Paul (Eldor A.) Collection

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https://hdl.handle.net/10217/100513

This digital collection includes articles by Eldor A. Paul, a Senior Research Scientist at the Natural Resource Ecology Laboratory and a Professor in the Department of Soil and Crop Sciences at Colorado State University, Fort Collins. Eldor has had a lifelong interest in teaching and research in both grassland ecology and agroecosystems, ranging from wheat fields in Canada, through corn-belt rotations in the Great Lakes region of the US, into the afforested systems in California and Colorado.

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Now showing 1 - 20 of 104

Open Access
14C Allocation in tree–soil systems
(Colorado State University. Libraries, 1994) Paul, Eldor A., author; Pregitzer, Kurt S., author; Horwath, William R., author; Heron Publishing, publisher
We studied whole-tree C allocation with special emphasis on the quantification of C allocation to roots and root respiration. To document seasonal patterns of C allocation, 2-year-old hybrid poplar trees greater than 3 m tall were labeled with 14CO2 in a large Plexiglas chamber in the field, in July and September. Climate and CO2 concentration were controlled to track ambient conditions during labeling. Individual tree canopy CO2 assimilation averaged 3.8 μmol CO2 m−2 s−1 (12.9 g C day−1 tree−1) in July and 6.2 μmol CO2 m−2 s−1 (9.8 g C day−1 tree−1) in September. Aboveground dark respiration was 12% of net daytime C fixation in July and 15% in September. Specific activity of root–soil respiration peaked 2 days after labeling and stabilized to less than 5% of maximum 2 weeks later. Low specific activity of root–soil respiration and a labeled pool of root C demonstrated that current photosynthate was the primary source of C for root growth and maintenance during the growing season. Root respiration averaged 20% of total soil respiration in both July and September based on the proportion of labeled C respired to labeled C fixed. In July, 80% of the recovered 14C was found above ground and closely resembled the weight distribution of the growing shoot. By September, 51% of the recovered 14C was in the root system and closely resembled the weight distribution of different size classes of roots. The finding that the distribution of biomass and 14C were similar verified that the C introduced during labeling followed normal seasonal translocation pathways. Results are compared to smaller scale labeling studies and the suitability of the approach for studying long-term C fluxes is discussed.
Open Access
A comparison of humic fractions of Chernozemic and Luvisolic soils by elemental analyses, UV and ESR spectroscopy
(Colorado State University. Libraries, 1974-11) Paul, E. A., author; Anderson, D. W., author; Russell, D. B., author; St. Arnaud, R. J., author; Agricultural Institute of Canada, publisher
The Ah horizons of a Brown–Dark Brown–Black–Dark Gray–Gray Luvisolic sequence of Canadian grassland and forest soils were studied. Clay-associated humus was present in greater proportions in the grassland than in forest soils, particularly in the grassland soils of the more arid regions. Amounts of alkali-pyrophosphate-extractable humus increased in the Brown to Gray Luvisolic sequence. Alkali-pyrophosphate-extractable humic acid (HA-A) contents were greatest in the Black and Dark Gray soils. C:H ratios, extinction coefficients at 280 nm (E280), and resistance to acid hydrolysis of the HA-A and clay-associated HA-B increased in the Brown to Gray Luvisolic sequence. Free radical concentrations were least for the Brown and Dark Brown soils, moderate for the Black and greatest for the Gray Luvisolic soils. The data suggest an increase in the proportion of aromatic components in the humic acids in going from the Brown to the Gray Luvisolic soils. An objective, multivariate similarity analysis based on 22 humus characteristics showed a relationship between soil zone or soil environment and the nature of the soil's humus.
