Endophytic N2 fixation in sweet potato: responses to N, P, and K inputs and visualization of 15N2 utilizing bacterial cells via Raman spectroscopy

• Published in: Biology and fertility of soils Vol. 59; no. 3; pp. 275 - 283
• Main Authors: Ueda, Risako, Yano, Katsuya
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.04.2023; Springer Nature B.V
• Subjects: Agriculture; Analytical methods; Bacteria; Biomedical and Life Sciences; Crop yield; Endophytes; Fertilizers; Fixation; Ipomoea batatas; Legumes; Life Sciences; Nitrogen fixation; Nitrogen isotopes; Nitrogen-fixing bacteria; Nitrogenation; Nutritional status; Potatoes; Raman spectroscopy; Saturation; Short Communication; Soil fertility; Soil Science & Conservation; Soybeans; Spectroscopy; Spectrum analysis; Sweet potatoes; Vegetables; Visualization; Stable isotope probing (SIP); Nitrogen fixation; Endophyte; Potassium; Phosphate; Resonance Raman spectroscopy
• ISSN: 0178-2762; 1432-0789
• DOI: 10.1007/s00374-023-01698-5

Fertilizer-N strongly limits non-leguminous crop yields; however, sweet potato ( Ipomoea batatas ) is an exception, likely due to its ability to acquire atmospheric N 2 via endophytic diazotrophs. Using Raman spectroscopy, we found that in 15 N 2 -fed sweet potato, some endophytic bacteria contained 15 N, providing direct evidence of N 2 fixation  in planta . To assess N 2 -fixing capability, pot experiments were conducted by varying N, P, and K fertilizer inputs. Sweet potato showed higher N content than the non-N 2 -fixing Ipomoea aquatica; additionally, it showed increased N content which was 1.4-fold higher than the fertilizer-N input. Its δ 15 N values were closer to those of N 2 -fixing soybean, with an estimated 11–56% of plant N derived from N 2 . The estimated amount of fixed-N in sweet potato was negligible without fertilizer-N; however, a gradual accumulation leading to an immediate saturation was observed with increasing fertilizer-N. During this state, increasing P supply linearly enhanced the capability, reaching 13 g N m −2 , comparable to that of legumes. However, K inputs affected neither N 2 fixation nor growth owing to strong K acquisition from the soil even without fertilizer-K. Our results indicate extensive N 2 fixation in sweet potato, depending on its nutritional status, particularly P; resonance Raman spectroscopy facilitates the visualization of active N 2 -fixing bacteria on a single-cell scale.