R. E. White

The influence of phosphate nutrition on H ion efflux from the roots of young rape plants

Changes in pH around the roots of young rape plants were studied using a nutrient film technique which allowed either part or all of the root system to be subjected to specific nutrient treatments. The rapidity and direction of change of pH was assessed by embedding absorbing roots in a thin layer of agar containing bromocresol purple. Measurements were also made with a pH microelectrode placed next to the roots.Phosphate-fed plants were deprived of phosphate when 14 days old. Patterns of pH changes round the deprived roots were the same as with phosphate-fed plants until the plants had been deprived of P for three days, when H ion efflux started in the terminal portions of the roots. The lengths of root producing acid and amounts of H ion both increased as the plants became more P deficient. Both P fed and P deprived roots produced HCO3 ions but the net amount of HCO3 ion produced by the P deficient roots fell as did nitrate uptake rates. Cation-anion balances measured at the end of the experiment showed that uptake of all anions and K decreased in the P deprived plants but uptake of Ca and Mg were little altered. This resulted in a smaller ratio of anions to cations absorbed which was reflected in the reduced HCO3 ion efflux.

Net mineralization and nitrification rates in a clay soil measured and predicted in permanent grassland from soil temperature and moisture content

SummaryNet mineralization of N and net nitrification in field-moist clay soils (Evesham-Kingston series) from arable and grassland sites were measured in laboratory incubation experiments at 4, 10 and 20°C. Three depth fractions to 30 cm were used. Nitrate accumulated at all temperatures except when the soil was very dry (θ=0.13 cm3 cm−3). Exchangeable NH4-ions declined during the first 24 h and thereafter remained low. Net mineralization and net nitrification approximated to zero-order reactions after 24 h, with Q10 values generally <1.6. The effect of temperature on both processes was linear although some results conformed to an Arrhenius-type relationship. The dependence of net mineralization and net nitrification in the field soil on soil temperature (10 cm depth) and moisture (0–15, 15–25, 25–35 cm depths) was modelled using the laboratory incubation data. An annual net mineralization of 350 kg N ha−1 and net nitrification of 346 kg N ha−1 were predicted between September 1980 and August 1981. The model probably overstressed the effect of soil moisture relative to soil temperature.

The effect of high solute concentrations on nitrification rates in soil

SummaryA short term nitrification assay (<18 h) was used to assess the effect of high concentrations of different solutes on the rate of nitrate production. High solute concentrations were found to inhibit nitrification and the degree of inhibition was related both to the osmotic pressure of the soil solution and the osmoticum used. Ammonium chloride, ammonium sulphate and sorbitol were used as sources of osmotic pressure. The results showed that, with ammonium salts, no inhibition was observed with pressures less than 2 atm. Above these values, the severity of the inhibition followed the order ammonium chloride>ammonium sulphate>sorbitol up to the maximum osmotic pressure studied (25 atm). With ammonium chloride, a pressure of 3.5 atm. was sufficient to cause a 90% inhibition of nitrification rate.The inhibition produced by mixtures of ammonium chloride and sorbitol, each mixture generating an osmotic pressure of 5 atm. in the assay, was also investigated. The results suggest that inhibition by Cl-ion is disproportionate to its contribution to the osmotic pressure of the soil solution.The recovery of the nitrification rate, following exposure to high osmotic pressure solutions, was also investigated. It was found that the recovery of the nitrification rate was only partial, with the extent of the recovery diminishing as the severity of the initial osmotic stress applied increased. These results suggest that both reversible and irreversible mechanisms are involved in the inhibition of nitrification.

