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Nitrifier denitrification is the pathway of nitrification in which ammonia (NH 3 ) is oxidized to nitrite (NO 2 − ) followed by the reduction of NO 2 − to nitric oxide (NO), nitrous oxide (N 2 O) and molecular nitrogen (N 2 ). The transformations are carried out by autotrophic nitrifiers. Thus, nitrifier denitrification differs from coupled nitrification–denitrification, where denitrifiers reduce NO 2 − or nitrate (NO 3 − ) that was produced by nitrifiers. Nitrifier denitrification contributes to the development of the greenhouse gas N 2 O and also causes losses of fertilizer nitrogen in agricultural soils. In this review article, present knowledge about nitrifier denitrification is summarized in order to give an exact definition, to spread awareness of its pathway and controlling factors and to identify areas of research needed to improve global N 2 O budgets. Due to experimental difficulties and a lack of awareness of nitrifier denitrification, not much is known about this mechanism of N 2 O production. The few measurements carried out so far attribute up to 30% of the total N 2 O production to nitrifier denitrification. Low oxygen conditions coupled with low organic carbon contents of soils favour this pathway as might low pH. As nitrifier denitrification can lead to substantial N 2 O emissions, there is a need to quantify this pathway in different soils under different conditions. New insights attained through quantification experiments should be used in the improvement of computer models to define sets of conditions that show where and when nitrifier denitrification is a significant source of N 2 O. This may subsequently render the development of guidelines for low-emission farming practices necessary.

In the current soil drying protocol of the 0.01 M calcium chloride (CaCl2) procedure, soils are oven dried at 40°C for 24 h. At this drying temperature, as well as at lower drying temperatures, a change of the actual soil nutrient element status cannot be excluded because microbes will be active during part of the drying period. However, a higher drying temperature may affect soil characteristics and soil processes and also lead to a misinterpretation of the soil nutrient element status. An explanatory study was conducted to get more insight into the effect of i) oven drying temperature and ii) the use of forced-air ventilation at low drying temperatures on nutrient elements extracted with the 0.01 M CaCl2 procedure. The goal of the study was to investigate the perspectives of optimization of the soil drying protocol of the 0.01 M CaCl2 procedure. Three moist test soils with different soil characteristics were oven dried at 20 and 40°C with and without forced air ventilation and at 70 and 105°C without fo...

A multinutrient soil extraction procedure in routine soil testing is attractive. Therefore, it has been suggested to convert conventional soil testing programs into a 0.01 M calcium chloride (CaCl2) multinutrient soil testing program using the relationship between test values of the 0.01 M CaCl2 extractant and those of the various conventional extractants. However, these relationships are often weak and an interpretation of the coefficient(s) is almost impossible. Therefore, a fundamental relationship has been deduced relating magnesium (Mg) extracted by conventional methods, (Mg-ext)a, with Mg extracted by the 0.01 M CaCl2 method (Mg-ext)CaCl2: (Mg-ext)a = [* (Mg-ext)CaCl2,t=t] [*(Mg-ext)CaCl2,t=t * (Q-re)CaCl2]. In this relationship, , , and are related to characteristics of the extraction procedure and Mg-fractions in the soils. The (Q-re)CaCl2 is the actual cation exchange capacity of the soil during the CaCl2 extraction. To test the usefulness of this relationship, 39 agricultural soils with widely differing soil characteristics were extracted with 0.01 M CaCl2 and seven conventional Mg extractants. For six conventional methods, the explained variance of the fundamental relationships was more than 0.92. The explained variance of the relationship among 0.01 M CaCl2 and the 0.1 N ammonium-lactate/0.4 N acetic acid extractant buffered at pH 3.75 was poor when the soils contained carbonates. We conclude that the derived fundamental relationship can be used for the design of a CaCl2 soil testing program for Mg. Preferably, this CaCl2 soil testing program should be validated in pot and field experiments.

The aim of this study was to investigate the effect of tissue nitrogen concentration, as a consequence of nitrogen supply rate, on the susceptibility of tomato plants to three pathogens. We varied tissue N concentration by supplying N at different rates by adding nitrate in different, exponentially increasing amounts to the nutrient solution on which the tomato plants were grown. Separate experiments were carried out to test susceptibility of tomato plants to the bacterial speck-causing Pseudomonas syringae pv tomato, to the wilt agent Fusarium oxysporum f.sp. lycopersici and to tomato powdery mildew caused by Oidium lycopersicum. The effect of tissue N concentration appeared to be highly pathogen-dependent: there was no effect on susceptibility to F. oxysporum, but susceptibility to P. syringae and O. lycopersicum increased significantly with increasing N concentration. We have previously demonstrated the opposite for susceptibility to Botrytis cinerea: decreasing susceptibility with increasing N concentration. The apparent contradictory effects are discussed in relation to the effect of N supply on both the nutritional value of the plant tissue to the pathogen and on the concentration of resistance-related compounds. We conclude that the effect of changing both characteristics on disease susceptibility is highly pathogen-specific and is probably dependent on differences in resource requirements of the pathogen or the sensitivity of the pathogen to plant resistance reactions or on both these factors.

