Adsorption efficiency of a continuous trapping system and its use for the collection of root exudates from cucumber

Root exudates comprising soluble low‐molecular‐weight organic compounds (LMWOCs) play a crucial role in the rhizosphere processes. Therefore, accurate determination of the composition and quantity of these compounds is of importance. A continuous trapping system (CTS) with XAD‐4 macroporous resin is widely used for collecting root exudates in hydroponics, and ion exchange resins are used in the separation of root exudates into neutral, acid and basic fractions. Here, we studied the adsorption equilibrium isotherms and dynamic adsorption properties of 27 standard LMWOCs on XAD‐4 resin. The adsorption isotherms of most LMWOCs matched the Langmuir isotherm equations. Furthermore, we investigated the adsorption efficiency of the CTS for LMWOCs and the recovery ratios during fractionation by ion exchange resins. The adsorption capacities of the CTS for LMWOCs which are non‐polar or having large non‐polar moieties were higher. The recovery ratios of most LMWOCs were larger than 80% during fractionation. The overall recovery ratios of LMWOCs were ca. 10, 20, and 30% for sugars, organic acids, and amino acids, respectively. Using this collection method, we present the composition and quantity of root exudates of cucumber (Cumumis sativus L.) at four different growth stages. The major components of root exudates were similar to previous works, whereas the quantities were different. The various collection methods may be the main reason for these differences. Therefore, it is necessary to study the overall recovery ratios when the compositions of root exudates via different collection methods are compared.     

Plasticity of high and low nutrient‐adapted grasses to added sulfur and nitrogen

Crop and native plants can be characterized as high and low nutrient‐adapted based on their expected response to native and applied nutrients. Our objective was to compare the plasticity of biomass allocation and tissue nutrient concentrations to added sulfur (S) and nitrogen (N) across a continuum of high and low nutrient‐adapted grasses, represented by barley (Hordeum vulgare), smooth brome (Bromus inermis), bluebunch wheatgrass (Pseudoroegneria spicata), and Idaho fescue (Festuca idahoensis). In our greenhouse study, treatments included two S sources (pyrite and gypsum), at 150 and 300 kg S ha^‐1, N at 50 kg ha^‐1, and a check. Shoot biomass of barley, smooth brome, and bluebunch wheatgrass was enhanced by S plus N. Shoot biomass of barley and smooth brome was greater with pyrite than with gypsum. Root biomass of smooth brome and bluebunch wheatgrass was greater with pyrite than with gypsum. Plant S concentrations of barley and Idaho fescue were enhanced by added S. Plant S concentrations in barley and smooth brome were greater with gypsum than with pyrite. Except for barley, plant S pools (shoot biomass x shoot S concentration) were enhanced with S plus N compared with no added nutrients. Nitrogen pools of barley, smooth brome, and bluebunch wheatgrass were higher with pyrite than with gypsum. Soil sulfate (SO₄) was greater when S or S plus N was added than without any added nutrients. For barley and smooth brome, soil sulfate tended to be lower with pyrite than with gypsum. For all soils, pH was lower with added S or added S plus N compared with unamended soils. While pyrite lowered soil pH, gypsum tended to increase soil pH. Overall, barley and smooth brome were highly plastic in responding to enhanced nutrient levels, bluebunch wheatgrass was relatively responsive, and Idaho fescue was least responsive.     

Sorption of HOC in soils with carbonaceous contamination: Influence of organic‐matter composition

