Assessing the influence of environmental drivers on soybean seed yield and nitrogen fixation estimates and uncertainties in the United States

Context: Soybean [Glycine max (L.) Merr.] is one of the major crops worldwide. Identification of environmental factors that improve both yield and N₂-fixation remain of high importance. Objective: The study aimed to i) assess the effect (estimate and uncertainty) of sulfur (S) fertilization on seed yield and N₂-fixation (as N derived from the atmosphere, Ndfa), and (ii) evaluate the influence of soil and weather variables on these estimates and uncertainties. Methods: Thirty-five studies from nine US states were analyzed, comparing no fertilization (Check) with S fertilization at planting (S), using a regression tree approach to assess environmental effects on yield and Ndfa. Results: For both treatments, precipitation from full-pod to full-seed explained 40 % of the yield variation. For the Check, [soil organic matter, SOM/(clay+silt)] was a secondary factor. For the S, seasonal precipitation above 73 mm resulted in the highest yield (4.9 Mg ha⁻¹), with 51 % Ndfa and 135 kg ha⁻¹ of fixed-N. Yield uncertainty, averaging 1.2 Mg ha⁻¹, was associated with soil clay content below 11 %. Vapor-pressure-deficit from full-bloom to full-pod influenced Ndfa, accounting for 40 % of its variation between treatments. For both treatments, the highest Ndfa (∼65 %) required vapor-pressure-deficit below 0.92 kPa. Soil clay was pivotal to the uncertainty in Ndfa, explaining 34 % and 40 % of the variation for Check and S, but with a reduction in uncertainty when soil clay was above 26 %. Conclusion: The main regulators of yield and Ndfa were precipitation, temperature, SOM, and soil texture. Sulfur fertilization moderately increased yield and Ndfa, especially in environments with high plant N-demand. Ndfa uncertainty was more related to crop growth factors, with high seed yield correlating with high Ndfa. Implications: Future research should focus on controlled studies to improve the knowledge of the identified soil and weather factors and their interplay with seed yield and Ndfa.