Generally, grasslands are considered as sinks for atmospheric CH4, and N input as a factor which reduces CH4 uptake by soils. We aimed to assess the short- and long-term effects of a wide range of N inputs, and of grazing versus mowing, on net CH4 emissions of grasslands in the Netherlands. These grasslands are mostly intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300-500 kg N ha-1 year-1. Net CH4 emissions were measured with vented, closed flux chambers at four contrasting sites, which were chosen to represent a range of N inputs. There were no significant effects of grazing versus mowing, stocking density, and withholding N fertilisation for 3-9 years, on net CH4 emissions. When the ground-water level was close to the soil surface, the injection of cattle slurry resulted in a significant net CH4 production. The highest atmospheric CH4 uptake was found at the site with the lowest N input and the lowest ground-water level, with an annual CH4 uptake of 1.1 kg CH4 ha-1 year-1. This is assumed to be the upper limit of CH4 uptake by grasslands in the Netherlands. We conclude that grasslands in the Netherlands are a net sink of CH4, with an estimated CH4 uptake of 0.5 Gg CH4 year-1. At the current rates of total N input, the overall effect of N fertilisation on net CH4 emissions from grasslands is thought to be small or negligible.

Methane (CH4) emissions from soils, representing the consequence of CH4 production, CH4 consumption and CH4 transport, are poorly characterised and show a large spatial variability. This study aimed to assess the determinants of field-scale spatial variability of CH4 emissions from wet grasslands on peat soil. Mean CH4 emission rates of a three-year experiment at 18 plots distributed over three sites in the nature preserve Nieuwkoopse Plassen on peat soil in the Netherlands were related to CH4 production and CH4 consumption capacities of soil layers, and to soil and vegetation characteristics. Spatial variability of CH4 emissions and possible determining factors was high. Annual CH4 emissions ranged from 3 to 37 g CH4 m-2 yr-1. Coefficients of variation (CV) of CH4 emissions were on average 37% among sites and 83% within sites. Most important determinants of spatial variability were CH4 production capacity (average: 211 ng CH4 g-1 dry soil h-1; CV: 131%) and aboveground biomass of sedges (Carex spp.) (average: 0.45 g dm-2; CV: 127%) (P < 0.01). Sedges may affect CH4 emissions by stimulating CH4 transport from anaerobic layers to the surface via their vascular system and/or by serving as substrate for methanogens. For extrapolation of CH4 emissions to larger areas, best results will be obtained by using factors that are easy to determine, like vegetation.

The aim of this study was to investigate the effect of nitrogen availability on susceptibility of tomato leaves to the fungal pathogen Botrytis cinerea. Plants with varying nitrogen availability were grown by adding N daily in exponentially increasing amounts to a nutrient solution at different rates. Leaves of plants grown at low nitrogen availability had a high leaf C/N ratio (21 g g-1) and were about 2.5 times more susceptible to primary lesion formation by B. cinerea compared to plant grown at high nitrogen availability, which had a low leaf C/N ratio (11 g g-1). Leaf C/N ratio accounted for 95% of variation in susceptibility. This relationship between C/N ratio and susceptibility persisted when plants were grown with exponential P addition and optimal N supply, and was thus independent of plant growth rate or related factors. We could not explain the effect of nitrogen availability by variation in the most obvious N-based resistance compound α-tomatine because more susceptible leaves with a high C/N ratio contained more α-tomatine. These leaves also contained more soluble carbohydrates. The level of soluble carbohydrates correlated positively with susceptibility, independent of the growth method. We therefore suggest that the effect of N availability on susceptibility must be explained by variation in levels of soluble carbohydrates and speculate about the role of these carbohydrates in the infection process.

The area of wet grasslands on peat soil in the Netherlands is slowly increasing at the expense of drained, agriculturally used grasslands. This study aimed (i) to assess the contribution of wet grasslands on peat soil to methane (CH4) emissions, and (ii) to explain differences among sites and between years in order to improve our understanding of controlling factors. For these purposes, a field study was conducted in the period 1994–1996 in the nature preserve “Nieuwkoopse Plassen”, which is a former peat mining and agricultural area. Net CH4 emissions were measured weekly to monthly with vented closed flux chambers at three representative sites, and at ditches near these sites. Three-years average of CH4 emissions was 7.9 g CH4 m−2 yr−1 for Drie Berken Zudde, 13.3 for Koole, and 20.4 for Brampjesgat. Ditches near the sites emitted 4.2–22.5 g CH4 m−2 yr−1. The time-course of CH4 emissions for all experimental sites and years was fit with a multiple linear regression model with ground water level and soil temperature as independent variables. Lowering or raising the ground water level by 5 cm could decrease or increase CH4 emissions by 30–50%. Therefore, ground water level management of these grasslands should be done with care.

Waste products can be diverted from landfill and used as fertilizer when the nutrient availability in these products can be reliably determined. The aim of this study was to determine whether aerobic incubation in an internal-loop airlift bioreactor is a reliable method for assessing plant-available N and P in organic waste products. Published results of pot experiments with ryegrass were used as a reference for available N and P. The incubation in the airlift reactor was also compared with conventional incubation in polyethylene bags. Samples of eight different organic products were incubated for 20 d in airlift reactors maintained at pH 5. The plant-available N fraction was determined by measuring inorganic concentrations in the reactor solution, and the plant-available P fraction was determined by extraction of P from the solution using iron oxide-coated filter paper. The available N fraction in the organic products as determined in the airlift reactor was closely related (R 2 = 0.73) with the N uptake by ryegrass (Lolium perenne L.). For P, there was no relationship between the P extracted in the airlift reactor and the P uptake by ryegrass, The incubation in the airlift reactors to assess plant-available N in organic waste products was more rapid than the pot experiment with ryegrass and was more reliable than the conventional incubation in polyethylene bags. We recommend that plant-available N and P be determined in different airlift reactors, because of possible pH-related P dissolution and because the iron oxide-coated filters contain NH 4 + which may hamper determination of N.

