Root Zone Of Wheat Research Articles

Monitoring soil moisture content in the root zone of winter wheat with multi-angle multispectral imagery

ABSTRACT Exploring the impact of different monitoring angles from unmanned aerial vehicle (UAV) on the monitoring accuracy of soil moisture content (SMC) is crucial for precision irrigation. To this end, experiment was conducted to monitor the SMC of winter wheat at different growth stages under different irrigation treatments in Yangling, Shaanxi, China. At a solar zenith angle of 45°, multispectral remote sensing data from a UAV were collected at thirteen different monitoring zenith angles. Simultaneously, the SMC in the wheat root zone was measured. From the UAV multispectral images, spectral reflectance was extracted for the construction of vegetation indices. Then the correlation between the vegetation indices and the measured SMC was analyzed. With the vegetation indices as input variables, SMC monitoring models were constructed using Extreme Learning Machine (ELM), Random Forest (RF), and Back Propagation Neural Network (BPNN). The study also examined the effect of specific angles (hotspot and dark spot angles) on the estimation accuracy of the SMC at the nadir angle. The results indicated that different monitoring angles significantly impact the SMC estimation accuracy. Band reflectance and vegetation indices exhibited significant peak values and angular effects at the monitoring zenith angle of 45°. The models achieved the optimal inversion accuracy at the hotspot angle (at a monitoring zenith angle of 45° in the solar principal plane), and the accuracy was ranked as follows: BPNN (R2 = 0.71; RMSE = 1.69) > ELM (R2 = 0.52; RMSE = 1.94) > RF (R2 = 0.48; RMSE = 2.10). By eliminating shadows at the nadir angle through the threshold of dark spot, inversion accuracy similar to the hotspot direction is achieved. This study provides a basis for appropriate selection of UAV flight angles for the monitoring of SMC in the root zone of winter wheat.

Spatial and temporal dynamics of the bacterial community under experimental warming in field-grown wheat.

Climate change may lead to adverse effects on agricultural crops, plant microbiomes have the potential to help hosts counteract these effects. While plant-microbe interactions are known to be sensitive to temperature, how warming affects the community composition and functioning of plant microbiomes in most agricultural crops is still unclear. Here, we utilized a 10-year field experiment to investigate the effects of warming on root zone carbon availability, microbial activity and community composition at spatial (root, rhizosphere and bulk soil) and temporal (tillering, jointing and ripening stages of plants) scales in field-grown wheat (Triticum aestivum L.). The dissolved organic carbon and microbial activity in the rhizosphere were increased by soil warming and varied considerably across wheat growth stages. Warming exerted stronger effects on the microbial community composition in the root and rhizosphere samples than in the bulk soil. Microbial community composition, particularly the phyla Actinobacteria and Firmicutes, shifted considerably in response to warming. Interestingly, the abundance of a number of known copiotrophic taxa, such as Pseudomonas and Bacillus, and genera in Actinomycetales increased in the roots and rhizosphere under warming and the increase in these taxa implies that they may play a role in increasing the resilience of plants to warming. Taken together, we demonstrated that soil warming along with root proximity and plant growth status drives changes in the microbial community composition and function in the wheat root zone.

Rice residue recirculation enhances mobilization and plant acquisition of soil inorganic phosphorus by increasing silicon availability in a semi-arid Inceptisol

Massive amounts of rice straw (RS) provide a significant problem for in situ and instant management at a low cost. On the other hand, mobilizing soil phosphorus (P) from inorganically fixed pools may increase its effectiveness over time. To address both of these issues at once as well as to decide the optimum rate of RS for P mobilization, this study was carried out to determine whether the use of RS + P solubilizing microbes (PSMs) could solubilize a sizable portion of the soil's fixed P and affect P transformation, silicon (Si) concentration, organic acid (OA) concentration, and enzyme activity to increase wheat yield. Depending on the soil temperature, the application of RS (at 12 Mg ha−1) + PSM could solubilize 3.40–3.66% of the inorganic P. Over control, it minimized the hysteresis impact by 6–8%. The soils of wheat included the acids oxalic acid, citric acid, formic acid, malic acid, and tartaric acid. At maturity stage of wheat, application of RS (at 12 Mg ha−1) + PSM + 75%P raised the activity of dehydrogenase, alkaline phosphatase activity, cellulase, and peroxidase by 1.77, 1.65, 1.87, and 1.82 times above control in soil, respectively. It also boosted Si concentration in the soil increased by 58% over control. Wheat grain yield was 40 and 18% higher under RS (at 12 Mg ha−1) + PSM + 75%P application than under control and 100% P application. Additionally, it increased root volume, length, and P uptake by 2.38, 1.74, and 1.62 times above control, respectively. According to path analysis, P solubilisation by Si and OAs considerably increased P availability in the wheat root zone. Therefore, cultivators could be advised to use RS (at 12 Mg ha−1) + PSM + 75% P of mineral P fertilizer to save 25% P fertilizer without reducing yield.

