Tolerancia de plantines de distintos genotipos de lechuga (Lactuca sativa L.) al riego con agua de alto contenido en sales

Horticultural production is carried out under irrigation in the area of Santa Rosa, La Pampa province, where available water sources are highly concentrated in salts. The aim of this study was to evaluate the effect of irrigation with high salt content water on growth, superoxide dismutase (SOD) activity and malondialdehyde concentration, in seedlings of 11 lettuce genotypes. The experiment was performed in a greenhouse, where the seedlings were irrigated for 54 days with two different kinds of water, one from rain as a control and another one of high salinity. Irrigation with high salinity water caused a decrease of 11, 12 and 87% in dry weight yield for Bermella, Grand Rapid and Boltar genotypes, respectively, and a vigor reduction of 2% for Ice 15975 and 3% for Grand Rapid. A the same time, irrigation treatment with high salinity resulted in plant malondialdehyde concentration  increases of 56, 98 and 138% for Bermella, Ice 15975 and Grand Rapid, respectively. In the case of the last two genotypes, their response was associated with a SOD activity decrease. The rest of the genotypes did not show changes in malondialdehyde concentration. These results evidenced a differential response among the assayed lettuce varieties, and that malondialdehyde concentration can be used to select genotypes tolerant to irrigation with water of high salt content at seedling stage. DOI: http://dx.doi.org/10.19137/semiarida.2017(02).5966La producción hortícola en la zona de Santa Rosa, La Pampa, se realiza bajo riego, el agua disponible para tal fin tiene un alto contenido de sales. El objetivo de este trabajo es evaluar la respuesta del crecimiento, la actividad de la SOD y la concentración de malondialdehido, al riego con agua salina en 11 genotipos de lechuga. Se trabajó en invernáculo, y los plantines durante 54 días fueron regados con dos calidades de agua, de lluvia y con alta salinidad. Los genotipos Grand Rapid, Bermella y Boltar mostraron una reducción del peso seco del 12, 11 y 87% respectivamente y en Grand Rapid e Ice 15975 la reducción del vigor fue del 3 y 2% respectivamente. En Ice 15975, Gran Rapid y Bermella hubo un incremento de la concentración de malondialdehido del 98, 138 y 56% respectivamente ante el riego con agua salada. En Ice 15975 y Grand Rapid esta respuesta se asocia a una disminución de la actividad de la SOD. El resto de los genotipos no expresó cambios en la concentración de malondialdehido. Estos resultados evidencian una respuesta diferencial de los genotipos en estudio, y la concentración de malondialdehido puede usarse para seleccionar genotipos de lechuga tolerantes al riego con agua con alto contenido de sales al estado de plantines. DOI: http://dx.doi.org/10.19137/semiarida.2017(02).596

Efecto de sustancias orgánicas solubles del suelo sobre la absorción de hierro en plántulas de girasol Effect of soluble soil organic substances on iron absorption by sunflower seedlings

La materia orgánica del suelo es uno de los recursos naturales que afecta la calidad del suelo. El uso y manejo del suelo pueden hacer disminuir su contenido afectando de este modo sus propiedades fisico-químicas y el crecimiento de las plantas. El objetivo de este trabajo fue evaluar la importancia del carbono orgánico soluble (COS) y de ácidos fúlvicos (AF), ambos afectados por el laboreo, en la absorción de hierro por plantas de girasol. El COS estuvo ausente en el suelo con agricultura y el AF disminuyó en un 57%. Plántulas de girasol crecidas en solución nutritiva aumentaron su contenido de hierro en hojas 18,1 y 6,7 veces con el agregado de 30 mg L-¹ y 40 mg L-¹ de COS y AF, respectivamente. Mayores concentraciones de COS y AF en la solución nutritiva mejoraron la absorción de hierro y evitaron su precipitación en las raíces.Soil organic matter (SOM) is one of the most important natural resources that affect soil quality. Soil uses and management may decrease SOM content affecting physical and chemical soil properties and consequently plant growth. The aim of this study was to evaluate the effect of soluble organic carbon (SOC) and fulvic acids (FA), both affected by tillage, on iron uptake by sunflower seedlings. Soluble organic carbon disappeared in the soil under cultivation and FA was reduced 57%. Sunflower seedlings growing in hydroponic solution increased iron absorption and translocation. Thus, iron content in leaves increased 18.1 and 6.7 times with the addition of 30 mg L-¹ and 40 mg L-¹ of SOC and FA, respectively. Increasing SOC and FA concentration in nutrient solution improved iron uptake and also prevented iron precipitation in roots