The economical and operational aspects of multinutrient extractants make them attractive for soil testing programs. However, the value of a multi element extractant is primarily determined by the relationship between the amount of nutrient extracted and crop response. To determine the perspectives of the 0.01M calcium chloride (CaCl2) extraction procedure as a multinutrient extractant, this paper reviews literature on the soil chemical, analytical and plant nutritional aspects of CaCl2 solutions as a soil extractant. Recent decades, CaCl2 solutions were common single nutrient extractants in plant nutritional and soil chemical research but the amount of nutrient extracted was sensitive for differences in sample treatment and extraction procedure. Therefore, a 0.01M CaCl2 procedure should be standarized to obtain a robust procedure. Calibration studies between conventional soil extraction procedure and the 0.01M CaCl2 procedure show fairly good relationships. A first step to develop a multielement ...

Contamination of soils with heavy metals becomes more and more a problem in many countries all over the world. In areas where metal contaminated soils are used for food crop production, metals relatively mobile within the plant, such as cadmium (Cd) and zinc (Zn) can easily come into the food chain with great risks for human health. Since bioavailability of heavy metals in soils varies with soil and plant characteristics, e.g., mineralogical and organic matter properties of the soil and plant metal susceptibility, prediction of heavy metal uptake by plants by the common soil and plant chemical analysis techniques is often unreliable. Recently, the use of biomarkers has been suggested to be a suitable technique complementing chemical soil analysis. Therefore, the usefulness of the biomarker phytochelatin (PC), a non‐protein thiol, specifically induced in plants suffering from heavy metal stress, was tested. Maize (Zea mays L.) plants were exposed to excess copper (Cu) or Cd in nutrient solution sy...

Summary Soil and plant testing programs are still based on ‘trial and error’ methods and lack scientific underpinning in terms of relevant soil chemical and plant nutritional processes, and are site-specific. The programs are valuable when the objective is to diagnose and predict deficiencies of plant nutrients. The programs are less valuable for refined fertilizer strategies, like Integrated Nutrient Management, which are essential in the near future to satisfy changing agricultural, environmental, economic and legislative boundary conditions. A more scientific approach to soil and plant testing programs appears desirable. To reduce undesirable side-effects of fertilization on the environment, more emphasis should be placed on fertilizer type and on timing and method of application.

Aerobic grasslands may consume significant amounts of atmospheric methane (CH4). We aimed (i) to assess the spatial and temporal variability of net CH4 fluxes from grasslands on aerobic sandy soils, and (ii) to explain the variability in net CH4 fluxes by differences in soil moisture content and temperature. Net CH4 fluxes were measured with vented closed flux chambers at two sites with low N input on sandy soils in the Netherlands: (i) Wolfheze, a heather grassland, and (ii) Bovenbuurtse Weilanden, a grassland which is mown twice a year. Spatial variability of net CH4 fluxes was analysed using geostatistics. In incubation experiments, the effects of soil moisture content and temperature on CH4 uptake capacity were assessed. Temporal variability of net CH4 fluxes at Wolfheze was related to differences in soil temperature (r2 of 0.57) and soil moisture content (r2 of 0.73). Atmospheric CH4 uptake was highest at high soil temperatures and intermediate soil moisture contents. Spatial variability of net CH4 fluxes was high, both at Wolfheze and at Bovenbuurtse Weilanden. Incubation experiments showed that, at soil moisture contents lower than 5% (w/w), CH4 uptake was completely inhibited, probably due to physiological water stress of methanotrophs. At soil moisture contents higher than 50% (w/w), CH4 uptake was greatly reduced, probably due to the slow down of diffusive CH4 and O2 transport in the soil, which may have resulted in reduced CH4 oxidation and possibly some CH4 production. Optimum soil moisture contents for CH4 uptake were in the range of 20 – 35% (w/w), as prevailing in the field. The sensitivity of CH4 uptake to soil moisture content may result in short-term variability of net atmospheric CH4 uptake in response to precipitation and evapotranspiration, as well as in long-term variability due to changing precipitation patterns as a result of climate change.

Heavy metal contaminated soils often show increased levels of more than one metal, e.g. copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb) or nickel (Ni). In case such soils are used for crop production, prediction of yield reduction or quality decline due to heavy metals in the soil is inadequate when based only on chemical soil analysis. The use of biomarkers such as phytochelatins (PC), non-protein thiols specifically induced in plants upon exposure to heavy metals, may be an additional tool or diagnostic criterion in heavy metal research and in practice. In the present work, Cu and Cd uptake and induction of PC synthesis are studied with hydroponically grown maize and wheat plants exposed to mixtures of the two metals. We observed a close positive relationship between the concentrations of Cd and PC in the plant shoot material. A decreased shoot concentration of Cd after addition of Cu, due to metal competition at common root absorption sites, coincided with lower shoot PC levels. Also differences in metal uptake and xylary metal transport among the two plant species were reflected in corresponding differences in PC concentration. The observed direct relationship between shoot PC concentration and the degree of metal-induced growth inhibition makes the use of PC promising for the purpose tested for.

A better knowledge of the plant-availability of nitrogen (N) and phosphorus (P) in organic products may help to improve the efficient use of these products as fertilizers. In the present study, availability indices for N and P of nine widely differing organic products obtained by different fractionation methods were compared with the plant uptake of N and P from these products. The fractionation methods included CaCl2 extraction, thermal fractionation (heating of organic products), and pepsin extraction, for N, and extraction with diluted sulphuric acid, P-Bray-I, P-Olsen, and extraction using an iron oxide coated filter paper, for P. The results of pot experiments with ryegrass using a double-pot technique (Janssen, 1990) over 62 (N experiment) and 93 days (P experiment) were used as reference for plant-availabe N and P. The 0.01 M CaCl2 extractable inorganic N reasonably predicted plant-available N only in organic products with a high inorganic N fraction. Thermal fractionation and pepsin extraction provided a reasonable index for mineralizable N in organic products having a high fraction of mineralizable N. Of the P fractionation methods, the extraction using iron oxide coated filter paper was the best indicator of plant-available P in the products.

