Lentil nitrogen fixation response to fertilizer and inoculant in the northern Great Plains

Abstract

Lentil (Lens culinaris Medikus) production in the semiarid northern Great Plains of the United States has increased dramatically over the past two decades. Lentil in rotation provides agroecosystem benefits of more efficient water use, pest cycle disruption, and biological nitrogen (N) fixation. Increasing N fixation could alleviate soil acidification and groundwater impairment, decrease N fertilizer costs, and increase lentil seed yield. Despite widespread farmer adoption of lentil in the region, little is known about the benefits of fertilizer or inoculant type on N fixation. The aim of this study was to determine how nutrients (potassium (K), sulfur (S), and foliar‐applied micronutrients) and rhizobial inoculant types (seed‐coat powder and granular) influence N fixation of lentil. The study was conducted at two field sites in Montana from 2019 to 2021. Fixed N amounts were calculated using both an N difference approach and 15N natural abundance method. N fixation was highly responsive to climatic conditions and soil characteristics. The amount of N fixed did not respond to K fertilization, likely because soil test K levels were sufficient. In a moderately dry year at a site with low soil sulfate‐S, fertilizer application of 5.6 kg S ha−1 increased N fixed by 40%. Inoculated lentil fixed more N than uninoculated lentil in two site‐years, but there were no differences in N fixed between inoculant types. Inoculation response was not related to field cropping histories with legumes. The study shows that S fertilization and rhizobial inoculation have potential to increase lentil N fixation amounts in the northern Great Plains.

Inoculant and fertilizer effects on lentil in the US northern Great Plains

“…The largest response occurred in 2019 at Sidney, MT, where the control yielded only 34% of the infurrow inoculant treatment. In a companion N fixation study conducted at Bozeman and Havre (Baber et al, 2023), inoculation increased N fixed amounts by lentil at Havre in 2019 and 2021, consistent with inoculation effects on yield at that location. However, amounts of N fixed were not different in either year at Bozeman, whereas inoculation increased yield at that location in 2019 (p = 0.04).…”

“…The seed-coat application method was favored at Havre in 2019 and 2021, while in-furrow inoculant was favored at Bozeman and Moccasin in 2020. Further, N fixation amounts did not differ between in-furrow and seed-coat inoculant treatments at Bozeman and Havre in any year (Baber et al, 2023). The lack of agreement between differences in yield (plot scale) and N fixation (measured on ∼0.6 m 2 of biomass) may be due to scale and associated differences in error terms and/or lack of representativeness of the biomass sampled area.…”

“…In turn, N fixation can be increased directly by feeding rhizobia with S (Zhao et al, 1999) or indirectly by greater biomass, allowing more carbohydrates to be mobilized to root nodules, thereby enhancing N fixation (Scherer et al, 2006). Although the study herein and the N fixation companion study were not designed to assess mechanisms, results from the N fixation study suggest an indirect S effect on biomass allowing more carbon transport to nodules, thereby increasing N fixation (Baber et al, 2023).…”

“…Seed protein concentration responses coincided with yield response only in one experiment (Hettinger 2019). The N 2 fixation amounts at Bozeman and Havre corresponded directly with seed protein concentration differences from inoculation, with no N-fixed differences at Bozeman, and positive benefits at Havre in 2019 and 2021 (Baber et al, 2023). Seed protein concentration for in-furrow inoculant was 8.3 g kg −1 greater for three experiments (p ≤ 0.03, Tables 4 and 6), but 11 g kg −1 less than seed coat inoculation for one experiment (p = 0.01, Havre 2019).…”

“…Due to potential "dilution" of seed protein when yield increases, N-yield (yield × N fraction) might be expected to be influenced by N fixation, even when seed protein is not. Baber et al (2023) found N-yield increased 12 kg N ha −1 at Bozeman in 2020 due to S fertilization. At Havre in 2020, seed protein concentration increased without a corresponding increase in N fixation (Baber et al, 2023).…”