Variation in the rate of N mineralization in a Yolo soil profile was studied using in vitro incubation methods. Negligible amounts of NH4‐N were recovered from leachates, indicating that the rate of N mineralization was the primary factor controlling N availability. During a 13‐week incubation at 25°C, 42% of the total estimated N mineralized was derived from the surface soil (0 – 18 cm), whereas 58% was contributed from the 18‐ to 108‐cm depths. Cumulative N mineralization in the 0‐ to 18‐cm and 18‐ to 36‐cm depth samples followed first‐order kinetics and was linear with respect to the square root of the incubation time. The mineralization rate constant differed more than twofold between the 0‐ to 18‐ and the 18‐ to 36‐cm depth samples. At lower depths (36 – 72 and 72 – 108 cm) the relationship between cumulative N mineralization and the square root of time was curvilinear.Interactive effects of soil temperature and moisture were also examined in an experiment with four incubation temperatures (15, 20, 25, and 30°C) in factorial combination with six soil moisture levels (0.1, 0.3, 0.7, 2, 4, and 10 bars). There was a significant moisture × temperature interaction; N mineralization increased above that expected from additive effects at 30°. Multiple regression was used to generate an equation that predicted net N mineralization as a function of soil temperature and moisture.The apparent effect of soil water content on N mineralization depended on experimental procdure. When soil water content was varied by adding water to air‐dry soil without complete equilibration, N mineralization declined linearly with water content. In contrast, there was a sharp decline between 0.3‐ and 2‐bar treatments and a more gradual decline at higher matric suctions when soil was equilibrated by pressure‐membrane desorption before incubation. These results indicate that both quantity and distribution of soil water affect in vitro estimates of N mineralization.

1991. A method for measuring hyphal nutrient and water uptake in mycorrhizal plants. Can. J.Bot. 69: 87-94.A new system was designed that permits examination of long distance transport of water and nutrients through mycorrhizal hyphae without the architectural, nutritional, and physiological differences associated with comparing mycorrhizal and nonmycorrhizal plants. The "rhizobox" system consists of a rectangular box with a chamber where mycorrhizal plants are grown and an outer chamber where hyphae proliferate. The two chambers are separated by root-excluding screens and an air gap. Two slightly different experiments examined hyphal transport. The first experiment demonstrated the difficulties of comparing water use by mycorrhizal and nonmycorrhizal plants because of dissimilarities in plant architecture. The second experiment avoided the problem by comparing intact mycorrhizal plants with plants where hyphae passing to the outer chamber were severed. In the outer chamber, a 5 rnM solution of RbCl was injected. Intact mycorrhizal plants transpired 35% more water than plants with severed hyphae in 16 h. The source of transpired water was the outer chamber, as suggested by lower soil moisture in the outer chamber and a higher Rb content in intact plants. This demonstrates an active role in water and nutrient transport by mycorrhizal hyphae, since plants were of a similar nature except for hyphal access to the outer chamber. ., et SHACKEL, K. 1991. A method for measuring hyphal nutrient and water uptake in mycorrhizal plants. Can. J. Bot. 69 : 87-94. Un nouveau systkme a Ct C dCveloppC afin d'examiner le transport sur de longues distances de l'eau et des nutriments dans les hyphes mycorhiziens, en excluant les diffkrences architecturales, nutritionnelles et physiologiques inCvitables lorsqu'on compare des plantes mycorhiziennes et non mycorhiziennes.Le ccrhizobox* est constituC d'une boite rectangulaire comportant une chambre oh se dCveloppent les plantes mycorhiziennes, adjacente a une autre chambre ou se dCveloppent les hyphes.Les deux chambres sont stparCes par des tamis qui retiennent les racines et entre lesquels on trouve un espace d'air. Le transport dans les hyphes a Ct C CtudiC dans deux expkriences 1Cgbrement diffkrentes. La premikre dCmontre les difficultks qui existent comparer l'utilisation de l'eau par des plantes mycorhizCes ou non mycorhiztes, a cause des diffkrences dans l'architecture des plantes. La deuxikme expirience contourne le problbme en comparant des plantes mycorhiziennes intactes avec des plantes dont les hyphes passant a la chambre extCrieure ont Ct C coupks. On injecte une solution de RbCl 5 mM dans la chambre extCrieure. Les plantes dont les hyphes restent intacts montrent, sur une pCriode de 16 h, une transpiration 35% suptrieure a celles dont les hyphes ont Ct C coupCs. La source d'eau transpirke se trouve dans la chambre exterieure, tel que suggtrC par une humidit6 plus basse dans la chambre externe et un contenu en Rb plus ClevC dans les plantes intactes. Ceci dCmontre un r6le ac...

