La Cantera Oriente is a shallow freshwater volcanic water body located at an altitude of 2 270 m above sea level in the Ecological Reserve of San Angel Pedregal of Mexico City （Mexico）. In order to ensure the conser...La Cantera Oriente is a shallow freshwater volcanic water body located at an altitude of 2 270 m above sea level in the Ecological Reserve of San Angel Pedregal of Mexico City （Mexico）. In order to ensure the conservation of its biological heritage including zooplankton, the present work was undertaken to quantify the seasonal changes in the diversity and density of rotifers and the selected physico- chemical variables during 2013-2014. Qualitative analysis of the zooplankton samples yielded 68 rotifer species which represented 24 genera in 15 families. Brachionus calyciflorus Pallas, 1766, B. quadridentatus Hermann, 1783, Polyarthra vulgaris Carlin, 1943, Lecane closterocerca （Schmarda, 1859） and Keratella cochlearis （Gosse, 1851） were the most common species. Preston plots of species frequency-density revealed that as many as 30% of the rotifer taxa were dominant throughout the year. The species with high population densities were Brachionus quadridentatus, Lecane closterocerca, Keratella cochlearis, and Lepadellapatella; their peak densities were 2 000, 1 000, 180 and 90 ind./L, all occurring in summer. Canonical correspondence analysis showed that Platyias quadricornis was related to the concentration of phosphates available in the environment and the conductivity, while B. quadridentatus was positively correlated with chlorophyll-a. The trophic status of the lake was eutrophic based on Chl-a content but oligotrophic with relation to the Braehionus： Trichocerca ratio.展开更多
In the subtropical highlands of Central Mexico, where the main crop is maize （Zea mays）, the conventional practice （CP） involves tillage, monoculture and residue removal, leading to soil degradation and unsustaina...In the subtropical highlands of Central Mexico, where the main crop is maize （Zea mays）, the conventional practice （CP） involves tillage, monoculture and residue removal, leading to soil degradation and unsustainable use of natural resources and agricultural inputs. Conservation agriculture （CA） has been proposed as a viable alternative in the region, based on reduction in tillage, retention of adequate levels of crop residues and soil surface cover and use of crop rotation. This study began in 2009 when the highlands of Central Mexico suffered from a prolonged drought during vegetative maize growth in July-August, providing an opportunity for the on-farm comparison of CA with CP under severe drought conditions which 21 climate change models projected to become more frequent. Under dry conditions, CA resulted in higher yields and net returns per hectare as early as the first and second years after adoption by farmers. As an average of 27 plots under farmers＇ management in 2009, the maize yields were 26% higher under CA （6.3 t ha-1） than under CP （5.0 t ha-l）. 2010 was close to a normal year in terms of rainfall so yields were higher than in 2009 for both practices; in addition, the yield difference between the practices was reduced to 19% （6.8 t ha-1 for CA vs. 5.7 t ha-1 for CP）. When all the 2009 and 2010 observations were analyzed in a modified stability analysis, CA had an overall positive effect of 3 838 Mexican Pesos ha-1 （320 $US ha-1） on net return and 1.3 t ha-1 on yield. After only one to two years of adoption by farmers on their fields, CA had higher yields and net returns under dry conditions that were even drier than those predicted by the analyzed 21 climate change models under a climate change scenario, emission scenario A2.展开更多
文摘La Cantera Oriente is a shallow freshwater volcanic water body located at an altitude of 2 270 m above sea level in the Ecological Reserve of San Angel Pedregal of Mexico City （Mexico）. In order to ensure the conservation of its biological heritage including zooplankton, the present work was undertaken to quantify the seasonal changes in the diversity and density of rotifers and the selected physico- chemical variables during 2013-2014. Qualitative analysis of the zooplankton samples yielded 68 rotifer species which represented 24 genera in 15 families. Brachionus calyciflorus Pallas, 1766, B. quadridentatus Hermann, 1783, Polyarthra vulgaris Carlin, 1943, Lecane closterocerca （Schmarda, 1859） and Keratella cochlearis （Gosse, 1851） were the most common species. Preston plots of species frequency-density revealed that as many as 30% of the rotifer taxa were dominant throughout the year. The species with high population densities were Brachionus quadridentatus, Lecane closterocerca, Keratella cochlearis, and Lepadellapatella; their peak densities were 2 000, 1 000, 180 and 90 ind./L, all occurring in summer. Canonical correspondence analysis showed that Platyias quadricornis was related to the concentration of phosphates available in the environment and the conductivity, while B. quadridentatus was positively correlated with chlorophyll-a. The trophic status of the lake was eutrophic based on Chl-a content but oligotrophic with relation to the Braehionus： Trichocerca ratio.
基金Supported by a scholarship from the Mexican National Science Commission(CONACYT)the CGIAR Research Program on Climate Change Agriculture and Food Security(CCAFS)the Mexican Secretariat of Agriculture,Livestock,Rural Development,Fisheries and Food(SAGARPA)
文摘In the subtropical highlands of Central Mexico, where the main crop is maize （Zea mays）, the conventional practice （CP） involves tillage, monoculture and residue removal, leading to soil degradation and unsustainable use of natural resources and agricultural inputs. Conservation agriculture （CA） has been proposed as a viable alternative in the region, based on reduction in tillage, retention of adequate levels of crop residues and soil surface cover and use of crop rotation. This study began in 2009 when the highlands of Central Mexico suffered from a prolonged drought during vegetative maize growth in July-August, providing an opportunity for the on-farm comparison of CA with CP under severe drought conditions which 21 climate change models projected to become more frequent. Under dry conditions, CA resulted in higher yields and net returns per hectare as early as the first and second years after adoption by farmers. As an average of 27 plots under farmers＇ management in 2009, the maize yields were 26% higher under CA （6.3 t ha-1） than under CP （5.0 t ha-l）. 2010 was close to a normal year in terms of rainfall so yields were higher than in 2009 for both practices; in addition, the yield difference between the practices was reduced to 19% （6.8 t ha-1 for CA vs. 5.7 t ha-1 for CP）. When all the 2009 and 2010 observations were analyzed in a modified stability analysis, CA had an overall positive effect of 3 838 Mexican Pesos ha-1 （320 $US ha-1） on net return and 1.3 t ha-1 on yield. After only one to two years of adoption by farmers on their fields, CA had higher yields and net returns under dry conditions that were even drier than those predicted by the analyzed 21 climate change models under a climate change scenario, emission scenario A2.
