Objectives Crop straws are rich in potassium and mostly return to the field after harvest. In this paper, we studied the effect of straw K on replenishing soil K supply capacity and storage.

Methods A long-term straw return experiment in a wheat-rice rotation field was conducted in Qianjiang City, Hubei Province. The experiment was 13 years old when soil samples were collected in 2018. The treatments included no straw return to the field and no fertilizer (CK), straw return to the field at the rate of 6000 kg/hm² per season and no fertilizer (RS), fertilizer without straw return (NPK), fertilizer and straw return at the same rate in RS (NPKS). The plant K and soil K content in different fractions were extracted and analyzed by chemical methods, and the apparent K balance was calculated. The process of soil K release under normal and exhausting conditions was determined by continuous immersion method using sodium tetraphenyl boron and organic acid, respectively.

Results Under the annual straw K input of 220 kg/hm², the soil K apparent balance in RS and SNPK was abundant, but a deficit was recorded in CK and NPK. RS and NPKS increased non-exchangeable K contents by 85.6–157.6 mg/kg and exchangeable K by 12.3–18.5 mg/kg in soil. The increased non-exchangeable K was mainly distributed in interlays (site i), followed by the surface (site p) of the clay minerals. However, the ratio of K at p, e, and i sites did not change significantly (P>0.05). Extracted by sodium tetraphenyl boron and organic acid, soil K was released rapidly first and then slowly, according to the first-order kinetic model. The release rate of soil K in the rapid stage was in the order RS>NPKS>CK>NPK, and that in the slow stage was RS>CK>NPKS>NPK. The K released in RS and NPKS treatments was 166.6 and 81.1 mg/kg more than NPK treatment in the rapid stage and 71.4 and 27.9 mg/kg higher in the slow stage. Extracted with an organic acid, the release rate of soil K in both rapid and slow stages were in the order NPKS>RS>NPK>CK. Soil K in NPKS and RS was 16.3 and 14.5 mg/kg higher than NPK treatment in the rapid stage and 46.2 and 111.8 mg/kg higher than that in the slow stage.

Conclusions Straw returning could supplement the soil K storage and change the balance from deficit to surplus state. Long-term straw returning could increase both the exchangeable and non-exchangeable K content. The increased K is mainly absorbed in the inter-layers of clay minerals, and small amounts on the mineral surface. The kinetics of K release proves that straw returning increases the supply intensity and capacity of soil K to crops in a rice-wheat rotational system.