Soybean nitrogen fixation - an evaluation of methods

Soybean has become an increasingly interesting crop for organic farmers in the dry region of eastern Austria. Via its N2 fixation soybean may contribute to the sustainability of organically managed farming systems. The relative contribution of N2 fixation to total plant N uptake is critical for the overall benefit of soybean in these systems but is so far unknown for the dry region of eastern Austria. A reliable assessment of the overall benefit of soybean requires an accurate measurement of the amount of N2 fixed. Various methods are available to measure symbiotic N2 fixation by field-grown legumes. Isotope-based methods are currently most commonly employed. They give a time-integrated measure of the total amount of N fixed over the entire period of plant growth. An alternative and simpler method is the quantification of ureides in xylem sap. Ureides are specifically associated with N2 fixation and allow an indirect measure of N2 fixation. Due to potential limitations of each of the methods, such as choice of appropriate reference plant in isotope-based techniques or drought effects on xylem ureide concentrations, the proposed study aims at testing a novel technique and evaluating established techniques for measurement of N2 fixation by well-watered and droughted soybeans. For the quantification of the beneficial effect of soybean on soil N economy, knowledge about the quantity of below-ground N is a prerequisite. The relative importance of N in legume roots and nodules as well as N released from roots during growth has only quite recently been highlighted by isotope-based studies. For soybean, only scant information is available about the proportion of N allocated below-ground. A further aim of the proposed study is the quantification of below-ground N allocation by field-grown soybean and the effect of water limitation thereupon. Based on this background, the following four objectives have been formulated: 1. Quantification of N2 fixation by soybeans grown organically under east Austrian climatic conditions. 2. Evaluation of methods to measure N2 fixation and their applicability under growth conditions limited by drought. 3. Quantification of below-ground N allocation by organically grown soybean and the effect of water limitation thereupon. 4. Development of a simplified method for the estimation of N2 fixation for east Austrian conditions. The results of this study will provide the basic scientific information on N 2 fixation and below-ground N allocation of organically grown soybean under east Austrian conditions. These data may subsequently be used in calculations on N-economies of soybean crops in the wider region of East Central Europe.