycling. Like N, it is hypothesized that at the wet end of Earth the S cycle is dominated by biologically mediated redox reactions. As rainfall declines to near 0, and as plants and microbes reach negligible levels, the S cycle shifts to dominantly inorganic processes, and the entire cycle undergoes a fundamental transformation. This rainfall gradient therefore provides a template to illustrate the uniqueness of our biotically-controlled planet, and to better interpret the geochemistry of our nearby terrestrial planet Mars, which has large surficial sulfate accumulations.
Broader Impacts: The rainfall gradient to be used is comprised of a series of sites that are part of the NSF funded CZEN network (Critical Zone Exploration Network). The project will benefit from the previous work done at these sites, some of which is critical to the success of this project (hydrology and geochemistry). The proposed work will also ultimately provide new data and information to the CZEN network database. Additionally, CZEN researchers will be collaborators on the project, strengthening the functioning of this novel initiative in geosciences. The project will largely involve graduate students, and will therefore provide training and opportunities for young scholars. However, the broad range of scientific expertise involved will make this an especially exciting and dynamic learning environment for everyone involved. Secondly, this research will further PI Amundson?s collaboration with Chilean collaborators and aid in international exchange. Finally, this proposal has been submitted to Emerging Topics in Biogeochemical Cycles because it is perceived that this project encompasses a climatic breadth that will allow us, in the end, to illustrate via general science and education outlets, the truly unique ? but alterable ? role of life on the history of our planet. Sulfur, along with N and some other elements, bears the unmistakable stamp of life. It is only by examining the big picture, via the sweep of Earth?s climate (proposed here), or from space (Sagan et al.1993. Nature 365:715-721), that it is possible to convey the uniqueness and fragility of Earth (Amundson. 2008. Nature Geoscience 1:5-6).
Sulfur is a key soil-derived element required for plant physiology, and bears chemical similarity to another essential element: nitrogen. However, the geographical distribution of sulfur in soils, and how climate controls the inputs and losses from soils, is not well known. In this project, we examined a series of soils and plants from the wet tropics of Puerto Rico to the hyperarid deserts of Chile, and developed an initial insight into sulfur geochemistry, and how it is both similar and dissimilar to nitrogen. The research is relevant to identifying landscapes with potentially low in sulfur, as well as providing more information about how polluted landscapes (from coal burning) may react to prolonged sulfur additions. Briefly, we found that microorganisms in soils of both marshes, and regions that receive pulses of rainfall (such as the semi-arid regions in California) can produce reduced sulfur gases (from the use of sulfur instead of oxygen in metabolic functions). Preliminary analyses indicates that sulfur may be the preferred reduced gases by microbes, even though chemically the element nitrogen should be the preferred reduced gas that is produced by the microbes. This interesting observation, along with results from laboratory studies by others, suggests that when sulfur is reduced to sulfide, it may inhibit nitrogen trace gas production below levels that might otherwise be expected. Since some nitrogen trace gases have very high greenhouse gas capabilities, this may be an important regulation of this production to the atmosphere. We also found that the amount and stable isotope composition (ratio of heavy to light sulfur atoms) varies with climate, and secondarily with other environmental variables such as soil age. In most soils, sulfur content declines systematically with depth, and the sulfur becomes more enriched in the heavier sulfur atom due to the microbial release of lighter sulfur as sulfate, which is either used by plants or leached to groundwater. We developed a preliminary global map of soil sulfur contents that may be useful to compare to other elemental patterns. Additionally, the preliminary map can help develop hypotheses that can be tested by future research on the both the local accuracy of these projections, and the mechanisms that control the patterns.