Open Access
Acid hydrolysis of easily dispersed and microaggregate-derived silt- and clay-sized fractions to isolate resistant soil organic matter
(Colorado State University. Libraries, 2006-08) Conant, R. T., author; Paustian, K., author; Paul, Eldor A., author; Plante, A. F., author; Six, J., author; British Society of Soil Science, publisher
The current paradigm in soil organic matter (SOM) dynamics is that the proportion of biologically resistant SOM will increase when total SOM decreases. Recently, several studies have focused on identifying functional pools of resistant SOM consistent with expected behaviours. Our objective was to combine physical and chemical approaches to isolate and quantify biologically resistant SOM by applying acid hydrolysis treatments to physically isolated silt- and clay-sized soil fractions. Microaggegrate-derived and easily dispersed silt- and clay-sized fractions were isolated from surface soil samples collected from six long-term agricultural experiment sites across North America. These fractions were hydrolysed to quantify the non-hydrolysable fraction, which was hypothesized to represent a functional pool of resistant SOM. Organic C and total N concentrations in the four isolated fractions decreased in the order: native > no-till > conventional-till at all sites. Concentrations of non-hydrolysable C (NHC) and N (NHN) were strongly correlated with initial concentrations, and C hydrolysability was found to be invariant with management treatment. Organic C was less hydrolysable than N, and overall, resistance to acid hydrolysis was greater in the silt-sized fractions compared with the clay-sized fractions. The acid hydrolysis results are inconsistent with the current behaviour of increasing recalcitrance with decreasing SOM content: while %NHN was greater in cultivated soils compared with their native analogues, %NHC did not increase with decreasing total organic C concentrations. The analyses revealed an interaction between biochemical and physical protection mechanisms that acts to preserve SOM in fine mineral fractions, but the inconsistency of the pool size with expected behaviour remains to be fully explained.
Open Access
Agriculture's role in greenhouse gas mitigation
(Colorado State University. Libraries, 2006-09) Paul, Eldor A., author; Sheehan, John, author; Antle, John M., author; Paustian, Keith, author; Pew Center on Global Climate Change, publisher
This report describes opportunities for U.S. agriculture to contribute to reducing net greenhouse gas emissions. The Pew Center on Global Climate Change was established by the Pew Charitable Trusts to bring a new cooperative approach and critical scientific, economic, and technological expertise to the global climate change debate.
Open Access
Analytical determination of concentric carbon gradients within stable soil aggregates = Détermination analytique de gradients concentriques de carbone au sein d’agrégats stables de sol
(Colorado State University. Libraries, 1998-08) Paul, Eldor A., author; Smucker, Alvin J. M., author; [ISSS-AISS-IBG-SICS], publisher
Soil aggregation dynamics directly control agricultural production and reduce environmental contamination by convection-dispersion sequestrations of most ions. Greater containment and longer residence times of most plant nutrients, pesticides, and water would better sustain most agricultural production systems without polluting nearby groundwater supplies. In short, the large surface areas associated with the plethora of porosities within each natural soil aggregate provide dynamically interactive areas for chemical sequestration. Once known, it is these active/inactive centers which can be modified to improve plant productivity and water quality.
Open Access
Analytical determination of soil C dynamics = Détermination analytique de la dynamique du carbone du sol
(Colorado State University. Libraries, 1998-08) Haile-Mariam, Shawel, author; Collins, Harold P., author; Paul, Eldor A., author; [ISSS-AISS-IBG-SICS], publisher
The significance and possible management of soil organic C (SOC) in ecosystem functioning, global change and sustainable agriculture is best determined through a knowledge of its dynamics. This requires analytically determined measurements of SOC pool sizes and flux rates. The amount and quality of plant residues inputs, biotic activity, site characteristics and management are reflected in the size of the pools and their turnover rates. Some constituents are decomposed during periods of weeks; some persist for centuries and millenia. Fractionation of the soil and the use of tracers such as 14C and 13C makes possible the determination of the dynamics of the pools involved such that more meaningful estimates of the role of SOC in the many functions in which it plays a role can be calculated.
