S. A. Barber

Sensitivity of simulated phosphorus uptake to parameters used by a mechanistic-mathematical model

SummaryChemical methods for determination of soil P available to plants do not take into account the effect of roots on P uptake by the plant. Moreover, the relative significance of root parameters, as compared to soil supply parameters in determining P uptake, is unknown. Simulation models have been verified for P uptake by corn (Zea mays L.) and soybeans (Glycine max L. Merr.). The objective of this paper was to use the Cushman simulation model, which has 11 plant and soil parameters, for a sensitivity analysis of the parameters involved in P uptake. Initial parameter values were those obtained from soybeans grown in Raub (Aquic Argiudoll) silt loam. Phosphorus uptake was simulated with each parameter changed independently, from 0.5 to 2.0 times initial value while all the other parameters remained constant. In addition, P uptake was simulated where interrelated parameters were changed according to their dependence. Root growth rate and root radius were the most sensitive parameters influencing P uptake. Soil P supply parameters were more sensitive than root physiological uptake parameters. Phosphorus concentration in soil solution affected P uptake more than the diffusion coefficient and buffer power. Reduction of root radius while root volume was maintained constant by increasing root length increased P uptake. Where both soil volume and root volume were kept constant, reduction of root radius to the size of root hairs or mycorrhizal hyphae gave the greatest P uptake.

Potassium and phosphorus uptake by corn genotypes grown in the field as influenced by root characteristics

SummaryRoot parameters of three corn (Zea mays L.) genotypes influencing P and K uptake were investigated in solution culture and field experiments. The data for these parameters were used to simulate P and K uptake by plants grown in the field using the Claassen-Barber model5. Root characteristics for ion influx, maximum rate of influx,Imax; Michaelis-Menten constant,Km; and minimum concentration of solution below which no further net influx occurs,Cmin were determined in solution culture. These kinetic parameters varied 2 to 3 fold among genotypes. Variations among genotypes were different for K than for P.Three corn genotypes were grown in the field and harvested 47, 54 and 68 days after emergence. Yield and root surface per plant increased about 3 fold during this time. At 47 days, 2/3 of the total root surface was in the top soil whereas 3 weeks later, it was less than 50%. Genotypes differed in distribution of roots between the topsoil and subsoil as well as in root surface per unit of shoot.K uptake predicted by the Claassen-Barber model was 2 to 3 times the observed. The overprediction could be related to high root density (length of root per unit soil volume) which indicated that competition between roots occurred that was not considered in the simulation model. The predicted P uptake (y) was correlated (r=0.91) to observed uptake (x) byy=0.98+0.67x, indicating underprediction of P uptake. The presence of root hairs may have been the cause of the underprediction. The calculated contribution of the subsoil to the observed uptake was 10% for K and 1% in the case of P. It was concluded that the plant parameters used to simulate nutrient uptake were rated accurately when allowance was made for root competition and presence of root hairs.

A numerical solution of whole plant nutrient uptake for soil-root systems with root hairs

SummaryRoot hairs increase phosphorus, P, uptake over that due to the plant root alone. A mechanistic model using 16 parameters was developed to describe this process. The model was verified with an experiment using six species that varied widely in root hair length, density and radius. A sensitivity analysis was conducted and the results are included to illustrate the situations where root hairs contribute significantly to P uptake. Length of root hair, root hair density and root hair radius all influenced predicted P uptake with root hair length being particularily significant.

Plant age and the phosphorus uptake characteristics of trimmed and untrimmed corn root systems

SummaryThe influx of P per meter of corn (Zea mays L.) root for corn grown in nutrient culture was measured by following the rate of loss of P from solution over a 4 to 6 hour period. Experiments were conducted on corn grown for 12, 14, 28, 35, 43, 52, and 80 days. The maximum P influx, Km(the Michaelis constant), P efflux, and the minimum concentration to which the plants reduced the solution P level were measured on plants with untrimmed and trimmed roots. Reducing root length by trimming did not alter P uptake characteristics of corn roots. Trimming was done one or two days prior to the measurement of P flux.Stage of plant growth or plant age affected P flux characteristics of the roots. The maximum P flux into the root at high P concentrations, Vmaxwas greatest at 28 days and then decreased so that at 80 days (2 weeks after tasseling) Vmaxwas 4% of the rate at 28 days. Plant age had very little effect on Kman indicator of the relation between P concentration in solution and uptake relative to VmaxThe values calculated for P efflux from the roots and the values measured for the minimum P level to which the corn plant could reduce the P level both decreased with plant age. The minimum P level decreased from 0.3 μM to 0.1 μM.Although flux of P into the root decreased with age, trimming the roots of older plants did not increase the P flux into the remaining roots indicating that P flux was regulated in the root rather than by the demand of the shoot for P.

