Objectives Based on a long-term fertilizer experiment, we analyzed soil potassium (K) apparent balance, the K absorption site in minerals and the desorption dynamics of K as affected by long-term application of chemical fertilizer combined with different organic fertilizers, to clarify the mechanism of the beneficial effect differences of organic fertilizers in improving soil K fertility.

Methods The long-term fertilization experiment of peanut-sweet potato rotation system was established in 2007, including five treatments: no fertilization (CK), merely chemical fertilizer (NPK), chemical fertilizer combined with commercial organic fertilizer (NPK+CM), pig manure (NPK+PM), and straw (NPK+RS). At sweet potato harvest in 2022 (the 15^th year of the experiment), the tuber yield and whole biomass were investigated, plants were sampled to analyze K content. The soil K apparent balance was calculated. At the same time, soil samples were collected for analysis of the soil K content in different forms, and adsorbed at different clay mineral sites by using continuous leaching method. The release characteristics of K was studied using organic acid and Sodium tetraphenyl boron as extractants.

Results In this peanut-sweet potato rotation system, the annual apparent balance of soil potassium under CK was in deficit, while under the four fertilizer treatments were in surplus. Compared with CK treatment, NPK+PM and NPK+RS increased non-exchangeable K by 260 and 188 mg/kg, exchangeable K by 72.3 and 75.0 mg/kg, and water-soluble K (H₂O-K) by 45.0 and 71.7 mg/kg, respectively (P<0.05). The interlayer (i-site), wedge edge (e-site) and surface (p-site) were the three main K adsorption sites. Compared with CK, NPK and NPK+CM treatment did not significantly changed the K adsorption amount and the ratio; NPK+PM and NPK+RS increased the p-site K content by 25 and 36 mg/kg, e-site K content by 58 and 66 mg/kg; NPK+PM treatment increased the i-site K content by 132 mg/kg; NPK+PM increased the K proportion at the e-site, while NPK+RS increased K proportion at p- and e-site but decreased that at i-site. In the K release experiment, NPK and NPK+CM treatment did not impact the rapid and total K release amount and rate regardless extractants; NPK+PM was recorded the highest rapid and total K release amount, which were significantly higher than NPK+RS under organic acid extraction, and had no significant difference from NPK+RS under sodium tetraphene boron extraction.

Conclusions The long-term combination of chemical fertilizer with pig manure or rice straw could improve the contents of non-exchangeable, exchangeable and water-soluble K in soil effectively. Chemical fertilizer did not change the ratio of adsorbed K at the main clay sites, while chemical fertilizer combined with pig manure significantly increased the interlayer K content, and chemical fertilizer combined with straw increased that at the wedge edge and mineral surface. Merely applying chemical fertilizer would not affect the bioavailability of soil K, long-term combining application of chemical fertilizer with pig manure or straw could increase the bioavailability of soil K, and the combination of chemical fertilizer with pig manure would be more effective than the combination of chemical fertilizer with straw under soil K deficient condition.