Detailed information about structure and composition of organic sorbents is required to understand their impact on sorption capacity and sorption kinetic of organic pollutants. Therefore, the chemical composition of organic material from 18 geosorbents was investigated by solid‐state ¹³C nuclear‐magnetic‐resonance (NMR) spectroscopy. Structural parameters such as aromaticity, polarity, and alkyl‐C content were related to the Freundlich sorption exponent (1/n) and the sorption coefficient . The geosorbents included three natural and four combusted coals (carbonaceous material), three Histosols, five mineral soils from Germany containing inputs of technogenic carbonaceous material, derived from industrial activities, and four non‐contaminated mineral soils from Germany. Equilibrium sorption was measured for five hydrophobic organic compounds and analyzed with the solubility‐normalized Freundlich sorption isotherm. With increasing maturation degree, the proportion of polar constituents decreases from the natural soils to the coals. In contrast to the non‐polluted mineral soils, the soils with technogenic input are characterized by high aromaticity and low polarity. A positive correlation between sorption coefficient and aromaticity was found. The Freundlich exponent (1/n) is negatively correlated with the aromaticity, denoting an increase of adsorption processes with increasing aromaticity. Likewise, the contribution of partitioning decreases. This sorption mechanism predominates only if the organic matter in the samples contains a high proportion of polar compounds.     

Chemical changes in the rhizosphere of metal hyperaccumulator and excluder Thlaspi species

Rhizosphere processes involved in hyperaccumulation and exclusion of metals are still largely unknown. Therefore, we conducted a rhizobag experiment on contaminated and non‐contaminated soils to investigate the chemical changes in the rhizosphere of the hyperaccumulators Thlaspi goesingense and T. caerulescens, and the metal‐excluder T. arvense. Root growth was restricted to the rhizobags separated by a nylon membrane (7 μm \x 25 μm mesh size) from surrounding bulk soil. Depletion of labile Zn in rhizosphere could not explain the amount of metals accumulated in T. caerulescens, whereas the difference in EDTA‐extractable Zn even exceeded total plant uptake. Substantially increased DOC in T. arvense rhizosphere indicates alleviation of phytotoxicity by formation of metal‐organic complexes. Hyperaccumulation and depletion of labile Zn in the rhizosphere was observed for T. goesingense grown on contaminated soil. On non‐contaminated soil, Zn was accumulated but labile Zn in rhizosphere was not changed. Nickel present in background concentrations in both soils was accumulated by T. goesingense only when grown on non‐contaminated soil. In contrast, labile Ni in the rhizosphere was increased in both soils, suggesting a general tendency of Ni mobilization by T. goesingense.     

Transformation of organic rhizodepositions by rhizosphere bacteria and its influence on the availability of tertiary calcium phosphate

This study assesses the influence of saccharides in the rhizodeposition on the phosphate solubilizing ability of rhizosphere bacteria. Water‐soluble rhizodeposits were analyzed of ¹⁴C‐labeled pea plants (Pisum sativum, cv. ‘Grapis’) which were grown at two different levels of P‐nutrition. The sugars produced were fed in vitro either as single compounds or as synthetic mixtures to three bacterial strains and the ability of the bacteria to mobilize Ca₃(PO₄)₂ was measured. The relative glucose proportion of pea exudates decreased under P deficiency while the content of galactose, ribose, xylose and fucose increased. In vitro feeding of single sugars and sugar mixtures showed that the ability of Pseudomonas fluorescens (PsIA12) to dissolve tertiary calcium phosphate was lower with pentoses and the mixed sugars of the P‐deficient plants than with glucose. On the other hand, the shifted sugar pattern observed under P deficiency increased the phosphate mobilization ability of Pantoea agglomerans (D5/23) and Azospirillum sp. (CC 322) considerably. This observation can only partly be explained by the acidification of the nutrient medium. Bacteria also produced different carboxylic anions depending on sugar supply. In addition to low‐molecular mono‐, di‐, and tricarboxylic acids which are known as P‐solubilizing substances, sugar acids also played an important role in cultures D 5/23 and CC 322.     

