Abstract
An Investigation of the Factors Controlling the Terrestrial Sulfur Cycle
by
Simona Andreea Yi-Balan
Doctor of Philosophy in Environmental Science, Policy, and Management
University of California, Berkeley
Professor Ronald Amundson, Chair
Sulfur (S), like nitrogen (N), is an essential macronutrient for life on Earth. Its deficit in soils decreases primary productivity, but its excess can impair ecosystem health. Unlike N cycling however, for which both the natural and the human-impacted cycles have been well studied, most research on S has focused on S pollution. My thesis addressed S cycling in pristine terrestrial systems, to understand the potential effects of global change on these essential functions. I used chemical analyses and stable isotopes to investigate the impact of climate, vegetation, topography, parent material and landscape age on the natural terrestrial S cycle in comparison to that of N.
I examined the S content and isotopic composition (as δ34S values) in soils and vegetation at 11 sites spanning broad gradients of climate globally. Soil S content generally increased with mean annual precipitation (MAP), but was uncorrelated with mean annual temperature (MAT). Soil and plant δ34S values increased with increasing MAP and MAT. MAP and MAT together accounted for about half of the observed variability in folial δ34S values, and for over a quarter of the observed variability in soil δ34S. These S patterns resembled those of soil N, known from previous studies. The difference between the δ34S values of soils and atmospheric inputs increased significantly, but weakly, with MAP, suggesting greater biological S isotope fractionation in wetter climates.
To directly explore the impact of vegetation, topography and parent material on soil S biogeochemistry, I collected soil, plant, pore water and precipitation samples from the wet tropical Luquillo Experimental Forest, Puerto Rico. Topography impacted S cycling by influencing soil redox conditions, while vegetation and parent material had a minimal impact. Pore water data suggested the co-occurrence of at least three major S-fractionating processes: plant uptake, mineralization and dissimilatory bacterial sulfate reduction (DBSR). This complex biogeochemical cycling appeared to be driven by the high rainfall. I modeled soil isotopic fractionation assuming advective transport of organic matter through the soil profile. This model worked well for N, but failed to describe S transformations, revealing a decoupling of the N and S biogeochemical cycles in these soils due to biotic processes.
I found a similar decoupling of S from N cycling on a chronosequence of marine terraces in Santa Cruz, California, where I investigated the impact of landscape age. I propose that two factors account for this apparent greater redox sensitivity of S compared to N isotopes. First, S is in less biological demand, and thus more readily fractionated by redox reactions, unlike N, which might be fully consumed for plant and microbial cellular metabolism during biological processes. Second, S may experience several reduction-reoxidation cycles due to its retention in soils via adsorption on iron and aluminum oxides, unlike N, which is easily lost from soils once reduced to gaseous form. In the deeper soil layers, processes that deplete the heavy S isotope (such as DBSR) dominated in the youngest soils, while processes that enrich the soil in the heavy isotope (such as mineralization) dominated in the older soils. Furthermore, pore water data revealed a division in soil processes with depth in the older soils, with large fluctuations in sulfate concentration and isotope fractionation near the surface (likely due to DBSR), but little change below the well-developed argillic horizons. My data showed no significant effect of phosphorus limitation on S cycling in the older soils. Rather, age impacted soil S content and δ34S values mostly due to changes in hydrology, including the development of a water restrictive argillic horizon, with increasing soil age.
In summary, my results showed that, of the factors examined, rainfall effects modified by landform characteristics are the most important controls on S biogeochemistry that dictate the types and rates of processes. S cycling should, therefore, most directly respond to the changes in rainfall predicted to occur due to global change. Specifically, a significant decrease in rainfall in many regions may reduce soil S content and the extent of its biological cycling.
