Biomedical subjects

Weixin Cheng

Publications and source records attributed to Weixin Cheng.

3 recordsLinked to original sources

PubMed2006

Influences of canopy photosynthesis and summer rain pulses on root dynamics and soil respiration in a young ponderosa pine forest.

Our first objective was to link the seasonality of fine root dynamics with soil respiration in a ponderosa pine (Pinus ponderosa P. & C. Lawson) plantation located in the Sierra Nevada of California. The second objective was to examine how canopy photosynthesis influences fine root initiation, growth and mortality in this ecosystem. We compared CO2 flux measurements with aboveground and belowground root dynamics. Initiation of fine root growth coincided with tree stem thickening and shoot elongation, preceding new needle growth. In the spring, root, shoot and stem growth occurred simultaneously with the increase in canopy photosynthesis. Compared with the other tree components, initial growth rate of fine roots was the highest and their growing period was the shortest. Both above and belowground components completed 90% of their growth by the end of July and the growing season lasted approximately 80 days. The period for optimal growth is short at the study site because of low soil temperatures during winter and low soil water content during summer. High photosynthetic rates were observed following unusual late-summer rains, but tree growth did not resume. The autotrophic contribution to soil respiration was 49% over the whole season, with daily contributions ranging between 18 and 87%. Increases in soil and ecosystem respiration were observed during spring growth; however, the largest variation in soil respiration occurred during summer rain events when no growth was observed. Both the magnitude and persistence of the soil respiration pulses were positively correlated with the amount of rain. These pulses accounted for 16.5% of soil respiration between Days 130 and 329.

Carbon Dioxide↗

PubMed2005

Measuring tree root respiration using (13)C natural abundance: rooting medium matters.

Tree root respiration utilizes a major portion of the primary production in forests and is an important process in the global carbon cycle. Because of the lack of ecologically relevant methods, tree root respiration in situ is much less studied compared with above-ground processes such as photosynthesis and leaf respiration. This study introduces a new (13)C natural tracer method for measuring tree root respiration in situ. The method partitions tree root respiration from soil respiration in buried root chambers. Rooting media substantially influenced root respiration rates. Measured in three media, the fine root respiration rates of longleaf pine were 0.78, 0.27 and 0.18 mg CO(2) carbon mg(-1) root nitrogen d(-1) at 25 degrees C in the native soil, tallgrass prairie soil, and sand-vermiculite mixture, respectively. Compared with the root excision method, the root respiration rate of longleaf pine measured by the field chamber method was 18% higher when using the native soil as rooting medium, was similar in the prairie soil, but was 42% lower if in the sand-vermiculite medium. This natural tracer method allows the use of an appropriate rooting medium and is capable of measuring root respiration nondestructively in natural forest conditions.

Carbon Isotopes↗

PubMed1999

Rhizosphere feedbacks in elevated CO(2).

Understanding rhizosphere processes in relation to increasing atmospheric CO(2) concentrations is important for predicting the response of forest ecosystems to environmental changes, because rhizosphere processes are intimately linked with nutrient cycling and soil organic matter decomposition, both of which feedback to tree growth and soil carbon storage. Plants grown in elevated CO(2) substantially increase C input to the rhizosphere. Although it is known that elevated CO(2) enhances rhizosphere respiration more than it enhances root biomass, the fate and function of this extra carbon input to the rhizosphere in response to elevated CO(2) are not clear. Depending on specific plant and soil conditions, the increased carbon input to the rhizosphere can result in an increase, a decrease, or no effect on soil organic matter decomposition and nutrient mineralization. Three mechanisms may account for these inconsistent results: (1) the "preferential substrate utilization" hypothesis; (2) the "priming effect" hypothesis; and (3) the "competition" hypothesis, i.e., competition for mineral nutrients between plants and soil microorganisms. A microbial growth model is developed that quantitatively links the increased rhizosphere input in response to elevated CO(2) with soil organic matter decomposition. The model incorporates the three proposed mechanisms, and simulates the complexity of the rhizosphere processes. The model also illustrates mechanistically the interactions among nitrogen availability, substrate quality, and microbial dynamics when the system is exposed to elevated CO(2).

Journal Article↗