Rhizodeposition and net release of soluble organic compounds by pine and beech seedlings inoculated with rhizobacteria and ectomycorrhizal fungi

Summary A lysimetric experiment was performed in a greenhouse to evalute root deposition and net release of soluble organic compounds after 1 and 2 years from pine and beech seedlings inoculated with an ectomycorrhizal fungus (Laccaria laccata) and/or rhizobacteria (Agrobacterium radiobacter for beech and Agrobacterium sp. for pine). Total C compounds released in the rhizosphere of both plants increased after inoculation with the bacteria or ectomycorrhizal fungus. The rhizobacteria increased root and plant growth and rhizodeposition, but the mycorrhizal fungi appeared to increase only root deposition. Soluble C compounds, collected after 2 years, represented only 0.1–0.3% of the total C compounds released into the rhizosphere, and were modified by inoculation with the microorganisms. After inoculation with the bacteria, levels of sugars and amino acids decreased in pine and beech rhizospheres, whereas organic acids increased, especially in the pine rhizosphere. In the rhizosphere of mycorrhizal beeches, sugar and amino acids increased, and organic acids differed from those released from non-mycorrhizal beeches. In the mycorrhizal pine rhizosphere, however, all compounds decreased. Following dual inoculations, mycorrhizal colonization increased, no effect on plant growth was observed, and virtually no organic acids were detected.     

Effect of crop growth on the distribution and mineralization of soil sulfur fractions in the rhizosphere

The effect of plant growth on the mineralization of organic matter and distribution of soil S fractions (plant available SO₄^2—, adsorbed SO₄^2—, carbon‐bonded S, ester‐bonded S, and residual‐S) in the rhizosphere was studied in a greenhouse experiment using a rhizobag technique. In this study wheat, oilseed rape and radish were grown on two soils, a Haplic Acrisol and a Hortic Anthrosol. Significant differences between S fractions in the rhizosphere and non‐rhizosphere were determined in dependence on soil type and crop species. In all cropped treatments lower amounts of ester‐bonded S and higher levels of residual‐S were found in the rhizosphere than in the non‐rhizosphere, while the amount of carbon‐bonded S fractions was similar. These results indicate firstly, that the arylsulfatase activity was higher in the rhizosphere than in the non‐rhizosphere and secondly, that mass flow of SO₄^2—‐S to the rhizosphere increased after mineralization of residual‐S. Compared to the non‐vegetated soil, the ester‐bonded S fraction of wheat and oilseed rape decreased in the rhizosphere revealing that the mineralization of organic S in the rhizosphere is related to the crop type.     

Effect of buckwheat (Fagopyrum esculentum) on soil‐phosphorus availability and organic acids

As a cover crop, buckwheat (Fagopyrum esculentum) may increase soil‐P availability. Buckwheat was grown in low‐P and P‐fertilized field plots, and organic anions were measured in rhizosphere soil. Soil‐P availability was not affected by buckwheat, but the concentration of rhizosphere tartrate^2– was significantly higher (p < 0.005) in low‐P vs. P‐fertilized plots. This suggests that organic‐anion root exudation may have a role in buckwheat‐rhizosphere P dynamics.     

Urea nitricphosphate for cool‐season grass production

Abstract

Significant losses of nitrogen (N) can occur via volatilization of ammonia (NH₃) when non‐incorporated broadcast applications of urea or urea‐containing fertilizers are made. This study was conducted to determine the efficacy of urea nitricphosphate (UNP) as an N and phosphorus (P) source for cool‐season grasses and to evaluate NH₃ volatilization potential of UNP as compared to urea under laboratory conditions. A three‐year field study compared UNP to ammonium nitrate (AN) and urea at 56 and 112 kg N/ha for tall fescue (Festuca arundinacea Schreb.) and smooth brome (Bromus inermis Leyss.). Brome yields were significantly higher from UNP as compared to urea for one of the three years. No such differences occurred with fescue. Nitrogen uptake was significantly higher from UNP as compared to urea for one year each for brome and fescue. Phosphorus uptake by brome was significantly higher from UNP as compared to urea for two years. Laboratory incubation studies showed significantly lower NH₃ volatilization from UNP than from urea after seven days, but no significant differences after 14 days. The delay in NH₃ volatilization was due to the diffusion and subsequent hydrolysis of urea immediately below the soil zone initially influenced by the UNP. The reduction in NH₃ volatilization at the early time could partially be attributed to an inhibition of urea hydrolysis and significantly lower soil pH values for UNP as compared to urea in the upper 30 mm of soil cores. The general conclusion from the field and laboratory work was that UNP is a suitable N source for cool‐season grasses, with the primary potential benefit being delayed NH₃ volatilization as compared to urea.     