Draft Genome Sequence of Bacillus velezensis Strain Krd-20 with Antifungal Activity, a Biotechnologically Important Isolate from Wheat Root Zone.

ABSTRACTBacillus velezensis Krd-20 strain with antifungal activity was isolated from the wheat rhizosphere. This strain is used to suppress fungi of the Fusarium sp. when growing oyster mushroom (Pleurotus ostreatus). Genome assembling resulted in 44 contigs with a total length of 3939663 bp were obtained, the GC content is 46.4%.

Spike Formation Is a Turning Point Determining Wheat Root Microbiome Abundance, Structures and Functions.

Root selection of their associated microbiome composition and activities is determined by the plant’s developmental stage and distance from the root. Total gene abundance, structure and functions of root-associated and rhizospheric microbiomes were studied throughout wheat growth season under field conditions. On the root surface, abundance of the well-known wheat colonizers Proteobacteria and Actinobacteria decreased and increased, respectively, during spike formation, whereas abundance of Bacteroidetes was independent of spike formation. Metagenomic analysis combined with functional co-occurrence networks revealed a significant impact of plant developmental stage on its microbiome during the transition from vegetative growth to spike formation. For example, gene functions related to biofilm and sensorial movement, antibiotic production and resistance and carbons and amino acids and their transporters. Genes associated with these functions were also in higher abundance in root vs. the rhizosphere microbiome. We propose that abundance of transporter-encoding genes related to carbon and amino acid, may mirror the availability and utilization of root exudates. Genes related to antibiotic resistance mechanisms were abundant during vegetative growth, while after spike formation, genes related to the biosynthesis of various antibiotics were enriched. This observation suggests that during root colonization and biofilm formation, bacteria cope with competitor’s antibiotics, whereas in the mature biofilm stage, they invest in inhibiting new colonizers. Additionally, there is higher abundance of genes related to denitrification in rhizosphere compared to root-associated microbiome during wheat growth, possibly due to competition with the plant over nitrogen in the root vicinity. We demonstrated functional and phylogenetic division in wheat root zone microbiome in both time and space: pre- and post-spike formation, and root-associated vs. rhizospheric niches. These findings shed light on the dynamics of plant–microbe and microbe–microbe interactions in the developing root zone.

Salt Dynamics and Budgeting in the Root Zone of Wheat under Irrigated Saline Environment

Plant growth is adversely affected by salt deposition in the root zone and requires judicious irrigation water management for enhancing productivity in an irrigated saline environment. To study the salt deposition and subsequent leaching of salts below the root zone of wheat, an experiment with four wheat cultivars (viz. three salt-tolerant KRL-210 (V), KRL-1-4 (V), and 12one salt non-tolerant HD- 2894 (V) were taken for the experiment under artificially prepared 4-1-1-1irrigation water with salinity levels of 4 dS m(S), 8 dS m(S) and 12 dS m(S) besides the 234-1varying salinity of the groundwater from 1.45 to 1.7 dS m(S) during rabiseasons of 2009-10 and 12010-11 at the research farm of Indian Agricultural Research Institute (IARI), New Delhi, India. -1 The soil salinity (EC) was 0.2 dS mbefore the conduction of the experiment during rabi 2009-1:2-1 10 and varied from 0.26 to 0.95 dS mduring rabi2010-11. The total salt-induced in soil was 70.15 -1-1 t haand 55.6 t hafor rabi2009-10 and 2010-11 respectively by saline irrigation water. The salt -1deposition in the crop root zone was observed at maximum (i.e., 17.04 and 22.97 t ha during rabi2009-10 and 2010-11, respectively) for Streated plots and minimum in Streatment levels. 41The production functions for wheat varieties were developed and the coefficient of 2determination (R) was 0.98 to 0.99 and 0.94 for salt-tolerant and salt non-tolerant varieties, respectively. Moreover, it was also estimated that the maximum salt was deposited on the top soil layer (15cm) and the leaching of salts from S, S, Sand Streatments levels was 65%, 63%, 123,452%, and 48% salts, respectively from the root zone. However, this study would assist in the computation of leaching requirements for enhancing productivity in irrigated saline environments.

