[Objectives]The study aimed to clarify rice's response to magnesium(Mg)fertilizer in the main rice-producing areas of Fujian Province,and determine the appropriate Mg fertilizer level under different soil-exchangeable Mg contents.[Methods]A total of 596 soil samples were collected across Fujian Province after rice harvest in 2019,and the exchangeable Mg content(ExMg)was determined.In the 16 cities/regions of Fujian,19 field experiments of magnesium fertilizer rates and 10 field demonstration experiments of magnesium fertilizer were conducted in 2020 and 2021.The tested Mg fertilizer rates were MgO 0(CK),15(Mg1),30(Mg2)and 45(Mg3)kg/hm2.Each demonstration experiment included applying MgO 30 kg/hm2(+Mg)and a no Mg application control(CK).The maximum return to magnesium(MRTMg)was used to determine the recommended Mg fertilizer dosage for rice cultivation in Fujian.[Results]The average ExMg in Fujian paddy fields was 72.1 mg/kg,with 74%of the soil sampling points below 80 mg/kg.Rice yield and incomes increased with increasing Mg fertilizer application rates.Compared to the CK treatment,rice yield significantly increased by 4.9%,7.0%,and 7.2%,and rice net profit increased significantly by 4.3%,5.5%,and 4.6%under the Mg1,Mg2,and Mg3 treatments,respectively.The optimal economic dosage of Mg fertilizer in Fujian was MgO 31.6 kg/hm2.The critical threshold for exchangeable Mg in Fujian rice soils was 80 mg/kg.When exchangeable Mg was<80 mg/kg,80-120 mg/kg,and>120 mg/kg,the optimal economic MgO dosage was 33.2,27.3,and 15.8 kg/hm2,respectively.[Conclusions]Mg deficiency in Fujian rice soils is widespread.Application of Mg fertilizer can significantly enhance rice yield and economic benefits.Soil exchangeable Mg supply should be considered to determine the appropriate Mg fertilizer level for rice production.
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