Urine patches are considered to be important sites for nitrous oxide (N2O) production through nitrification and denitrification due to their high concentration of nitrogen (N). The aim of the present study was to determine the microbial source and size of production of N2O in different zones of a urine patch on grassland on peat soil. Artificial urine was applied in elongated patches of 4.5 m. Four lateral zones were distinguished and sampled for four weeks using an intact soil core incubation method. Incubation of soil cores took place without any additions to the headspace to determine total N2O production, with acetylene addition to determine total denitrification (N2O+N2), and with methyl fluoride to determine the N2O produced through denitrification. Nitrous oxide production was largest in the centre and decreased towards the edge of the patch. Maximum N2O production was about 50 mg N m−2 d−1 and maximum denitrification activity was 70 mg N m−2 d−1. Nitrification was the main N2O producing process. Nitrous oxide production through denitrification was only of significance when denitrification activity was high. Total N loss through nitrification and denitrification over 31 days was 4.1 g N per patch which was 2.2% of the total applied urine-N.

Nitrate leaching from a double-cropping forage system was measured over a 2-year period (June 1994–May 1996) in the Northwest region of Portugal using ceramic cup samplers. The crops were grown for silage making and include maize (from May to September) and a winter crop (rest of the year) consisting of a mixture of cereals and Italian ryegrass. The experiment was performed on two different sites with a history of many years under the same crop and fertiliser management, but differing in the amounts of N applied as fertiliser and by regular cattle slurry applications. The annual nitrate leaching losses measured ranged from 154 to 338 kg N ha-1. These amounts lead to annual mean concentrations between 22 and 41 mg \(NO_3^ - \)-N L-1 in the drained water. The coarse textured soils (sandy loams) and the climatic conditions of the region with more than 600 mm of drainage concentrated between October and March, tended to promote the leaching of all the nitrate-N left in the soil after the maize crop plus the N released by mineralization during the winter period. On these soils, the minimum amount of drainage (necessary to provide the complete leaching of all the nitrate-N in the soil profile in the end of summer), seems to be between 300 and 400 mm. The winter crops removed important quantities of N (83–116 kg N ha-1) but, due to their late establishment in autumn they did not succeed in taking up the nitrate-N left in the soil after the maize crop. Approaches for reducing the nitrate leaching losses in this particular system are discussed.

Nitrous oxide (N2O) in soils is produced through nitrification and denitrification. The N2O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N2O or N2. Aim of the study was to quantify N2O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N2O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N2O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH3F). Nitrous oxide production over the 2 year period was on average 34 kg N ha-1 yr-1 for mown treatments that received no N fertiliser and 44 kg N ha-1 yr-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N2O production to reach 81 kg N ha-1 yr-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N2O production in the 0–40 cm layer. The N2O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N2O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N2O. In conclusion, N2O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N2O producing process.

Methane (CH4) is the most important greenhouse gas next to CO2 and as such it contributes to the enhanced greenhouse effect. Peat soils are often considered as sources of CH4. Grasslands on the other hand are generally considered to be a net sink for atmospheric CH4. The aim of this study was twofold: (i) to quantify the net CH4 emission of intensively managed grasslands on peat soil in the Netherlands; and (ii) to assess the effects of grassland management, i.e. drainage, nitrogen (N) fertilization, and grazing versus mowing, on CH4 emission rates. Net CH4 emissions were measured weekly or biweekly for one year with vented closed flux chambers at two sites, one with a mean ground water level of 22 cm below surface and one with a mean ground water level of 42 cm. On each site there were three treatments: mowing without N application, mowing with N application, and grazing with N application. The dominating species was perennial ryegrass (Lolium perenne L.). Net CH4 emissions were low, in general in the range of -0.2 to 0.2 mg CH4 m-2 d-1. In the relatively warm summer of 1994, consumption of atmospheric CH4 peaked at 0.4 mg m-2 d-1. On an annual basis, the sites were net consumers of atmospheric CH4. However, the consumption was small: 0.31 to 0.08 kg CH4 ha-1 yr-1. Effect of mean ground water level was significant, but small. There were no significant effects of withholding N fertilization for some years and grazing versus mowing on net CH4 emissions. We conclude that grassland management of intensively managed grasslands on peat soil is not a suitable tool for reducing net CH4 emissions.

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N2O) emission from grassland. During cold and dry conditions in early spring, emission of N2O from both ammonium (NH4+) and nitrate (NO3−) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N2O and total denitrification losses from NO3− containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N2O and via denitrification, respectively. Emissions of N2O and total denitrification losses from NH4+ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N2O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N2O emission from AS was much smaller than the effect of using NH4+ fertilizer instead of NO3− fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N2O from NO3− fertilizers but not from NH4+ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N2O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha−1 to 3.1% for a dressing of 300 kg N ha−1. In another experiment, N2O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N2O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N2O emission from intensively managed grasslands.

H+ production due to N uptake in a mature Scots pine stand subjected to high NH4 + deposition was previously estimated to amount to approx. 2.2 kmol ha-1 y-1. The question whether H+ transfers related to N mineralization (ammonification and nitrification) offset or corroborate this proton production is investigated in the present research. To determine N mineralization, soil cores were used of which both ends were closed with layers of ion exchange resin (IER) to prevent influx and efflux of ions. The effect of liming on N mineralization and the resulting H+ production was investigated in 7 incubation periods of each ca. 8 wk. Because of its high mobility NO3 accumulated in both IER layers at the expense of that in the incubated forest floor and mineral soil. Net N mineralization in the soil cores as a whole amounted to 40 and 77 kg N ha-1 in 384 d in the control and limed plots, respectively. In both treatments ca. 65% of mineralized N was nitrified. H+ production due to N mineralization amounted to approx. 1.2 kmol ha-1 y-1 in the control and limed plots. Liming reduced the amount of C in the forest floor, but not forest floor mass, because of an increased mixing with mineral particles.

