Norbert Claassen

Organic phosphorus compounds as a phosphorus source for higher plants through the activity of phosphatases produced by plant roots and microorganisms

SummaryThe efficiency of phosphatases produced by clover, barley, oats and wheat was investigated in soils treated with sodium glycerophosphate, lecithin and phytin. Root exudates of aseptically grown clover were also examined for the breakdown of different organic P compounds in order to test the efficiency of plant-produced phosphatases. In general, the plants were able to use P from all the organic sources used in the study almost as efficiently as inorganic sources. Dry-matter yield, P uptake, acid and alkaline phosphatase activity and microbial population were increased in all the P treatments. Organic P enhanced alkaline phosphatase activity. Lecithin increased fungal, and phytin bacterial growth. There was no alkaline phosphatase activity in the asepticallly grown clover root exudates. Phosphatase released in aseptic culture after 4 weeks of clover growth was able to efficiently hydrolyse sodium glycerophosphate, lecithin and phytin. The amount of organic P hydrolysed in this and in the soil experiment surpassed plant uptake by a factor of 20. This suggests that the limiting factor on plant utilization of organic P is the availability of hydrolysable organic P sources.

Phosphorus efficiency of plants

Föhse et al. (1988) have shown that P influx per unit root length in seven plant species growing in a low-P soil varied from 0.6×10-14 to 4.8×10-14 mol cm-1s-1. The objective of this work was to investigate the reasons for these differences. No correlation was found between P influx and root radius, root hairs, cation-anion balance and Ca uptake. However, when root hairs were included in mathematical model calculations, the differences of P influx could be accounted for. These calculations have shown that in soils low in available P, contribution to P uptake by root hairs was up to 90% of total uptake.The large contribution of root hairs to P uptake was partly due to their surface area, which was similar to that of the root cylinder. However, the main reason for the high P uptake efficiency of root hairs was their small radius (approx. 5×10-4 cm) and their perpendicular growth into the soil from the root axis. Because of the small radius compared to root axes, P concentration at root hair surfaces decreased at a slower pace and therefore P influx remained higher. Under these conditions higher Imax (maximum influx) or smaller Km values (Michaelis constant) increased P influx. The main reasons for differences found in P influx among species were the size of Imax and the number and length of root hairs. In a soil low in available P, plant species having more root hairs were able to satisfy a higher proportion of their P demand required for maximum growth.

Phosphorus efficiency of plants I. External and internal P requirement and P uptake efficiency of different plant species

Plant species differ in their P efficiency,i.e. the P content in soil needed to reach their maximum yield. The differences in external P requirements can be atributed to either a lower internal P requirement for optimum growth or higher uptake efficiency of the plant. The objective of this research was to investigate the reasons for different P efficiencies of seven plant species.Onion, ryerass, wheat, rape, spinach, tomato and bean were grown in a P-deficient subsoil fertilized with 0, 2, 5, 10, 20, 40 and 80 mg P 100 g−1. All species showed a strong yield increase due to P fertilization. To reach 80% of maximum yield onion and tomato needed 17 and 11 mg P 100 g−1 respectively, corresponding to a soil solution concentrations of 6.9 and 5.7 μmol P l−1, whereas ryegrass, wheat and rape needed about 5 mg P 100g−1 corresponding to only 1.4 μmol P l−1 in soil solution. These differences in external P requirement cannot be explained by differences in their internal P requirement since onion, with the highest external P requirement, only contained 0.14% P in the shoot at 80% of maximum yield, while wheat, as the most P efficient species, contained 0.28%.P efficiency was related to the uptake efficiency of the plant which is determined by both root-shoot ratio and absorption rate per unit of root (influx). Species of low efficiency such as onion, tomato and bean had low influx rates and low root-shoot ratios, whereas species of medium to high efficiency had either high influx rates (rape and spinach) or high root-shoot ratios (ryegrass and wheat). The combination of both high influx rate and high root-shoot ratio was not found in any of the species studied.

Mobilization of phosphate in different soils by ryegrass supplied with ammonium or nitrate

Mobilization of soil P as the result of plant-induced changes of soil pH in the vicinity of plant roots was studied. Seedlings of ryegrass were grown in small containers separating roots from soil by a 30-μm meshed nylon screen which root hairs could penetrate but not roots. Two soils were used, a luvisol containing P mainly bound to calcium and an oxisol containing P mainly bound (adsorbed) to iron and aluminum. Plant-induced changes of soil pH were brought about by application of ammonium-or nitrate-nitrogen. After plants had grown for 10 d the soil was sliced in thin layers parallel to the root mat which had developed on the screen, and both soil pH and residual P determined. Mobilization of P was assessed by P-depletion profiles of the rhizosphere soil.Soil pH at the root surface decreased by up to 1.6 units as the result of ammonium N nutrition and it increased by up to 0.6 units as the result of nitrate N nutrition. These changes extended to a distance between 1 and 4 mm from the root surface depending on the type of soil and the source and level of nitrogen applied. In the luvisol, compared to zero-N treatment, P mobilization increased with the NH4-induced decrease in pH, whereas the NO3-induced pH increase had no effect. In contrast, in the oxisol a similar pH decrease caused by NH4 nutrition had no effect, whereas the pH increase caused by NO3 increased markedly the mobilization of soil P. It is concluded that in the luvisol calcium phosphates were dissolved by acidification, whereas in the oxisol adsorbed phosphate was mobilized by ligand exchange.