Open Access
Applicability of the carbon-dating method of analysis to soil humus studies
(Colorado State University. Libraries, 1967-09) McCallum, K. J., author; Rennie, D. A., author; Paul, Eldor A., author; Campbell, C. A., author; Williams and Wilkins Co., publisher
The organic fraction of soil is known to be composed of the soil biomass, partially decomposed plant and animal residues, and the materials commonly referred to as humic substances. Knowledge of the persistence of these fractions in soil is vital to the understanding of their contribution to soil fertility and soil genesis. Much information concerning the biochemistry of the humus materials also could be obtained through a knowledge of the mean residence times of the various organic fractions.
Open Access
Automated analysis of 15N and 14C in biological samples
(Colorado State University. Libraries, 1989) Paul, Eldor A., author; Harris, D., author; Marcel Dekker, Inc., publisher
An automated method for the simultaneous analysis of total N, total C, 15N and 14C in small plant and soil samples is described. A commercial C-N analyser - continuous flow isotope ratio mass spectrometer (ANCA-MS) has been extended to also measure CO2 and collect 14CO2 produced by sample combustion. Samples containing 20 - 200 μg N and up to 5 mg C can be analysed directly with no sample preparation other than drying and fine grinding. The precision of total elemental analysis is comparable to that by conventional methods. The average standard deviation of 15N analyses of plant material at natural abundance was ±1 ‰. This is accurate enough for all 15N studies except those using natural abundance and possibly long term studies of soil organic matter. Recovery of 14C in test samples was 100%. The instrument can be operated by graduate students under supervision and operating costs are primarily for sample cups, combustion catalyst and quartz tubes.
Open Access
Behavior of free amino acids in soil
(Colorado State University. Libraries, 1960-03) Putnam, H. D., author; Schmidt, E. L., author; Paul, Eldor A., author; Soil Science Society of America, publisher
The behavior of a mixture of amino acids in a soil environment was studied. Extractions were made with 80% ethanol. The extract was concentrated and then was analyzed for amino acids by gradient elution chromatography. After 1 hour of soil contact in the cold, at least some of each amino acid could be recovered, but the extraction was not efficient. Replicate soil flasks to which amino acids had been added were incubated at 28° C. under conditions that allowed for both CO2 and amino acid analysis of the same flask. After 24 hours, substantial degradation had occurred but at least trace amounts of each of the added amino acids except threonine could still be detected. Beta alanine appeared on the 24-hour chromatogram although it was not among the amino acids added initially. Results of both chromatographic analysis and CO2 collection suggested that nearly all of the added amino acids were degraded by the end of 96 hours. Separate studies using microbiological assay failed to confirm the persistence of threonine in soil as reported in the literature. The possibility that the beta alanine found in the soil environment was formed from aspartic acid decarboxylation was explored, but large additions of aspartic acid to soil did not result in substantial increases in beta alanine.
Open Access
Biological and molecular structure analyses of the controls on soil organic matter dynamics
(Colorado State University. Libraries, 2008-09) Magrini, K., author; Follett, R. F., author; Conant, R., author; Paul, Eldor A., author; Morris, S. J., author; Lomonosov Moscow State University, Department of Chemistry, publisher
The dynamics of soil organic carbon (SOC) are controlled by the interaction of biological, physical, and chemical parameters. These are best measured by a combination of techniques such as long-term field sites with a C3↔C4 plant switch. Acid hydrolysis and 14C- dating measure the mean residence time (MRT) of the resistant fraction. Long-term incubation allows the in situ biota to identify and decompose the labile SOC components. Statistical analysis (curve fitting) of the CO2 release curves, determines the pool size and of the two labile fractions (1). The effect of chemical structure is measured with pyrolysismolecular beam mass spectrometry (py-MBMS). The dynamics of charcoal, clay and silt are measured with both 13C and 14C.