Summary To investigate Cd binding in the root system as a possible cause of differential Cd distribution in maize inbred lines, Cd present in ‹soluble› and ‹insoluble pools› in roots of a ‹shoot Cd excluder› (B73) and a ‹non-shoot Cd excluder› (H98) was determined after extraction of the roots in HEPES buffer solution (pH 6.5). At a similar root Cd concentration (approximately 70 μg Cdg-1dry wt.), about 49 % of the Cd concentration in roots of H98 but 25 % of that in roots of B73 was removed during extraction (‹soluble pooh›). The acid-soluble thiol concentrations in roots of the ‹shoot Cd excluder› B73 were much lower compared with those of the ‹non-shoot Cd excluder› H98. Exposure of B73 and H98 plants to the same Cd concentration in solution resulted in nearly equal acid-soluble thiol concentrations in the root extracts. Binding characteristics of Cd to structural components of the roots (‹insoluble pool›) were similar for both maize inbreds. The relationship between transport of Cd from roots to shoots and amounts of Cd complexed to acid-soluble thiol compounds in the ‹soluble pool› of ‹non-shoot Cd excluders› compared with ‹shoot Cd excluders› is discussed.

To assess the mineral composition of plants growing in pure fly ash, grasses growing on lysimeters filled with alkaline, neutral, or acid fly ash were sampled several times in a 6‐year period. The samples were analyzed for elements essential for plants and animals as well as non‐essential, but environmentally significant, trace elements. Grasses were also sampled from ash dumps that were 20 and 30 years old. Fly ash is not a proper source of plant macronutrients N, P, and K. Plant growth on the alkaline fly ash can be influenced for some time by the high salinity of that ash. Grasses growing on unweathered fly ash were found to be high in Al, B, Co, Fe, Mo, Ni, Pb, and Se. Concentrations of several elements declined in time but levels of B, Fe, Mo, and Ni were still elevated in grasses on both fly ash dumps. All concentrations, except Al, were lower than toxicity levels for plants as found in literature. In plants growing on fresh fly ash concentrations of Mo, Pb, and Se can exceed the maximum to...

In order to investigate the physiological basis of the differential Cd distribution and the degree of variation of this Cd distribution among maize inbred lines, six inbreds designated earlier as ‘shoot Cd excluders’ (B73, H99, and H96) and ‘non-shoot Cd excluders’ (B37, H98, and N28) were grown in nutrient solution culture at different external Cd levels or at different pH. The characterization of the inbreds according to their shoot/root partitioning of Cd was consistent, independent of pH or level of Cd supply. The Cd concentrations in the plants were highest at the highest pH of the solution cultures. Generally, there was a positive correlation between the Cd concentrations in shoots and xylem exudates. It was shown that the Cd concentration in the roots is particularly important in the Cd distribution process. Above a ‘critical’ internal Cd concentration in the roots, specific for each inbred, the ability to retain Cd is strongly diminished. It is concluded that structural and/or physiological characteristics of the roots are involved in Cd partitioning.

Genotypic variation in uptake and distribution of cadmium (Cd) was studied in 19 inbred lines of maize (Zea mays L.). The inbred lines were grown for 27 days on an in situ Cd-contaminated sandy soil or for 20 days on nutrient solution culture with 10 µg Cd L-1. The Cd concentrations in the shoots showed large genotypic variation, ranging from 0.9 to 9.9 µg g-1 dry wt. for the Cd-contaminated soil and from 2.5 to 56.9 µg g-1 dry wt. for the nutrient solution culture. The inbred lines showed a similar ranking for the Cd concentrations in the shoots for both growth media (r2=0.89). Two main groups of inbreds were distinguished: a group with low shoot, but high root Cd concentrations (shoot: 7.4±5.3 µg g-1 dry wt.; root: 206.0±71.2 µg g-1 dry wt.; ‘shoot Cd excluder’) and a group with similar shoot and root Cd concentrations (shoot: 54.2±3.4 µg g-1 dry wt.; root: 75.6±11.2 µg g-1 dry wt.; ‘non-shoot Cd excluder’). The classification of the maize inbred lines and the near equal whole-plant Cd uptake between the two groups demonstrates that internal distribution rather than uptake is causing the genotypic differences in shoot Cd concentration of maize inbred lines. Zinc (Zn), a micronutrient chemically related to Cd, showed an almost similar distribution pattern for all maize inbred lines. The discrepancy in the internal distribution between Cd and Zn emphasizes the specificity of the Cd distribution in maize inbred lines.