Verification of a mathematical model by simulating potassium uptake from soil

SummaryThis work develops the mathematical models suggested by various authors to simulate nutrient uptake of plants from soil. The simulation is based on ion transport from the soil to the roots by mass flow and diffusion and on Michaelis-Menten kinetics of nutrient uptake from soil solution by plant roots. For this purpose a differential equation is numerically integrated. Inter-root competition is allowed for by the choice of the boundary conditions. The integration procedure used makes it possible to take into account a variable buffer power which depends on soil solution concentration.The model calculates the change of nutrient concentrations in soil as a function of distance from the root surface for preestablished periods of time. Furthermore, the rate of uptake and the quantity of nutrients taken up per cm of root length is obtained. If the growth function of the root is known, nutrient uptake of a growing root system can be calculated.In order to verify the model two experiments were made:1.Potassium distribution was measured in a soil in the vicinity of rape roots under three different K levels. The calculated values agreed with the measured data.2.Potassium uptake of maize plants was measured in pot experiments with three different soils at two K levels each. Calculated K uptake agreed satisfactorily with measured K uptake. It is therefore concluded that the theoretical conception of the model is realistic and that the parameters have correctly been measured. The model thus appears to be useful to simulate such aspects of nutrient uptake of plants from soil which cannot be measured.ZusammenfassungDie Arbeit hat das Ziel, die von verschiedenen Autoren vorgeschlagenen Rechenmodelle zur Beschreibung der Nährstoffaufnahme aus dem Boden zu entwickeln. Sie basieren auf dem Nährstofftransport vom Boden zur Wurzel durch Massenfluss und Diffusion sowie auf der Kinetik der Nährstoffaufnahme von Wurzeln aus der Bodenlösung nach der Michaelis-Menten-Kinetik. Dabei wird eine Differentialgleichung numerisch integriert. Die Konkurrenz der Wurzeln um Mineralstoffe wird durch die Wahl der Randbedingungen berücksichtigt. Das gewählte Integrationsverfahren ermöglicht die Einbeziehung einer variablen, von der Konzentration der Bodenlösung abhängigen Pufferung.Das Modell errechnet die Änderung der Konzentration von Mineralstoffen im Boden mit zunehmender Entfernung von der Wurzeloberfläche in beliebigen Zeitspannen. Weiterhin wird die Aufnahmerate und die Nährstoffaufnahme pro cm Wurzel ermittelt. Its die Wachstumsfunktion der Wurzeln bekannt, so kann auch die Nährstoffaufnahme eines wachsenden Wurzelsystems errechnet werden.Zur Verifizierung des Modells wurden zwei Versuche durchgeführt:1.Ein Boden wurde auf drei K-Gehalte gebracht und die Verteilung der K-Konzentration in der Umgebung von Rapswurzeln gemessen. Die Modellrechnung ergab eine gute Übereinstimmung zwischen der gemessenen und der errechneten Konzentrationsverteilung im wurzelnahen Boden.2.In einem Gefässversuch mit drei Böden und je zwei K-Stufen wurde die K-Aufnahme von Maispflanzen gemessen. Auch hier stimmete die gemessene K-Aufnahme mit dem Ergebnis der Modellrechnung befriedigend überein Hieraus wird der Schluss gezogen, dass die theoretischen Vorstellungen, die dem Modell zugrundeliegen, realistisch sind und die verwendeten Parameter richtig gemessen wurden. Das Rechenmodell erscheint daher geeignet, um auch solche Aspekte der Mineralstoffaufnahme aus dem Boden zu untersuchen, die man nicht messen kann.

