Greater contribution of low-nutrient tolerance to sorghum and maize growth under combined stress conditions with high aluminum and low nutrients in solution culture simulating the nutrient status of tropical acid soils

Aluminum is usually regarded as the determining factor for plant growth in acid soils and nutrient deficiencies are often additional growth-limiting factors in tropical acid soils. Taking into account the potential interactions between Al toxicity and nutrient deficiencies, the present study investigated sorghum (Sorghum bicolor Moench [L.]) and maize (Zea mays L.) cultivar differences for: (1) Al tolerance (relative growth in a one-fifth strength nutrient solution [low-nutrient medium, ionic strength: 4.5 mmol L?1] with Al and without Al), (2) low-nutrient tolerance (relative growth in a low-nutrient medium compared with growth in a full-strength nutrient solution), (3) combined tolerance (relative growth in a low-nutrient medium containing Al compared with a full-strength medium lacking Al). The goal of the present study was to identify the predominant growth-limiting factor using a solution culture medium that simulates the nutrient status of tropical acid soils. Differential Al tolerance among 15 cultivars of sorghum and 10 cultivars of maize in short-term assays (2.5 or 20 µmol L?1 AlCl3 in 0.2 mmol L?1 CaCl2 at pH 5.0 or 4.9, respectively, for 24 h) was positively correlated with Al tolerance in long-term cultures (11.1 or 42.6 µmol L?1 soluble Al in the low-nutrient medium at pH 4.5 or 4.3, respectively, for 29 days). However, the level of Al tolerance in the short-term assays was not correlated with the combined tolerance, suggesting that a short-term screening technique may not be practically useful for estimating cultivar adaptation to a combination of stress factors in tropical acid soils. In sorghum, a less Al-tolerant plant species, higher Al tolerance was associated with less Al absorption by the roots and greater K translocation into the shoots. In maize, a more Al-tolerant plant species, there was no correlation between the accumulation or transport of elements and Al tolerance. Standardized partial regression coefficients suggested that low-nutrient tolerance contributed more to combined tolerance than Al tolerance under most conditions (except for Al-sensitive sorghum at 42.6 µmol L?1 AlCl3). A greater combined tolerance was associated with a higher K shoot concentration in sorghum and a higher Ca shoot level in maize. Plant nutritional characteristics linked to low-nutrient tolerance should be evaluated as an important strategy for plant production in tropical acid soils, both for Al-tolerant plant species and for Al-sensitive plant species under low-Al conditions

A base-level stratigraphic approach to determining Holocene subsidence of the Ganges-Meghna-Brahmaputra Delta plain

Relative sea level history, which is the result of the combined effects of land subsidence, sediment supply and absolute sea level history may be reconstructed from preserved sediment thicknesses. However, variations in the preserved sediment thicknesses between different sedimentary environments strongly limit the accuracy of this type of geological approach, particularly in fluvial channelized systems, such as delta plains. To address this, we apply three different and independent stratigraphic approaches to the case of the Ganges–Brahmaputra–Meghna Delta (GBMD). Our approach has been made possible by a broad dataset of geological archives we have collected, which includes more than 400 hand-drilled stratigraphic wells, 198 radiocarbon ages, and river seismic reflection data (255 km of high-resolution multichannel seismic images). The seaward gradient of accommodation and the limit of the subsiding delta plain are estimated, assuming that the delta is near or at the base-level, which is considered to be the relative sea-level. First, a statistical analysis of the variability of preserved sediment thicknesses is used to derive the average pattern of accommodation from the Holocene isopach. Secondly, the preserved sediment thicknesses are analyzed by geomorphotectonic domains to estimate an average pattern of accommodation. Thirdly, the burial history of the seismically imaged last glacial incision of the Brahmaputra River is reconstructed. Results suggest that the variability of preserved sediment thicknesses can be up to 35% in a delta plain between river channel and flood plain deposits for the same relative sea-level history. Taking this variability into consideration, the Holocene relative sea-level history of the GBMD and the most likely pattern of subsidence are determined. Results provide evidence of moderate Holocene subsidence over the delta, gently increasing seaward from <0.2 mm/yr landward of the Hinge Zone, which can be considered as the northern limit of the subsiding delta plain, to 2 ± 0.7 mm/yr in the middle fluvial delta to 4 ± 1.4 mm/yr in the lower tidal delta. This enables us to construct the first millennial-scale map of subsidence pattern on the GBMD in which uncertainties on subsidence rates are provided. This map may aid in evaluating the negative impact that human modification may have on subsidence and relative sea level in the GBMD, and thereby help to determine better sustainable coastal management practices for the GBMD and other large delta plains

