Nitrogen fixation by soybean in the Pampas relationship between yield and soil nitrogen balance

Soybean [Glycine max [L.] Merrill] is the major grain crop in the Pampas. Its productivity strongly depends on soil nitrogen availability and biological nitrogen fixation because nitrogen fertilizers are not usually applied. Scarce local information has been generated about biological nitrogen fixation potential of soybean crops or to the soil nitrogen balance during its growing season. A meta-analysis was performed of locally published results from 10 field experiments in order to estimate average values of nitrogen fixation from the atmosphere, crop yield and soil nitrogen balance in the region and, also, to fit a regression model relating yield and fixed nitrogen. Results were compared with a nitrogen balance calculated using a model fitted to worldwide data. Nitrogen fixation in aboveground biomass was assessed by isotopic methodologies in all the experiments and an estimation of fixed nitrogen in roots was performed. Grain yield varied from 662 kg of dry matter [DM] ha -1 for non nodulating isolines to 4260 kg DM ha -1 for commercial varieties. Nitrogen fixation accounted for 20 to 55 percent of the plant nitrogen. In average, 40 percent of the soybean nitrogen was fixed from the atmosphere, rounding 109 kg N ha -1. Shoot biomass and grain yield were positively correlated to fixed nitrogen [R 2 0.69, P = 0.05]. The slope of the regression of the amount of fixed nitrogen against yield showed that, in average, the crop fixed from the atmosphere 52 kg of N t -1 DM grain produced. By harvest approximately 60 kg of N t -1 DM were extracted from the agricultural system. Consequently, the apparent soil nitrogen balance was slightly negative and as higher the yield, the more negative the nitrogen balance. For an average soybean yield in the Pampas [2600 kg ha -1, 14 percent water] the soil nitrogen balance can be estimated to be -18 kg N ha -1 yr -1. Estimation of fixed nitrogen with the model adjusted to worldwide data was higher than with the local model and the calculated nitrogen balance turned to positive, so this later model seemed not to be applicable to pampean conditions. Future research is needed for the evaluation and the development of management practices and crop rotations for minimizing the negative soil nitrogen balance of the region.

Bibliographic Details

Main Authors:	Di Ciocco, Cesar Augusto, Penón, E., Coviella, Carlos Eduardo, López, S., Díaz Zorita, Martín, Momo, Fernando Roberto, Alvarez, Carina Rosa
Format:	Texto biblioteca
Language:	eng
Subjects:	BIOLOGICAL NITROGEN FIXATION, SOIL NITROGEN BALANCE, SOYBEAN, GLYCINE MAX,
Online Access:	http://ceiba.agro.uba.ar/cgi-bin/koha/opac-detail.pl?biblionumber=46468
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