Open Access
Carbon allocation, belowground transfers, and lipid turnover in a plant–microbial association
(Colorado State University. Libraries, 2012-09) Paul, Eldor A., author; Schultz, David J., author; Calderón, Francisco J., author; Soil Science Society of America, publisher
Radioactive tracers were used to study the C allocation to coarse and fine roots, aboveground plant tissues, mycorrhizal lipids, belowground respiration, and soil in a mycorrhizal association. Sorghum bicolor (L.) Moench was grown in soil with a nonmycorrhizal microbial inoculum with and without Glomus clarum, a mycorrhizal inoculant. Fifty-one-day-old mycorrhizal (M) and nonmycorrhizal (NM) plants were subjected to a 3-h exposure to 14CO2 and sequentially harvested after 52, 54, 57, 64, and 76 d. Mycorrhizal plants assimilated 21% more 14C than NM plants, even though they were slightly smaller in size. They also had a higher percentage and absolute allocation of 14C to root tissue, belowground respiration, and soil. Mycorrhizal roots had a higher content of total lipids and total fatty acids. The fungal fatty acid 16:1ω5, usually associated with arbuscular mycorrhizal fungi, comprised up to 29.5% of the total fatty acid content of M roots, while NM roots had only trace levels of this molecule. Thin-layer chromatography was used to separate the fatty acids extracted from the roots. The 14C of the various components was determined by radiography. The 14C mean residence time (MRT) of the mycorrhizal fatty acid 16:1ω5 was calculated at 7.1 d. The monoenoic, saturated, and total fatty acids had MRTs ranging from 11.1 to 14.3 d. The lipids of NM roots incorporated less 14C label. This underscores the difference in the lipid C cycle between the M and NM roots. Translocation of the 14C to soil was 6.3% of the photosynthesized C in the M plants relative to only 2.4% in the NM plants, giving an indication of its movement into the mycorrhizal hyphae as well as to the soil.
Open Access
Carbon and nitrogen mineralization kinetics in soil previously amended with sewage sludge
(Colorado State University. Libraries, 1989-01) Paul, Eldor A., author; Boyle, Michael, author; Soil Science Society of America, publisher
Microbial mineralization rates of organic carbon (C) and nitrogen (N) were determined on the same sludge-amended and nonamended soil samples. The purpose of this integrated approach was to high-light the long-term dynamics of N release with C stabilization in sludge-affected soil. Three application rates of digested municipal sludge, check, 45 Mg ha−1 and 180 Mg ha−1, were incorporated into field plots annually for 8 years, with no addition during the subsequent 3 years. Barley was grown on the site each spring of the 11 years. In an 87-week laboratory incubation experiment conducted on soil samples collected 3 years after the last sludge addition, N and C mineralization rates (kn, kc) increased with sludge application rate. Soil nitrogen mineralization potentials (No) increased with sludge application, unlike carbon mineralization potentials (Co) which did not correlate with sludge application. The C/N ratio of the mineralized organic matter decreased with sludge application rate. Three years after field incorporation of sludge, decomposition of the organic fraction can be described as a set of two first-order rate reactions. One fraction is characterized by a large stable element (high No, Co and low kn, kc); the second fraction consists of a smaller labile portion which is characterized by low No, Co and high kn, kc values. The microbial biomass decreased to less than half of its original amount after 20 weeks of incubation in all soil treatments.
Open Access
Carbon economy of soybean-rhyzobium-glomus associations
(Colorado State University. Libraries, 1985-11) Paul, E. A., author; Harris, D., author; Pacovsky, R. S., author; The New Phytologist, publisher
Carbon uptake and allocation in plants that were largely dependent on microbial symbionts for N and P was compared to that in plants given inorganic fertilizer. Soybeans (Glycine max L. Merr.) were grown in sterilized soil and were either left uninoculated, or were inoculated with Rhizobium japonicum (Kirschner), or both R. japonicum and Glomus fasciculatum (Thaxter sensu Gerd.). Uninoculated plants were given N and/or P fertilizer at rates required to produce plants similar in size to inoculated plants. Carbon flows to plant parts, root nodules and vesicular-arbuscular mycorrhizas were measured in six- and nine-week-old plants by determining the distributions of 14C after pulse labelling with 14CO2. Root nodules in non-mycorrhizal plants utilized 9% of total photosynthate; this was increased to 12% in nodulated, mycorrhizal plants. Mycorrhizas used 17% of the total photosynthate of six-week-old plants; this fell to 8% after nine weeks. Rates of 14CO2 fixation in leaves of nodulated or nodulated plus mycorrhizal plants were up to 52% higher than in plants without microbial symbionts. Part of the increase was due to higher specific leaf area in plants colonized by symbionts, but other factors such as source-sink relationships, starch mobilization and leaf P concentrations were also involved in the host-plant adaptations to the C demand of the microbial endophytes.
