The nitrogen cycle in shallow water sediment systems of rice fields Part III: The influence of N-application on the yield of rice

Fertilizer application to rice-fields in the river-deltas in the Mediterranean area is a potential menace for wildlife protection, through eutrophication.Fertilizer use shows a trend of increasing rates of N application. A rate for N of 200 kg ha^–1 has become normal and a rate of 400 kg ha^–1 has already been recorded.Denitrification causes large losses of N with the result that more fertilizer is applied. This is especially true for the Camargue (S-France), where N is applied long before the rice (Aryza sativa) can take it up.Therefore we have tried to develop techniques which need the application of smaller amounts of N which are used more efficiently. In order to do this we tried to establish a N budget for rice-fields.Experiments were therefore set up in the field (plots of 550 m²) and in pots (2–3 l). Our results suggest that a late application of N (e.g. when the rice shows signs of N-deficiency by becoming yellowish), but at lower concentrations (70 kg ha^–1) can produce the same ultimate yield. The introduction of carp without any further input of N produced the same final yield.The N budget shows that 15±1.5 g m^–2 of N is needed for a normal crop. N losses due to denitrification may be as high as 12.2–13.6 g m^–2 of N. The input by irrigation water may provide up to about 20% of the input; N fixation is negligible. We estimate that 25–50% of the N missing in the budget comes from minderalization of the organic N pool in the soil. Denitrification may render part of this pool bio-available by oxidation. In sum, this work has revealed some surprising effects with potentially important consequences for farming practice and, in consequence, for conservation.     

A synaptic model for the kindling effect

In this work we present a model of the kindling effect based on the Hopf bifurcation for a system of ordinary differential equations. The model shows how quantitative changes in the physiological parameters at the microscopic, synaptic scale, produce the afterdischarge which is a macroscopic effect at the neuronal network scale. The presynaptic mechanisms are based on the vesicular hypothesis, or more generally on the quantal theory of synaptic transmission. The postsynaptic processes rely on Granit's law. This model gives a consistent framework which organizes and explains several experimental observations.     

Synaptic Integration in Electrically Coupled Neurons

Interactions among chemical and electrical synapses regulate the patterns of electrical activity of vertebrate and invertebrate neurons. In this investigation we studied how electrical coupling influences the integration of excitatory postsynaptic potentials (EPSPs). Pairs of Retzius neurons of the leech are coupled by a nonrectifying electrical synapse by which chemically induced synaptic currents flow from one neuron to the other. Results from electrophysiology and modeling suggest that chemical synaptic inputs are located on the coupled neurites, at 7.5 μm from the electrical synapses. We also showed that the space constant of the coupled neurites was 100 μm, approximately twice their length, allowing the efficient spread of synaptic currents all along both coupled neurites. Based on this cytoarchitecture, our main finding was that the degree of electrical coupling modulates the amplitude of EPSPs in the driving neurite by regulating the leak of synaptic current to the coupled neurite, so that the amplitude of EPSPs in the driving neurite was proportional to the value of the coupling resistance. In contrast, synaptic currents arriving at the coupled neurite through the electrical synapse produced EPSPs of constant amplitude. This was because the coupling resistance value had inverse effects on the amount of current arriving and on the impedance of the neurite. We propose that by modulating the amplitude of EPSPs, electrical synapses could regulate the firing frequency of neurons.     

The nitrogen cycle in shallow water sediment systems of rice fields. Part 1: The denitrification process

Denitrification causes important losses of N-fertilizer in rice-fields, where high temperature and high production of organic matter favour denitrification losses. Two techniques have been used to quantify the denitrification losses: the ¹⁵N technique, which can be used to quantify the amount finally incorporated, and the acetylyne inhibition technique which is a direct measure of the quantities lost.Both techniques were applied in enclosures (diameter = 44 cm) in the field while moreover bio-assays in 3 l glass beakers were carried out. In all experiments where nitrate was added we found a rapid decrease of nitrate; usually about 30–50% of the nitrate that disappeared was recovered as N₂O. As in one experiment, in which we measured the N₂O disappearance rate as well, the N₂O itself decreased at a rather constant rate of 20% per day, a correction must be made for this N₂O decrease in the calculations of the nitrate disappearance rate. Although we have only one series in which the decrease of N₂O was measured, the mathematical analysis indicates that as much as 80% of the N-fertilizer is actually lost. This figure is in full agreement with the ¹⁵N experiments; if the ¹⁵N was applied early only about 7% was recovered in soil and plants, while if it was applied later (after 7 weeks) about 20% was incorporated.Denitrification rate could be fitted on an negative exponential regression line; the rate constant increased during the summer. It is suggested that organic matter caused this increase.During denitrification considerable quantities of nitrite appear, which later on disappear again by processes still unknown; the nature of the available organic matter may be important for this nitrite production.With N-serve we tried to inhibit NH₃ oxidation. In this way we tried to prevent the considerable N losses and to demonstrate that the nitrite produced in our experiments was not derived from NH₃ oxidation. N-serve, however, had very little influence. It is probably inactivated by absorption onto the sediments.From these results it is suggested that the efficiency of N-application may be considerably increased by using low doses of N-fertilizer, but applied late in the growing season, e.g. 7 weeks after sowing. This favours environmental protection as well.     

The nitrogen cycle in shallow water sediment systems of rice fields Part II: Fractionation and bio-availability of organic nitrogen compounds

The organic N-pool in soils provides an important part of the N metabolized by rice. As this pool is, however, very large compared to the yearly uptake, — which may involve a few percent only — direct evidence of its importance is difficult to establish. We tried therefore to distinguish different fractions with supposedly different availability. We did not succeed in developing a satisfactory fractionation scheme, but improved existing fractionation schemes by applying a sequential extraction using an alkaline extraction followed by an acid one; the residue of the latter was still an important portion of the total.These three fractions were measured both in pot cultures and in an experimental field with different N applications schemes, before and after rice growth. The NaOH-extractable fraction nearly always increased during rice growth, except in the last growth period in the rice field. The acid-extractable fraction decreased in all experiments, sometimes a little, sometimes considerably. The residual fraction usually showed a decrease except in the second, dry period in the field, when there was a stark increase. Probably processes occur shifting the ratios between the fractions in a way different from uptake by rice.The NaOH-extractable N showed a small increase when ammonia was used in the pot cultures, but decreased when nitrate was used. Although about 80% of this nitrate is denitrified, rice growth was stimulated by nitrate application, probably because of its oxidative action on organic matter. The same effect was found with H₂O₂; this compound caused the alkaline and acid extractable fractions to decrease as well. We think that nitrate exerts its favourable role on N uptake for rice growth mainly by its oxidative capacity. Ammonia does the same, after it is — automatically — oxidized to nitrate in the oxygenated water of the rice field.