Objectives Soil magnesium (Mg) deficiency was one of the main limiting factors for pepper production. The effect of different Mg fertilizer application amounts on pepper yield and soil exchangeable Mg content was studied, and the mechanisms was also explored.

Methods A field experiment was conducted in east of Guizhou Province during 2018 and 2019, using chilli pepper (Capsicum annuum var. conoides) as test material. Five Mg application levels were set up, including: 0, 22.5, 45, 67.5, and 90 kg/hm², denoted as Mg0, Mg22.5, Mg45, Mg67.5, andMg90, respectively. Chilli pepper yield, yield components, leaf net photosynthetic rate (Pn), leaves chlorophyll content, shoot Mg concentration and soil exchangeable Mg content were measured.

Results Soil Mg application significantly increased the yield of pepper, with the Mg67.5 and Mg90 treatments showing the best results. In 2018 and 2019, the yields of chilli pepper significantly increased by 20.7%−40.6% and 14.8%−18.0%, respectively, compared to the Mg0 treatment. The yield increase was mainly attributed to improvements in the number of fruits per plant and single fruit weight. Soil Mg fertilization enhanced the Pn, chlorophyll content, and Mg content of the plants during the flowering and fruit-setting stage (a critical growth period). In 2018 and 2019, the Pn were increased by 41.8%−72.8% and 27.3%−71.3%, respectively, total chlorophyll content increased by 23.3%−37.4% and 10.6%−16.2%, respectively, compared to the Mg0 treatment. During the flowering and fruit-setting stage, plant magnesium concentration increased by 57.3%−74.7% and 29.8%−69.8%, respectively. At harvest, plant magnesium concentration increased by 24.7%−78.0% and 17.6%−42.0%, respectively. The 0−20 cm soil layer exchangeable Mg content under Mg-treated plots in 2018 and 2019 was 58.1−79.4 mg/kg and 62.1−101.7 mg/kg, respectively, representing significant increases of 26.8%−52.9% and 34.9%−121.0%, compared to the Mg0 treatment. In 2018, there were no significant differences in exchangeable Mg content among treatments in the 20−40 cm and 40−60 cm soil layers. In 2019, the exchangeable Mg content in the 20−40 cm and 40−60 cm soil layers under Mg treatments was 64.0−92.6 mg/kg and 70.9−97.0 mg/kg, respectively, showing significant increases of 18.3%−71.3% and 11.9%−53.0%, compared to Mg0 treatment. Regression analysis revealed that the Pn during the flowering and fruit-setting stage, as well as the yield of chilli pepper, exhibited a highly significant linear positive correlation with the exchangeable Mg concentration in the 0−20 cm soil layer at harvest. The Mg content in the fruit of chilli pepper. reached a plateau value of 1.95 g/kg when the soil exchangeable Mg concentration reached 74.2 mg/kg.

Conclusions Mg fertilization can effectively improve the exchangeable Mg concentration in 0−20 cm soil layer, providing sufficient Mg nutrition for chilli pepper during the whole growing period. As a result, Mg fertilization can enhance the net photosynthetic rate and chlorophyll content of leaves, and maintain leaf greenness during flowering and fruit setting stage, and therefore increase the chilli pepper yield and fruit Mg content. Applying Mg 67.5 kg/hm² is recommended for high chilli pepper yield and maximum fruit Mg content, as well as the high soil exchangeable Mg content in the tested area.