Four lettuce (Lactuca saliva L.) varieties ('Benita’, ‘Wendy’, ‘Mirena’, and ‘Jacky') were grown for 43 days in a pH‐controlled complete nutrient solution without cadmium (Cd) or with either 0.01 or 0.03 mg Cd/1 and with NH4 or NO3 as the form of nitrogen nutrition. Cadmium did not affect dry matter yield. ‘Wendy’ had a significantly higher total dry matter production when grown on NO3 compared to NH4, whereas growth of ‘Mirena’ was best on NH4 (P

Two experiments are described in which tomato plants (Lycopersicon esculentum L. var Ailsa Craig) were grown in water culture supplied with 10–300 μM Mn. Toxicity symptoms associated with a yield reduction were observed only in treatments in excess of 50 μM Mn indicating that this species is relatively tolerant of high Mn supply. Dark brown/black spots appeared first in the cotyledons. Similar symptoms were observed in the leaves, progressively from the oldest leaf. Manganese concentration in the shoot tissues ranged from 286 to 4240 μg. g‐1 dry weight. The high Mn concentration values found in the shoot tissues of the toxic plants indicate that Mn was highly mobile in the xylem as confirmed by xylem sap analysis. The concentrations of both Ca and Mg were lower in the smaller Mn toxic plants. Not only was uptake of Ca and Mg retarded but so also was the distribution of Ca and Mg to the younger tissues as illustrated by measurements of Ca and Mg concentrations along a leaf age sequence. This is in...

For the set up of a multinutrient 0.01 M calcium chloride (CaCl2) soil testing program a conversion from conventional soil testing programs to a CaCl2 program has been proposed in literature. Such conversion should be based on the relationship between test values of the conventional method and the CaCl2 method. For magnesium (Mg) it was shown in earlier work that the conversion could be improved when the actual cation exchange capacity (CEC) of the soil, CECact, was taken into account. However, determination of CECact necessitates an extra analytical procedure. The goal of this study was to test a procedure for estimating CECact of a soil. In this procedure, CECact was calculated as the summation of the estimated charge of organic carbon (C) and clay in the soil at pHact, the actual pH of the soil. A series of 39 test soils representing agricultural soils in The Netherlands was used to derive the pH dependency of the negative charge of organic C and clay. The following relationship was found: CECact: [M(1)x 0.0624] [M(2)x (0.295-D(2)pHact)]. In this relationship, M(1) and M(2) represent clay and organic C in g kg-1 dry soil, respectively, and D(2)pHact the difference in negative charge of organic C at pHact and pH 8.1. The pHact equals pH measured in 0.01 M CaCl2. The relationship was tested on another dataset of 38 agricultural soils. There was good agreement between the calculated and measured CECact (R2=0.89). It was concluded that the procedure can be used for estimation of CECact.

Dairy farming systems are a major source of nitrous oxide (N2O) emission. The aim of our study was to explore the possibilities to reduce N2O emissions from intensively managed dairy farming systems using a system analysis and literature data. Intensively managed grasslands are the dominant source of N2O emission in dairy farming systems. There are several measures to mitigate N2O emission from these grasslands, categorized in three broad groups: improved N fertilizer management, improved grassland management and improved management of livestock production. A reduction of N2O emission from intensively managed dairy farming systems up to 70% may be achieved through implementation of a package of measures to improve nitrogen (N) management, including refined N fertilizer application, reduction of NH3 volatilization, restricted grazing, use of maize silage, and the use of clover as an alternative for N fertilizer.

To establish the structural and physiological characteristics related to the genotypic variation in Cd distribution between maize inbred lines (‘shoot Cd excluders’ and ‘non-shoot Cd excluders’), shoot and root morphological parameters were studied on plants grown in nutrient solution. Furthermore, the xylem sap composition and the desorbability of Cd from roots of these inbreds have been compared. No relationship between the morphological characteristics of either shoot (specific leaf area and leaf area ratio) or root (specific root length, specific surface area, and average diameter) and Cd distribution could be assessed. Cadmium concentrations in the xylem exudates from ‘non-shoot Cd excluders’ were higher than those from ‘shoot Cd excluders’, but not related to citrate and malate concentrations. The absolute and relative amounts of Cd desorbed from roots of ‘shoot Cd excluders’ were about twice as high compared to those of the ‘non-shoot Cd excluders’, especially at the lowest Cd concentration in solution. The absence of a relationship between shoot or root morphological parameters and Cd partitioning and the differences between both groups in the amounts of Cd desorbed, even at similar root Cd concentrations, indicate that the differential Cd distribution between ‘shoot Cd excluders’ and ‘non-shoot Cd excluders’ may be related to the observed differences in root Cd concentration, desorption characteristics and binding capacity of Cd inside and/or outside the root and its distribution within the roots.

A pot experiment was conducted with a coarse‐textured calcareous soil (pH‐H2O 8.3) to study the effect of single and combined application of N and S fertilizers on soil pH, Fe, Mn, Zn, and P mobilization, and on growth and micronutrient uptake by com (Zea mays L.). Increasing amounts of elemental sulfur were mixed with the soil. To stimulate S oxidation, the treated soils were incubated for six weeks at field capacity. Nitrogen was applied as NH4NO3 (100, 200, and 400 mg N/kg). After six weeks, dry matter yields were recorded and shoots were analyzed for Fe, Mn, Zn, and P. At the end of the experiment, soil pH and the DTPA‐extractable micronutrients were determined. The results showed that: a. Soil pH was decreased by 0.2, 0.5, and 0.9 unit as a result of increasing S applications. b. Applied sulfur and N fertilizer had increased the availability of micronutrients to following crops. c. Application of N and/or S resulted in increased dry matter yields. d. Manganese uptake tended to be higher as a...