Particulate organic matter, carbohydrate, humic acid contents in soil macro- and microaggregates as affected by cultivation

Cultivation is known to strongly affect not only soil structure but also organic substances responsible for aggregation. This research was conducted to study the effect of cultivation on the distribution of soil macro- and microaggregates as related to changes in soil organic matter content in a Typic Haplustoll, located in central Cordoba, the principal peanut (Arachis hypogaea L.) growing area of Argentina. Samples were taken from A or Ap horizons at (1) an undisturbed and (2) a cultivated site for determination of the aggregate fractions by the wet sieving method. The mineral associated organic carbon (MOC) and particulate organic carbon (POC), in soils and their aggregates, were separated by dispersion and sieving procedures. Carbohydrate content in soil and in aggregate fractions was determined by dilute acid (CHda) and hot water (CHhw) extraction, whereas humic (HA) and fulvic acid (FA) were extracted by the NaOH method. The results indicated that the total macroaggregates (> 250 μm) content was 1.7 times lower in the cultivated than in the undisturbed soil. The large macroaggregates (2800-2000 μm) were the most affected, decreasing a 92% due to cultivation compared with undisturbed. In contrast, the microaggregate (250-53 μm) content was twice that high in the cultivated than in the undisturbed soil. The concentrations of OC, POC, CHda, CHhw were all reduced substantially by cultivation, with the microaggregates showing an almost complete loss of its POC content. The destruction of these transient organic cementing agents was assumed to have contributed to the collapse of the macroaggregates. This has resulted in exposure of POC, making it more available to rapid oxidation and microbial attack. There were indications suggesting POC and CHhw contents to be valuable as indicators of soil structure degradation due to exhaustive cultivation practices. Although it is well know that humic substances are chemically and structurally much more stable than nonhumic substances but our results showed a surprising decrease humic substances under continuous cultivation.Fil: Bongiovanni, Marcos D.. Universidad Nacional de Río Cuarto; ArgentinaFil: Lobartini, Juan Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; Argentin

Effect of soluble soil organic subtances on iron absorption by sunflower seedlings

La materia orgánica del suelo es uno de los recursos naturales que afecta la calidad del suelo. El uso y manejo del suelopueden hacer disminuir su contenido afectando de este modo sus propiedades fisico-químicas y el crecimiento de lasplantas. El objetivo de este trabajo fue evaluar la importancia del carbono orgánico soluble (COS) y de ácidos fúlvicos(AF), ambos afectados por el laboreo, en la absorción de hierro por plantas de girasol. El COS estuvo ausente en el suelocon agricultura y el AF disminuyó en un 57%. Plántulas de girasol crecidas en solución nutritiva aumentaron su contenidode hierro en hojas 18,1 y 6,7 veces con el agregado de 30 mg L-1 y 40 mg L-1 de COS y AF, respectivamente. Mayores concentraciones de COS y AF en la solución nutritiva mejoraron la absorción de hierro y evitaron su precipitación enlas raícesSoil organic matter (SOM) is one of the most important natural resources that affect soil quality. Soil uses andmanagement may decrease SOM content affecting physical and chemical soil properties and consequently plant growth.The aim of this study was to evaluate the effect of soluble organic carbon (SOC) and fulvic acids (FA), both affectedby tillage, on iron uptake by sunflower seedlings. Soluble organic carbon disappeared in the soil under cultivation andFA was reduced 57%. Sunflower seedlings growing in hydroponic solution increased iron absorption and translocation.Thus, iron content in leaves increased 18.1 and 6.7 times with the addition of 30 mg L-1 and 40 mg L-1 of SOC and FA,respectively. Increasing SOC and FA concentration in nutrient solution improved iron uptake and also prevented ironprecipitation in roots.Fil: Bongiovanni, Marcos Darío. Universidad Nacional de Río Cuarto; ArgentinaFil: Lobartini, Juan Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; Argentin

