ABSTRACT
Environmental responses of stomatal conductance (gs) as basic information for a photosynthesis-transpiration-coupled model have been increasing under global warming. This study identified the impact of gs behavior under different soil water statuses and temperatures in rice, maize, millet, and sorghum. The experiments consisted of various soil moisture statuses from flooding to drying and combination of soil moisture status and temperature. There was a reduction in shoot biomass of maize and sorghum caused by decreasing of gs, photosynthesis (A), and transpiration (E) in early imposed waterlogging without dependent temperature, whereas millet and rice were dependent on temperature variation. The effect of gradual soil drying, gs, A, and E of maize, millet, and sorghum were caused by low temperature, except rice. The impact of the combination of various soil water statuses and temperatures on gs is important for the trade-off between A and E, and consequently shoot biomass. However, we discovered that an ability to sustain gs is essential for photo assimilation and maintaining leaf temperature through evapotranspiration for biomass production, a mechanism of crop avoidance in variable soil water status and temperature.
ABSTRACT
In tropical mountainous areas, soil degradation and yield decrease have been anticipated due to conversion from shifting to continuous cultivation and the introduction of cash crops. In our previous report, we quantified the decrease in maize yield under continuous cultivation in farmers' fields in Laos. In this report, we focused on soil nutritional conditions under continuous cultivation in the farmers' fields. For the purpose, twelve soil properties were investigated over two years from three sample sites in each of the 40 farmers' fields with the duration of continuous cultivation varying from 1 to 30 years. Total carbon (TC), total nitrogen (TN), available phosphorus, exchangeable potassium, and exchangeable calcium in the soil decreased with increasing duration of continuous cultivation in the sloped fields. These soil nutrients decreased to around half of the initial content in these 30 years. However, the decreasing rates of TC and TN were negligible in the flat fields. Other soil properties such as clay and exchangeable magnesium were not related to the duration of continuous cultivation in both sloped and flat fields. The reduction in maize yield was mainly explained by TC, but the determination coefficient was only 0.24. Although further analysis is required to quantify the effect of soil nutrients on maize production, the development of integrated soil management would be necessary in the sloped fields for sustainable crop production in the study site.
ABSTRACT
Saltpetre (KNO3; potassium nitrate) is one of the major ingredients of gunpowder. Simplex saltpetre (total 126 samples) together with gunpowder (total 93 samples) commodities which are currently prevailing in local markets as used for wildlife hunting and rocket festivals (local name in Lao: Boun Ban Fai) were collected from throughout Lao PDR. Dual isotopic composition of nitrates (δ15N, δ18O) was analyzed by the microbial denitrification method. Binary plotting of the data was conducted to characterize the mode of formation. In Lao PDR, almost all the commodities were imported from neighbouring Thailand, and to a lesser extent from Israel. Binary plotting of δ15N vs. δ18O of nitrates shows their intrinsic origin manufactured by Haber-Bosch and subsequently Ostwald processes. We observed an inverse correlation (δ15Nnitrate: up to + 12â ; δ18Onitrate: down to + 15â ), deviating from the reservoir values (free air) of δ15N (dinitrogen; up to 0â ) and δ18O (oxygen; +23.5â ), interpreted this as an indication of isotope fractionation during manufacture. The most plausible interpretation for this is the fractionation associated with formation of NO following NO2 and finally NO3 products. A nearly comparable inverse relationship is also observed for nitrates in pyrotechnics manufactured in China.