Tall fescue aluminum tolerance is affected by neotyphodium coenophialum endophyte

Roots of endophyte‐infected (E+) tall fescue (Festuca arundinacea Schreb.) exude more phenolic‐like reductants than roots of endophyte‐free (E‐) plants when mineral stressed. Phenolic compounds are efficient chelators of aluminum (Al) and may influence Al tolerance in many plant species. The objective of our study was to determine if enhanced release of phenolic compounds by roots of E+ plants contributes to Al tolerance in tall fescue. Two cloned genotypes (DN2 and DN11) of tall fescue infected with their naturally occurring fungal endophyte Neotyphodium coenophialum (Morgan‐Jones and Gams) Glenn, Bacon and Hanlin and their noninfected isolines were grown in nutrient solutions at 0 μM Al (Al‐) and at 640 μM Al (Al+) under controlled environment conditions. Root and shoot dry matter (DM) of endophyte‐infected tall fescue was greater in E+ than E‐ plants by 57% and 40%, respectively, when plants were grown without Al. Endophyte infection did not affect root and shoot DM of tall fescue grown with Al but relative (to Al‐treatment) reduction in root and shoot DM was greater in E+ than E‐ plants. In response to Al stress, more Al (47%) and P (49%) could be desorbed from root surfaces of E+ than E‐ plants. Aluminum concentrations in roots of E+ plants were 35% greater and P concentrations were 10% less than those determined in roots of E‐plants. No differences in mineral concentrations were observed in shoots, regardless of endophyte status, or Al level in nutrient solution. Roots of E+ plants increased pH of both Al‐ and Al+ nutrient solutions to a greater extent than roots of E‐ plants in a 48 h interval. Our results show that more Al can be sequestered on root surfaces and in root tissues of endophyte‐infected tall fescue than in plants devoid of endophyte. Aluminum sequestration was greater on root surfaces and in root tissues of E+ than E‐ plants of a given tall fescue genotype. Our results suggest that increased exudation of phenolic‐like compounds from roots of endophyte‐infected tall fescue may be directly involved in Al tolerance and serves as a mechanism for widespread adaptability and success of endophyte‐tall fescue associations.     

Sugar analysis with the shaffer‐somogyi micro‐analysis,high performance liquid chromatography and enzymatic analysis in crop samples

Abstract

To evaluate the reliability of the Shaffer‐Somogyi (SS) micro‐analysis of reducing sugars, extracts of 14 dried crop samples were analyzed before and after hydrolysis in 0.05 N ^H ₂SO₄with this method, with High Performance Liquid Chromatography (HPLC) and with an enzymatic glucose and fructose assay. The values, obtained with the SS micro‐analysis were for many samples higher than those, obtained with HPLC, suggesting that other compounds than sugars, present in certain plant tissues, respond in this non‐specific method. Enzymatic analysis tended to give lower values for sugar content than HPLC. It is recommended, that routine analysis of crop samples with the SS micro‐analysis is preceeded by analysis with HPLC to assess the contribution of non‐sugars to the outcome of the former.     

Developmental aberrations in seeds of boron deficient sunflower and recovery

Sunflower (Helianthus annuus L.) cv. Modern grown in refined sand at deficient (0.033 mg L^‐1) boron (B) developed visible symptoms of low B accompanied by marked depression in growth, dry matter, tissue B, flower head size, and seed weight. The B deficient seeds showed a marked decrease in non‐reducing sugars and contents of oil and starch whereas in leaves reducing sugars accumulated. Except for slight increase in leaf B and flower size, resumption of sufficient B (0.33 mg L^‐1) to B deficient plants from the day of anthesis could not appreciably alter the growth, dry matter, head size and seed weight of deficient plants. Apart from this, non‐reducing sugar content in seeds increased on resupplying B to deficient plants. A significant decrease in non‐reducing sugars and starch content in B sufficient seeds by withdrawing B from the day of anthesis indicate a specific role of B in production and deposition of reserve in the seeds of sunflower.     