Effect of soil moisture and phosphate level on root hair growth of corn roots

SummaryRoot hairs have been shown to enhance P uptake by plants growing in low P soil. Little is known of the factors controlling root hair growth. The objective of this study was to investigate the influence of soil moisture and P level on root hair growth of corn (Zea mays L.). The effect of volumetric soil moistures of 22% (M0), 27% (M1), and 32% (M2) and soil (Raub silt loam, Aquic Argiudoll) P levels of, 0.81 (P0), 12.1 (P1), 21.6 (P2), 48.7 (P3), and 203.3 (P4) μmol P L−1 initially in the soil solution, on shoot and root growth, P uptake, and root hair growth of corn was studied in a series of pot experiments in a controlled climate chamber. Root hair growth was affected more by soil moisture than soil P. The percentage of total root length with root hairs and the density and length of root hairs on the root sections having root hairs all increased as soil moisture was reduced from M2 to M0. No relationship was found between root hair length and soil P. Density of root hairs, however, was found to decrease with an increase in soil P. No correlation was found between root hair growth parameters and plant P content, further suggesting P plays a secondary role to moisture in regulating root hair growth in soils. The increase in root hair growth appears to be a response by the plant to stress as yield and P uptake by corn grown at M0 were only 0.47 to 0.82, and 0.34 to 0.74, respectively, of that measured at M1 across the five soil P levels. The increase in root hair growth at M0, which represents an increase of 2.76 to 4.03 in root surface area, could offset, in part, the reduced rate of root growth, which was the primary reason for reduced P uptake under limited soil moisture conditions.

Soil phosphorus after 25 years of cropping with five rates of phosphorus application

Abstract Rate of change of levels of various soil P forms as affected by P fertilizer application and crop removal is important for determining long term P fertilization policy. A rotation‐fertility experiment conducted for 25 years at the Purdue Agronomy Farm, West Lafayette, IN has provided data for calculating changes in soil P levels. Average annual P applications ranged from 0 to 54 kg/ha. Soil samples taken initially and after 25 years were analyzed for resin exchangeable P, Bray Pl, and total P. A linear relation between P applied minus P removed by cropping and Bray Pl indicated that Bray Pl increased one μg/g for every 17 kg net P/ha added. When P application on some plots was stopped for up to eight years, resin and Bray Pl levels decreased. The rate of decrease was greater the higher the P level at the time P application ceased. Increase in soil P level by fertilizer addition and decrease in P level by cropping appeared to be reversible processes.

Root growth and phosphorus and potassium uptake by two corn genotypes in the field

In a field study P and K uptake by two corn (Zea mays L.) genotypes which differed in root growth was investigated. The effect of differences in root growth on P and K uptake was assessed using a mechanistic-mathematical model which describes nutrient uptake by growing plant roots in soil. Nitrogen was applied at 0 and 227 kg ha−1 to Pioneer 3732 and B73xMo17 corn grown on Raub silt loam (fine-silty, mixed, mesic Aquic Argiudoll) and at 227 kg N ha−1 to these two genotypes on Chalmers silt loam (finesilty, mixed, mesic, Typic Haplaquoll). Root growth and P and K uptake by the two corn genotypes was measured 31, 47, 75 and 91 d after planting on the Raub and 31, 47, 61 and 75 d after planting on the Chalmers soil.Root growth and P and K uptake by B73xMo17 was greater than that of Pioneer 3732 on N-fertilized Raub soil. On Chalmers soil the difference in root growth between the two genotypes resulted in an increase in K but not P uptake. The higher soil P level of the Chalmers appears to have offset possible differences in P uptake due to root size. There were no differences between the two genotypes in either the percentage of roots with root hairs, or the density or length of root hairs. Phosphorus and K uptake calculated with the simulation models for both corn genotypes on both soils over each of three growth periods agreed with observed P (Y = 0.68X + 1.71; r = 0.944**) and K (Y = 0.88X + 15.52; r = 0.928**) uptake. Differences in P and K uptake between B73xMo17 and Pioneer 3732 resulted primarily from the difference in root growth in the topsoil. A high correlation was found between root surface area and P (r = 0.893**) and K (r = 0.928**) uptake by both corn genotypes on both the soils.

