Sedgwick Reserve

30 Jun, 05 PM - 29 Jun, 05 PM

Research Objectives: My research focuses on the comparison of N cycling in exotic annual and native perennial grassland systems, and on determining the relative importance of different microbial processes in contributing to N loss from each grassland type. My specific questions are: (1) Are stands of nonnative annual grasses more ?leaky? with regard to N than are stands of native perennial grasses? (2) Which microbial processes are important in contributing to N loss from each stand? (3) How does N fertilization affect the ?leakiness? of the stand and the microbial processes contributing to N loss? Study Sites: It is often difficult to determine whether an invasive plant has altered soil properties and microbial activity after becoming established, or if its success in a given spot is due to pre-established differences in soil properties or nutrient availability. I have had the advantage of using seeded multi-species experimental plots and monoculture plots to study microbial activity without the confounding factor of heterogeneous soils. As a complement to the study of microbial processes in these plots, I would like to use naturally occurring stands of perennial grasses at Sedgwick to gain a better understanding of soil microbial processes under natural field conditions. A description of the three plot types follows: (1) Multi-species Experimental Plots: In 1997, 34 3x3 m2 experimental plots of seeded annual and perennial grasses were established at Sedgwick Reserve. Measurement of soil microbial processes and N cycling is currently underway in these plots. The experimental plots lie along Figueroa Creek and utilize approximately 3.5 ha of agricultural fields. Prior to their acquisition in 1997, these fields were continuously row-cropped to yield soil uniformity. (2) Monocultures: In the fall of 2000, Eric Seabloom established experimental monocultures (1 m2) composed of 25 species selected from 5 distinct functional groups as well as bare ground plots on uniform soils at Sedgwick Reserve. These plots have been maintained as single-species stands for the past two years. In the proposed research, I will make use three native perennial grass plots (Bromus carinatus, Elymus glaucus, and Nassella pulchra) and five exotic annual grasses (Avena barbata, Bromus madritensis, Bromus hordeaceus, Bromus diandris, and Hordeum murinum). (3) Naturally Occurring Stands: Three pairs of established, naturally occurring stands of nonnative annual-dominated and native perennial-dominated grasses have been chosen in the Figueroa Creek drainage at Sedgwick Reserve. The plot pairs have similar slope, aspect, oak canopy cover, and soil type, and were set up by Stanley Harpole as observational plots in 2001. Methods: To answer my first question, I am measuring N lost as either NO3- or DON in groundwater, and N gases lost as either N2O or N2. In January of 2002, lysimeters were installed in all 16 Experimental Plots. Throughout the rainy season, water samples are collected and N content measured using a Lachat flow-injection autoanalyzer. Beginning this fall, I will measure actual denitrification and N gas production rates using the static-core acetylene inhibition technique and the analysis of N2O by gas chromatography (GC) as described by Groffman et al. (1999). This involves taking soil cores in gas-tight PVC tubes, adding acetylene gas to 10% of the headspace in each core, and then sampling and analyzing the headspace several times over the course of a 6 hour incubation. To answer the second question, I am examining the mechanisms behind N loss from annual and perennial systems by quantifying rates of soil microbial processes (nitrification and denitrification) and determining their relative importance in contributing to N loss from each system. In situ rates of N mineralization and nitrification may be quantified together using the intact-core technique with paired soil samples taken in PVC tubes. One sample is brought back to the lab, extracted, and analyzed for NH4+ and NO3- immediately. After four weeks, incubated soils are also analyzed. The rate of N mineralization is calculated as the final concentration minus the initial concentration of inorganic N. Rates of denitrification may vary widely on spatial and temporal scales, and the combined use of several methods for the study of microbial processes can provide a more complete picture of N cycling in soils (e.g. Clein and Schimel 1995, Fierer et al. 2001). In addition to the techniques mentioned above, I plan to use intact-core 15N isotope pool dilution as an additional measure of N cycling in the monoculture plots. After injecting 15NH4+ into the monoculture plots, short-term rates of uptake by plants and microbes and longer term partitioning will be measured using techniques similar to those used by Jackson et al. (1989) and Schimel et al. (1989). In addition, I will continue measuring the nitrification and denitrification potentials of soils using the assays developed by Tiedje et al. (1989), and I plan to initiate in-lab incubations of soils for the determination of microbial carbon (C) respired vs. N mineralized. To address the final question, I am comparing the amounts of N lost from fertilized plots to amounts lost from control plots. Interactions between soil microbes and plants can often be complex, making it difficult to predict the relative importance of microbial mechanisms behind N ?leakiness? from each grassland system. Therefore, N fertilization is being used as a tool in the Experimental Plots to better understand process interactions. Time schedule: After 1st rains (Oct. or Nov. 2003): 1st sampling of soils for in-lab denitrification analyses, placement of in situ soil cores for N min. measurement. Following each rain event during the 2003-2004 rainy season: Collection of groundwater from lysimeters for analysis April 2004 (height of growing season, water availability high): 2nd sampling of soils for in-lab analyses, placement of in situ soil cores for N min. measurement. June 2004 (end of growing season, beginning of summer drying period): 3rd sampling of soils for in-lab analyses, placement of in situ soil cores for N min. measurement. I will continue this project through the 2003-2004 winter-spring rainy season. I hope to write up the results from data collected at Sedgwick Reserve during the summer of 2004.

Visit #2131 @Sedgwick Reserve

Approved

Under Project # 1706 | Research

Nitrogen dynamics in native perennial and non-native annual grasslands in California

research_scientist - University of California, Santa Barbara

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Sophie Parker Jun 30, 2003 - Jun 29, 2004 (366 days)

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