Soil moisture and nitrogen content influence wheat yield through their effects on the root system and soil bacterial diversity under drip irrigation

AbstractSoil properties and plants influence soil microbial community diversity, and their role in nutrient cycling could influence plant productivity; however, it is not clear the relationships between soil bacterial communities, soil moisture, and nitrogen (N) content, and whether these associations influence plant productivity. In this study, we investigated how the soil microbial community affects root zone productivity in winter wheat. We also verified whether productivity can be improved by water adjustment and N management in drip irrigation field experiments (I0, natural condition; I1, jointing to heading stage S1 55%–80%θf, field capacity, anthesis to mature stage S2 70%–80%θf; I2, S1 55%–90%θf, S2 70%–90%θf; and I3, S1 55%–100%θf, S2 70%–100%θf) and under different N application rates (N0, N1, N2, and N3 represent 0, 150, 200, and 250 kg N ha−1). Soil moisture and N application rate significantly influenced water use efficiency (WUE), N use efficiency (NUE), yield, and soil bacterial abundance and diversity in the wheat root zone. N application decreased the richness indices in un‐irrigated soils (I0), whereas it initially increased, and then decreased the indices in irrigated soils (I1, I2, and I3). The primary factor influencing the dominant bacterial taxa was soil moisture content (SMC), followed by root length density (RLD). The root system and soil bacterial diversity strong influenced wheat yield. The I1N1 treatment resulted in the highest root weight density (RWD), WUE, and NUE, and thus increased wheat yield; furthermore, it had the least influence on the diversity of soil bacteria. Our work suggests that the response of bacterial communities to soil moisture and nitrogen content largely regulates crop production in agro‐ecosystems, and that regulating soil moisture and nitrogen fertilizer use may provide a sustainable solution for maintaining farmland productivity.

Modeling the temporal distribution of water, ammonium-N, and nitrate-N in the root zone of wheat using HYDRUS-2D under conservation agriculture.

In the current study, the temporal distribution of both soil water and soil NO3-N under several conservation agriculture (CA) practices during the wheat crop growth were characterized by HYDRUS-2D model. Treatments comprised of conventional tillage (CT), permanent broad beds (PBB), zero tillage (ZT), PBB with residue (PBB+R) and ZT with residue (ZT+R). Hydraulic inputs of the model, comprising the measured value of Kfs, α and n, obtained as the output of Rosetta Lite model were optimized through inverse modeling. Model predicted the daily change in soil water content (SWC) of the profile during the simulated period (62-91 DAS) with good accuracy (R2 = 0.75; root mean squared error (RMSE) = 0.038). In general, soil water balance simulated from the model showed 50% lower cumulative drainage, 50% higher cumulative transpiration along with higher soil water retention, in PBB+R than CT. Reported values of the first-order rate constants, signify nitrification of urea to NH4-N (μa) (day-1) nitrification of NH4-N to NO3-N (μn) (day-1) and the distribution coefficient of urea (Kd-in cm3 mg-1) were optimized through inverse modeling. Later they were used as solute transport reaction input parameters of the model, to predict the daily change in NO3-N of the profile with better accuracy (R2 = 0.83; RMSE = 4.62). Since NH4-N disappears fast, it could not be measured frequently. Therefore, not enough data could be generated for their use in the calibration and validation of the model. Results of simulation of daily NO3-N concentration indicated a higher concentration of NO3-N in the surface layer and its leaching losses beyond the root zone were relatively lesser in PBB+R, than CT, which resulted in less contamination of the belowground water. Thus, the study clearly recommended PBB+R to be adopted for wheat cultivation in maize-wheat cropping system, as it enhances the water and nitrogen availability in the root zone and reduce their losses beyond the root zone.