The uptake of Cd by lettuce cultivars was studied in soil and water culture experiments (16 and 8 cultivars, respectively). In the soil experiment, sandy soils from both a Cd-contaminated and a non-contaminated area of the Netherlands were used; the water culture experiment included 3 Cd levels (0, 0.03 and 0.1 mg/litre). Fresh and dry yields of shoots and roots were hardly affected by Cd in either of the experiments. The lettuce cultivars showed a close similarity in shoot Cd concentration, ranging from 8.23 to 10.72 micro g/g DW for the Cd-contaminated soil and from 3.96 to 8.83 micro g/g DW in the water culture at the highest Cd level tested. These results contradict published data on a large genotypic variation in Cd concentration in lettuce [see John, M. K. et al, Environmental Pollution (1976) 10, 163-173]. Based on the narrow range in shoot (and root) Cd concentration between cultivars, it is concluded that it is unwarranted to use lettuce as a pilot plant in comparative physiological investigations on Cd uptake and distribution. (Abstract retrieved from CAB Abstracts by CABI’s permission)

To study possible adaptive mechanisms inbred lines from three populations of Plantago major from sites that were found to differ in P availability were compared. In a pot experiment the growth and P uptake either in the presence or absence of Glomus fasciculatum was determined. Under these P-limited conditions it was shown by partitioning the relative growth rate (RGR, in mgg-1 day-1) in the components root weight ratio (RWR, in grootsg plant -1 ), specific P uptake rate (SPUR, in μmol P g plant -1 day-1), and P-efficiency (PEFF, in mg μmol P-1), that the increase in RGR of mycorrhizal infected plants was related to an increase in SPUR, and a decrease in RWR and PEFF. P. major ssp. major had a lower RGR (related to a lower PEFF and SPUR) and a higher RWR than P. major ssp. pleiosperma. In a second experiment three inbred lines were compared upon P depletion in a nutrient solution. The P. major ssp. major line had a lower RGR and higher RWR, and a higher accumulation of P in the roots than the P. major ssp. pleiosperma lines under optimal growing conditions. There were no differences among the inbred lines in the relative contribution of inorganic P to the total P concentration in the shoot. The results are discussed in relation to the characteristics of the habitats of the investigated P. major populations.

Results are reported for tomato (Lycopersicon esculentum L. var. Ailsa craig) and wheat (Triticum aestivum L. cv. Mara) which demonstrate that increasing concentrations of Mg in the plant raises plant tolerance to M n toxicity.

In experiments with pea cv. Rondo grown hydroponically for 6 wk with nitrate as the only N source, an acidic nutrient uptake pattern was observed. When effectively nodulated plants were fixing nitrogen, however, cation absorption exceeded anion absorption resulting in an alkaline ion uptake pattern. Maintaining ambient temp. at 13 deg C allowed for comparison of the effects of N acquisition on nutrient absorption and proton or hydroxyl/bicarbonate excretion by the roots in plants with similar DM yield and N content.The amount of excreted alkalinity or acidity for both nitrate-supplied and nitrogen-fixing plants corresponded well with the respective excess absorption of nutritive anions or cations. Physiological and agronomic consequences of the alkaline nutrient uptake pattern and acidity generation by nitrogen-fixing legumes are discussed. (Abstract retrieved from CAB Abstracts by CABI’s permission)

In a pot trial, maize cv. Prior and sugarbeet cv. Polyrave were given N as urea or ammonium. There was no significant difference between DM producton in either spp. with the different N sources, although plant N content was increased with N supplied as ammonium. Root cumulative net proton extrusion closely agreed with calculated values for net absorption of cations, indicating that urea was absorbed intact, at a rate sufficient for growth. Xylem exudate was free from urea and NH3-N suggesting metabolism in the roots. Urea appeared to be assimilated other than by enzymic breakdown. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Ricinus communis L. was used to test the Dijkshoorn-Ben Zioni hypothesis that NO(3) (-) uptake by roots is regulated by NO(3) (-) assimilation in the shoot. The fate of the electronegative charge arising from total assimilated NO(3) (-) (and SO(4) (2-)) was followed in its distribution between organic anion accumulation and HCO(3) (-) excretion into the nutrient solution. In plants adequately supplied with NO(3) (-), HCO(3) (-) excretion accounted for about 47% of the anion charge, reflecting an excess nutrient anion over cation uptake. In vivo nitrate reductase assays revealed that the roots represented the site of about 44% of the total NO(3) (-) reduction in the plants. To trace vascular transport of ionic and nitrogenous constituents within the plant, the composition of both xylem and phloem saps was thoroughly investigated. Detailed dry tissue and sap analyses revealed that only between 19 and 24% of the HCO(3) (-) excretion could be accounted for from oxidative decarboxylation of shoot-borne organic anions produced in the NO(3) (-) reduction process. The results obtained in this investigation may be interpreted as providing direct evidence for a minor importance of phloem transport of cation-organate for the regulation of intracellular pH and electroneutrality, thus practically eliminating the necessity for the Dijkshoorn-Ben Zioni recycling process.

