Final Technical Report

Abstract

OAK-B135 This study took place at the Nevada Desert FACE (Free Air CO2 Enrichment) Facility at the Nevada Test Site, where effects of elevated atmospheric CO2 on a desert ecosystem are being studied. One hundred sixty-eight minirhizotrons (clear plastic tubes) were installed to a depth of 1m in the soil in the control and elevated CO2 plots. Tubes were installed from a suspended platform to avoid soil compaction and disturbance. Tubes were placed under individuals of two dominant shrub species, Larrea tridentata and Ambrosia dumosa, and along systematic transects across the plots. Specialized video cameras were inserted down the tubes at 4 week intervals to provide images of plant root systems on the upper side of the tube. A ratcheting mechanism assured consistent imaging of the same precise locations during each sampling period. Images were collected every 4 weeks from December 1997 to January 2001, after which the images were too degraded from repeated camera abrasion on the tubes for adequate analysis. Over 100,000 video images were analyzed and the appearance, growth, and disappearance of 23,634 individual fine roots (<2 mm diameter) were tracked over time, totaling 125,679 root observations and measurements. Elevated CO2 did not have an effect on the timing of seasonal patterns of fine root growth or turnover (mortality). There were no consistent effects of elevated CO2 on fine root length standing crop, production, or turnover except standing crop was consistently lower under the elevated CO2 treatment across the community transects. The specific root length (m/g of root dry weight) found to be higher for Larrea and Ambrosia under elevated CO2 treatments. Procedures were developed to translate the length measurements taken from minirhizotron images to biomass estimates per unit soil volume, utilizing these specific root length measurements. While few differences in fine root length were apparent as a result of elevated CO2 treatment, conversion to biomass units indicated that elevated CO2 led to decreases in fine root biomass, production, and turnover. This was an unexpected result since many elevated CO2 studies have shown increases in below ground biomass allocation. No differences were found in fine root carbon or nitrogen concentrations, but lower biomass turnover under elevated CO2 implies lower rates of C and N cycling through fine root turnover. Funds from this interagency agreement also allowed the development of improved software for image analysis, which will benefit other researchers using minirhizotrons to study below ground responses to elevated CO2 or other treatments.