Boron nutrition, intracellular transport, and knife-cut disease in sunflower

The present study was conducted to investigate in sunflower the processes of boron (B) uptake, intracellular compartmentation, and xylem translocation in response to B supply, ranging from deficiency to incipient toxicity, and to short-term changes in B supply. The experiments were conducted with two sunflower genotypes, selected on the basis of their susceptibility to knife-cut disease. It appears that the roots of the susceptible genotype of sunflower were more sensitive to low B contents in the solution media than the shoots. The decrease in root dry weight in high-B treatments could also indicate it was more sensitive to B toxicity. Though root dry weight decreased, the shoot/root dry-weight ratio was smaller in the resistant genotype, suggesting that this genotype would have a substantially larger root volume, capable of supporting the B demand of its shoots. The B contents in the water-insoluble residue (WIR) of roots were similar for all genotypes and treatments. In contrast, the B concentration in WIR of leaves reached values near saturation only when B started to accumulate in the cell sap (CS) of roots to the level as detected in CS of leaves. The critical values of B concentrations in shoot tissues would then be established after the B requirement for cell walls was satisfied and a proper metabolic B content in CS of roots was reached. Uptake efficiency (UE) values less than 1.00, detected as a result of treatments with high concentrations of B in the nutrient solution, suggested the presence of an exclusion mechanism that restricted B accumulation. The high UE value obtained with low-B treatments indicated that mechanisms other than mass flow had played a role in providing the acquired B.Fil: Fuertes, M. E.. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Orioli, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiarida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiarida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; Argentin

Plant Uptake of Iron Chelated by Humic Acids of Different Molecular Weights

Mobilization of iron (Fe) chelated by humic acids (HA) of low (HA10,000) and high molecular weight (HA100,000) fractions and its uptake by plants were investigated in growth experiments with sunflower seedlings. The iron chelates (labeled with 59Fe) contained in dialysis bags (mw. cutoff=3500) were placed in minus iron Hoagland solutions as the Fe source and at the same time fulvic acid (FA), EDTA, and low and high molecular weight HA fractions were added in the solutions as mobilizators. Characterization of FA, HA10,000, and HA100,000 were performed by infrared spectroscopy and chemical analysis, e.g., total acidity, COOH, and phenolic‐OH content. Roots and leaves were harvested, dried, and ground for Fe activity determination. Iron contents and pH in the nutrient solutions were measured before and after treatments. The supply of Fe to the plants was apparently sufficient, because no Fe deficiency has been detected in the test plants but during the whole absorption period, the pH of the nutrient solution was about 4.5. The Fe contents in leaves indicated that part of the Fe was rapidly transported from roots to leaves. Judging from the Fe contents in leaves, it was assumed that the small size HA10,000 and EDTA were the most efficient in affecting transport of Fe from root to leaf tissue. FA, HA10,000, and especially HA100,000 were unable to penetrate the dialysis bags and, hence, were effective in Fe mobilization only after the Fe, dissociated from the Fe‐HA chelate, has passed the dialysis membrane into the nutrient solutions. In contrast, the small size EDTA was expected to have penetrated the dialysis bags, permitting mobilization of chelated Fe by ligand exchange inside the bags, and transporting the Fe to the roots. The results suggested that the humic substances used in this study were able to form with the Fe3+ ion complexes that maintained the iron available to the sunflower plants. In the chemical form of Fe.L, where L was FA o HA, the iron within the bags or in solution or in the roots free space, was available for exchange reactions with the natural sunflower plant chelators for its transport to the leaves.Fil: Bocanegra, M. P.. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Orioli, Gustavo Adolfo. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentin

