Abstract: Potassium is deficient in most of China’s arable lands. The application of organic amendments, such as biochar, is an effective way to increase soil potassium content. The research on the interactions between long-term and annual low amount of biochar returning and potassium application on brown soil potassium, crop productivity, and soil potassium balance is not sufficient. We carried out a long-term field experiment on maize cropland, which followed a two-factor randomized block design with two factors including biochar (C) and potassium fertilizer (K). There were two levels of biochar addition (0 t·hm^-2 and 2.625 t·hm^-2; denoted as C₀ and C₁, respectively), and two potassium fertilizer levels (0 kg·hm^-2 and 60 kg·hm^-2; denoted as K₀ and K₁, respectively). Biochar and potassium fertilizer had different effects on different forms of potassium across soil layers. In 0-20 cm soil layer, the content of water-soluble potassium, slowly available potassium, available potassium, and total potassium in C₁ treatment was significantly increased by 57.14%, 17.25%, 23.48%, and 8.31% compared with C₀. On the contrary, the content of soil readily available potassium in C₁ treatment was 8.31% lower than that in C₀ treatment. The contents of soil readily available potassium, slowly available potassium, and available potassium in K₁ treatment were 16.61%, 16.42%, and 44.25% higher than those in K₀ treatment, respectively. Compared with C₀K₀ treatment, C₁K₁ significantly increased the contents of water-soluble potassium, slowly available potassium, and available potassium by 72.72%, 77.45%, and 36.19%, respectively. Biochar had a significant effect on soil water-soluble potassium content, which was contrary to that in 0-20 cm soil layer. At K₀ and K₁ levels, soil water-soluble potassium content in C₁ was significantly reduced by 8.65% and 42.57% compared with C₀, respectively. The contents of soil readily available potassium, slowly available potassium, and available potassium in K₁ treatment increased by 13.33%, 10.84%, and 19.63%, respectively, compared with K₀. The content of water-soluble potassium only in K₁ treatment decreased by 40.35% compared with K₀ at C₁ level. Compared with C₀K₀ treatment, soil water-soluble potassium content in C₁K₁ was significantly decreased by 45.51%, while available potassium content was significantly increased by 24.26%. Both biochar and potassium fertilizer could significantly increase dry weight of maize organs and total dry weight, and the potassium absorption of different organs and the whole plant. Both biochar and potassium fertilizer could significantly increase maize yield, with 8.66% higher in C₁ than in C₀, 10.10% higher in K₁ than K₀, and 19.76% higher in C₁K₁ than in C₀K₀. The application of biochar could significantly improve the absorption and utilization rate of potassium fertilizer, resulting in surplus in soil potassium. Our results indicated that the long-term return of low amount of biochar and potassium fertilizer application year by year increased potassium availability in the surface soil, improved the dry matter accumulation and potassium absorption of maize, alleviated the negative potassium balance in the soil, and enhanced maize production. The comprehensive analysis showed that the combined application of biochar and potassium fertilizer is an effective fertilization management mode to improve the effectiveness of soil potassium and increase maize yield.

Key words: biochar, potassium fertilizer, maize, soil potassium, yield.