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1.
The tomato (Solanum lycopersicum L.) cultivar Micro‐Tom (MT) is widely used in physiological studies, but the effects of nitrate ( ) and ammonium ( ) ratios ( : ratios) and, in particular, the effects of the accompanying ions in sources are unknown. To determine whether the accompanying ions in sources influence toxicity, the effects of : ratios on the physiology, electrolyte leakage index, nutrition, and dry weight were studied using hydroponics. The sources were ammonium chloride (NH4Cl) or ammonium sulfate [(NH4)2SO4], and five : ratios were used: 100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100. The source was calcium nitrate [Ca(NO3)2], and the nitrogen (N) concentration was 15 mmol L?1. The results indicate that NH4Cl or (NH4)2SO4 can be used in studies on toxicity because the accompanying ions did not influence the tomato plants. In addition, : ratios of 100 : 0 and 75 : 25 resulted in the highest dry weight of tomato plants, whereas ratios of 25 : 75 or 0 : 100 were toxic.  相似文献   

2.
The application of mineral nitrogen (N) fertilizers is one of the most important management tools to ensure and increase yield in agricultural systems. However, N fertilization can lead to various ecological problems such as nitrate () leaching or ammonia and nitrous oxide emissions. The application of N stabilizers (i.e., inhibitors) combined with urea fertilization offers an effective option to reduce or even prevent N losses due to their regulatory effect on ammonium () and release into the soil. The present field experiment therefore aimed at soil N speciation dynamics after urea spring fertilization (225 kg N ha?1) in the presence of a urease inhibitor (UI), a nitrification inhibitor (NI), both inhibitors (UI+NI) or when no inhibitor was applied at all. The study focused on the distribution of N species among soil matrix and soil solution. Plant cultivation was completely omitted in order to avoid masking soil N turnover and speciation by plant N uptake and growth dynamics. Application of UI clearly delayed urea hydrolysis in the top soil, but a complete hydrolysis of urea took place within only 10 days after fertilization (DAF). Nitrification was significantly reduced by NI application, leading to higher and lower concentrations in treatments with NI. Due to sorption of to the soil matrix a significantly larger fraction of was always detected in the soil extracts compared to soil solution. However, while in soil extracts the impact of NI application was less apparent and delayed, in soil solution a quick response to NI application was observed as revealed by significantly increased soil solution concentrations of . Because of the “asymmetric” soil phase distribution soil solution was predominant over only initially after fertilization even in inhibitor treatments (≈ 8 to 10 DAF). Nevertheless, inhibitor application tended towards closer ratios of to concentration in soil solution and hence, might additionally affect concentration dependent processes like plant N uptake and root development. Despite cold spring conditions urea application along with UI and/or NI did not indicate a limited supply of plant available and .  相似文献   

3.
Soil nitrogen (N) availability and pH are two determinants affecting plant growth, both of which are influenced by long‐term N deposition. However, the physiological mechanism of plants response to the changes in soil N availability and pH are not fully understood. To investigate the response of Picea asperata to both factors, seedlings of P. asperata were exposed to 50 or 1000 µM NH4NO3 with pH 5 or pH 7. In the current study, P. asperata, regardless of N availability and pH in growth medium, exhibited invariably a preference. Lower root biomass, root : shoot mass ratio, total root length and area, and root vitality were detected in high N condition compared to those in low N supply, corresponding well to lower net influxes of and at the root surface in both pH treatments. These results indicate that P. asperata may employ an active‐forge strategy to exploit nutrient resources for growth under low N availability, probably by increased below‐ground carbon allocation and net influxes of and . Although low pH, to some extent may generate more malondialdehyde, P. asperata would enhance pH tolerance by increased detoxification, i.e., antioxidant enzymes (peroxidase), free proline and soluble protein as well as improved carbohydrate status (i.e., soluble sugar and starch).  相似文献   

