Anine Smith M.S. defense
GDPE student Anine Smith M.S. defense
Increased nitrogen deposition is currently an important driver of plant species composition change in terrestrial ecosystems globally. General effects of increased soil nitrogen supply caused by deposition include increased production, decreased species richness and increased dominance of invasive plants.
These effects may vary depending on the degree of N limitation in a particular ecosystem, but because most terrestrial plant communities are at least partially N-limited, increased production is almost always expected to result. Increased production is often accompanied by community change.
Shifts in community structure can occur through changes in basic ecosystem conditions (such as soil properties) or through alterations in competitive interactions, potentially disrupting feedbacks that maintain an ecosystem at a given fertility level. Although there has been a substantial amount of research on the effects of increased N on communities and ecosystems, most studies add large and, relative to natural inputs, unrealistic amounts of N.
Thus, responses are often immediate and drastic. This “two-point” approach, comparing control to high-N plots, provides little information about the levels of N inputs at which responses first occur which is more important for managing and mitigating the effects of increased N deposition in a proactive rather than reactive manner. The overarching goal of this study was to identify response thresholds to N addition in Northern Great Plains grasslands that differed markedly in soil fertility.
At present, N inputs into these ecosystems are relatively low (3.68 kg N/ha/yr) but these are expected to increase with plans for the construction of several fossil fuel based energy production facilities regionally. Over two years we assessed responses to increased N inputs (from 2.5 to 100 kg N/ha) in soils, leaf tissue, plant community composition, and aboveground net primary production (ANPP) in northern mixed prairie grasslands that varied 3-fold in ANPP.
Despite the three sites’ close geographic proximity in Wind Cave and Badlands National Parks in South Dakota, these three sites encompassed the range of productivity found across the northern Great Plains and thus my results will enable us to better forecast both ecosystem and community responses to increased fertilization in this understudied region.
After two years of fertilization (with and without water additions) at levels ranging from 0 to 100 kg N/ha, I found significant effects from increased N inputs on ANPP when nitrogen levels exceeded 68 kg N/ha/year and effects on leaf tissue nitrogen, soil nitrogen content and N mineralization rates, particularly when levels exceeded 45 kg/ha/year. Alterations in soil and leaf nitrogen content and production tended to have linear responses and remain consistent across sites.
Absolute responses to nitrogen were greatest in the grasslands with highest ANPP, but significant responses occurred even in low ANPP sites (Badlands). No significant and consistent effects were found on total species richness, and community metrics such as evenness and diversity indices, plant cover by functional group or canopy cover. This experiment was conducted during two years (2010-2011) with record high rainfall amounts and as a result, I detected no differences between watered and unwatered plots in terms of soil moisture or productivity.
Thus, I assume that water limitation during this time period was alleviated, allowing me to focus solely on nitrogen limitation. My results suggest that though higher production sites may have a greater absolute biomass response, low fertility and low production environments can be quite responsive to nitrogen addition as well. Community change may occur more slowly with these northern grasslands which show little response to two years of increased N inputs.
Event Contact: Jeri Morgan can be reached at (970) 491-4373
Sponsored by the Graduate Degree Program in Ecology.
|Calendar Name:||All University Events Calendar|
|Event Category:||Dissertation & Thesis Defenses|
|Start Time:||01:00 PM|
|End Time:||02:00 PM|
|Event Begins On:||Friday, September 21, 2012|
|Event Ends On:||Friday, September 21, 2012|
|Submitter's Name:||Jeri Morgan|