tion rates and greater immobilization generated by surface residues (Rice and Smith, 1984).
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.Additional key words: 15 N, Glycine max, isotopic dilution, nodulation and non-nodulation isolines, tillage system, yield. ResumenComunicación corta. Fijación biológica y balance de nitrógeno de soja en la región pampeana La fijación biológica de nitrógeno (BNF) es de importancia central para el balance de N en cultivos de soja (Glycine max) y varios autores sugieren un balance negativo en condiciones de alto rendimiento. Pocos estudios han medido a la fecha la contribución de la BNF al N del suelo en la región pampeana. Nuestro objetivo fue el de cuantificar la BNF utilizando tres metodologías de medición diferentes (dos metodologías isotópicas usando como cultivo de referencia sorgo o una isolínea no nodulante de soja y el método de la diferencia en el contenido de N de la isolínea de soja no nodulante y la nodulante) y estimar el balance de N en el cultivo de soja bajo labranza convencional y siembra directa. El estudio fue realizado durante 2004-2005 y los tratamientos se realizaron utilizando un diseño en bloques completamente al azar con tres réplicas en parcelas de 3 × 7 m. La estimación de la BNF basada en las tres metodologías estudiadas, varió entre el 45% y el 58% del contenido total de N de las plantas, equivalente a 94 a 123 kg N ha -1 . Dependiendo de la metodología empleada para estimar la BNF, el balance de nitrógeno varió entre -7 a 22 kg ha -1 . Con un rendimiento promedio en grano de 1.61...
A simple and fast method was developed for the determination of traces of Fe, Mn, Cu, Cr, Ni, Zn and Mo in endive using wavelength-dispersive x-ray fluorescence spectrometry. Plants were grown in different sewage sludge compositions for 60 days in greenhouse conditions. Samples were compacted and analyzed after drying, milling and homogenization. The method was validated using certified standard reference materials. The results were compared with those obtained by atomic absorption spectrometry. Good agreement for all elements was obtained. The detection limits were found to be Fe 1, Mn 3, Cu 2, Cr 5, Ni 1, Zn and Mo 3 µg g −1 . These results permitted the evaluation of the potential use of sewage sludge as an agricultural amendment.
RESUMENEl pensamiento sistémico contribuyó al origen del pensamiento agroecológico con aportaciones desde la teoría general de sistemas y la cibernética como su base teórica primigenia. Así, la agroecología en los años 70 del siglo pasado pudo erigirse como una nueva forma de pensar y hacer ciencia en relación con el quehacer agrícola al intentar ir más allá del enfoque reduccionista predominante. El estudio de la realidad agrícola contemporánea exige planteamientos nuevos que permitan comprender sus dimensiones, para lo cual los principios basados en el modelo input-output que funciona bajo un controlador cibernético resultan insuficientes. En consecuencia, el propósito de este manuscrito fue analizar la evolución del pensamiento sistémico y su trascendencia en el pensamiento agroecológico, al brindar a éste último herramientas teórico-conceptuales para estudiar fenómenos agrícolas actuales a través de los sistemas complejos, y con ello, involucrar aspectos poco explorados en su abordaje tradicional como la historia, la política y la cultura.PALABRAS CLAVE: Agricultura; Agroecosistema; Pensamiento sistémico; Pensamiento complejo; Sistemas complejos ABSTRACT Systems thinking contributed to the origin of agroecological thought, which had as primordial theoretical basis the contributions from the general systems theory and cybernetics. Thus, agroecology in the 70s of the last century could establish itself as a new way of thinking and doing science related to agricultural activity to try to go beyond the dominant reductionist approach. However, the study of contemporary agricultural reality demands new approaches to understand it in its many dimensions. Accordingly, the purpose of this study was to analyze the evolution of systems thinking and its significance in the agro-ecological thinking by providing theoretical and conceptual tools to study current agricultural phenomena through complex systems, including unexplored aspects in its traditional approach as history, politics and culture. KEYWORDS: Agriculture; Agroecosystems; Systemic thinking; Complex thinking; Complex systems INTRODUCCIÓN La actividad científica vinculada al estudio de la realidad agrícola se ha realizado fundamentalmente desde el paradigma analítico, el cual ha predominado en la ciencia desde el siglo XVII (Morin 2007). Esto es, el
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