Producción de cebolla determinada por la densidad de plantas, nivel de nitrógeno y pérdida de área foliar

Onion (Allium cepa) is an important horticulture crop because of its value as a food with a long shelf life being a relatively non-perishable product. It is very helpful to understand the growth response of the seeded onion crop to conduct appropriate field practices in attaining the highest or optimum yields. A three year field experiment was conducted using a variety of onion Valcatorce INTA, in a randomized block design with five replicates. Treatments were two plant densities and three rates of N application. The bulb growth followed a classical sigmoid curve. During the rapid growing period, the crop had the greatest leaf area (LA) with at least six leaves per plant. Increasing plant density increased yield in kg/ha, but decreased bulb size. Defoliating 40 to 60% of the LA had a significant impact on bulb production only at early growth stages. Late in the growing period, the remaining LA was apparently large enough for producing sufficient amounts of metabolites to feed new leaves, increasing their photosynthesis efficiency for the benefit of bulb production.La cebolla es un importante cultivo debido a su valor como alimento con una vida relativamente larga siendo así un producto no perecedero. Es muy útil entender las respuestas del crecimiento del cultivo de cebolla para realizar las prácticas a campo que conduzcan a un mayor y óptimo crecimiento. Se realizó un experimento a campo usando una variedad Valcatorce en un diseño de bloques al azar con cinco replicas a dos densidades de siembra y tres niveles de fertilización nitrogenada. El crecimiento del bulbo siguió una clásica curva sigmoidea. Durante el rápido período de crecimiento, el cultivo tuvo la mayor área foliar (AF) con al menos seis hojas por planta. Con el aumento de la densidad de plantas aumentó la producción en kg/ha pero disminuyó el tamaño del bulbo. Defoliaciones del 40 al 60% del AF tuvieron un fuerte impacto en la producción de bulbos sólo en los primeros estadios de crecimiento. En los últimos estadios, el AF remanente fue aparentemente lo suficientemente grande para producir metabolitos que alimentaron nuevas hojas, aumentando la eficiencia de fotosíntesis para la producción de bulbos.Fil: Siliquini, O. A.. Universidad Nacional de la Pampa. Facultad de Agronomía; ArgentinaFil: Orioli, Gustavo Adolfo. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Determinación del intervalo hídrico óptimo en usoles de la región semiárida pampeana = Determination of least limiting water range in ustolls of the semarid pampas

En la región semiárida pampeana el intervalo hídrico óptimo (IHO) no ha sido utilizado para valorar cambios en la calidad del suelo relacionados al manejo. El objetivo del trabajo fue caracterizar y evaluar el IHO en un suelo bajo uso en vegetación natural con gramíneas perennes (VN) y bajo uso agrícola (A). El estudio se desarrolló en Paleustoles petrocálcicos de la planicie con tosca de la Región Semiárida Pampeana Central. En un suelo VN y otro bajo uso A, y en estrato superior, medio e inferior del perfil respectivamente, se determinó, arcilla, limo, porosidad total, contenido de carbono, nitrógeno y el IHO. El uso A condicionó los valores del IHO en los dos primeros estratos y presentó mayores diferencias entre profundidades con respecto al suelo en VN. Los valores promedios del IHO para el suelo VN fueron 0,14; 0,15 y 0,15 cm3 cm-3 y en el suelo A de 0,06, 0,01 y 0,18 cm3 cm-3 en estratos superior, medio e inferior respectivamente. En ambos usos, el límite húmedo del IHO fue el contenido hídrico en capacidad de campo. El límite seco en el suelo A lo representó la curva de resistencia a la penetración, en cambio en el suelo VN el límite seco correspondió a la curva de punto de marchitez. Se observó que el IHO correlacionó con carbono y nitrógeno en el estrato superior e inferior, mientras que en el estrato medio del perfil esta relación no fue importante. En general se puede inferir que el intervalo hídrico óptimo fue un indicador sensible que reflejó los cambios en el sistema poroso del suelo bajo el uso agrícola con respecto a un suelo de similar textura bajo vegetación natural.The Least Limiting Water Range (IHO) has not been used in the semiarid Pampas to describe the soil quality changes related to management. The objective of this work was to characterize and evaluate the IHO in soils under natural grassland vegetation (VN) and agriculture (A). The study was carried out on petrocalcic Paleustolls in the calcic plains of the central semiarid Pampas. Clay and silt, carbon and nitrogen contents, total porosity and IHO were determined on samples from upper, medium, and lower soil profile layers of a VN and an A soil. Agriculture affected IHO in the upper and medium layer, and showed more differences among layers than VN. Average IHO values for VN were 0.14, 0.15 y 0.15 cm3 cm-3, and 0.06, 0.01 y 0.18 cm3 cm-3 in the A soil for the upper, medium and lower layers, respectively. In both soils the IHO moist limit was determined by field capacity moisture contents, whereas the dry limit was defined by the penetration resistance curve in A and by the permanent wilting point moisture in VN. A positive relation between IHO and total soil carbon and nitrogen was observed for the upper and lower depth layers, while in the medium depth layer this relationship was not important. In general terms, it is concluded that the Least Limiting Water Range was a sensitive indicator which reflected the changes in the soil porous system under agricultural use compared to a soil with similar texture under natural vegetation.EEA AnguilFil: Fernandez, Romina. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Anguil; ArgentinaFil: Quiroga, Alberto Raul. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Anguil; Argentina. Universidad Nacional de La Pampa. Facultad de Agronomía; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Noellemeyer, Elke. Universidad Nacional de La Pampa. Facultad de Agronomía; Argentin