Root-associated microbiomes of wheat under the combined effect of plant development and nitrogen fertilization

BackgroundPlant roots assemble microbial communities both inside the roots and in the rhizosphere, and these root-associated microbiomes play pivotal roles in plant nutrition and productivity. Although it is known that increased synthetic fertilizer input in Chinese farmlands over the past 50 years has resulted in not only increased yields but also environmental problems, we lack a comprehensive understanding of how crops under elevated nutrient input shape root-associated microbial communities, especially through adjusting the quantities and compositions of root metabolites and exudates.MethodsThe compositions of bacterial and fungal communities from the roots and rhizosphere of wheat (Triticum aestivum L.) under four levels of long-term inorganic nitrogen (N) fertilization were characterized at the tillering, jointing and ripening stages. The root-released organic carbon (ROC), organic acids in the root exudates and soil organic carbon (SOC) and soil active carbon (SAC) in the rhizosphere were quantified.ResultsROC levels varied dramatically across wheat growth stages and correlated more with the bacterial community than with the fungal community. Rhizosphere SOC and SAC levels were elevated by long-term N fertilization but varied only slightly across growth stages. Variation in the microbial community structure across plant growth stages showed a decreasing trend with N fertilization level in the rhizosphere. In addition, more bacterial and fungal genera were significantly correlated in the jointing and ripening stages than in the tillering stage in the root samples. A number of bacterial genera that shifted in response to N fertilization, including Arthrobacter, Bacillus and Devosia, correlated significantly with acetic acid, oxalic acid, succinic acid and tartaric acid levels.ConclusionsOur results indicate that both plant growth status and N input drive changes in the microbial community structure in the root zone of wheat. Plant growth stage demostrated a stronger influence on bacterial than on fungal community composition. A number of bacterial genera that have been described as plant growth-promoting rhizobacteria (PGPR) responded positively to N fertilization, and their abundance correlated significantly with the organic acid level, suggesting that the secretion of organic acids may be a strategy developed by plants to recruit beneficial microbes in the root zone to cope with high N input. These results provide novel insight into the associations among increased N input, altered carbon availability, and shifts in microbial communities in the plant roots and rhizosphere of intensive agricultural ecosystems.

Effectiveness of complex inoculation of spring wheat with N[2] -fi xing bacteria Azospirillum brasilense and mold-antagonist Chaetomium cochliodes

Aim. To study the specifi cities of complex inoculation of spring wheat roots with the bacteria of Azospirillum genus and Chaetomium cochliodes Palliser 3250, and the isolation of bacteria of Azospirillum genus, capable of fi xing atmospheric nitrogen, from the rhizospheric soil, washed-off roots and histoshere. Materials and meth- ods. The phenotypic features of the selected bacteria were identifi ed according to Bergi key. The molecular the polymerase chain reaction and genetic analysis was used for the identifi cation the bacteria. Results. It has been demonstrated that during the introduction into the root system of spring wheat the strain of A. brasilensе 102 actively colonizes rhizospheric soil, root surface and is capable of penetrating into the inner plant tissues. Conclusions. The soil ascomucete of C. cochliodes 3250 promotes better settling down of Azospirillum cells in spring wheat root zone, especially in plant histosphere which induces the increase in the content of chlorophyll a and b in the leaves and yield of the crop.

Chemical Forms and Phytoavailability of Copper in Soil as Affected by Crop Residues Incorporation

Preceding crops as a source of organic matter are important sources of micronutrient and can play an important role in the soil fertility and soil cycling of micronutrients. In addition to the role of the organic matter in increasing the concentration of micronutrients in soil solution, attention should be also paid to the role of the kind and the quantity of the root’s exudates released in response to the incorporation of different plant residues in the rhizosphere. Present research was conducted with the objective of studying the effect of the kind of preceding crops: Trifolium (Trifolium pretense L) and Sorghum (Sorghum bicolor L) on chemical forms of copper (Cu) in solid phases of a calcareous soil in a completely randomized block field experiment with split plot (3 m × 5 m) arrangement, consisting of 3 replications and 3 treatments. After incorporation of the residue, wheat (genotype back cross) was planted. After harvesting the wheat, soil samples were collected from root zone of wheat. Selected soil properties and chemical forms of Cu were determined in the solid phases of the soil samples. Incorporation of plant residues significantly increased the concentration of DTPA-extractable Cu, in the soil. The highest effect was obtained for Trifolium treatment. Incorporation of plant residues decreased the carbonate-bound Cu (Cu-Carb) fraction in the solid phase and increased oxide-bound Cu (Cu-Ox) as compared to the control (fallow treatment). Fraction of organic-bound Cu (Cu-Org) in the soil increased with incorporation of plant residues as compared with the fallow treatment. Trifolium was the most effective in increasing Cu-Org. Cu-Ox and Cu-Residual (Cu-Res) forms showed a significant negative correlation and Cu-Org showed a significant positive correlation with the concentration of DTPA-extractable Cu. Incorporation of Trifolium residues decreased the fraction (%) of Cu-Carb and Cu-Ox (less soluble forms) and consequently increased the fraction (%) of Cu-Org which in turn elevated the concentration of DTPA-extractable Cu. Trifolium was the most effective in increasing the phytoavailability of Cu in soil.

