Objectives The study compared the effects of straw return or biochar of maize on the changes of soil acidity and exchangeable capacity at the same application rate of NPK in the field micro-plots, thus providing a theoretical reference in rational utilization of straw and biochar for the alleviation of soil acidification.

Methods A located micro-plot experiment was conducted for four consecutive years in the Experimental Station of Institute of Plant Nutrition and Fertilizer, Shenyang Agricultural University. Six treatments were setup, including no fertilizer (CK), nitrogen, phosphorus and potassium (NPK), biochar (C), NPK with biochar (CNPK), straw (S), and NPK with straw (SNPK). The nutrient inputs in treatments of NPK, CNPK and SNPK were all N 225 kg/hm², P₂O₅ 112.5 kg/hm² and K₂O 112.5 kg/hm², the straw input in S treatment was 4500 kg/hm² and the biochar input in C treatment was 1500 kg/hm². Changes in active acid, exchangeable acid, CEC and base-exchangeable ions in soils were evaluated.

Results The application of biochar or straw enhanced soil pH, and decreased the contents of exchangeable acid and exchangeable aluminum in soil, but did not affect the contents of exchangeable H⁺ in all the treatments after continuous experiment of four years. The C and the S treatments increased soil pH by 0.55 and 0.45 units, respectively; but the CNPK and SNPK treatments only increased soil pH by 0.31 and 0.13 units relative to the pH before the experiment (6.05), respectively. Soil pH value in the CNPK was significantly higher than that in the SNPK, but no significant difference in soil exchangeable acid contents was observed between the two treatments. The total base-exchangeable ions in all treatments were significantly higher than those in the control. The C treatment significantly increased the amounts of base-exchangeable ions, exchangeable Ca²⁺ and Mg²⁺ compared to the S treatment. Under equal amounts of straw and NPK nutrients, the amounts of base-exchangeable ions in the CNPK and SNPK treatments were increased by 17.6% and 15.1% compared with CK, respectively, suggesting that straw carbonization returning was greatly beneficial to increasing the soil CEC, which were increased by 1.68 cmol/kg in the C treatment, 2.52 cmol/kg in the CNPK treatment, 1.53 cmol/kg in the S treatment and 2.30 cmol/kg in the SNPK treatment compared with the CK, respectively, and the CNPK exhibited the highest soil CEC.

Conclusions When equal amounts of straw and NPK nutrients were applied, both the biochar and straw returning could reduce soil acidity and the contents of exchangeable aluminum in exchangeable acid, and increase the accumulation of base-exchangeable ions and exchangeable capacity in soil, indicating the improvement effects of biochar in alleviating soil acidity were very remarkable.