Open Access
Carbon flow in plant microbial associations
(Colorado State University. Libraries, 1981-07-24) Kucey, R. M. N., author; Paul, Eldor A., author; American Association for the Advancement of Science, publisher
Measurement of the distribution of the photosynthesis product in the symbiotic association of a legume, a mycorrhizal fungus, and nitrogen-fixing bacteria showed that the fungus incorporated 1 percent of the photosynthesis product and respired 3 percent. The nodules of a 5-week-old plant utilized 7 to 12 percent of the photosynthesis product. The legume compensated in part for the needs of its microbial partners through increased rates of photosynthesis.
Open Access
Carbon isotope ratios of Great Plains soils and in wheat-fallow systems
(Colorado State University. Libraries, 1997-07) Peterson, G. A., author; Lyon, D., author; Halvorson, A. D., author; Leavitt, S. W., author; Paul, Eldor A., author; Follett, R. F., author; Soil Science Society of America, publisher
The purposes of this study were to improve knowledge of regional vegetation patterns of C3 and C4 plants in the North American Great Plains and to use δ13C methodology and long-term research sites to determine contributions of small-grain crops to total soil organic carbon (SOC) now present. Archived and recent soil samples were used. Detailed soil sampling was in 1993 at long-term sites near Akron, CO, and Sidney, NE. After soil sieving, drying, and deliming, SOC and δ13C were determined using an automated C/N analyzer interfaced to an isotope-ratio mass spectrometer. Yield records from long-term experimental sites were used to estimate the amount of C3 plant residue C returned to the soil. Results from δ13C analyses of soils from near Waldheim, Saskatchewan, to Big Springs, TX, showed a strong north to south decrease in SOC derived from C3 plants and a corresponding increase from C4 plants. The δ13C analyses gave evidence that C3 plant residue C (possibly from shrubs) is increasing at the Big Springs, TX, and Lawton, OK, sites. Also, δ13C analyses of subsoil and topsoil layers shows evidence of a regional shift to more C3 species, possibly because of a cooler climate during the past few hundreds to thousands of years. Data from long-term research sites indicate that the efficiency of incorporation of small-grain crop residue C was about 5.4% during 84 year at Akron, CO, and about 10.5% during 20 year at Sidney, NE. The 14C age of the SOC at 0- to 10-cm depth was 193 year and at 30 to 45 cm was 4000 yr; 14C age of nonhydrolyzable C was 2000 and 7000 year for these same two respective depths. Natural partitioning of the 13C isotope by the photosynthetic pathways of C3 and C4 plants provides a potentially powerful tool to study SOC dynamics at both regional and local scales.