NITROGEN FIXING BACTERIA OF SPRING WHEAT ROOT ZONE

The paper presents the study of active nitrogen fixation bacteria of genera Azotobacter, Azospirillum, Bacillus, Flavobacterium, Enterobacter and Pseudomonas isolated from root zone of spring wheat plants. The ability of selected diazotrophs to form associative systems with spring wheat was investigated. The most significant increase of molecular nitrogen fixation activity in root zone of plants was observed under the Azospirillum species background.

Effects of salt accumulation on the leaf water potential and transpiration rate of pot-grown wheat with a controlled saline groundwater table

We investigated the effects of continuous salt accumulation in the root zone of wheat. Leaf water potential and transpiration rate were measured in plants grown in an experimental pot system. Plants were grown in a controlled-environment glasshouse in sand-filled pots, in which the groundwater table was maintained at a desired depth. Saline water was recharged from the base, providing constant water content in the unsaturated root zone. At 52 days after sowing, the water table was set at a depth of 47 cm from the soil surface. Groundwater and initial soil water were replaced with saline water with electrical conductivities (EC) of 0.2 dS m−1 (Control [C]), 8 dS m−1 (EC8) and 16 dS m−1 (EC16). The daily transpiration rate, leaf water potential (ψL ) and EC of the soil water at a depth of 45 cm were measured during the experiment. Stomatal behavior was expressed using the transpiration ratio E t /E tc (the ratio of the transpiration rates of EC8 and EC16 to that in the control). Our results indicated that ψL and E t /E tc gradually decreased with increasing soil water EC as a result of transpiration and water uptake by the roots. The final ψL values of EC8 and EC16 were –4.0 MPa and –7.0 MPa, respectively, and the final E t /E tc values of EC8 and EC16 were 0.3 and 0.2, respectively. E t /E tc decreased with decreasing ψL , and induced stomatal closure. The E t /E tc of the wheat was 0.5 when ψL was –2.5 MPa and 0.3 when ψL was –3.0 MPa. Although a reduction in ψL from –1.5 MPa to –3.0 MPa caused a steep decline in E t /E tc , a subsequent reduction of ψL to –7.0 MPa caused only a minor reduction. This relationship between stomatal behavior and salt accumulation is similar to that reported in previous studies for plants grown in drought conditions. Changes in the transpiration ratio were expressed in an empirical equation as a function of ψL .

Nitrogen Fertilization on Uptake of Soil Inorganic Phosphorus Fractions in the Wheat Root Zone

Plant nutrient uptake from the soil is dependent on fertilizers applied, soil chemicals, and other factors. The purpose of this study is to quantify the effects of applied N fertilizers on P uptake by winter wheat (Triticum aestivum L.) and on the change of soil pH in the root zone related to reductions of inorganic P fractions in the rhizosphere soil. An experiment was conducted using different forms of N fertilizers (i.e., NH4+–N and NO3−–N) with three N concentrations (0, 100, and 300 mg kg−1) applied in a calcareous soil. Biomass and total N uptake of the plant increased with the N concentrations and NH4+–N fertilizer resulted in a greater biomass than NO3−–N nutrition. Total P uptake in the plant was also higher with NH4+–N fertilizer than with the NO3−–N nutrition. Compared with the zero N treatment, the soil pH around the roots decreased by 0.30 and 0.65 units, respectively, with N treatments of 100 and 300 mg kg−1 of NH4+–N fertilizer. The amount of soil inorganic P fractions in the root zone decreased with increasing NH4+–N applied. The NO3−–N treatments reduced rhizosphere acidification and had a less impact on the soil inorganic P fractions. The results suggest that enhancing rhizosphere acidification attributable to applications of NH4+–N fertilizer can increase P availability in calcareous soils for plant uptake.