Potassium availability in relation to soil moisture II. Calculations by means of a mathematical simulation model

SummaryIn order to study the influence of soil moisture on the availability of potassium a simulation model was used. The model is designed to describe the transport of a nutrient from the soil to plant roots and its distribution around a root. From a pot experiment, the measured K uptake of onion plants, grown in soil under different moisture levels, agreed satisfactorily with the calculated K uptake. The model is therefore regarded as a valid means of quantifying the dynamics of K in the soil around plant roots.Calculations from a loess soil have shown that decreasing water content resulted in- a strong decrease of K transport from the soil to the root,- a faster decrease of the K concentration at the root surface and therefore- increasingly steep gradients of the K concentration around the root With the root density found in this experiment the K concentration of the moist soil (θ∼ 0.4) decreased almost equally in the total soil volume whereas in the dry soil (θ ∼ 0.1) not much change occurred in the middle between two roots.Therefore, the rate of K uptake per unit of root decreased much faster in the dry than in the moist soil. Calculations for sandy and loess soils, which have different water tension curves, have shown that the availability of K in the sandy soil is much more sensitive to changes in water tension than in the loess soil.The simulation technique can thus be used to analyzed the influence of single factors on the availability of K and to estimate the extent of this influence.ZusammenfassungUm den Einfluss des Wassergehaltes des Bodens auf die Verfügbarkeit von Kalium zu untersuchen, wurde ein Rechenmodell angewendet, das den Transport eines Nährstoffs vom Boden zur Wurzel und dessen Verteilung in der Umgebung der Wurzel beschreiben soll. An einem Gefässversuch mit Zwiebelpflanzen bei unterschiedlicher Bodenfeuchte ergab die Rechnung eine befriedigende Übereinstimmung mit der gemessenen K-Aufnahme der Pflanzen. Daraus wird geschlossen dass das Modell realistisch genug ist, um auch die K-Dynamik im wurzelnahen Boden zu quantifizierenSolche Rechnungen haben an einem Lössboden gezeigt, dass abnehmendem Wassergehalt- der K-Transport aus dem Boden zur Wurzel stark abnimmt,- dei K-Konzentration an der Wurzeloberfläche rascher sinkt, und daher- zunehmend steilere K-Konzentrations-Gradienten in Wurzelnähe enstehen. Bei der gegebenen Wurzeldichte sinkt die K-Konzentration des feuchten Bodens im gesamten Volumen nahezu gleichmässig ab, während sie im trockenen Boden in der Mitte zwischen zwei Wurzeln nur wenig abnimmt. Die K-Aufnahmerate pro Einheit Wurzel sinkt daher in trockenem Boden viel rascher als in feuchtem Boden ab. Rechnungen an Sand- und Lössböden, die sich durch ihre Wasserspannungskurve deutlich unterscheiden, zeigen sinngemäss, dass ein Sandboden in seiner K-Verfügbarkeit auf Änderungen der Wasserspannung viel empfindlicher als ein Lössboden reagiert.Die Modellrechnung ermöglicht es demnach, die Wirkung einzelner Faktoren der K-Verfügbarkeit zu erkennen und in ihrem Ausmass abzuschätzen.

Phosphorus Efficiency of Cabbage (Brassica oleraceae L. var. capitata), Carrot (Daucus carotaL.), and Potato (Solanum tuberosumL.)

Plant species and genotypes within the same species may differ in phosphorus efficiency. The objective of this research was to study phosphorus efficiency of cabbage (Brassica oleraceae L.), carrot (Daucus carota L.), and potato (Solanum tuberosum L.) and to quantify the contribution of morphological root characteristics to P uptake of the plant species. An experiment was conducted in a glasshouse with six P levels: 0, 12, 27, 73, 124 and 234 mg P kg−1 soil, and with six replications. Cabbage attained 80% of its maximum yield already at the level of no P supply, whereas carrot and potato reached only 4 and 16% of their highest yields respectively at this level of P supply. This indicated that cabbage was P-efficient compared to carrot and potato. Root/shoot ratio (cm root g−1 shoot d. m.) increased in the order of cabbage < carrot < potato, and was enhanced at lower P levels. Root hair length was not affected by P level, and averaged 0.22, 0.03 and 0.18 mm for cabbage, carrot, and potato, respectively. Predicting P uptake by a mechanistic simulation model revealed that root hairs contributed about 50% to the total P uptake of cabbage and potato, but only 0.3% to that of carrot. The relationship between the observed P uptake and the predicted P uptake of the plants revealed that model parameters explained nearly 4/5th of the total P uptake of carrot and potato, but only 2/5th of that of cabbage. This showed that the P uptake of cabbage was strongly under-predicted, whereas that of carrot and potato was predicted well. Therefore, it was hypothesised that cabbage may have the ability to mobilise and take up soil P additionally by other root mechanisms such as exudation of organic acids.

