Soybean profitability and yield component response to nitrogen fertilizer in Iowa

Abstract

Nitrogen fertilizer application to soybean [Glycine max (L.) Merr.] in Iowa, USA, has shown inconsistent results. We performed a study in central Iowa (2015 and 2016) to investigate the effect of N fertilizer rate (0, 45, 90, 135 kg N ha −1) and application timing (planting, flowering, pod setting) on soybean yield, yield components, and to calculate the economic net return to N fertilizer. Results showed a positive effect of N fertilizer on soybean yield and yield components both years. Seed and aboveground biomass dry weight were positively correlated to N fertilizer, and both were 17% greater than No-N treatment. Nitrogen fertilizer rate that significantly increased seed and aboveground biomass was 135 kg N ha −1 regardless of application timing (2015), or at planting (2016). Moreover, the same N fertilizer addition applied at planting benefitted seed and aboveground biomass N accumulation only in 2016 (avg. 32.00 and 34.68 g N uptake m −2 , respectively), both 1.5-times higher than No-N treatment. Favorable environmental conditions during 2016 lead to hand-measured yield difference of 22% compared to 2015. Economic net return analysis showed that the additional revenue from increased yield attributed to supplemental N fertilization offset the application cost, resulting in net return gains between US$5.83 to $281.89 ha −1 (all treatments except 45 kg N ha −1 on 2015). This study highlights the importance to parse out soybean yield in its components, and the need to quantify yield gains from N fertilizer additions in economic terms which shed some light on any tradeoffs. Abbreviations: BNF, biological nitrogen fixation; DOY, day of the year; NHI, nitrogen harvest index; TC, total carbon; TN, total nitrogen. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Modelling symbiotic biological nitrogen fixation in grain legumes globally by LPJ-GUESS

“…5a). This may be partly due to the fact that LPJ-GUESS at this point does not model soybean hulls, which account for~15-20% of the total above-ground dry matter at harvest in the U.S. soybean rain-fed cropping system (Córdova et al, 2020). The observed increase in shoot and leaf biomass due to water supply was 19% and 21%, respectively.…”

“…In addition, due to the difficulties in measuring both nodules and roots in the field directly, in many studies the observed BNF rates were determined from plant above-ground biomass. Excluding the root contribution to the whole plant BNF rates most likely result in an underestimation of N fixation (Córdova et al, 2019(Córdova et al, , 2020: N associated with nodules and roots in soybean and faba bean may account for 20-40% of the total N accumulation at mid-flowering phase (Unkovich and Pate, 2000;Khan et al, 2003).…”

“…In the field trials a specific threshold of soil N concentration above (below) which N fixation is inhibited (stimulated) is hard to measure. In addition, the timing of N application remains a challenge (Córdova et al, 2020). Some studies reported that applying N fertilizer at planting as starter N can increase yield gains because of sufficient soil available N to stimulate early season soybean growth (Pikul et al, 2001;Osborne and Riedell, 2011;Gai et al, 2017).…”

Modeling symbiotic biological nitrogen fixation in grain legumes globally with LPJ-GUESS (v4.0, r10285)

“…5a). This may be partially due to the fact that LPJ-GUESS at this point does not model soybean hulls, which account for ∼ 15 %-20 % of the total aboveground dry matter at harvest in the US soybean rain-fed cropping system (Córdova et al, 2020). The ob-served increase in shoot and leaf biomass due to water supply was 19 % and 21 %, respectively.…”

“…For example, in the CROPGRO model (Boote et al, 2008), N fixation in soybean starts in the second trifoliolate stage and continues until the end of physiological maturity, whereas it ceases at the middle of the seed-filling period in the EPIC model (Cabelguenne et al, 1999). Much experimental evidence has indicated that the N fixed by legumes varies widely among crop growth stages, with the largest BNF rate observed between the late vegetative phase and the early seed-filling period (Santachiara et al, 2017;Córdova et al, 2020;Ciampitti et al, 2021). In this study, a specific function, similar to the temperature response function, is thus implemented in the BNF scheme to represent the variation of N fixation with the course of the legume life cycle:…”