Weather Conditions and Biostimulants Influence Nitrogen Acquisition from Different Sources by Soybean Plants

This study aimed to determine the influence of weather conditions (air temperature, precipitation and insolation) on the quantity of nitrogen taken up by soybean plants whose cultivation included an application of two biostimulants: Asahi and Improver, which have been approved for sale. An isotopic dilution method was used which involved an application of mineral fertilisers enriched with the isotope 15 N (5%) to detect the quantity of nitrogen fixed from the atmosphere, acquired from the soil and taken up from the fertiliser. Microplots of 1 m 2 , organised to form larger units, were planted with soybean. The impact of meteorological conditions on the amount of nitrogen fixed by plants as influenced by the experimental biostimulants was estimated using regression trees based on the C&RT algorithm in STATISTICA 13.3. This procedure yielded regression trees which revealed that, irrespective of the test biostimulant, the quantity of nitrogen fixed from the atmosphere was mainly influenced by the air temperature in July, as indicated by the first and most significant branching of the tree. The poorest fixation of atmospheric nitrogen in plants was observed when the average 24-h air temperature in July was higher than 20.9 °C, the quantities being 20.61, 31.33 and 30.49 kg, respectively, in the control, Asahi- and Improver-treated plots. The superior nitrogen uptake from fertiliser, from 10.64 (for the control) to 14.98 kg (in the Improver-amended units), was found when the air temperatures recorded in July and June did not exceed, respectively, 20.9 and 13.15 °C, and the daily rainfall in July was up to 5.65 mm. The regression tree model associated with the quantity of nitrogen acquired by soybean plants from soil indicates that, just like atmospheric nitrogen and nitrogen taken up from fertiliser, the average daily air temperature in July was the major factor determining the first branching of the tree. When this temperature went beyond 20.9 °C, the lowest uptake of nitrogen from soil was found for control plants.