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prerequisite for the calculation of the proportion of a legume's nitrogen derived from the atmosphere (%Ndfa) using the N natural abundance technique (NA) is the determination of the B value (δN value for a legume when completely dependent on N fixation for growth). With this objective, Kabuli-type chickpea seeds were inoculated with Mesorhizobium ciceri, strain ISC-6, and an indigenous strain mixture (ISM) from the Guadalquivir Valley (southern Spain); and mayor-equina-type faba bean seeds were inoculated with Rhizobium leguminosarum strain ISL-19. Both legumes were grown in perlite with an N-free nutrient solution and in a growth chamber; uninoculated plants were also grown under the same conditions. Samples of the inoculated plants were collected at full flowering and maturity, analyzing total N and N content of the nodules, root, shoot and seeds. The seeds sown and uninoculated plants at wilting were also analyzed. The uninoculated chickpea and faba bean plants showed strong N discrimination in the remobilization of reserve proteins from the cotyledons to the shoot. Only 52% of the N from the chickpea seed and 46% from the faba bean seed were translocated to the shoot. The B value for both inoculated species (−1.44‰ in chickpea and −1.33‰ in faba bean) demonstrated the existence of a significant isotopic discrimination of N in the fixation of atmospheric N. The B value did not show significant differences between full flowering and seed maturity for either species, which allowed field sampling to be performed for the calculation of %Ndfa between both growth stages. Each species presented different N discrimination for the different parts analyzed. In chickpea, strong discrimination was observed between the nodules (+3.11‰) and the rest of the plant (−1.67‰); this discrimination was less pronounced in faba bean and always with negative values between the underground parts (nodules and root, with a mean value of −0.75‰) and the shoot (−1.71‰). The B value obtained was −1.80‰ in chickpea and −1.71‰ in faba bean. However, given the importance of the amount of seed N in chickpea and faba bean, seed N contribution should be considered in the calculation of the B value using a mass balance model which considers 50% of this amount. In this case, the adjusted B values would increase to −2.05‰ and −1.89‰ in chickpea and faba bean, respectively. [ABSTRACT FROM AUTHOR]

Little research has examined the influence of tillage system on root growth in wheat grown on rainfed Vertisols. A 3-year field study (2003, 2004 and 2005) was carried out on a typical Vertisol (southern Spain), to determine the effects of tillage system on root growth in spring wheat ( Triticum aestivum L) grown in continuous rotation with faba bean ( Vicia faba L), within the framework of the long-term “Malagón” experiment started in 1986. Tillage treatments were no-tillage (NT) and conventional tillage (CT), and the experiment was designed as a randomized complete block with three replications. The following parameters were measured: above-ground biomass, grain yield, root length density (RLD), root biomass (RB) and root N content. In the topmost 10 cm of soil, higher values were found under CT than under NT for RLD in the rainiest year (20.2 km m−3 vs. 9.6 km m−3 respectively) and for RB (512 kg ha−1 vs. 261 kg ha−1 respectively) in all study years. In deeper layers, no difference was recorded between the two tillage systems. Greater wheat root development in the upper soil layer under CT may reflect the greater soil penetration resistance found in the topmost 10 cm under NT. Root separation using a sieve with a 0.5 mm mesh screen led to a marked underestimation of RLD and RB, with values up to three times higher when using a 0.2 mm mesh screen. Mean wheat root N content in the topmost 30 cm of soil accounted for over 80% of total root N content. The highest grain yield was observed under NT, since this system provided greater water storage in the soil profile in the mostly dry study years. [ABSTRACT FROM AUTHOR]