Nitrogen fixation by soybean in the pampas: Relationship between yield and soil nitrogen balance

DI CIOCCO, C.; PENÓN, E.; COVIELLA, C; LÓPEZ, S.; DÍAZ ZORITA, M.; MOMO, F.; ALVAREZ, R.

Mar del Plata

Congreso; XIX Congreso Latinoamericano y XXIII Congreso Argentino de la Ciencia del Suelo. Latinoamérica unida protegiendo sus suelos; 2012

Asociación Argentina de la Ciencia del Suelo - Sociedad Latinoamericana de la Ciencia del Suelo

A meta-analysis of published results from 10 field pampean experiments was performed in order to estimate average nitrogen fixation by soybean, soil nitrogen balance and to fit a regression model relating yield and fixed nitrogen in the region. 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. In average, 40 % of the soybean nitrogen was fixed from the atmosphere, rounding 109 kg N ha-1. 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 % 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.