The application of Fe, Cu or Zn at 20 mu g g-1 soil to eight Spanish soils resulted in 7-20% inhibition of urease activity. The inhibitory effect was most pronounced in light-textured soils, and Cu was the most effective metal. These elements did not inhibit microbial production of urease in two soils. Cu, but not Zn and Fe, markedly retarded urea hydrolysis and reduced the loss of urea-N by ammonia volatilisation in a loamy sand. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Nutrient uptake and biological dinitrogen fixation were studied using effectively nodulated pea plants which had been grown hydroponically for 6 wk in nutrient sol. of pH 4.0, 5.5 or 7.0. The pH of the root medium was kept constant by automatic titration and the temp. maintained at 13 deg C throughout. Although the N content of the plants grown at pH 4.0 and 7.0 was lower in comparison with that of plants of the pH 5.5 treatment, no N deficiency symptoms were observed. About the same number of nodules was formed in each of the treatments but they were smallest in the pH 5.5 treatment. Nevertheless, the total amount of dinitrogen fixed/plant at pH 5.5 was larger than that at higher or lower acidity. This was due to a higher nitrogenase activity/unit of nodule wt., which could be ascertained by an in vivo acetylene reduction test. In all cases more nutritive cations than anions were absorbed resulting in net proton extrusion by the roots. Both cation and anion uptake and hence the extent of proton release were affected substantially by ambient acidity. Plants grown at pH 7.0 generated 2.3 times as much acidity as plants grown at pH 4.0. The symbiotic dinitrogen fixing process was repressed by raising the temp. to 25 deg which resulted in no DM production and proton release and accelerated the onset of maturation. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The dry matter yields and total nitrogen contents in the vegetative shoots of pea plants (Pisum sativum L.), infected with an effective or ineffective strain of Rhizobium leguminosarum, and supplied with increasing amounts of 15N‐labelled nitrate fertilizer, were studied in a pot experiment with a sandy soil. In the ineffective strain treatment both dry matter yield and shoot nitrogen concentration increased considerably as the amount of 15N‐labelled nitrate fertilizer was increased from 0 to 400 mg N/pot. When plants had been infected with an effective strain the dry matter yield was independent of the nitrate application rate as was the tissue nitrogen concentration which was high in all cases, indicating that nitrogen supply had been sufficient. Only application of 100 mg N/pot in the effective strain treatment resulted in a slightly depressed nitrogen content of the shoot, indicating a suboptimal collaboration of nitrate uptake and assimilation, and dinitrogen fixation. Calculation of the amo...

Summary Electrical charge distribution and partitioning of nitrogenous compounds in xylem exudates of dinitrogen-fixing and nitrate-supplied pea plants (Pisum sativum L.) were compared. Nitrate-supplied plants exuded considerably more xylem sap than dinitrogen-fixing plants. In the xylem saps of nitrate-supplied plants 91% of the total cationic charge was balanced by inorganic anions. This small excess inorganic positive charge was compensated by aspartate, citrate, and malate. Total cation concentration in the xylem sap of dinitrogen-fixing plants was twice that of nitrate-supplied plants (70.0 versus 36.6 meq l-1), while an excess cation over inorganic anion concentration of 21.6 meq l-1 was found. This charge gap was compensated for 80% by aspartate and the remaining charge was balanced by citrate, malate, and succinate. In both treatments total cations were balanced within 3% by inorganic anions plus carboxylates. Total nitrogen concentration in the xylem saps of dinitrogen-fixing plants was 2.7 times as high as that of nitrate-supplied plants (174 versus 65 mmol l-1). The relative distribution of nitrogen over the various compounds differed for the two treatments. In nitrate-supplied plants 65% of the nitrogen was translocated in reduced form. Asparagine and glutamine contributed for 45 and 13%, respectively, to the long distance nitrogen transport. The respective contributions of these amides in the xylem sap of dinitrogen-fixing plants were 70 and 16%, while 11% was transported as aspartate. In both treatments very low concentrations of other amino acids were detected and all xylem sap samples were shown to be free from ureides. Evaluation of data about the distribution of nitrate reductase activity over the plant organs, the fraction of absorbed nitrate reduced in the roots, and the relation between hydroxyl/bicarbonate excretion (= excess anion uptake) and nitrate uptake, allowed the conclusion that in nitrate-supplied pea plants no xylem-phloem cation recirculation is necessary for the regulation of electroneutrality in the root tissue.

Various aspects of proton balance in plants are reviewed including the balance in plants taking their nitrogen supply from various sources, recirculation of nutrients in plants, the H ion status of plants, lime-induced chlorosis and the effects of soil acidification and ammonia deposition. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The growth of nitrate‐supplied and dinitrogen‐fixing pea plants was studied in a pot experiment with a sandy soil in a pH‐H?O range from 3.4 to 5.6. Optimum growth in both treatments occurred at pH 5.0. At low pH, N2‐plants yielded significantly less than NO3‐plants. Planting of nodulated seedlings did not enhance yield in comparison with sowing in inoculated soil, indicating that nodulation was not the most sensitive process in restricting yield. Comparison of the nitrogen contents of shoots of planted and sown N2‐plants allowed the suggestion that the synthesis of nitrogenous compounds was also not limiting yield. At low pH, root growth was severely reduced in dinitrogen‐fixing plants in comparison with nitrate‐supplied plants. This difference could be explained by the influence of the form of nitrogen nutrition on the cation‐anion uptake pattern of the plant and the resulting pH‐shift in the rhizosphere. It is to be expected that in an acid soil under field conditions the indirect effect of ni...

Fe-efficiency reactions' are induced in the roots of dicotyledonous plants as a response to Fe deficiency. The role of phloem Fe in the regulation of these reactions was investigated. Iron travels in the phloem of Ricinus communis L. as a complex with an estimated molecular weight of 2400, as determined by gel exclusion chromatography. The complex is predominantly in the ferric form, but because of the presence of reducing compounds in the phloem sap, there must be a fast turnover in situ between ferric and ferrous (k approximately 1 min(-1)). Iron concentrations in R. communis phloem were determined colorimetrically or after addition of (59)Fe to the nutrient solution. The iron content of the phloem in Fe-deficient plants was lower (7 micromolar) than in Fe-sufficient plants (20 micromolar). Administration of Fe-EDTA to leaves of Phaseolus vulgaris L. increased the iron content of the roots within 2 days, and decreased proton extrusion and ferric chelate reduction. The increase in iron content of the roots was about the same as the difference between iron contents of roots grown on two iron levels with a concomitantly different expression of Fe-efficiency reactions. We conclude that the iron content of the leaves is reflected by the iron content of the phloem sap, and that the capacity of the phloem to carry iron to the roots is sufficient to influence the development of Fe-efficiency reactions. This does not preclude other ways for the shoot to influence these reactions.