Modeling magnesium, phosphorus and potassium uptake by loblolly pine seedlings using a Barber-Cushman approach

The Barber-Cushman mechanistic nutrient uptake model, which has been utilized extensively to describe and predict nutrient uptake by crop plants, was evaluated for its ability to predict K, Mg, and P uptake by loblolly pine (Pinus taeda L.) seedlings. Sensitivity analyses were also used to investigate the impact of changes in soil nutrient supply, root morphological, and root uptake kinetics parameters on simulated nutrient uptake. Established experimental techniques were utilized to define the 11 parameters needed to model uptake by 1-0 seedlings of K, Mg, and P from a modified A horizon soil (Lilly series). Model predictions of K and P uptake over a 180-d growth period were underestimated by 6 and 11%, respectively. Estimates of Mg uptake were underestimated by 62%. While the level of agreement between predicted and observed K and P values was quite acceptable, analysis of parameter values and results of sensitivity analyses both indicated that the model underestimation of Mg uptake was the result of applying an Imax value developed under relatively low Mg concentration to a situation in which the functional Imax would be much higher due to the dominance of passive versus active uptake. Overall results of sensitivity analyses indicate that under the circumstances investigated, Imax, was the primary variable controlling plant uptake of K, Mg, and P. The dominance of this term over others was due to the relatively high Cli values for all three nutrients. Reducing (-50%) or increasing (+ 100%) other soil supply, root morphological, and remaining root uptake kinetics values did not substantially alter model estimates of nutrient uptake.

Comparison of root and root hair growth in solution and soil culture

Abstract Aerated solution culture is frequently used for studying plant growth. Few comparisons have been made of root growth in solution with that found in soil. The objective of this study was to compare root growth and root hair development in these two mediums. Corn (Zea mays L.) grown in aerated solution at two temperatures (18 and 25°C) and three P concentrations (2, 10, and 500 μmol L‐1) was compared with that in three soils, Raub (Aquic Argiudoll) and two Chalmers (Typic Haplaquoll) silt loams, in a controlled climate chamber over 21d. Corn plant weight and root growth were similar in solution culture and Raub soil when grown at an air and soil temperature of 18°C. At 25°C both yield and root growth were greater in Raub soil, even though P uptake by corn was 7‐fold greater in solution culture. The same difference was found when corn grown at 25°C in solution culture at 3 different P concentrations was compared with that grown in Chalmers soil at two P levels. Percentage of total root length with r...

GROWTH REQUIREMENTS FOR NUTRIENTS IN RELATION TO DEMAND AT THE ROOT SURFACE

Summary Nutrient influx across the soil-root interface (rate of uptake per unit of root surface area) is influenced by the demand of the plant for the nutrients required for additional growth. Size of influx will depend on the relation between area of active root surface and the demand of the shoot. Nutrient demand per unit of root, surface varies with many factors including plant species, cultivars within species, nutrient, plant age, root age, nutrient status of the plant, and light and temperature conditions. Interactions between these factors as well as changes within the plant to compensate for a low rate of supply of a particular nutrient make specific characterization of nutrient influx difficult. Average nutrient influx is usually greatest during the first two weeks of growth and then decreases as the plant matures. While effect of root age on influx differs with nutrient and plant species, influx of some nutrients continues even after root suberization. Split-root experiments indicate that increasing requirement of phosphate and potassium by the plant per unit of root does not increase influx until there is a reduction in the concentration of these nutrients in the shoot. Control of influx within the plant appears to reside in the shoot rather than the root. Limited data indicate that 60 per cent or more of the roots need to be absorbing at their maximum rate in order that influx of phosphate or potassium does not limit plant growth. Plants have a capacity to compensate for deficiencies and when nutrients become limiting they may increase root size relative to shoot demand, increase root hair density and length.