Completed Research Projects

The Changing Seasonality of Tundra Plant-Soil Interactions
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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.

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.

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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