Objective To investigate the effects of soil application of selenium-enriched organic fertilizer on selenium accumulation and fatty acid in the kernel of ' Lvling ' walnuts, as well as to evaluate the in vitro antioxidant capacity of selenium-enriched walnut kernels, thereby providing a scientific basis for the cultivation of selenium-enriched walnuts.

Methods Based on the 20-year old 'Lvling' walnut trees, selenium-enriched organic fertilizer was applied by strip trenching along the outer edge of the tree crown during the shell hardening period and the oil accumulation period respectively. Nuts were harvested at the ripening period, and kernel quality traits and in vitro antioxidant capacity were determined. Correlation analyses were conducted to examine the relationships between selenium content, kernel components, and antioxidant activity.

Results The selenium content in walnut kernels showed a trend of first increasing and then decreasing with the increase of the amount of selenium-enriched organic fertilizer application. In both application periods, the highest kernel selenium content was observed at an application rate of 2 kg/plant, beyond which selenium content declined. Selenium uptake by the kernels was more efficient when fertilizer was applied during the shell hardening stage than during the oil accumulation stage, with a maximum selenium content of 0.252 mg/kg in the former, compared with 0.103 mg/kg in the latter. Kernel antioxidant capacity increased to varying degrees with increasing selenium content. However, the enhancement of hydroxyl radical scavenging capacity, O₂⁻ radical scavenging capacity, and ABTS cation radical scavenging capacity were not significant when the application rate was more than 2 kg/plant. The selenium content in kernels was significantly positively correlated with soluble sugars, starch, soluble proteins, linoleic acid, linolenic acid, hydroxyl radical scavenging ability, ABTS cation radical scavenging ability, and DPPH radical scavenging capacity ability, and was highly significantly negatively correlated with oleic acid.

Conclusion Application of selenium-enriched organic fertilizer at 2 kg/plant during the hardening period resulted in a selenium content of 0.252 mg/kg in the kernels of 'Lvling'. This treatment significantly increased the selenium content in the kernels, and represented the most economical fertilization strategy. Moreover, it increased the contents of soluble sugars, starch, and linoleic acid, leading to a notable improvement in kernel quality.