Low levels of phosphorus and high levels of aluminum are important soil acidity factors for the growth of higher plants; however, very little is known about their effects on the soil rhizobia. The present study was conducted to determine the relative effects of acidity, P, and Al on rhizobia. Tolerance of low pH (4.5), low P (5-10 µM), and high AI (50 µM) was assessed for 10 strains of cowpea rhizobia by detailed growth studies in defined liquid media. Tolerances to these factors were determined for 65 strains of cowpea rhizobia and Rhizobium japonicum by a rapid method based on attainment of turbidity from a small inoculum. Strains varied in response. Low P (as compared with 1,000µM) limited total attainable population density to 5 X 10 7 cells/ml, and slowed the growth of some strains. Acidity generally increased lag time or slowed growth of most strains, and stopped growth of about 50% of them. Tolerance of acidity did not necessarily entail tolerance of Al. Aluminum (50 µM) increased the lag time or slowed growth of almost all strains tolerant of low pH. It virtually stopped growth of 40% of the strains. With our system the rhizobia had to make 1,000-fold growth in the stress media before they could significantly raise pH and precipitate Al. A valid rapid screening can be based on ability to attain visible turbidity in culture under acid or Al stress, so long as initial density is small («10 5 cells/ml). The cowpea rhizobia tended to have more tolerance to Al than R. japonicum and overall Al was a more severe stress than low pH or low P.

Dissolved phosphate was mixed with topsoil samples, and the decline in solution phosphate concentration (P) was followed for 200–300 days by periodically shaking and extracting subsamples with 1 or 10 mM CaCl2 (1:10).During the first 20–40 days, (P) declined faster in soil suspensions that were being shaken than it did in undisturbed soil at 0.1 bar moisture. After 40 days of reaction, shaking time had little effect.The slow fixation had first‐order kinetics with respect to (P). The relative rate was faster in an Andept than in three Oxisols. It was unaffected by lime, though lime increased the strength of adsorption.Equilibrium was achieved at 50 days in an Andept and 100–200 days in three Oxisols. At equilibrium, the amount of adsorbed phosphate remaining labile was estimated from values of (P), using 6‐day adsorption isotherms. Labile phosphate so estimated amounted to 30–50% of the added phosphate, implying that the residual value of phosphate added to these soils should be substantial and permanent except for removal by crops and erosion.Desorption isotherms diverged from adsorption isotherms less markedly with increasing time after phosphate addition, as if the slow reaction caused much of the apparent hysteresis.

Production of chickpea (Cicer arietinum L.) in semiarid and coastal areas may be limited by the salt sensitivity of the chickpea symbiosis. Accordingly, this study was done to analyze effects of salt on the symbiosis by comparing the NaCl tolerance of chickpea‐specific strains of Rhizobium with tolerance of the chickpea plant grown either with combined N or dependent on symbiotic N2‐fixation. Effects of supplemental N on the salt response of symbiotic chickpea were also tested. In greenhouse sand cultures, growth of chickpea was depressed by NaCl at only 20 mM concentration unless mineral N was provided. With no added NaCl 22 strains of rhizobia were all effective, producing plant yields comparable with NH4NO3 control treatments; but with NaCl added at 75 mM only 1 strain did significantly better than controls with no N or inoculum. Poor symbiotic performance was not due to salt limitation of growth of rhizobia. Rhizobial growth rates determined by viable counts in yeast mannitol medium were unaffected by NaCl at 120 mM and only moderately depressed by 250 mM. Effects of supplemental N were studied in a solution culture experiment with four levels of NaCl (0, 15, 23, and 31 mM) combined factorially with four different N treatments: NH4NO3 absent, present continuously, present only during the first 31 days, or present only after onset of vigorous fixation at 31 days. The growth of both N‐fertilized and symbiotic plants was inhibited by salt. Ammonium nitrate moderated salt stress most strongly during the later period of growth (after 31 days). Growth inhibition of N‐fertilized plants was associated with excess Cl accumulation in shoots, which occurred regardless of N treatment. Additional effects could account for the greater inhibition in symbiotic plants. Salt delayed nodulation, and N‐treated plants showed greater retention of Na in roots and lower concentrations of Na in shoots than completely symbiotic plants The data indicate a clear need for greater salt tolerance in chickpea. And since the nature of the salt response changed markedly with N‐source, selection of cultivars and testing of management procedures should be done with both N‐fertilized and symbiotic plants.