Effects of Long-Term Biochar Application on Potassium in Maize Rhizosphere Soil

To investigate the effects of long-term biochar application on different forms of potassium (K) content in maize rhizosphere soil and maize growth, two biochar application rates (B0: 0 t ha⁻¹ yr⁻¹, B1: 2.625 t ha⁻¹ yr⁻¹) and two K fertilizer application rates (K0: 0 kg ha⁻¹ yr⁻¹, K1: 60 kg ha⁻¹ yr⁻¹) to create four treatments (B0K0, B0K1, B1K0, B1K1). In this long-term field trial, we investigated various forms of K in the maize rhizosphere soil, together with soil physicochemical properties and maize growth indicators. Results indicate that biochar significantly increased microbial biomass carbon (MBC), cation exchange capacity (CEC), and electrical conductivity (EC) in the rhizosphere soil, while also improving rhizosphere soil pH. Compared with the treatment without biochar, biochar application significantly increased the content of water-soluble potassium (WSK), exchangeable potassium (EK), and non-exchangeable potassium (NEK) in the rhizosphere soil by 18.57% (2021) and 11.18% (2022), 13.49% (2021), and 11.43% (2022), 14.65% (2021), and 17.06% (2022), respectively. Maize roots were more developed, and plant height, stem diameter, and leaf area index were significantly increased. With above-ground dry weight and K uptake significantly increasing by 13.87% (2021) and 12.04% (2022), and 41.84% (2021) and 43.87% (2022), respectively. Compared with B0K0, the B1K1 treatment—which combined biochar with K fertilizer—exhibited the highest K content in all forms within the rhizosphere soil, along with the greatest maize aboveground dry weight and K uptake. This study demonstrates biochar’s potential in meeting crop root K demands, laying the foundation for its application in enhancing soil K fertility.