Soil bacterial community response to sulfadiazine in the soil–root zone

Sulfonamide antibiotics reach soil via manure and adversely affect microbial diversity. Clear effects of these bacteriostatic, growth‐inhibiting antibiotics occur in the presence of a parallel input of microbial activity stimulating manure. Natural hot spots with already increased soil microbial activity are located in the rhizosphere, comprising microorganism such as Pseudomonas with plant growth promoting and pathogenic strains. The hypothesis was therefore that the antibiotic activity of sulfonamides is promoted in the rhizosphere even in the absence of manure, followed by shifts of the natural plant‐specific microbial community structure. This was evaluated by a laboratory experiment with Salix fragilis L. and Zea mays L. After 40 d of incubation, sub‐areas such as non‐rhizosphere soil, rhizosphere soil and plant roots were sampled. Effects on microbial community structure were analyzed using 16S rRNA gene fragment patterns of total bacteria community and Pseudomonas. Selected exoenzymes of N‐, P‐, and C‐cycling were used to test effects on microbial functions. Compared to the factors soil sub‐area and sulfadiazine (SDZ) content, plant species had the largest influence on the bacterial community structure and soil exoenzyme activity pattern. This was also reflected by an up to 1.5‐fold higher acid phosphatase activity in samples from maize‐ compared to willow‐planted soil. We conclude that antibiotic effects on the bacterial community structures are influenced by the antibiotic concentration and root influence.     

Arbuscular mycorrhizal fungus and rhizobacteria affect the physiology and performance of Sulla coronaria plants subjected to salt stress by mitigation of ionic imbalance

Salt stress has become a major menace to plant growth and productivity. The main goal of this study was to investigate the effect of inoculation with the arbuscular mycorrhizal fungi (AMF; Rhizophagus intraradices) in combination or not with plant growth‐promoting rhizobacteria (PGPR; Pseudomonas sp. (Ps) and Bacillus subtilis) on the establishment and growth of Sulla coronaria plants under saline conditions. Pot experiments were conducted in a greenhouse and S. coronaria seedlings were stressed with NaCl (100 mM) for 4 weeks. Plant biomass, mineral nutrition of shoots and activities of rhizosphere soil enzymes were assessed. Salt stress significantly reduced plant growth while increasing sodium accumulation and electrolyte leakage from leaves. However, inoculation with AMF, whether alone or combined with the PGPR Pseudomonas sp. alleviated the salt‐induced reduction of dry weight. Inoculation with only AMF increased shoot nutrient concentrations resulting in higher K⁺: Na⁺, Ca²⁺: Na⁺, and Ca²⁺: Mg²⁺ ratios compared to the non‐inoculated plants under saline conditions. The co‐inoculation with AMF and Pseudomonas sp. under saline conditions lowered shoot sodium accumulation, electrolyte leakage and malondialdehyde (MDA) levels compared to non‐inoculated plants and plants inoculated only with AMF. The findings strongly suggest that inoculation with AMF alone or co‐inoculation with AMF and Pseudomonas sp. can alleviate salt stress of plants likely through mitigation of NaCl‐induced ionic imbalance, thereby improving the nutrient profile.     