4.
Nitrification inhibitors (NIs), DCD (dicyandiamide), and DMPP (3,4‐dimethylpyrazole phosphate), in combination with urea (UR) and ammonium sulfate nitrate (ASN) fertilizers were studied under contrasting soil textures (sand, loam, and clay) from cultivated soils collected in Brazil and Germany. Soil samples were incubated over 50 days and the content of ammonium ( ), nitrate ( ), and soil pH were measured periodically. Applied NIs delayed the nitrification process across all soil textures. Correlation analysis indicated that combining ASN with NIs resulted in higher content and efficiency in delaying the nitrification process with high N‐conversion rate (r = –0.82). The combination of ASN+ DMPP increased the efficiency of the N‐conversion rate (r = –0.86) due to H+ release in soil, while UR+DCD (r = –0.50) had an efficiency of the N‐conversion rate similar to UR (r = –0.42). All the NIs had a better performance in reducing formation in sandy soils as compared to the loam and clay textured soils. Use of DMPP with an N fertilizer results in a soil pH decrease and can be an option to increase the efficiency of the N‐conversion rate, reducing N losses in soil. Overall, our results suggest that NIs have a better performance in reducing formation in sandy soils as compared to that of the loam and clay textured soils. Use of DMPP with ASN results in a soil pH decrease and can be an option to reduce N losses in soil.  相似文献   

5.
Phosphorus (P) concentrations in needles and leaves of forest trees are declining in the last years in Europe. For a sustainable forest management the knowledge of site specific P nutrition/availability in forest soils is vital, but we are lacking verified simple methods for the estimation of plant available P. Within this study, four soil P extraction methods [water ( ), double‐lactate (Plac), citric acid (Pcit), and sodium bicarbonate ( )], as well as total P content of the soil (Ptot) were tested to investigate which method is best correlated with foliar P concentrations of spruce [Picea abies (L.) H. Karst.] and beech [Fagus sylvatica (L.)]. Mineral soil samples from 5 depth levels of 48 forest sites of the Bavarian sample set of the second National Forest Soil Inventory (BZE II) were stratified according to tree species (spruce and beech) and soil pH (pH < 6.2 and > 6.2), covering the whole range of P nutrition. The extractable amount of P per mass unit of soil increased in the order << Plac < < Pcit, decreased with soil depth, and was higher in soils with pH < 6.2. Citric acid extracted up to 10% of Ptot in acidic soils. Whereas Pcit delivers adequate regression models for P nutrition in the case of spruce (R2 up to 0.53) and beech (R2 up to 0.58) for acidic soils, shows good results for spruce growing on acidic soils (R2 up to 0.66) and for beech on soils with pH > 6.2 (R2 up to 0.57). Plac produces adequate models only for beech on high pH soils (R2 up to 0.64), while did not produce acceptable regression models. Ptot seems suitable to explain the P nutrition status of beech on acidic (R2 up to 0.62) and alkaline soils (R2 up to 0.61). Highest R2s are obtained mostly in soil depths down to 40 cm. As and Pcit showed good results for both investigated tree species, they should be considered preferentially in future studies.  相似文献   

6.
Sewage sludge incineration‐ash (FB‐I) represents a potential alternative phosphorus (P) fertiliser with a high concentration of P, although with relatively low crop availability. In this study, we investigated two P‐solubilisation approaches (acidification and P mobilisation by citrate) to enhance plant P uptake from the FB‐I ash in a pot study by using various biotic strategies: (1) a pre‐treatment of ash with a Penicillium bilaiae inoculum, (2) an isogenic line of wheat that excretes citrate from the root tip, (3) nitrogen (N) provided as combined with nitrification inhibitor dicyandiamide (DCD). All strategies were tested combined with each other and with different methods for ash application: (1) completely mixed within the top one third of soil in a pot, or (2) applied as distinct band at 10 cm depth. Triple super phosphate (TSP) at a rate of 15 mg P kg?1 soil per pot was sufficient to support maximum shoot growth. Ash mixed into the first top third part of soil in the pot at a rate of 180 mg P kg?1 soil (equivalent to 60 mg P kg?1 soil throughout the pot) significantly increased the soil water‐extractable P and the subsequent shoot P uptake and shoot biomass for both wheat lines and microbial pre‐treatment to support maximum plant performance. Shoot P concentration in these treatments was further enhanced when the plants received and DCD, although not leading to a significant increase in shoot biomass. The citrate secretion by the root tips and pre‐inoculation with P. bilaiae of the ash did not influence plant growth. In conclusion, root‐zone soil acidification by nutrition is regarded as a promising strategy to improve the fertilising effect of such alternative P fertilisers originating from urban waste streams.  相似文献   