Open Access
Changes in ecosystem carbon following afforestation of native sand prairie
(Colorado State University. Libraries, 2013-09) Paul, Eldor A., author; Stromberger, M. E., author; Morris, S. J., author; Drijber, R., author; Hellerich, J., author; Mellor, N. J., author; Soil Science Society of America, publisher
Determining the dynamics of carbon (C) as a function of vegetation and residue inputs is important for predicting changes in ecosystem functions and the global C cycle. Litter and soil samples were analyzed from plantations of eastern red cedar (Juniperous virginiana) and ponderosa pine (Pinus ponderosa) and native prairie at the Nebraska National Forest to evaluate the impact of different types of land management on soil C contents and turnover rates. Total soil C to a depth of 1 m was greatest in the cedar stands. Pine ecosystems stored more C in the tree biomass and litter but lost more native prairie C from the soil. The soil 13C content showed 82% of the original, and prairie C remained under cedars compared with ∼45% under pine. Soil cation contents were greatest overall in cedar soils and lowest in pine. The C content in cedar soils was strongly related to Ca content. Differences in microbial community fatty acid profiles were related to vegetation type, and nutrients explained ∼60% of the variation in profiles. Our research indicates that changes in soil C and nutrient content following conversion from prairie to forest are dependent on tree species planted, characteristics of the plant litter, and cation cycling in the plant–soil system.
Open Access
Chemical differences in soil organic matter fractions determined by diffuse-reflectance mid-infrared spectroscopy
(Colorado State University. Libraries, 2011-03) Collins, Harold P., author; Reeves, James B., author; Calderón, Francisco J., author; Paul, Eldor A., author; Soil Science Society of America, publisher
We performed mid-infrared (MidIR) spectral interpretation of fractionated fresh and incubated soils to determine changes in soil organic matter (SOM) chemistry during incubation. Soils from four sites and three depths were processed to obtain the light fraction (LF), particulate organic matter (POM), silt-sized (silt), and clay-sized (clay) fractions. Our results show that the LF and clay fractions have distinct spectral features regardless of site. The LF is characterized by absorbance at 3400 cm−1, as well as between 1750 and 1350 cm−1 The clay fraction is distinguished by absorption near 1230 cm−1, and absorption at 780 to 620 cm−1 The POM, like the LF, absorbs at the broad peak at 1360 cm−1 High SOM soils are characterized by absorbance at 1230 cm−1, a band for aromatics, possibly associated with resistant C. Soils from different sampling depths have specific spectral properties. A band at 1330 cm−1 is characteristic of shallow depths. Because of their low organic matter (OM) content, the deeper samples are characterized by mineral bands such as quartz, clays, and carbonate. Spectroscopic data indicates that the clay fraction and the LF suffered measurable chemical transformations during the 800-d incubation, but the POM and silt fraction did not. As the LF decomposes, it loses absorbance at 3400, 1223, and 2920 to 2860 cm−1 The band at 1630 cm−1 increased during incubation, suggesting a resistant form of organic C. The clay fraction suffered changes that were opposite to those of the LF, indicating that LF decomposition and clay decomposition follow different chemistries.
Open Access
Comparative diversity of ammonia oxidizer 16S rRNA gene sequences in native, tilled, and successional soils
(Colorado State University. Libraries, 1999-07) Kowalchuk, George A., author; Bruns, Mary Ann, author; Stephen, John R., author; Paul, Eldor A., author; Prosser, James I., author; American Society for Microbiology, publisher
Autotrophic ammonia oxidizer (AAO) populations in soils from native, tilled, and successional treatments at the Kellogg Biological Station Long-Term Ecological Research site in southwestern Michigan were compared to assess effects of disturbance on these bacteria. N fertilization effects on AAO populations were also evaluated with soils from fertilized microplots within the successional treatments. Population structures were characterized by PCR amplification of microbial community DNA with group-specific 16S rRNA gene (rDNA) primers, cloning of PCR products and clone hybridizations with group-specific probes, phylogenetic analysis of partial 16S rDNA sequences, and denaturing gradient gel electrophoresis (DGGE) analysis. Population sizes were estimated by using most-probable-number (MPN) media containing varied concentrations of ammonium sulfate. Tilled soils contained higher numbers than did native soils of culturable AAOs that were less sensitive to different ammonium concentrations in MPN media. Compared to sequences from native soils, partial 16S rDNA sequences from tilled soils were less diverse and grouped exclusively within Nitrosospira cluster 3. Native soils yielded sequences representing three different AAO clusters. Probes for Nitrosospira cluster 3 hybridized with DGGE blots from tilled and fertilized successional soils but not with blots from native or unfertilized successional soils. Hybridization results thus suggested a positive association between the Nitrosospiracluster 3 subgroup and soils amended with inorganic N. DGGE patterns for soils sampled from replicated plots of each treatment were nearly identical for tilled and native soils in both sampling years, indicating spatial and temporal reproducibility based on treatment.