The influence of nitrate nutrition on H+ efflux by young rape plants (Brassica napus cv. emerald)

SummaryChanges in pH around the roots of young rape plants (Brassica napus cv. emerald) were studied using a nutrient film technique that allowed part or whole of the root system to be subjected to specific nutrient treatments. The rapidity and direction of pH change was assessed by imbedding absorbing roots in a thin film of agar containing bromocresol purple. When nitrate-fed plants were deprived of all sources of nitrogen at 15 or 17 days old, the release of H ions from the roots was immediate and uniformly distributed over the root length. When nitrate was witheld from half of the root system of nitrate-fed plants, the roots deprived of nitrate immediately started to produce H ions even though the nitrate-fed half of the root system continued to supply the whole of the plant with nitrate. However, the rate of H ion production in plants partly supplied with NO3 was less than in plants completely deprived of NO3. It is suggested that malate produced in the shoots, following nitrate reduction, may be redistributed to the roots deprived of nitrate. There, HCO3 produced by the decarboxylation of the malate masks some of the expected H ion efflux.

Components of the nitrogen cycle measured for cropped and grassland soil-plant systems

SummaryInputs and outputs to the N balance of a clay soil catchment (Evesham and Kingston series) under grassland and cereals at Wytham near Oxford were measured over 2 years. Soil mineral N (NH4+NO3) was measured to 1 m depth at intervals of 2 to 8 weeks. The frequency distribution of these values was approximately log-normal and the geometric mean was used as an estimate of central tendency. Overall, soil mineral N tended to decrease during the study period, but marked fluctuations were observed in autumn (October–November) and early spring (February–March) in the grassland due to mineralization of soil organic N, and in the arable soil in April–May following the application of N fertilizer to the spring barley and winter wheat.N lost by leaching, including a little surface runoff, was calculated from the NO3 concentration of the catchment drainage and the volume of drainage. The estimate of N leached using concentrations unweighted for flow rate was only 14 per cent less than that based on flow-weighted concentrations. The differences in the uptake of N by cereals and grass between fields were explicable partly in terms of soil type and partly in terms of the timing and amounts of fertilizer added. The results are discussed in the context of steady-state equilibrium of N in the soil-plant system. However, an N balance could not be struck because N input due to mineralization, and N outputs due to gaseous losses and immobilization of N in the soil and root biomasses, were not measured and could not be accurately estimated.

Modelling growth responses of soil nitrifiers to additions of ammonium sulphate and ammonium chloride

SummaryFollowing the addition of 0–75 μmole N g−1 as ammonium chloride or ammonium sulphate to a sandy loam soil the nitrate formed was measured daily for a period of 15–17 days. The nitrate produced as a function of time was described using the Monod equation for microbial growth. An optimisation technique is described for obtaining, from the nitrification time course data, the maximum specific growth rate, the affinity constantant and an index limited by the concentration of ammonium in soil solution. Additions of more than 7.3 μmoles N g−1 soil as ammonium chloride were found to inhibit nitrification. The inhibition was interpreted as being caused by osmotic pressure or by chloride ion. A similar effect was not found with ammonium sulphate, because the salt concentration in the soil solution was restricted by the precipitation of calcium sulphate. The model developed was capable of accounting for nitrate production in the soil under non-steady state conditions of substrate concentrations and nitrifier biomass.

A theoretical consideration of the implications of cell clustering for the prediction of nitrification in soil

SummaryThe time course of nitrate production is different in different soils; in some soils, nitrate is produced at a constant rate while in others, this rate increases with time, often exponentially. Mechanistic models, based on the Monod equations, cannot account for a constant rate of nitrification. All such mechanistic models make the implicit assumption that the nitrifying organisms are distributed uniformly as single cells throughout the soil volume, while in reality, the cells might be expected to occur in small clusters formed by repeated cell division. This paper examines the effects of allowing the ammonium oxidising cells to occur in evenly distributed clusters of cells of equal volume. One effect of clustering would be the lowering of soil pH around the cluster, caused by differences in the rates of acid production and diffusion. The effects of this pH depression were examined using a mathematical model.In general, it was found that the effect of clustering was to reduce the rate of ammonium oxidation. In extreme cases, in which the fraction of the soil volume occupied by the cells was assumed to be small, the model predicted a constant rate of ammonium oxidation with time in contrast to the increasing rate with time predicted by a model based on a uniform single cell distribution. The clustering model was therefore capable of reproducing the different time courses of ammonium oxidation reported in the literature. The differences between the time courses of ammonium oxidation predicted using the two different assumptions was affected by the initial pH of the soil. This observation suggested a possible experimental test of the clustering hypothesis.