7.
This paper evaluated, in a laboratory incubated soil, the properties of digestate as a nitrogen fertilizer in comparison with sewage sludge, compost and urea, this last as a typical mineral fertilizer. The incubation period lasted for 90 d and during this time, pH, CO2 and evolution were measured. The maximum concentration of nitrate was reached in the incubated microcosm fertilized by urea (133 mg kg−1 after 62 d), and that of digestate was very similar (113 mg kg−1). Soil treated with compost showed a slower nitrate evolution. A significantly negative correlation was detected between cumulative nitrogen nitrified at the end of the trial, and the values of the C:N ratio of the biomasses used (compost, sludge and digestate) (mg kg−1 vs . C:N, r = –0.94, n = 3, p < 0.05), and between the alkyl‐C content at the end of the experiment (mg kg−1 vs . alkyl‐C, r = –0.95, n = 3, p < 0.05). As expected, pH decreases and soil respiration (CO2 evolution) were also well correlated with the content of nitrate. Considering that about 90% of the nitrogen content in the digestate is short acting, the results obtained indicate that the nitrogen rate of mineralization in digestate is very similar to that of urea, confirming that digestate could replace traditional mineral fertilizers.  相似文献   

8.
Alkaline soil is widely distributed and cultivated throughout the agricultural regions of the world. Organic carbon (OC) concentrations in alkaline soil are often small, partly because of the limitations of a high pH on the productivity of crops and pastures together with the effects of high pH on the chemistry of soil OC. Soil pH is often hypothesized to be a major factor in regulating OC turnover in agricultural soil, but there are few detailed studies on the effects of high pH on carbon cycling in alkaline soil. Sodium, K+, Mg2+ and Ca2+ are the major cations in alkaline soil, whereas Cl?, , and are the major anions. The effect of different combinations of these cations and anions on soil pH and OC is not well described in the literature. The objectives of this study were to evaluate the effect of cations and anions on soil pH and to quantify the dissolution of OC in relation to these changes in pH. The results showed that (Na+, K+ and Mg2+) salts and CaCO3 dominate in the pH range 7.0–8.5, and salts of Na+ and K+ dominate above pH 8.5. The amount of dissolved OC (DOC) increased significantly as pH increased. Therefore, the presence of large concentrations of and not only increased pH but also promoted the dissolution of soil OC. The concentration of Ca2+ modified this effect; large concentrations of Ca2+ increased the adsorption and reduced the concentration of DOC.  相似文献   