Valores umbrales de algunos indicadores de calidad de suelos en molisoles de la región semiárida pampeana = Threshold values of some soil quality indicators in the semiarid pampas

Los objetivos del trabajo fueron evaluar los efectos de variaciones en los contenidos de materia orgánica sobre propiedades edáficas e identificar indicadores simples y de fácil determinación que sirvan para caracterizar el efecto del uso en Molisoles de la Región Semiárida Pampeana. El estudio se desarrolló en Paleustoles petrocálcicos y Haplustoles énticos de la planicie con tosca de la Región Semiárida Pampeana Central. Se seleccionaron sitios en planteos agrícolas (A) y en situaciones de vegetación nativa de Prosopis caldenia (VN). Con el fin de caracterizar los indicadores de calidad de suelo se determinó: arcilla+limo (a+l), distribución de tamaño de agregados, inestabilidad estructural (IIE), peso volumétrico de los agregados (PVA), densidad aparente máxima (DAM), susceptibilidad a la compactación (SC), humedad crítica (HC), materia orgánica total (MO) y sus fracciones, pH, cationes intercambiables y capacidad de intercambio catiónico (CIC). Las relaciones entre MO y sus fracciones con las variables físicas fueron más estrechas en suelos A que en VN. Un umbral cercano a 30 g kg-1 de MO fue determinado, por encima del cual los suelos no serían susceptibles a compactarse. Los suelos A presentaron mayor proporción de macroagregados (> 8 mm), pero también valores más elevados de PVA que en suelos VN. Los indicadores DAM, SC tuvieron estrechas relaciones con MO en suelos A, y fueron indicadores significativos de calidad de uso de los suelos. El PVA que mostró estrecha relación con la MO y a+l se podría proponer como indicador de calidad física debido a su sensibilidad para discriminar suelos bajo distintas prácticas de uso. Es importante destacar que la determinación de este indicador es sencilla, rápida y económica.The objective of the present study was evaluate the effects of variations in the organic matter contents on soil properties and identify simple y easily determinable indicators that could be useful to monitor soil quality in the semiarid Pampa. The work was carried out in petrocalcic Paleustolls and entic Haplustolls of the Calcic Plain of the semiarid central Pampas. Sites under agricultural use (A) versus native «Caldenal» vegetation (NV) were chosen. In order to characterize soil quality indicators; soil texture (c+l), aggregate size distribution, structural instability (IIE), volumetric aggregate weight (VAW), maximum bulk density (MBD), compaction susceptibility (CS), critical moisture (CM), total organic matter (OM) and its fractions, pH, exchangeable cations, and cation exchange capacity (CEC) were determined. The relationships between OM and its fractions with the physical variables were more significant in soils under A than under NV. A threshold value of approximately 30 g kg-1 OM was determined above which soils were not susceptible to compaction. Agricultural soils had higher proportions of macroaggregates (> 8mm) and higher VAW than soils under NV. Although MBD, CS, and CM showed good relationships with OM in soils under A and were meaningful indicators of soil physical quality, their use is limited since the Proctor test is not a simple method. Instead, VAW could be proposed as a soil physical quality indicator since its determination is simple, economical, fast, sensitive, and its values were highly correlated with differences in OM.EEA AnguilFil: Fernandez, Romina. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Anguil; ArgentinaFil: Quiroga, Alberto Raul. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Anguil; Argentina. Universidad Nacional de La Pampa. Facultad de Agronomía; ArgentinaFil: Alvarez, Cristian. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Anguil; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Noellemeyer, Elke. Universidad Nacional de La Pampa. Facultad de Agronomía; Argentin

