University of California, Santa Barbara

Post-Doc, Earth Research Institute

About

My research addresses the impacts of seasonality and disturbance on biogeochemical processes. I ask questions at the ecosystem scale, with emphasis in two directions: (1) ecosystem biogeochemical response to global change, such as the feedbacks between climate change, carbon (C), nitrogen (N) and phosphorus (P) cycles and vegetation communities, and (2) microbial community structure and function, with the purpose of understanding the relationship between soil microbial community and ecosystem processes.

Many of my research questions investigate temporal variability, to understand how biotic and abiotic seasonality interacts with disturbance or climate drivers. I use a combination of field sampling and field and laboratory experimental manipulation, followed by biogeochemical and molecular analysis and post-hoc statistical modeling. The majority of my research has been in arctic landscapes, but my questions are broadly applicable to any ecosystem, and I have applied my research in high and low arctic tundra, boreal forest, temperate grasslands and Mediterranean ecosystems.

1. ECOSYSTEM RESPONSE TO GLOBAL CHANGE

Nutrient fluxes and pools are indicators of ecosystem health and ecosystem feedback to global change. I analyze these fluxes, pools and their drivers to understand controls on ecosystem processes, and the biotic and abiotic mechanisms that are involved in regulating them. For example:

ENVIRONMENTAL CHANGES ALTER THE CARBON CYCLE OF HIGH ARCTIC ECOSYSTEMS
Thule, Greenland
As the soil active layer deepens in permafrost soils, thaw dynamics move down the soil profile, potentially creating hotspots of microbial activity and dissolved organic carbon (DOC) production. This DOC may be exported from the landscape throughout the summer and fall, determining ecosystem C budgets and supporting aquatic productivity. I am investigating seasonal soil solution C and nitrogen (N) dynamics in Thule, Greenland as part of a NSF-funded project with Josh Schimel, Sean Schaeffer, Jeff Welker and Adam Csank.


SPATIAL AND TEMPORAL INFLUENCES OF THERMOKARST FAILURES ON SURFACE PROCESSES IN ARCTIC LANDSCAPES
North Slope, Alaska
Permafrost thaw on upland arctic slopes results in large erosional scars, releasing soil nutrients to aquatic systems, and exposing large, potentially unstable C pools. These scars succeed to unusually productive shrublands with age, which may represent a temporary mid-successional community or a shift to enhanced shrub growth across the Arctic. However, in the strongly nutrient-limited tundra the source of nutrients for this enhanced plant growth is a mystery. As part of a large interdisciplinary NSF-funded project, through my post-doc with Josh Schimel at UCSB, I developed a research program that uses biogeochemical and stable isotope analyses to investigate how permafrost thaw impacts soil nutrient cycling along a recovery chronosequence. This research is in collaboration with Andrés Baron, Michelle Mack and Ted Schuur.


SPRING THAW IN THE TUNDRA: THE QUALITY AND QUANTITY OF 'LITTER TEA'
Toolik Lake and North Slope, Alaska
In collaboration with Josh Schimel, Sean Schaeffer and Claudia Boot I am investigating the quantity and quality (carbon/nitrogen/phosphorus stoichiometry, dissolved organic carbon UV and NMR) of compounds leached from litter, under snow and at thaw, and the impact of these compounds on soil nutrient cycling and soil microbiall activity in different tundra vegetation types and at different stages of thermokarst recovery.


THE IMPACT OF DEEPENED SNOW ON WINTER AND THAW BIOGEOCHEMISTRY IN THE LOW ARCTIC TUNDRA
Daring Lake, NWT, Canada
Winters are long and cold in the Arctic, yet winter soil microorganisms are biologically active in frozen soils, and more temperature-sensitive than the summer community. Arctic winters are warming more rapidly than the rest of the globe. Deeper snow - a consistent climate prediction for the Arctic - impacts ecosystem processes, Arctic nutrient budgets and global C cycling by further warming Arctic winter soils. Cold-season soil processes set the stage for the next growing season, by controlling the size of the soil nutrient release to plants at thaw. Variation in this spring release is critical, as nutrient-limitation controls plant growth and thus ecosystem C budgets, yet there is little information about the processes that occur in northern ecosystem cold seasons. During my Ph.D. research with Paul Grogan, I illustrated that warmer soil under deeper snow altered microbial physiologies, and substantially enhanced the flush of C, N and phosphorus (P) to the soil under the melting snow pack. I tested the mechanism behind this flush: warmer winter soils under deeper snow were more important than spring snow melt water for this spring flush of nutrients.

THE SEASONALITY OF COLD SEASON DECOMPOSITION
Toolik Lake and Healy, Alaska
Winter decomposition rates are different in the early winter, the deep cold of mid-winter, and during the dramatic thaw season. Because climate change has seasonality, I am investigating the relative importance of changes in winter precipitation (deeper snow) on each of these sub-seasons with Josh Schimel, Seeta Sistla, Sue Natali, Ted Schuur and Jeff Welker. Preliminary research on soil enzyme activity and biogeochemistry has begun in boreal and low-Arctic sites.


2. MICROBIAL COMMUNITY STRUCTURE AND FUNCTION

Soil microbial communities are staggeringly diverse and complex, and are largely responsible for all or part of several global nutrient cycles. After a century studying individual species, microbial ecologists are just beginning to understand how microorganisms interact with their environment, other individuals, and other species. And although soil microbial communities have long thought to be redundant as a result of their vast diversity, it has recently become clear that when soil microbial communities change in the face of disturbance both function and physiology may change, so that resulting changes to ecosystem processes become unpredictable. My research investigates the process of soil microbial community change and the impact on ecosystem processes:

MICROBIAL COMMUNITY SEASONALITY
Daring Lake, NWT, Canada
With collaborators Paul Grogan, Sam Banerjee, and Steve Siciliano, and using a combination of traditional microbiology (epifluorescent microscopy, PLFA) and a molecular method (quantitative PCR) I found that Arctic soil microbial communities vary seasonally and that this variation is strongly correlated with ecosystem biogeochemical processes. New bacterial and fungal growth occurs at sub-zero temperatures and crashes with snow melt. This crash occurs at the same time that soil nutrients drop, and when numbers of microbial predators (protozoa, actinomycetes) are enhanced. So do seasonal microbial community fluctuations drive seasonal biogeochemistry, or the other way around? I look forward to future elucidation of community and process relationships using tools I have started to develop, such as structural equation modelling and metagenomics.


MICROBIAL COMMUNITY RESPONSE TO FREEZE AND THAW
Toolik Lake, Alaska and Thule, Greenland
I am also investigating the predictability of microbial community response to repeated freezing and thawing in Arctic tundra soils using soil microbial genomic data combined with environmental and process data, together with Josh Schimel, Sean Schaeffer and Claudia Boot. In addition, we are characterizing microbial communities that are active during freeze and thaw, in Thule, Greenland.

MICROBIAL COMMUNITY SUCCESSION
North Slope, Alaska
As with above-ground ecology, soil microbial succession after disturbance may follow a progression of stages, and a mid-succession peak in diversity, which is related to their changing environment and their community interactions. I am investigating this using pyrosequencing of the bacterial and fungal DNA along the chronosequence of thermokarst recovery described above with Josh Schimel and Matt Wallenstein. One of my career research goals is to be able to describe to students these successional interactions, and the contribution of these interactions to those that occur at the ecosystem level.

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