9.
Rapeseed (Brassica napus L.) is a crop requiring high levels of nitrogen (N) fertilizer for growth and to optimize yield and seed quality. To limit the environmental pollution associated with intensive N fertilizer use, rapeseed–clover (Trititcum incarnatum L.) mixtures were grown in lysimeters under low N conditions (100 kg N ha−1). Considering the high sulfur (S) requirements of both rapeseed and clover, two inputs of S fertilizer (30 and 60 kg S ha−1) were applied. The effects S input on the agronomic performance of rapeseed in mixture and monocrops considered as reference, the N2‐fixing capacity of clover, and the leaching of nitrate and sulfate were monitored. This study showed that the N2‐fixing capacity (%Ndfa) of clover was improved (1.3‐fold) when it was grown in mixture with rapeseed at S60. However, irrespective of the type of cropping (monocrops or mixtures) and S application level (30 or 60 kg S ha−1), the biomasses and total N and S contents of both plants were not significantly different, nor was the rapeseed seed quality. Moreover, the yield of rapeseed grown in mixture at S60 was significantly lower than the yield of rapeseed grown as a monocrop (331.5 ± 9.8 versus 380.8 ± 3.5 g DW m−2, respectively). The results demonstrate that, in our field conditions, rapeseed mixed with clover required only 30 kg S ha−1 to maintain yield and seed quality, despite the high S needs of both plants. More surprisingly, compared to the rapeseed monocrop, the rapeseed–clover mixture led to an increase in N (‐N) and S (‐S) leaching during the early winter period of cultivation.  相似文献   

10.
The lack of reliable estimation of water demand in drought study has been an important obstacle in efforts for characterizing the variability of water consumption and its effects on drought monitoring and prediction, particularly over water‐stressed regions. This study evaluated the performance and the contribution of three modified water demand estimates in drought modeling, including Penman–Monteith (PM) method, dual‐source potential evapotranspiration model (2S PET model), and climatically appropriate for existing conditions precipitation ( ). The results show that Standardized Moisture Anomaly Index based on shows the highest correlations with different types of drought evidence, demonstrating that the as a water demand metric performs the best in drought modeling over water‐stressed regions. Standardized Precipitation Evapotranspiration Index based on 2S PET model performs better than that based on PM equation. Although the 2S PET model is physically superior to PM equation over agricultural or water‐stress regions where the plant canopy is not “closed,” the limitations of using PET as water demand estimates in drought study still exist. Theoretically speaking, the actual water demand in non‐humid regions should be highly associated with the climatically averaged water supply capability (precipitation, P ), and the contributions of water demand and supply to drought indices are generally balanced in any region, which is realistically reflected by the relationships between P and . On the contrary, the magnitude and contribution of PET 2S and PET PM were always higher than P over water‐stressed regions, which could result in a water imbalance and generate more systemic errors in drought identification. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

11.
Translocation of nitrogen to the shoot of young bean plants after uptake of NO and NH by the root Phaseolus vulgaris plants (var. nana, cv. Saxa) at the primary leaf stage (without nodules) were fed during 6 hours with 15NO and 15NH, respectively. 24 hours after the absorption period more 15N from the absorbed NO was translocated from the root to the shoot. The presence of NH in the nutrient solution enhanced the translocation of 15NON, probably by an inhibition of nitrate reductase. NH4-+15N is mainly retained in the root by a high incorporation into the root protein. It can be concluded that nitrogen from newly absorbed NO is not retained and used for protein synthesis in the root according to the root's potential to synthesize protein. Nitrate reduction in the root is considered to be the limiting factor. This is supported by the fact that withdrawal of NO in the nutrient solution prior to the 15N-experiment increased NOtranslocation to the shoot as a consequence of a lowered level of nitrate reductase. In an experiment with 14NOsupply to the roots and 15NOapplication to the primary leaves (infiltration method) a considerable amount of 15N was translocated from the leaves to the roots. This indicates that an insufficient NOreduction in the root can be substituted by a retranslocation of reduced N-compounds from leaves to the roots. The proportion of NO reduced in the root influences also the pattern of primary distribution of nitrogen in the shoot of plants at the 4 leaf stage. At a concentration of 0,2 meq/l NO in the nutrient solution as compared to 20 meq/l NO during 10 hours a relative higher amount of 15N was transported from the root to the younger, growing leaves i.e. via the phloem to metabolic sinks.  相似文献   