Potassium availability in relation to soil moisture I. Effect of soil moisture on potassium diffusion, root growth and potassium uptake of onion plants

SummaryThe objective of this research is to evaluate the influence of soil water content on- the mobility of potassium in soil,- plant growth and- K uptake of plants. The mobility of K increased with soil moisture. Increasing the volumetric water content (θ) from the 0.1 to 0.4 resulted in a rise of the effective diffusion coefficient (De) by a factor of about 10. This is mainly due to the increase of the tortuosity or impedance factor with higher soil moisture.In order to relate K mobility in soil to the availability of K for plant uptake, onion plants were grown in special containers under constant water content in the range of 0.1 to 0.4 cm3 H2O cm−3 of soil. Results are- both K content and growth of the plants increased with soil moisture,- water content below θ=0.1 reduced root growth- K inflow per unit of root surface increased with soil moisture. Maximum rate of inflow occurred with θ=0.25 in the soil used. It is therefore concluded that soil moisture affected K availability by affecting both K mobility and root growth.ZusammenfassungDie Arbeit hat das Ziel, den Einfluss des Wassergehaltes des Bodens auf- die Mobilität der Kaliumionen im Boden,- das Pflanzenwachstum und- die K-Aufnahme Zu bestimmen. Hierzu wurden einerseits Messungen der Mobilität von Kalium im Boden durch-geführt. Sie ergaben eine Erhöhung des effektiven Diffusionskoeffizienten (De) mit ansteigendem volumetrischen Wassergehalt (θ). De nahm um mehr als das Zehnfache zu während θ von 0,1 auf 0,4 anstieg. Dies ist der Erhöhung des Tortuositäts-oder Widerstands-faktors mit steigendem Wassergehalt zuzuschreiben. Um zu prüfen, in welchem Masse die Diffusionsbedingungen im Boden die Pflanzenverfügbarkeit von Kalium beeinflussen, wurde ein Vegetationsversuch durchgeführt. Hierzu wurden Zwiebelpflanzen in speziellen Versuchsgefässen bei konstanten Wassergehalten zwischen 0,1 und 0,4 cm3 H2O/cm3 Boden kultiviert. Die Ergebnisse sind:- K-Konzentration und Ertrag der Pflanzen wurden mit zunehmedem Bodenwassergehalt erhöht.- Der Wassergehalt des Bodens beeinflusste das Wurzelwachstum; unter θ=0,1 nahm die Wurzellänge stark ab.- Die K-Aufnahmerate eines Wurzelabschnitts stieg mit dem Wassergehalt an; bei θ=0,25 war die maximale Aufnahmerate in diesem Boden erreicht. Bie niedrigem Wassergehalt des Bodens wird die Kalium-Verfügbarkeit demnach beeinträchtigt sowohl durch den Rückgang der Mobilität von Kalium im Boden als auch die Verringerung des Wurzelwachstums.

The use of the ingrowth core method for measuring root production of arable crops - influence of soil conditions inside the ingrowth core on root growth.

The ingrowth core method can be used to measure root gross growth (i.e. root production). A mesh bag filled with root free soil is buried into the root zone. After about 14 days, the bag is pulled out and root length inside the core can be determined. An objection against this method is the inability to obtain the same soil conditions inside the bag as outside, which can result in different root growth pattern in the ingrowth core compared to the bulk soil. To study this, mesh bags were buried in a stand of oilseed rape and were filled with soil at different nitrate, phosphate, moisture, and bulk density levels. Results showed that root growth was only influenced by a high nitrate content and a high soil density in the cores, which resulted in higher and lower root length densities (RLD), respectively. In a long-term ingrowth experiment similar root length densities in the cores and in the bulk soil were measured, indicating that there were no root growth enhancing or impeding conditions inside the ingrowth cores. The conclusion is drawn, that the ingrowth core method gives reliable results, provided the N content and the soil density inside the bags are comparable to the bulk soil.

Root production and root mortality of winter barley and its implication with regard to phosphate acquisition

Winter barley was grown in a long-term fertilizer experiment (14 years) using two P treatments: (i) no P fertilization over the whole time (−P) and (ii) an annual fertilization of 44 kg P ha−1 (+P). The objective of the study was to investigate the influence of the P supply on total root production and root mortality (i.e., root turnover) and to assess the benefit of a more rapid root turnover on P acquisition. Shoot development and grain yield was reduced in the ‘−’ treatment, whereas the standing root system had nearly the same size as in the ‘+P’ treatment. Gross root growth was measured using the ‘ingrowth core method’. Mesh bags filled with root-free soil were buried into the rooting zone (0–30 cm) and root growth into the bags over periods of 2–3 weeks was determined. Assuming that no root mortality occured inside the bags during this short period, root length in the bags will be a measure of total root production. Total root production between April and June exceeded the size of the standing root system by a factor of 2 to 3 and was significantly higher at P deficiency. Root mortality as the difference between total root production and the size of the standing root system was also increased at P shortage. P uptake was calculated by using a mechanistic transport and uptake model. Calculations based on gross root growth and root mortality resulted in a higher uptake than calculations based on the development of the standing root system, although the length of the active roots were the same in both calculations. This was due to a better exploitation of undepleted soil areas by the growing root system. The root renewal by a continuous root growth and root mortality is discussed as a mechanism of P uptake efficiency.