The eVect of land use on the biogeochemistry of small tropical rivers and their estuaries was studied using the Kallada River and Ashtamudi estuary located in the State of Kerala, India, as a model system. Water, suspended matter and sediments collected during the monsoon and intermonsoon periods in 2002 and 2003 were analyzed for dissolved nutrients (nitrate, nitrite, phosphate, silicate) and for phytoplankton abundance and composition, amino acid contents and stable carbon (C)) and nitrogen (N) isotope ratios. Seasonal and spatial variations of dissolved nutrients and suspended matter along the course of the river point to distinct diVerences in the C and N sources that are controlled by hydrology, geology and land use. Unusually low concentrations of dissolved silicate and suspended matter suggest low erosion rates of the Precambrian basement rocks and the Wrm lateritic soils in non-agricultural areas. Most dissolved nutrients and suspended particulate organic matter originated from fertilized agricultural soils. The biogeochemistry of sedimentary organic matter indicates that most of the Kallada River load is deposited in the upper Ashtamudi estuary, while the middle and lower parts have a stronger marine inXuence. The spatio-temporal variation of dissolved and particulate river Xuxes clearly indicates an eVect of land use and land cover on the biogeochemistry of the Kallada River. While the phosphate yield was high (6 £ 10 3 mol km ¡2 year ¡1 or 185 kg km ¡2 year ¡1 ), the N yield was relatively low (10 £ 10 3 mol km ¡2 year ¡1 or 141 kg km ¡2 year ¡1 ), which is unlike the situation in many other densely populated regions of tropical Asia.

Oxyfluorfen, a diphenyl-ether herbicide is being used to control annual and perennial broad-leaved weeds and sedges in a variety of field crops including onion. The present study was aimed to investigate the dynamics and field persistence of oxyfluorfen in onion plant, bulb and soil under Indian tropical conditions. Application of four rates of oxyfluorfen viz., 200, 250, 300 and 400 g AI ha(-1) as pre-emergence gave good weed control in field experiment with onion. The oxyfluorfen residue dissipated faster in plant than in soil respectively, with a mean half-life of 6.1 and 11.2 days. Dissipation followed first-order kinetics. In laboratory column leaching experiments, 17 percent of the applied oxyfluorfen was recovered from the soil and indicates its solubility in water and mobility in sandy clay loam soil was low. A sorption study revealed that the adsorption of oxyfluorfen to the soil was highly influenced by the soil organic carbon with the Koc value of 5450. The study concludes that the dissipation of oxyfluorfen in soil and onion was dependent on the physico-chemical properties of the soil and environmental conditions.

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Direct seeded rice (DSR) is gaining momentum in India due to acute labour shortage during the peak period of transplanting and shortage of water. Weeds are the major biological constrain in DSR and its management contributes a major share in cost of production, as crop and weed emerge simultaneously and exerts competition right from the beginning of the crop. Field experiments were conducted in rabi 2013 and 2014 to study the effect of new herbicide combination, bispyribac sodium + metamifop 14% SE on weed control in DSR and their residual effect on succeeding greengram. Results revealed that the post-emergence (POE) application of herbicide combination, bispyribac sodium + metamifop 14% SE at 70 g ha-1 with wetter recorded significantly lower total weed density (25.78 and 24.19 plants m-2 respectively, during 2013 and 2014), total weed biomass (24.89 g m-2 and 34.56 g m-2 respectively, during 2013 and 2014) and higher weed control efficiency Original Research Article