Isabel Casar

Forest-to-pasture conversion influences on soil organic carbon dynamics in a tropical deciduous forest

On a global basis, nearly 42% of tropical land area is classified as tropical deciduous forest (TDF) (Murphy and Lugo 1986). Currently, this ecosystem has very high deforestation rates; and its conversion to cattle pasture may result in losses of soil organic matter, decreases in soil fertility, and increases in CO2 flux to the atmosphere. The soil organic matter turnover rate in a TDF after pasture conversion was estimated in Mexico by determining natural abundances of13C. Changes in these values would be induced by vegetation changes from the C3 (forest) to the C4 (pasture) photosynthetic pathway. The rate of loss of remnant forest-soil organic matter (fSOM) was 2.9 t ha−1 year−1 in 7-year-old pasture and decreased to 0.66 t ha−1 year−1 by year 11. For up to 3 years, net fSOM level increased in pastures; this increment can be attributed to decomposition of remnant forest roots. The sand-associated SOM fraction was the most and the silt-associated fraction the least depleted. TDF conversion to pasture results in extremely high rates of loss of remnant fSOM that are higher than any reported for any tropical forest.

Carbon isotopic composition of Mexican honey

SUMMARYWe characterized Mexican honey from the main production regions, using the internal stable carbon isotope ratio (ISIRA) of the honey and its protein. We found the average δ13C given by White for American honey is very close to the corresponding value for Mexican honey even though clear evidence that C4 and CAM plants are visited by bees. We found adulteration of Mexican commercial samples using the criterium of −1.00 ‰ difference also given by White. Two pyrolysis methods were tested to optimize the accuracy of the carbon isotope ratio analysis of honey. We found that the quartz method showed smaller variation in the data due to a complete combustion of the honey. The honey protein was isolated and purified using the protein dialysis procedure recommended by the Association of Official Analytical Chemists. We found that the direct precipitation method without dialysis is not useful for samples which are adulterated by more than 50%.

Influence of the Spatial Non-Uniformity of the Grating on the Beam Energy Exchange of Thick Sillenite Crystals

The influence of the variation of the phase and magnitude of the complex light modulation along sample thickness on the gain in a thick crystal of BTO in two-wave mixing was obtained.

Energy Exchange in Thick Photorefractive Crystals of BTO with Optical Activity at High Modulation Depth

The effect of the spatial non-uniformity of the photorefractive grating and the polarization angle of the recording light beams on the gain in thick crystals of BTO with optical activity were obtained.

Beam Energy Exchange Dependence on Grating Period in Bismut Silicate (Bi 12 SiO 20 ) with Optical Activity and Linear Birefringence under Strong Nonlinear Regime

We calculated the energy exchange in BSO, considering different grating periods, optical activity, birefringence, absorption, polarization angle, nonlinear conditions and high dc fields. Large beam energy exchange may occur in spite of absorption.

Variation of the Magnitude and Phase of the Recorded Complex Refraction Index along Waves Propagation in Sillenites

We calculated the complex recorded refraction index taking into account optical activity, absorption, birefringence, polarization angles, external d. c. field and the non-uniformity of the grating along sample thickness. A strong variation was found.

Influence of the Optical Acivity and Linear Birefringence on the Diffraction Efficiency of Bismut Silicate (Bi12 SiO20 ) Under Strong Nonlinear Regime

We calculated the diffraction efficiency for a BSO crystal, considering optical activity, strong non-linear conditions and high dc fields. A significant increase may occur depending on sample thickness, polarization angle and applied field.