Open Access
Comparison of nitrification rates in blueberry and forest soils
(Colorado State University. Libraries, 2002-01) Paul, Eldor A., author; Ravenscroft, John, author; Serce, Sedat, author; Throop, Philip A., author; Hanson, Eric J., author; American Society for Horticultural Science, publisher
Highbush blueberries (Vaccinium corymbosum L.) are long lived perennial plants that are grown on acidic soils. The goal of this study was to determine how blueberry cultivation might influence the nitrification capacity of acidic soils by comparing the nitrification potential of blueberry soils to adjacent noncultivated forest soils. The net nitrification potential of blueberry and forest soils was compared by treating soils with 15N enriched (NH4)2SO4, and monitoring nitrate (NO3--N) production during a 34-day incubation period in plastic bags at 18°C. Net nitrification was also compared by an aerobic slurry method. Autotrophic nitrifiers were quantified by the most probable number method. Nitrate production from labeled ammonium (15NH4+) indicated that nitrification was more rapid in blueberry soils than in forest soils from six of the seven study sites. Slurry nitrification assays provided similar results. Blueberry soils also contained higher numbers of nitrifying bacteria compared to forest soils. Nitrification in forest soils did not appear to be limited by availability of NH4+ substrate. Results suggest that blueberry production practices lead to greater numbers of autotrophic nitrifying bacteria and increased nitrification capacity, possibly resulting from annual application of ammonium containing fertilizers.
Open Access
Comparisons between P-fertilized and mycorrhizal plants
(Colorado State University. Libraries, 1986-01) Paul, Eldor A., author; Bethlenfalvay, G. J., author; Pacovsky, R. S., author; Crop Science Society of America, publisher
In experimentation with vesicular-arbuscular mycorrhizal (VAM) fungi, the availability of non-VAM control plants of equal size to VAM plants is a fundamental requirement. The purpose of this work was to determine nutrient regimes needed to achieve growth equivalence between VAM and non-VAM plants. Soybean [Glycine max (L.)Merr.] cv. Amsoy 71 and sorghum [Sorghum bicolor (L.) Moench] cv. Bok 8 plants were grown under controlled conditions in a soil (Josephine silty clay loam, mesic Typic Haploxerult) low in plant-available P. Soybeans were inoculated with one of four species and sorghum with one of two species of VAM fungi. Non-inoculated control plants received nutrient solutions that contained 0.0, 0.2, 0.4, or 1.0 mM P. while the growth of P-supplemented controls may be equivalent to VAM plants, an important question remains: Are these plants also equivalent in terms of such functional parameters as leaf development, dry matter partitioning, and nutrient assimilation? The objective of this experiment was to answer these questions. The response to VAM colonization was similar in both hosts, although less extensive colonization was observed in sorghum. Dry weight, leaf area, and P content increased exponentially with nutrient solution P level. Plants colonized with VAM fungi grew 3 to 6 times larger than the P-free controls but attained only 35 to 65% of maximum growth possible with high fertilizer P input. Host response to VAM colonization was equivalent to that of plants receiving between 0.12 and 0.22 mM P for phytomass, leaf area, and N content. Mycorrhizal plants contained less P, Mn, and root Fe but more Zn and Cu than comparable plants fertilized with P. It was concluded that P-treated, non-VAM plants differed physiologically and anatomically from VAM plants of equivalent size grown under P stress. It may therefore be necessary to establish the comparability of VAM plants and of "VAM-equivalent controls" separately for each plant parameter of interest. Even then, differential growth responses in VAM-host associations may prevent complete comparability between VAM and P-fertilized plants.