12.
Molybdenum (Mo), a plant micronutrient, is involved in nitrogen (N) cycling of global ecosystem, but little is known about its effect on soil N transformation especially the key processes nitrification and denitrification. A long‐term field experiment was carried out to investigate the effects of continuous sufficient soil available Mo on vegetable N uptake and soil N transformation. The experiment consisted of three treatments: control (CK), Mo deficiency (NPK), and Mo application (NPK+Mo). The results show that (1) after a 7‐year‐experiment, continuous Mo application significantly increased soil available Mo content. (2) Compared to the NPK treatment, NPK+Mo treatment showed an increase of 11, 18, and 8% in the cumulative crop yield, plant N uptake, and N fertilizer use efficiency. (3) With continuous Mo application, the soil , , microbial biomass N, and total N contents were reduced by 14, 29, 40, and 12%, the soil nitrate reductase (NR) and nitrite reductase (NiR) activities were reduced by 14 and 8%, as well as the potential denitrification activity (PDA) and gross nitrification rate (GNR) were decreased by 64 and 80%, respectively. Additionally, continuous Mo application decreased the abundance of ammonia‐oxidizing archaea (AOA) and increased the abundance of narG‐containing denitrifiers (narG) and nirK‐type nitrite reducers (nirK) significantly. The data suggest that a deficiency in soil available Mo may induce the risk of soil N accumulation and environmental N emission in vegetable soil, whereas continuous Mo application could mitigate this risk by increasing crop yield and N uptake and, by decreasing soil N residues, soil nitrification and denitrification.  相似文献   

13.
Thermal expansion of a wheat flour-water dough was measured with a dynamic mechanical analyzer (DMA) at a temperature scan range of 25 to 160°C, in 5°C/min increments. Dough water-absorption levels were increased from 50 to 70% (14% mb) in 4% increments. A standard breadbaking method was used, and loaf volume was measured for regression analysis. The thermal expansion pattern of flour-water dough during heating included four stages with changes in the thermal expansion coefficient: gas thermal expansion (GTE) (25–60°C), starch gelatinization-gluten matrix formation (GMF) (60–100°C), vapor pressure expansion (VPE) (100–120°C), and structure fixation-crust formation (SCF) (>120°C). The onset temperature (To) between each stage and the thermal expansion coefficient (Ce) of each stage were affected significantly by dough water content. The onset temperature () from GTE to GMF (the starting temperature of gelatinization of starch in dough) decreased from 68 to 55°C as water absorption increased from 50 to 70%. The thermal expansion coefficient () of flour-water dough during GMF was highly correlated (r2 = 0.886) to bread loaf volume. The ratio () of thermal expansion coefficient during the GMF stage to the coefficient during the GTE stage also was significantly correlated (r2 = 0.882) to baking volume. Thus, DMA measurement of dough thermal expansion has the potential to be a powerful method of predicting baking quality in cultivar screenings, baking simulations, and scale-up studies.  相似文献   

14.
Nitrogen (N) fertilizer use efficiency (NUE) in flooded paddy fields is relatively low. Many N fertilizer management options have been proposed to enhance NUE and minimize environmental damage. However, few investigations are focusing on the role of the characteristics of soil N transformations in regulating NUE and N losses in paddy fields. In this study, we test the role of soil N transformations on NUE and N losses under rice growth conditions in two paddy soils collected from Jiangxi (JX) and Sichuan (SC) in China. The N recoveries of applied 15N either as nitrate or ammonium in plant and soil, and N losses estimated by 15N balance were investigated in rice pot experiments using a 15N tracing technique. The results showed that gross nitrification rates in soil collected from JX were much lower than those in soils collected from SC either at 60% water holding capacity (WHC) or rice growth (flooding) conditions, which could be due to the difference in soil pH. The ‐N concentration in soil solution was maintained at a relatively high level for a long time period after N fertilizer application in the JX soil (41 d) compared to the SC soil (26 d), caused by different nitrification rates owing to different soil pH. The 15N uptake by rice in the JX soil (29–78%) was always significantly higher than that in the SC soil (22–54%), while N losses from the plant–soil system in the JX soil (17–21%) were always significantly lower than those from the SC soil (20–34%) at the same rice growth stage in the labeled 15N ammonium treatment. However, there were no significant differences in 15N uptake by rice and N losses in applied treatment between the two studied soils. These results indicate that nitrification, not denitrification, was the key process determining NUE and N losses in paddy soils. The results of the N application gradient experiment also indicated that higher amounts of N fertilizer should be applied for the same amount of N uptake, however, this caused higher N losses, in soils characterized by high nitrification rate (e.g ., the alkaline soil). Results highlighted that soil N transformations in particular nitrification rate provided a very good guideline for an optimized N management.  相似文献   

