Completed Research Projects
Arctic soils have large stores of carbon (C) and may act as a significant CO2 source with warming if decomposition rates increase. The key to
understanding tundra soil processes is nitrogen (N), though, as both
plant growth and decomposition are severely N limited. However, current
models of tundra ecosystems and their responses to climate change
assume that while N limits plant growth, C limits decomposer
microorganisms. In addition, N availability has been observed to be
strongly seasonal with relatively high concentrations early in the
growing season followed by a pronounced crash. We need to understand
the controls on this seasonality to predict Arctic System responses to
climate change, but several questions need answers: 1) What
causes the seasonal nutrient crash? 2) Does microbial activity switch
between C and N limitation with the crash? 3) How will a lengthening of
the growing season alter overall ecosystem C and N dynamics, as a
result of differential extension of the periods before and after the
nutrient crash? 4) What will be the larger impacts of these patterns on
the Arctic system?
We have been addressing these questions by: 1) Varying the length and
timing of the growing season in the field by advancing snow melt and
warming the ecosystem; 2) Establishing the fine scale seasonal
time-courses of soil N availability, plant N content, leaf expansion,
root growth and rhizodeposition, ecosystem respiration, microbial
biomass and enzyme activity; 3) Conducting lab experiments to determine
the extent to which microbial activity is limited by temperature, and C
and N availability before and after the crash; and 4) Refining models
developed for arctic ecosystems to better handle how plant and
microbial systems respond to N limitation. This
research is improving our understanding of the temporal effects of the
seasonality of nutrient availability and how it may change in a warming
Arctic with a lengthening growing season.
Controls on P availability & runoff in agricultural fields
Phosphorus (P) is an essential nutrient for plants and is often added
as fertilizer to farm fields. However, adding P to fields can be
problematic because P mined from phosphate rock is a non-renewable
resource—easily accessible sources are projected to peak in the year
2030 and, subsequently, become depleted. In addition, P added to
agricultural fields can runoff into surface waters and contribute to
harmful algal blooms, including the bloom that shut down the Toledo
municipal water supply in August 2014.
The application of P fertilizers is often inefficient, as much of the P
forms complexes with soil particles and is unavailable to plants. In
addition, some of the P from previous fertilizer applications remains
bound to soil particles from year to year, becoming “legacy P”. After
several years of fertilization, a soil may become saturated with P, at
which point the ability of soil particles to bind additional P becomes
negligible. At this point, P inputs to surface waters may
increase—either by erosion of soil particles into waterways, or by
runoff of dissolved P that does not bind to P-saturated soils.
The goal of our research, which began in summer 2016 and is ongoing, is
to determine if enhancing the activity of decomposer organisms during
the plant growing season could stimulate the mobilization and uptake of
legacy-P. Depleting the legacy P pool has the potential to help
decrease the amount of P that farmers need to apply to their crops or
can be leached into waterways.
Additionally, I was a member of the now-ended NSF funded Research
Coordination Network on Phosphorus Sustainability (2015-2017) (https://sustainablep.asu.edu).
This gave me the opportunity to participate in developing new syntheses
on P’s role in providing ecosystem
services/disservices (e.g. MacDonald et al. 2016) and the role of
legacy-P (e.g. Rowe et al. 2015). Beyond
providing opportunities for networking and publication, these syntheses
help to inform our research on P management.
Investigating nitrogen deposition effects on biological soil crust stability and biogeochemical cycling in drylands
Nitrogen
(N) deposition in the western US is rising and has already dramatically
affected terrestrial ecosystems. For example, N deposition has
repeatedly been shown to lower air quality, increase greenhouse gas
emissions, alter plant community composition, reduce water quality, and
significantly modify fire regimes. Accordingly, the effects of N
deposition represent one of our largest environmental challenges and
make difficult the National Park Service’s (NPS) important mission to
“preserve the scenery and the natural and historic objects and the
wildlife… unimpaired for the enjoyment of future generations.” Due to
increased population growth and energy development, the Four Corners
region has become a notable ‘hotspot’ for N deposition, however, our
understanding of how increased N deposition will affect these unique
ecosystems remains notably poor. We are conducting a multi-disciplinary
project to gather information that will help NPS safeguard the Four
Corner’s national parks, both now and into the future. We will use
field and laboratory techniques to help elucidate the ecosystem
consequences of N deposition to Arches National Park. This research is
being conducted in N fertilization plots that were set up in the Park
two years ago. We are using these plots to determine how increased N
inputs affect biological soil crust stability and biogeochemistry. In
particular, we are exploring how increased N inputs affect belowground
processes, including effects on soil aggregate stability, CO2 efflux,
and enzyme activity and biogeochemical cycling. We are also examining N
deposition interactions with cheatgrass (Bromus tectorum) invasion. We
are using field and laboratory approaches to assess the responses to as
well as the mechanisms behind N deposition effects on biocrust
functioning and cheatgrass density. This
research will provide increased information regarding N deposition
effects and to develop a rapid biocrust assessment useful to land
managers.