Neotyphodium coenophialum‐endophyte infection affects the ability of tall fescue to use sparingly available phosphorus

Neotyphodium coenophialum, (Morgan‐Jones & Gams) Glenn, Bacon & Hanlin, infected tall fescue (Festuca arundinacea Schreb.) plants perform better than non‐infected isolines on phosphorus (P)‐deficient soils. Our objective was to characterize growth and P uptake dynamics of tall fescue in response to endophyte infection and P source at low P availability in soil. Two tall fescue genotypes (DN2 and DN4) infected with their naturally occurring N. coenophialum strains (E+), and in noninfected (E‐) forms were grown in Lily soil (fine loamy siliceous, mesic Typic Hapludult) in a greenhouse for 20 weeks. Three soil P treatments were imposed: no P supplied (control) and P supplied as commercial fertilizer (PF) or as phosphate rock (PR) at the level of 25 mg P kg^‐1 soil. Interaction of tall fescue genotype and endophyte status had a significant influence on mineral element uptake suggesting high specificity of endophyte‐tall fescue associations. Endophyte infection did not affect root dry matter (DM) when no P was supplied but shoot DM was reduced by 20%. More biomass was produced and greater P uptake rate occurred in PR than PF treatment. Root DM was greater in E+ DN4 than E‐DN4 when supplied with either PF or PR. In contrast, endophyte infection did not affect root DM of DN2, regardless of P source. Relative growth rate (RGR) of E+ plants grown with PR was 16% greater than that of E‐plants. Endophyte infection did not improve growth or P uptake in PF treatment. When PR was supplied, P uptake rate was 24% greater in E+ DN2 than E‐ DN2, but endophyte infection did not benefit DN4. Phosphorus‐use efficiency was 6% less in E+ DN2 but 16% greater in E+ DN4 compared to E‐ plants, regardless of P source. Root exudates of E+ DN2, but not E+ DN4 solubilized more P from PR than those of E‐ plants. The correlation between root RGR and P uptake rate was relatively high for E‐ plants (r=0.76), but low for E+ plants (r=0.27) grown with PR. Results suggest that P uptake by E+ tall fescue might rely on mechanisms other than an increase in root biomass (surface area). Endophyte infection modified tall fescue responses to P source. This phenomenon was associated with modes of P acquisition which included enhanced activity of root exudates in releasing P from PR in E+ plants (DN2), and increased root biomass (DN4). The dominant means of P acquisition may be determined by a specific association of endophyte and tall fescue genomes. Endophyte‐tall fescue association plasticity contributes to widespread success of symbiotic in marginal resource conditions.     

EFFECTS OF CALCIUM AND SALINITY STRESS ON QUALITY OF LETTUCE IN SOILLESS CULTURE

Abstract

Fine fescues (Festuca spp.) are generally considered acid tolerant compared to other cool‐season turfgrasses. However, there is little information on aluminum (Al) tolerance of fine fescues at both the species and cultivar levels. The objectives of this study were to identy cultivars of fine fescues with superior ability to tolerate Al, and compare the Al tolerance of endophyte infected and endophyte‐free cultivars in Al tolerance. A total of 58 cultrvars of fine fescues belonging to five species or subspecies [14 hard fescue (F. longifolia Thuill), 25 Chewings fescue (F. rubra L. ssp. commutata Gaud), 15 strong creeping red fescue (F. rubra L. ssp. rubra), two slender creeping red fescue (F. rubra L. ssp. trichophylla), and two sheep fescue (F. ovina L.)] were selected from the 1993 National Fineleaf Fescue Test and screened under greenhouse conditions using solution culture, sand culture, and acid Tatum soil (Clayey, mixed, thermic, typic, Hapludult). The acid Tatum soil had 69% exchangeable Al and a pH of 4.4. An Al concentration of 640 μM and a pH of 4.0 were used in solution culture and sand culture screening. The grasses were seeded and grown for three weeks before harvesting. Aluminum tolerance was assessed by measuring relative root length, shoot length, root weight, shoot weight, and total dry matter. Differences in Al tolerance were identified at both the species and cultivar level based on relative growth were as follows: i) hard fescue and Chewings fescue were more Al tolerant than strong creeping red fescue; ii) within species or subspecies, significant differences were found among cultvars of Chewings fescue, strong creeping red fescue, slender creeping red fescue, and sheep fescue; whereas no difference was observed among the hard fescue cultivars; and iii) the cultivars containing endophyte exhibited greater Al tolerance compared the eudophyte‐free cultivars. The results indicate that fine fescues vary in Al tolerance and there is potential to improve Al tolerance with breeding and to refine their management recommendations regarding soil pH.     

