Samuel. Abraham

Bio: Samuel. Abraham is an academic researcher. The author has contributed to research in topics: Combustion & Aqueous solution. The author has an hindex of 1, co-authored 1 publications receiving 86 citations.

Evaluation of criteria for describing crop residue degradability in a model of carbon and nitrogen turnover in soil

Abstract: Good predictions of plant litter carbon and nitrogen turnover in soil depends heavily on a reliable quantification of litter quality. In this work, eight common agricultural crop residues were described by their intrinsic chemical properties and incubated in a sandy soil (15°C and −10 kPa water potential). Measured C mineralization varied greatly between the plant materials and was well predicted by a simulation model in which litter C is subdivided into three pools according to the results of stepwise chemical digestion (Van Soest analysis). Nevertheless, there was a significant lack of fit for some of the materials. This was caused by differences in the specific decay rates of holocellulose-like substances (kSPM) as subsequently estimated for each plant material by fitting the model to data on remaining holocellulose. Even though the model takes account of microbial N deficiency, the optimized kSPM values were significantly correlated with initial N (r2=0.93) but not with lignin concentration. To test the predictive value in our model of indices and quantities thought to be related to litter degradability, we investigated whether they were correlated with the kinetically defined pool of readily decomposable plant constituents as estimated by fitting the model to measured C mineralization rates. Neutral detergent-soluble C (Van Soest analysis) was best correlated with the estimated pool (r2=0.78) followed by water-soluble C (r2=0.69) and C digestible in vitro in rumen fluid (r2=0.66). Measured C to N ratio in holocellulose-like substances was highly correlated with the overall C to N ratio of the plant materials (r2=0.96). In the model, we describe the degradability of litter N on the basis of the measured C to N ratios of the litter pools. The simulated microbial N requirement is governed by a successive replacement of rapidly growing organisms with a low C to N ratio by more slowly growing organisms with a slightly higher C to N ratio, reflecting the commonly observed increase along the decay continuum in the fungal contribution to microbial activity. This model feature was supported by a measured tendency to an increasing biomass C to N ratio. The model gave unbiased simulations of N mineralization and microbial biomass N. This indicates that the descriptions of litter N availability and microbial N requirement in the model were reasonable. However, significant discrepancies between simulated and measured values occurred for some of the plant materials during the first few days of decomposition, emphasizing the need for more accurate knowledge for this very dynamic phase. Our results suggest that an a priori characterization of litter degradability is possible in our model by the use of stepwise chemical digestion for subdivision of litter C and N combined with measurements of initial N to set the decay rate constant of holocellulose-like materials.

Amino acid metabolism in rat brain studied with 14c-labelled glucose.

Abstract: FOLLOWING the studies of KILLAM and BAIN (1957) in which they showed that seizures induced by several hydrazide compounds could be prevented by administration of pyridoxine or y-aminobutyric acid (GABA) together with the finding that levels of free GABA were lowered in the brain and glutamic acid decarboxylase (GAD) activity was depressed, many other workers have studied the effect of a wide variety of drugs on the levels of free amino acids in the brain. Although in certain instances definite changes in the levels of some free amino acids in the brain did occur, it is difficult to correlate such changes with the altered behaviour of the animal such as convulsions or depressed activity (DE ROPP and SNEDEKER 1961 ; KAMRIN and KAMRIN 1961 ; SOEP and JANSSEN 1961). Neither has it been possible to show an unequivocal relationship between the level of free GABA in the brain and the behavioural response of the animal (BAXTER and ROBERTS 1961). Some measure of the rate of formation and utilization of the amino acids in the brains of animals treated with neurotoxic agents compared with normal animals might yield further information about brain metabolism accompanying gross changes in behaviour. BELOFF-CHAIN, CANTAZARO, CHAIN, MAW and POCCHIARI (1955) using brain slices incubated in vitro with [U-14C]glucose were the first to demonstrate the rapid formation of certain free amino acids from glucose. A similar rapid labelling of amino acids in brain from [U-14C]glucose has been shown to occur in vivo after an intravenous injection of isotopically labelled glucose (BUSCH, FUJIWARA and KEER 1960; VRBA 1962; VRBA, GAITONDE and RICHTER 1962) and in brains of cats perfused in situ with a simplified blood containing [U-14C]glucose (GEIGER, KAWAKITA and BARKULIS 1960). In the present work the aim was to measure the specific activity of the amino acids which quickly become labelled from [14C]glucose in normal animals and to study the effect of neurotoxic compounds on this labelling. The study was divided into three parts encompassing measurements of the levels and degree of labelling of amino acids in brain tissue with [U-14C]glucose as the radioactive source in (i) brain cortex slices incubated in vifro with and without added drugs (ii) whole brains of control and intoxicated rats given [U-14C]glucose intravenously and (iii) slices of brain cortex prepared from rats intoxicated in vivo and the slices incubated with [U-laC] glucose in vitro. The neurotoxic compounds chosen for comparative studies with control animals were two organo-metal compounds, triethyltin and triethyl-lead. Previous studies on these two substances have shown them to bring about very different behavioural