Recovery of saturated hydraulic conductivity under secondary succession on former pasture in the humid tropics

Landscapes in the humid tropics are undergoing a continuous change in land use. Deforestation is still taking its toll on forested areas, but at the same time more and more secondary forests emerge where formerly agricultural lands and pastures are being abandoned. Regarding soil hydrology, the extent to which secondary succession can recover soil hydrological properties disturbed by antecedent deforestation and pasture use is yet poorly understood. We investigated the effect of secondary succession on saturated hydraulic conductivity (Ks) at two soil depths (0-6 and 6-12 cm) using a space-for-time approach in a landscape mosaic in central Panama. The following four land-use classes were studied: pasture (P), secondary forest of 5-8 years of age (SF5), secondary forest of 12-15 years of age (SF12) and secondary forest of more than 100 years of age (SF100), each replicated altogether four times in different micro-catchments across the study region. The hydrological implications of differences in Ks in response to land-use change with land use, especially regarding overland flow generation, were assessed via comparisons with rainfall intensities.Recovery of Ks could be detected in the 0-6 cm depth after 12 years of secondary succession: P and SF5 held similar Ks values, but differed significantly (α = 0.05) from SF12 and SF100 which in turn were indistinguishable. Variability within the land cover classes was large but, due to sufficient replication in the study, Ks recovery could be detected nonetheless. Ks in the 6-12 cm depth did not show any differences between the land cover classes; only Ks of the uppermost soil layer was affected by land-use changes. Overland flow - as inferred from comparisons of Ks with rainfall intensities - is more likely on P and SF5 sites compared to SF12 and SF100 for the upper sample depth; however, generally low values at the 6-12 cm depth are likely to impede vertical percolation during high rainfall intensities regardless of land use.We conclude that Ks can recover from pasture use under secondary succession up to pre-pasture levels, but the process may take more than 8 years. In order to gain comprehensive understanding of Ks change with land use and its hydrological implications, more studies with detailed land-use histories and combined measurements of Ks, overland flow, precipitation and throughfall are essential.     

The gas phase oxidation of elemental mercury by ozone

The gas phase reaction between elemental mercury (Hg⁰) and ozone (0₃) has been studied in sunlight, in darkness, at different temperatures, and different surface-to-volume (s/v) ratios. At 0₃ concentrations above 20 ppm, a loss of Hg⁰ and a simultaneous formation of oxidized mercury (Hg(II)) was observed. The results suggest a partly heterogeneous reaction, with a gas phase rate constant of 3±2×10^?20 cm³ molec.^?1 s^?1 at 20 °C. This corresponds to an atmospheric Hg half-life of about one year at a mean global 0₃ concentration of 30 ppb.     

Amelioration of a hardsetting Alfisol through deep mouldboard ploughing, gypsum application and double cropping. III. Crop production and profitability

Deep mouldoard ploughing to 0.45 m, gypsum application (5 t ha⁻¹), and double cropping were evaluated, alone and combined, as ameliorants for a hardsetting red-brown earth (Alfisol). The double cropped treatment consisted of winter wheat (Triticum aestivum) and a summer forage crop consisting of sudax (Sorghum sudanese) intersown with cowpea (Vigna unguiculata). This paper describes the effect of these treatments on crop development, yield, crop-water relations and economic returns. Crop emergence was not affected by any treatment. Mouldboard ploughing significantly increased wheat grain yield from 1.86 to 2.15 t ha⁻¹, in 1984, but not in 1985. Yield was also increased from 5.18 to 5.68 t ha⁻¹ as a result of gypsum addition in 1985—after no significant difference in 1984. Double cropping significantly increased yield (by 0.6 t ha⁻¹) in 1985. Greater increases were obtained with the summer cropping phase where higher evaporative demand during the summer would have aggravated soil structural problems associated with hardsetting. Sudax-cowpea dry matter yields were increased from 13.6 to 17 t ha⁻¹ and from 13.0 to 17.5 t ha⁻¹ during summer 1984–1985 by gypsum and mouldboard ploughing, respectively. The higher yields are attributed to increased water storage and depletion, and reduced soil strength for the mouldboard ploughed and gypsum main treatments. No interactions were found between the main treatments, except at the end of the 1985 winter cropping phase when there was a negative interaction between mouldboard ploughing and gypsum application for dry matter, equivalent root length and yield. The yield increases associated with mouldboard ploughing did not persist beyond the second year of the experiment. Double cropping combined with mouldboard ploughing (Mb+DC) was the most profitable treatment, whereas mouldboard ploughing with gypsum (Mb+G) was the least profitable for the crops grown in this experiment.     

Linkage of early-onset familial breast cancer to chromosome 17q21

Human breast cancer is usually caused by genetic alterations of somatic cells of the breast, but occasionally, susceptibility to the disease is inherited. Mapping the genes responsible for inherited breast cancer may also allow the identification of early lesions that are critical for the development of breast cancer in the general population. Chromosome 17q21 appears to be the locale of a gene for inherited susceptibility to breast cancer in families with early-onset disease. Genetic analysis yields a lod score (logarithm of the likelihood ratio for linkage) of 5.98 for linkage of breast cancer susceptibility to D17S74 in early-onset families and negative lod scores in families with late-onset disease. Likelihood ratios in favor of linkage heterogeneity among families ranged between 2000:1 and greater than 10(6):1 on the basis of multipoint analysis of four loci in the region.     

New routes to early memories

Stimulation of one side of the olfactory system during training with odor-milk pairings in neonatal rats results in their ability to recall an odor memory by using the trained but not the untrained side of the brain. In 12-day-old rats, olfactory learning can be recalled by stimulation of either the trained or untrained side. The development of bilateral recall reflects the maturation of olfactory commissural pathways that provide access to the olfactory memory stored on the contralateral side. Furthermore, the commissural pathways need not be present at the time of memory formation but can establish new and specific access to already existing olfactory memories.