15.
《Soil Use and Management》2018,34(2):219-235
Spatial variation in soils is required to supply ameliorants and fertilisers in the Australian sugarcane industry. However, traditional approaches are cost‐prohibitive. We investigated how a digital soil mapping (DSM) approach could be used to identify management zones. First, ancillary data including electromagnetic induction and gamma‐ray spectrometry data were collected. Using fuzzy k‐means (FKM) clustering, two to six management zones were identified. A similar approach was used to cluster percentage yield variations (2014, 2015 and 2016). Using restricted maximum likelihood analysis of topsoil (0–0.3 m) and subsoil (0.6–0.9 m) physical (e.g. clay) and chemical (e.g. exchangeable sodium percentage [ESP], and exchangeable calcium and magnesium) properties, three zones were found to minimise the mean squared prediction error (). By comparison, the three zones obtained using the percentage yield variation only minimised for subsoil ESP, which suggested it had some influence on sugarcane yield and productivity. Different rates of gypsum were required to manage the moderately sodic topsoil ESP for each zone. This was similarly the case with lime to overcome deficiencies in exchangeable calcium and magnesium. The results were consistent with yield variance, suggesting the smaller yield in some zones was due to topsoil sodicity and strongly sodic subsoil with a greater clay content. We concluded that the DSM approach was successful in identifying soil management zones and can be used to improve soil structural stability and fertility. The zones also had ramifications for strip trials to determine yield increases by comparing variable rate applications of gypsum and lime.  相似文献   

16.
Nitrate () leaching can cause elevated concentrations of ‐N in water, which can have adverse impacts on water quality and human health. In grazed pasture systems, most of the ‐N leaching occurs beneath animal urine‐N deposits. The objective of this study was to investigate the effect of four different pasture species compositions [perennial ryegrass/white clover (P. ryegrass WC), tall fescue/white clover (T. fescue WC), Italian ryegrass/white clover (It. ryegrass WC) and perennial ryegrass/Italian ryegrass/white clover/red clover/chicory/plantain (Diverse)] on ‐N leaching losses from animal urine patches, and to examine the relative importance of root system architecture and seasonal activity to reduce ‐N leaching losses. The results show that ‐N leaching losses were 24–54% lower beneath It. ryegrass WC than other pasture species. Total dry matter (DM) yield in the season following establishment was 11–58% greater in the It. ryegrass WC pasture, while average winter daily N uptake rate of It. ryegrass WC over the two seasons was on average 58% greater than P. ryegrass WC and T. fescue WC. In the second season, the P. ryegrass WC and T. fescue WC pastures had up to 140 and 82% more roots between 0 and 40 cm depth, respectively, than the other pasture species compositions. These results suggest that in grazed pasture systems, high plant winter activity (plant growth/root metabolic activity) is more important than specific root architecture (e.g. deep roots) to reduce ‐N leaching losses.  相似文献   

17.
The objective of this study was to determine the ratio and amount of Fe II and Fe III iron in different parts of 20 and 40 day old bean plants grown in pots under normal and HCO-treatment. The Fe II and Fe III iron determination was carried out by a modification of a method described by Vogel (1969). The Fe II and Fe III concentrations in the plant varied according to its age, the plant part, the order of leaves and HCO-treatment. At the second sampling date, the lower total iron content in the lower leaf particularly under the HCO-treatment suggests that the supply of iron from the roots was restricted. The iron content of the different leaves was almost evenly divided into Fe II and Fe III at the first date. At the second date, most of the iron in the bud leaf was present as Fe II. Under HCO-treatment the Fe II content of the bud leaf and the flower was similar as in the corresponding parts of the normal green plants whereas the Fe III content was considerably lowered in these plant parts as result of the HCO-treatment. The results indicate a substantial retranslocation of iron from older to younger leaves and a higher Fe II/Fe III ratio in flowers and bud leaves particularly under HCO-induced chlorosis.  相似文献   