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Early Research
My graduate research was focused on understanding SOM decomposition and
nutrient availability in the Arctic tundra of Alaska. The Arctic is
warming faster than temperate zones, and its average temperature has
already risen significantly in the last 50 years. If the soils do warm
up and dry out a bit, accumulated SOM could decompose more rapidly,
increasing CO2
emissions. However, plant growth in the Arctic is limited by nutrient,
particularly N, availability. If SOM decomposes more rapidly, nutrient
availability could increase, elevating net primary productivity, and
serving as a negative feedback on increased soil CO2
emissions due to warming. Whether the tundra will be a source or sink
for C as it warms depends on the balance between increased CO2 losses from SOM decomposition, and increased plant productivity as a result of higher nutrient availability.
My research in the Arctic began with a study of the controls on SOM
decomposition (described in Weintraub and Schimel 2003). I used
long-term lab incubations and chemical fractionation to determine the
relationship between SOM decomposition and chemistry, and to quantify
the relative proportion of tundra SOM biologically available for
decomposition. I found that a large proportion of tundra SOM is
potentially mineralizable, despite the dependence of decomposition on
lignin breakdown, and that the accumulation of SOM in tundra soils is
the result of nutrient limitation and unfavorable conditions, not
chemical limitations to decomposition.
Since tundra SOM is potentially mineralizable, and plant
growth is typically N limited in the Arctic tundra, N availability is
likely to control changes in the C balance of the tundra in response to
warming. However, net N mineralization, the traditional measure of N
availability, is too low to account for the N taken up by plants in the
Arctic tundra. Others have shown that at least some tundra plants can
take up amino acids directly, but our understanding of the importance
of amino acids to tundra plants is still poor.
To assess the competitive partitioning of amino acids between
tundra plants and soil microbes, I added isotopically labeled amino
acids and NH4+ to intact cores of two tundra sedges. On all but one occasion both plant species took up more of an added amino acid than NH4+.
To determine the role of physiological uptake capacity in N
partitioning, I quantified the uptake rates of a variety of organic N
forms in excised roots and soil from a dominant tundra sedge. I
hypothesized that plants would best take up the compounds that had the
lowest uptake rates in soil. However, most compounds had similar uptake
rates in both plants and soil. These results indicate that individual
amino acids have the potential to be as important to these tundra
plants as NH4+, suggesting that the total free amino acid pool may be a more important N source to this plant than NH4+.
To assess amino acid availability in tundra soils I measured
amino acid concentrations throughout the growing season, and I measured
protease activity with and without added protein to determine the
controls on amino acid production by extra-cellular
enzymes. Total amino acid concentrations were usually higher than NH4+,
and concentrations of both dropped dramatically in all soils in the
middle of July, suggesting intense competition for N at the height of
the Arctic growing season. At this time, I also observed dramatic
increases in potential (with added substrate) protease activity and
soluble protein concentrations. Increased soil N demand apparently
stimulates microbial production of proteases, causing the activity of
these enzymes to exceed the availability of soil proteins, and become
substrate limited. It is likely that this increase in protease activity
also caused the pulse of soluble proteins that occurred at this time,
liberating them from the breakdown of larger, insoluble protein
complexes.
The research project I worked on as a post-doc was focused on
understanding the interactions between C availability to soil microbes,
soil enzyme dynamics, and nutrient availability. This work was done in
the sub-alpine coniferous forest at the Niwot Ridge LTER in the Front
Range of the Rocky Mountains. Dramatic shifts in the soil fungal
community from winter to summer were previously observed at this site.
The hypothesis I was testing is that these shifts are stimulated by
inputs of labile C to the soil associated with plant root growth at the
height of the growing season, resulting in the dominant C source to
soil microbes switching from soil organic matter decomposition to root
C inputs. To test this hypothesis, I tracked changes in the dynamics of
a variety of different microbial enzymes, and changes in labile soil C,
N, and P pools across the growing season. We predicted that if our
hypothesis was correct, there should be increases in soil C
availability at the height of the growing season. At this time, the
dominant soil enzymes should switch away from those that depolymerize
large molecules such as cellulose and lignin, to those that make N and
P available to soil microbes as soil C availability increases. Such a
change in soil enzyme activity should also correspond to changes in
soil N and P availability, and changes in the composition of fungal
community. We found, however, that rhizodeposition is high in the
spring, when the soils are still snow covered, and not at the height of
the growing season, and that there are large ephemeral populations of
microorganisms dependent upon this C. Microbial N acquisition from
peptide degradation increased with increases in microbial biomass when
rhizodeposition was highest. However, our data indicate that the
breakdown of cellulose, lignin, chitin, and organic phosphorus are not
affected by springtime increases in soil microbial biomass associated
with increases in rhizodeposition. We concluded that the priming of
soil C mineralization by rhizodeposition is due to the stimulation of
the microbial biomass and an increase in the breakdown of N rich
proteins, but not due to increases in the degradation of plant litter
constituents such as cellulose and lignin.
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