Sorption of atrazine, 2,4‐D,nitrobenzene and pentachlorophenol by urban and industrial wastes

This study was carried out to investigate the sorption properties of man‐made soil developed from sewage sludge, municipal wastes, brick and mortar debris, harbour sludge, sand fills, fly ash, and wastes from coking plants and coal mines. The composition of organic matter in the samples was analysed, and the sorption isotherms of four reference chemicals (nitrobenzene, atrazine, 2,4‐D, pentachlorophenol) were determined. Fly ash, which contains up to 89% of its carbon as Black Carbon, showed a strong affinity to all four chemicals. For the other waste materials, a strong correlation between the logarithm of the Freundlich adsorption constant, K_f, and the logarithm of organic carbon, C_o, was established (r = 0.85–0.96). This holds for the non‐ionic nitrobenzene and also, within a certain pH range depending on the pK_a of the compound, for the three ionizable organic compounds (atrazine: pH > 4; 2,4‐D: pH > 5; PCP: pH > 6). At pH near the pK_a value the sorption is sensitive to pH. There were no statistically significant differences between the waste materials and the natural soils in the relations between logK_f and logC_o for either ionic or non‐ionic chemicals. This result suggests that the method devised for estimating the sorption of organic chemicals in natural soils based on their content of organic carbon is equally valuable for the waste materials, with the exception of fly ash which contains a large amount of Black Carbon.     

Resin extraction of labile, soil organic phosphorus

In order to develop a method for estimating labile, soil organic phosphorus (P_o), macroporous anion-exchange and adsorbent (XAD) resins were tested for their ability to extract P_o from a permanent pasture soil. Experimental variables included: ionic form, soil-water resin ratio, addition of cation-exchange resin, extraction time, extraction temperature, continuous and stepwise extraction and soil pretreatment (chloroform and microwaves). The amounts of extracted P_o and P_i (inorganic phosphorus) showed little variation between the anion-exchange resins, when used in the bicarbonate form. In the chloride form, the amounts extracted were less and more variable. XAD resins extracted much less P_o and very little P_i. The macroporous anion-exchange resin, Lewatit MP500A in the bicarbonate form, was chosen for further studies. It extracted 21 mg P_o kg^?1 from air-dried soil and 8 mg P_o kg^?1 from moist, incubated soil. The specific amounts of P_o and P_i extracted were little affected by variations in the ratio between soil, water and resin, but increased with extraction time and temperature. Chloroform pretreatment of the soil mainly increased extracted P_i, whereas microwave pretreatment only increased extracted P_o. The magnitude of these increases was approximately constant irrespective of extraction temperature, indicating that the increases came from killed micro organisms.     

The suitability of pde‐solvers in rhizosphere modeling,exemplified by three mechanistic rhizosphere models

Mechanistic rhizosphere models are mathematical models that calculate the uptake of solutes (e.g. nutrients such as P or K) in a plant root and/or the concentration profiles of ions or microorganisms across the rhizosphere. They consist of partial differential equations (pde's) and their associated initial and boundary conditions, and often require numerical solutions due to non‐linearity. The suitability of the pde‐solver FlexPDE for rhizosphere modeling was evaluated by reproducing the outputs of three original rhizosphere models and comparing the results with the original model outputs. The models considered were NST 3.0 (Syring and Claassen, 1996), Kirk (1999), and Newman and Watson (1977). From visual comparison, the curves as produced by the original model and FlexPDE match very well. All calculated R²s (coefficients of determination) were equal to 1 with a rounding precision of three decimal places. Rescaling the time, using an upwind scheme in the presence of convection terms and controlling the relative error with the FlexPDE function errlimit were helpful in finding the most accurate numerical solution. These results showed that FlexPDE is a valuable tool in rhizosphere modeling.