18.
Nitrate Reduction to Ammonium in a Soil with Wastewater Irrigation Flooding with wastewaters including 48 mg/l NH-N, 15 mg/l organic N and 63 mg/l K15NO-N has led to strong nitrogen losses by denitrification from a sandy Cambisol. Beside this 15NH was formed also. Possible reasons of the nitrate reduction to ammonium are discussed and conclusions for practical wastewater irrigation are drawn.  相似文献   

19.
Mobilization of non‐exchangeable ammonium (NH ) by hyphae of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glumus mosseae was studied under controlled experimental conditions. Maize (Zea mays) and parsley (Petroselinum sativum) were grown either alone or in symbiosis with Glomus mosseae in containers with separated compartments for roots and hyphal growth. In one experiment, 15NH was added to the soil to differentiate between the native non‐exchangeable NH and the non‐exchangeable NH derived from N fertilization. Non‐exchangeable NH was mobilized by plant growth. Plant dry weight and N uptake, however, were not significantly influenced by mycorrhizal colonization of the roots. The influence of root infection with mycorrhizal fungus on the mobilization of non‐exchangeable NH was negligible. In the hyphal compartment, hyphal uptake of N resulted in a decrease of NH in the soil solution and of exchangeable NH . However, the NH concentration was still too high to permit the release of non‐exchangeable NH . The results demonstrate that, in contrast to roots, hyphae of VAM fungi are not able to form a non‐exchangeable‐NH depletion zone in the adjacent soil. However, under conditions of a more substantial depletion of the exchangeable NH in the mycorrhizal sphere (e.g., with longer growth), an effect of mycorrhiza on the non‐exchangeable NH might be found.  相似文献   

20.
The Hedley fractionation has become the preeminent measure for estimating the bioavailability of phosphorus (P) in soils. However, mechanisms underlying P extractability have never been tested. We hypothesize that P sequentially extracted by individual steps can either be referred to a specific mineral source (Hypothesis 1) or to its binding strength to minerals (Hypothesis 2). We prepared mineral‐P associations in the laboratory using various secondary mineral phases and P forms (orthophosphate, phytic acid, ribonucleic acid), which were then subject to the Hedley sequential extraction scheme (anion exchange resin in form, 0.5 M NaHCO3, 0.1 M NaOH, 1 M HCl, and concentrated HCl at 80°C). Extracts were analyzed for P as well as for the main mineral‐borne elements by inductively coupled plasma–optical emission spectroscopy (ICP–OES). In order to test if the observed mineral dissolution patterns match those of natural soils, we applied the Hedley fractionation to forest soils comprising various P stocks and measured in addition to extracted P also iron, aluminum, and calcium by ICP – OES. Phosphorus extractability from mineral‐P associations differed between P forms and mineral phases. Adsorbed P always contributed to several or all extracts, Hypothesis 1 was thus not tenable. Aluminum hydroxide, allophane, ferrihydrite, and goethite completely dissolved during Hedley fractionation from the third extraction step onwards. Successive mineral dissolution also occurred for the soil samples. Thus, extracted P represents partly desorbed P from various soil constituents and partly P co‐released upon dissolution of various minerals. Consequently, also Hypothesis 2 could not be confirmed, i.e., the sequential extraction is not suitable to assess different binding strengths between P forms and minerals. We conclude that the method hardly provides information for studies aiming at the mechanistic understanding of P bioavailability in soil.  相似文献   

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