Submitted to: Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/25/2008
Publication Date: 7/22/2009
Publication URL: parking.nal.usda.gov/shortterm/21111_DijkstraEcology2009.pdf
Citation: Dijkstra, F.A. 2009. Modeling the Flow of 15N After a 15N Pulse to Study Long-Term N Dynamics in a Semi-Arid Grassland. Ecology 90:2171-2182. Interpretive Summary: Nitrogen (N) is important for many ecosystem processes such as net primary productivity, plant species composition, and carbon sequestration. Despite the importance of N, it remains difficult to examine key processes of the N cycle. One way to study the N cycle in terrestrial ecosystems is by adding 15N (stable isotope of N that is rare in natural systems) to the soil and then tracing the 15N afterwards in different ecosystem compartments. A computer model was developed that simulates the flow of 15N in plants, microbes, and soil after addition of 15N to the soil. 15N Fractions measured in plants for five years after addition of 15N to a semi-arid grassland grown under ambient and elevated atmospheric CO2 conditions (368 vs. 720 ppm) in Colorado were simulated with the model. A sensitivity analyses was done to test how different processes affected 15N in plant biomass. The model accurately simulated the 15N measurements. The model showed different sensitivities for each process tested. In conclusion, this model provides a useful tool to better understand N cycling in terrestrial ecosystems.
Technical Abstract: Nitrogen (N) cycling in terrestrial ecosystems remains poorly understood. Progress in studying N cycling has been hindered by a lack of effective measurements that integrate processes such as denitrification, competition for N between plants and microbes, and soil organic matter decomposition over large time-scales (years rather than hours or days). Here I show how long-term measurements of 15N in plants, microbes, and soil after a one-time addition of 15N (labeled N) can provide powerful information about long-term N dynamics. I developed a simple dynamic model and show that fractions of labeled N in plant and microbial N pools (expressed as a fraction of total N in each pool) can change long after 15N application (= 5 years). These 15N dynamics are closely tied to the turnover times of the different N pools. The model accurately simulated the labeled N fractions in aboveground biomass measured annually during five years after addition of 15N to a semi-arid grassland system. I then tested the sensitivity of five different processes on labeled N fractions in aboveground plant biomass. Changing plant/microbial competition for N had very little effect on the labeled N fraction in aboveground biomass in the short- and long-term. Changing microbial activity (N mineralization and immobilization), N loss, or N resorption/re-translocation by plants affected the labeled N fraction in the short-term, but not in the long-term. Large long-term effects on the labeled N fraction in aboveground biomass could only be established by changing the size of the active soil N pool. Therefore, the significantly greater long-term decline in the labeled N fraction in aboveground biomass observed under elevated CO2 in this grassland system could have resulted from an increased active soil N pool under elevated CO2 (i.e., destabilization of soil organic matter that was relatively recalcitrant under ambient CO2 conditions). I conclude that short- and long-term labeled N fractions in plant biomass after a 15N pulse are sensitive to processes such as N mineralization and immobilization, N loss, and soil organic matter (de-)stabilization. Modeling these fractions provides a useful tool to better understand N cycling in terrestrial ecosystems.