Nitrate Enhancement of Nitrification Depth in Sediment/Water Microcosms

Streams draining agricultural catchments in loward England commonly have high nitrate concentrations. Laboratory microcosms were used to study nitrogen transformations in the sediment from such a stream. Solutions of either 0.5 mmol dm−3 calcium nitrate or calcium chloride (control) were passed once only over the sediment surface. After 5 days incubation, Eh in the nitrate treatment stabilized at approximately 500 mV at 5 mm below the water/sediment interface, and 285 mV at 10 mmn, whereas in the chloride treatment Eh was −200 mV to −250 mV at all measured depths (5 mm and below). Inorganic N mesurements of the seidment profiles showed that much more ammonium-N was being lost from the nitrate-treated sediment that the control. It was hypothesized that, when nitrate was present in the streamwater, the combined processed of nitrate diffusion and denitrification progressively consumed organic matter, thereby allowing dissolved oxygen to diffuse further into the sediment. A consequence of these processes would be a greater depth of nitrification and hence more ammounium loss.The plausibility of this hypothesis was evaluated by constructing a deterministic model which simulated the nitrate and ammonium profiles from the processes of diffusion, denitrification, nitrification, and ammonification. Kinetic parameters and diffusion impedance factor were determined from independent experiments. This calibrated model provided reasonable simulations of experimental data obtained from an additional experiment, which also showed that peaks in denitrification and nitrification activity were offset from each other by 2–3 mm, and that they migrated down the profile with increasing incubation time.

The effect of prolonged exposure to acetylene on denitrification in a laboratory stream sediment system

Abstract The effects of prolonged exposure to acetylene on denitrification activity in a sediment-water [0.5 mmol dm−3 Ca(NO3)2] system were studied in the laboratory to evaluate the possible use of the acetylene inhibition method for long-term in situ studies in streams. Denitrification accounted for 86% of the nitrate reduced in the acetylene treated system. Acetylene addition induced anaerobic conditions within the top 5 mm of sediment after 2–3 days incubation which did not occur in the control treatment. This was accompanied by an increased rate of nitrate reduction and nitrite production in the recirculating “stream water”. Analysis of the sectioned acetylene-treated sediment revealed much steeper nitrite and nitrate diffusion profiles than in the control. Enzyme assay showed that denitrification activity was 6 times higher than the initial level at the sediment-water interface of the acetylene treatment, and decreased with depth, whereas the control showed a much smaller peak at 5.5 mm depth. It was hypothesized that the results could be explained by heterotrophic degradation of acetylene, with subsequent utilization of the breakdown products by denitrifiers.

The effect of phosphate nutrition of young rape plants on nitrate reductase activity and xylem exudation, and their relation to H ion efflux from the roots

Levels of nitrate reductase activity (N.R.A.) were measured in shoots and roots of P sufficient and P deficient rape plants and changes in N.R.A. examined in relation to the onset of H ion efflux from the roots. Rates of xylem exudation were measured and the sap analysed for nitrate, amino-N and phosphate content.The optimum concentration of phosphate in the leaves for N.R.A. was about 0.7%. Both high and low concentrations of phosphate within the leaves inhibited N.R.A in those leaves. This inhibition of N.R.A led to the accumulation of nitrate in the older parts of the shoots of P sufficient plants. Less accumulation of nitrate occurred in the P deficient plants since nitrate uptake by the plants decreased before any fall in N.R.A.Xylem exudation rates halved within 18 hours of depriving the plants of phosphate, and, since the composition of the sap remained constant, this indicated a reduced flux of nitrate into the xylem. The rate of xylem exudation continued to fall and by the end of the experiment was approximately one tenth of the rate in the P sufficient plants.The onset of H ion efflux from the terminal portions of the root preceded any effect on N.R.A by 2 days.