Sedgwick Reserve

01 Jul, 05 AM - 30 Jun, 05 AM

The goal of my dissertation research is to quantify differences in nitrogen cycling dynamics between annual-dominated and perennial-dominated systems through time and over small spatial scales. I expect to determine if grass type produces distinct patterns in nitrogen availability and processing across the landscape and through the seasons, and I hope gain a better understanding of the mechanisms that generate these patterns. The broad question that I intend to address with my research is: How does grass type influence nitrogen cycling dynamics over space and time? To answer this question I have chosen to do my work in both naturally-occurring and experimentally planted annual and perennial grasslands at Sedgwick Reserve. My research involves taking soil samples and groundwater samples from different grassland plots at Sedgwick Reserve. I have taken samples for laboratory assays from three types of plots: (1) plots in naturally-occurring stands of annual and perennial grass, (2) mixed monotypic stands of grass, and (3) monoculture plots. All plots were established by Jim Reichman and Eric Seabloom between 1998 and 2000. Net nitrogen mineralization and nitrification were quantified using in situ intact core incubations of paired soil samples. Incubations have been performed in the mixed plots over the course of two years. Samples were incubated for a one month period in PVC tubes capped with resin cookies. Dates were chosen to get a good representation of nitrogen cycling during different seasons. The rate of net nitrogen mineralization was calculated as the final concentration of inorganic nitrogen in the soil core and the bottom resin cookie minus the concentration of inorganic nitrogen in the initial soil core. Net nitrification rates were determined by considering nitrate concentrations alone. When net nitrification rates are less than zero, the rate of immobilization was assumed to be greater than mineralization. Net immobilization occurs when microbial growth is nitrogen limited. To gain better insight into the importance of microbial immobilization in these grasslands, I plan to measure gross mineralization using the 15N isotope pool dilution technique. This entails taking more soil samples to the lab for 15N additions and 24 hour incubations. I intend to take these samples from the mixed monotypic stands of annual and perennial grasses. Several tools can be used to address the question of how soil properties and nitrogen cycling vary on small spatial scales in perennial and annual grasslands. To characterize the spatial pattern in nitrogen resources in California grasslands, I plan to conduct a geostatistical autocorrelation analysis to detect gradients in nitrogen pools and process rates to determine the scale over which these gradients occur. This type of analysis relies on the assumption that near neighbors (in this case, discrete soil samples) are more similar than far neighbors. For this analysis, I will take soil samples over evenly-spaced intervals along a grid in the mixed plots, and use data analysis methods similar to those employed by Schlesinger et al. (1996). As with the gross mineralization measurements, soil sampling for spatial analysis can take place in the mixed plots where I have taken the majority of my nitrogen process measurements. These plots contain both annuals and perennials. Photographs of the ground cover at each sampling site can later be used to map patterns in resource pools in each plot. I plan to take these photographs early in the growing season, when differences in grass phenology allow for easy mapping of annual and perennial cover. If timed correctly, samples taken for spatial analysis can be used for the gross mineralization incubations as well, and pattern in both resource availability and nitrogen processing will be elucidated. In addition to looking at nitrogen, it might be useful to analyze samples for an element or compound that is not actively used by plants and microbes, to see if spatial patterning may be driven by physical, rather than biological, processes. Creating a semivariogram will allow me to determine if there is a spatial component to the variability- in other words, if there is definable patchiness in nitrogen pools. I can use kriging to make estimates of points that are not sampled, and hopefully create an isopleth map of nitrogen resources to compare fine-scale patterns in annual-dominated and perennial-dominated systems. Schlesinger, W.H., J.A. Raikes, A.E. Hartley, and A.F. Cross. 1996. On the spatial pattern of soil nutrients in desert ecosystems. Ecology 77:364-374.

Visit #5957 @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 Jul 1, 2004 - Jun 30, 2005 (365 days)

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