The paleogene record of Himalayan erosion: Bengal Basin, Bangladesh

A knowledge of Himalayan erosion history is critical to understanding crustal deformation processes, and the proposed link between the orogen's erosion and changes in both global climate and ocean geochemistry. The most commonly quoted age of India–Asia collision is ~ 50 Ma, yet the record of Paleogene Himalayan erosion is scant — either absent or of low age resolution. We apply biostratigraphic, petrographic, geochemical, isotopic and seismic techniques to Paleogene rocks of the Bengal Basin, Bangladesh, of previously disputed age and provenance. Our data show that the first major input of sands into the basin, in the > 1 km thick deltaic Barail Formation, occurred at 38 Ma. Our biostratigraphic and isotopic mineral ages date the Barail Formation as spanning late Eocene to early Miocene and the provenance data are consistent with its derivation from the Himalaya, but inconsistent with Indian cratonic or Burman margin sources. Detrital mineral lag times show that exhumation of the orogen was rapid by 38 Ma. The identification of sediments shed from the rapidly exhuming southern flanks of the eastern–central Himalaya at 38 Ma, provides a well dated accessible sediment record 17 Myr older than the previously described 21 Ma sediments, in the foreland basin in Nepal. Discovery of Himalayan detritus in the Bengal Basin from 38 Ma: 1) resolves the puzzling discrepancy between the lack of erosional evidence for Paleogene crustal thickening that is recorded in the hinterland; 2) invalidates those previously proposed evidences of diachronous collision which were based on the tenet that Himalayan-derived sediments were deposited earlier in the west than the east; 3) enables models of Himalayan exhumation (e.g. by mid crustal channel flow) to be revised to reflect vigorous erosion and rapid exhumation by 38 Ma, and 4) provides evidence that rapid erosion in the Himalaya was coincident with the marked rise in marine 87Sr/86Sr values since ~ 40 Ma. Whether 38 Ma represents the actual initial onset of vigorous erosion from the southern flanks of the east-central Himalaya, or whether older material was deposited elsewhere, remains an open question

The Paleogene record of Himalayan erosion: Bengal Basin, Bangladesh

A knowledge of Himalayan erosion history is critical to understanding crustal deformation processes, and the proposed link between the orogen's erosion and changes in both global climate and ocean geochemistry. The most commonly quoted age of India-Asia collision is ~ 50 Ma, yet the record of Paleogene Himalayan erosion is scant -- either absent or of low age resolution. We apply biostratigraphic, petrographic, geochemical, isotopic and seismic techniques to Paleogene rocks of the Bengal Basin, Bangladesh, of previously disputed age and provenance. Our data show that the first major input of sands into the basin, in the > 1 km thick deltaic Barail Formation, occurred at 38 Ma. Our biostratigraphic and isotopic mineral ages date the Barail Formation as spanning late Eocene to early Miocene and the provenance data are consistent with its derivation from the Himalaya, but inconsistent with Indian cratonic or Burman margin sources. Detrital mineral lag times show that exhumation of the orogen was rapid by 38 Ma. The identification of sediments shed from the rapidly exhuming southern flanks of the eastern-central Himalaya at 38 Ma, provides a well dated accessible sediment record 17 Myr older than the previously described 21 Ma sediments, in the foreland basin in Nepal. Discovery of Himalayan detritus in the Bengal Basin from 38 Ma: 1) resolves the puzzling discrepancy between the lack of erosional evidence for Paleogene crustal thickening that is recorded in the hinterland; 2) invalidates those previously proposed evidences of diachronous collision which were based on the tenet that Himalayan-derived sediments were deposited earlier in the west than the east; 3) enables models of Himalayan exhumation (e.g. by mid crustal channel flow) to be revised to reflect vigorous erosion and rapid exhumation by 38 Ma, and 4) provides evidence that rapid erosion in the Himalaya was coincident with the marked rise in marine 87Sr/86Sr values since ~ 40 Ma. Whether 38 Ma represents the actual initial onset of vigorous erosion from the southern flanks of the east-central Himalaya, or whether older material was deposited elsewhere, remains an open question