Lixin Wang

Wang, L June 22nd, 2007

Soil biogeochemistry, aridity and plant adaptation responses in southern Africa savannas

Savannas cover about 20% of the Earth’s land area and 50% of Africa, crossing a wide range of climatic conditions. It remains unclear how the relative importance of water and nutrient limitations varies with the mean climatic conditions in the savanna ecosystems. We used the Kalahari T ransect (KT) in southern Africa as a representative savanna ecosystem and combined multiple tools to understand the variations in soil biogeochemistry and subsequent responses of plants on this immense rainfall gradient.

Keywords: Grass productivity, Fertilization experiment, Isotopes, Nitrogen, Soil profiles, Water

Preferences in nitrogen uptake across plant species and environments

Nitrogen is a limiting nutrient in most terrestrial and aquatic ecosystems; its uptake and availability have long been subjects for ecological research. Plant preference for the utilization of either ammonium or nitrate, has been documented in different plant species and ecosystems. However, the ecological origin of such preferences and how they are modified in a changing environment remain a mystery. Our two studies (an African savanna field site and a greenhouse location) addressed this issue using native African grasses from a geomorphologically homogeneous region, namely, the Kalahari Transect.

Keywords: Adaptation, C₄ plants, Enriched, Grass, Kalahari, Preferences, Nitrogen cycling, Savannas, Stable isotope

Remote sensing of nitrogen and carbon isotope compositions in terrestrial ecosystems

The stable isotopes have been frequently used to indicate processes occurring in soils, plants, and the atmosphere at scales from individual organisms to entire ecosystem. Remote sensing, on the other hand, is a powerful tool used to identify ecosystem patterns and processes at larger scales (e.g., from regional to global scales). A marriage of these two approaches would hold promise to allow for spatially continuous estimates of isotopic compositions, thus providing unprecedented information into the patterns and processes within the Earth’s ecosystems. To date, however, the combination of isotopic and remote sensing techniques is still in the exploratory stage. Two examples, one utilizing high spectral resolution remote sensing and the other employing high spatial resolution remote sensing, are suggested as possible approaches for the integration of stable isotope and remote sensing techniques.

Partition evaporation and transpiration in arid environemnts using OA-ICOS

The off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) uses a narrowband continuous-wave laser that systematically disrupts cavity resonances using a laser that is placed in an off-axis configuration with respect to the cavity. The OA-ICOS absorption cell effectively traps the laser photons so that they make thousands of passes on average before leaving the cell. As a result, the effective optical path length may be several thousands of meters using high-reflectivity mirrors and thus the measured absorption of light after it passes through the optical cavity is significantly enhanced. The OA-ICOS method provides a high frequency resolution LAS with low power requirements (~100 W) that operates at ambient temperatures without need for a reference gas. The OA-ICOS technique has the potential to greatly expand the use of continuous δ¹⁸O and δ²H fluxes measurements to address a wide range of ecohydrological research topics.

Keywords: Hydrogen, Isotope, Laser, Los Gatos Research, Oxygen, Water vapor

Other projects:

1. ¹⁵N patterns during secondary succession at Blandy Experimental Farm

2. Soil CO₂ flux and its abiotic controls during secondary succession in northern Virginia

3. Post-fire resource redistribution in desert grasslands

4. Form and function of grass ring patterns in arid grasslands: the role of abiotic controls

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