Allophane, aluminum, and organic matter accumulation across a bioclimatic sequence of volcanic ash soils of Argentina

An investigation was conducted to study Al fractions, organic matter accumulation, and their effects on physicochemical properties in soils derived from volcanic ash at the Andinopatagonian region in SW Neuquén, Argentina. Five sites were selected in a climatic biosequence, ranging from Nothofagus forest with a udic soil moisture regime at the footslope of the Andes mountain range to grass-shrub steppe with a xeric soil moisture regime in the hills and plains. The morphological characteristics showed young pedons, containing andic, mollic, and cambic horizons. A change from low bulk density from <0.82 Mg m-3 in Andisols under a udic regime to medium density values of ∼1.1 Mg m-3 in Xerands and Xerolls was observed. This was related to a lower rate of weathering probably coinciding with a lower content of allophane and higher clay crystallization and to a decrease in total soil organic carbon (Corg). The rate of soil weathering was estimated from acid oxalate extractable Al, which together with Corg significantly explained the bulk density variation (R 2=0.7091, p<0.00002). Soil acidity varied from moderately and strongly acidic in Andisols with a udic soil moisture regime to slightly acid and almost neutral in Xerands and Xerolls. The higher base concentration and low soil acidity in Xerolls were attributed to less leaching, due to lower mean annual precipitation. However, pHNaF and Corg were the variables which significantly explained the pHwater variation (R 2=0.7806, p<0.0000). This showed that soil acidity was closely related to Al activity and organic matter content, rather than to base content. The lower C/N ratios, in the surface horizons of the Xerands and Xerolls versus the Andisols with a udic soil moisture regime, suggested that the rate of mineralization was faster under warmer conditions with xeric moisture regime than with udic moisture regime. There was also a remarkable difference in the humic C/fulvic C ratio in A-horizons, which may reflect the input of vegetation residues indicating a sensible contribution of the cover of grass-shrub steppe to the formation of humic substances. The acid oxalate extractable Si and Al values reflected a low rate of weathering, which decreased when mean annual precipitation decreased and a xeric soil moisture developed being also higher under Nothofagus forest than under grass-shrub steppe, which reflects the possible effect of the mean annual precipitation gradient (2000-700 mm) and vegetation types on soil weathering. The differences in type and quantity of organic matter input, with a higher organic matter content and lower humic C/fulvic C ratio under Nothofagus, indicate a more reactive organic matter which may be somewhat responsible for the weathering degree. No significant relationship between acid extractable Al and Corg was found suggesting that organically bound Al is not dominant. The detection of higher allophane estimations in soils with the highest Corg levels, may indicate that the anti-allophanic hypothesis probably does not apply to these soils.Fil: Broquen, Patricia Cecilia. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; ArgentinaFil: Candan, Florencia. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; ArgentinaFil: Falbo, Gabriel. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; Argentin