Efecto de la resistencia mecánica del suelo sobre la densidad de raíces finas de Eucalyptus globulus

In two sites in Balcarce and Necochea counties, on eight trees, mechanical impedance of soil (RM) and fine roots (under 2 mm in diameter) density were determined in the first 50 cm under the soil surface and near the stem. A very high density of fine roots was found at 20 cm depth in both sites, while a low density of f ine roots was observed in soil zones with high RM values. These high values corresponded with a clayey horizon in Balcarce site and a till induced pan in Necochea site. Over 0.5 Mpa RM values the fine roots diminished. RM should be considered as an important tool to detect radical growth limitant conditions and the basis to select tillage during pre-plantation labors.El efecto de la resistencia mecánica del suelo sobre la densidad de raíces finas de Eucalyptus globulus fue determinado en el sudeste de la provincia de Buenos Aires, Argentina. Sobre cuatro árboles en cada una de dos parcelas de ensayo ubicadas en Balcarce y San Francisco, se determinó la resistencia mecánica del suelo (RM) y la densidad de raíces finas (DRF) menores a 2 mm, en los primeros 50 cm del perfil en la zona cercana al tronco. En ambos sitios se observó una alta concentración de raíces finas en los primeros 20 cm del suelo y una baja DRF en las zonas con mayor RM, que correspondieron a un horizonte arcilloso en Balcarce y en un piso de arado en San Francisco. Por encima de 0,5 Mpa se observó una importante disminución en la densidad de raíces finas. La medición de RM puede constituir una herramienta valiosa para identif icar impedimentos al desarrollo radical y sobre la base de ello, decidir la preparación del suelo previo a la plantación

Short communication. Biological fixation of nitrogen and N balance in soybean crops in the pampas region

Biological nitrogen fixation (BNF) is of key importance in the N balance of soybean (Glycine max) crops. A number of authors have suggested that a negative balance may occur under high yield conditions. Few studies have measured the contribution of BNF to soil N in the pampas region. The aims of the present study were to compare three BNF determination methods – two isotopic methods using sorghum or a non-nodulating soybean isoline as a reference crop, and one involving the calculation of the difference in N content between the nodulating and non-nodulating soybean isolines – and to estimate the N balance in soybean crops raised under conventional tillage and no tillage practices. The study was performed in 2004-2005; a complete randomised block design was used with three replicates (plot dimensions 3 × 7 m). The different methodologies estimated BNF to account for 45-58% of total plant N, equivalent to 94 to 123 kg N ha-1. Depending on the methodology for estimating the BNF the soil N balance varied between –7 and 22 kg N ha-1. With an average grain yield of 1,618 kg ha-1 and a BNF accounting for approximately 50% of total plant N (i.e.,115 kg N ha-1), the soil N balance was slightly positive (14 kg ha-1) and independent of the tillage practice. The tillage systems had no effect (P < 0.05) on the mass or number of nodules, shoot biomass production at the R1 or R6 growth stages, the N content, BNF, or grain yield. Since the present results were obtained using non-commercial soybean isolines, further research is required to determine the soil N balance when high yielding soybean crops are raised