Fate of 15N-urea applied to wheat-soybean succession crop

The wheat crop in São Paulo State, Brazil, is fertilized with N, P and K. The rate of applied N (0 to 120 kg.ha-1) depends on the previous grown crop and the irrigation possibility. The response of wheat to rates and time of N application and the fate of N applied to irrigated wheat were studied during two years. Residual N recovery by soybean grown after the wheat was also studied. The maximum grain productivity was obtained with 92 kg.ha-1 of N. The efficiency of 15N-urea utilization ranged from 52% to 85%. The main loss of applied 15N, 5% to 12% occurred as ammonia volatilized from urea applied on soil surface. The N loss by leaching even at the N rate of 135 kg.ha-1, was less than 1% of applied 15N, due to the low amount of rainfall during the wheat grown season and a controlled amount of irrigated water, that were sufficient to moisten only the wheat root zone. The residual 15N after wheat harvest represents around 40% of N applied as urea: 20% in soil, 3% in wheat root system and 16% in the wheat straw. Soybean recovered less than 2% of the 15N applied to wheat at sowing or at tillering stage.

Interactions of traffic and tillage applied to cotton on N movement below the root zone of a subsequent wheat crop

Although research has demonstrated the negative impact of traffic-induced soil compaction on crop productivity, knowledge is lacking regarding the interactive effects of equipment traffic and tillage systems, especially in regards to N management in conservationtilled multiple-cropping systems. The objective of this study was to examine the interaction of traffic and tillage systems applied to cotton ( Gossypium hirsutum L.) on N utilization and movement below the root zone of subsequent double-cropped wheat ( Triticum aestivum L.). A field study was initiated in 1987 on a thermic Typic Hapludult soil complex, utilizing a wide-frame tractive vehicle (WFTV) that allows for 6.1-m wide, untrafficked research plots to double-crop cotton with wheat. The experimental design was a split-plot with three replications. Main plots were: (1) conventional traffic (simulated with tractor); (2) no traffic (WFTV only). Subplots were tillage systems for cotton: (1) complete surface tillage without subsoiling (surface); (2) complete surface tillage and annual in-row subsoiling to 40-cm depth (subsoil); (3) complete surface tillage with once-only complete disruption of the tillage pan by subsoiling to a 50-cm depth on 25-cm centers in 1987 (complete); (4) strip-till where cotton was planted with in-row subsoiling into wheat residue. All tillage treatments were applied to the cotton and residual effects were observed in the wheat. In 1990–1991, fertilizer applications were made to wheat as 15N-labeled NH 4NO 3, and soil solution samples were collected (90-cm depth). While previous cotton tillage had no significant effect on wheat yields, traffic reduced wheat yields from 3427 to 2981 kg ha −1 in 1990. With no traffic, total fertilizer N recovery in the plant-soil system was increased by 20 and 10% in 1990 and 1991, respectively. The strip-till tillage treatment increased total fertilizer N recovery in wheat by 20% compared with other tillage systems in 1990. Surface tillage without subsoiling for cotton increased NO 3-N concentration below the root zone of wheat (90-cm depth) in both years. The data indicate that the tillage/traffic management system used for production of one crop in a doublecropping system was a major factor in reducing N losses and increasing fertilizer N recovery in the plant-soil system of the succeeding crop.

MICROORGANISMS IN THE ROOT ZONE IN RELATION TO TEMPERATURE

A study was made of the numbers and kinds of microorganisms in the root zone of wheat and soybeans grown in the greenhouse at three ranges of temperature: 55–60°, 70–75°, and 85–90 °F. Numbers of bacteria in the rhizosphere and rhizoplane of wheat decreased as the temperature increased, whereas numbers in the root-free soil and on soybean roots increased with increased temperature. The same relationships held for methylene blue reducing, glucose-fermenting, and ammonifying bacteria and those requiring amino acids for optimal growth. Generic analysis of bacterial isolates showed that the percentage of Gram-negative rod-forming organisms increased with a rise in temperature in root-free soil and on soybean roots but decreased on wheat roots. On the other hand, the incidence of Gram-positive and pleomorphic organisms decreased in soil and on soybean roots with higher temperature and increased on wheat roots.Fungal isolations from washed root segments showed a greater incidence of Mucor, Rhizopus, Rhizoctonia, and Gliocladium on soybeans at the high temperature whereas species of Fusarium and Cylindrocarpon were more prevalent at the low temperature. With wheat the most striking feature was the predominance of non-sporing dark species at the high temperature and of non-sporing hyaline types at the low temperature.The number of soil nematodes decreased in the rhizospheres of both plants with increased temperature, the effect being more pronounced with soybeans. With this crop all except one of the genera recognized decreased in number, whereas with wheat, the incidence of certain nematodes such as species of Helicotylenchus, Boleodorus, and Aphelenchoides did not change with temperature.The results suggest that temperature exerts a direct effect on the microorganisms in the root zone and an indirect effect by influencing plant growth.