Sustained effects of biochar application on physico-chemical properties, enzyme activities and quality of soil with continuous planting of cucumber

Objectives This study investigated the sustained effects of biochar application on properties of root zone soil continuously planted with cucumber, providing scientific evidences for biochar remediation of continuous cucumber planting soil and widely use in agriculture.

Methods A greenhouse experiment was conducted at Rugao Institute of Agricultural Science, Jiangsu province. Biochar was applied only once at rates of 0, 5, 10, 20, 30, and 40 t/hm², denoted as CK, C1, C2, C3, C4 and C5, respectively, and physical properties, nutrient contents and enzyme activities of root zone soils continuously planted with cucumber were analyzed in two consecutive years. The soil quality index (SQI) was used to evaluate the impact of different rates of biochar on quality of soil continuously planted with cucumber.

Results The changes of soil physico-chemical properties with different rates of biochar in the first season were consistent with that in the second season. Biochar application reduced the soil bulk density, while increased the soil porosity, saturated water content, field moisture capacity, saturated hydraulic conductivity, available phosphorus and organic matter content, compared with the control without biochar application. The contents of total nitrogen, nitrate nitrogen and ammonium nitrogen were greatest when biochar was applied at 30 t/hm². Compared with the control, biochar application reduced the mass content of < 0.25 mm soil aggregates, while increased the content of > 0.25 mm soil aggregates. The contents of 0.25–0.5 mm and 0.5–1 mm soil aggregates were greatest when biochar was applied at 40 t/hm². soil urease and catalase activity was greatest at biochar rates of 30 t/hm² and 20 t/hm², respectively, and showed the same trend of increasing at first and then decreased with increase rate of biochar in consecutive two cropping seasons. Cucumber yield in first and second seasons was 3.24 × 10⁴ kg/hm² and 6.18 × 10⁴ kg/hm², respectively, and was highest when biochar was applied at 30 t/hm². The analysis of SQI indicated that soil quality change in the order of C4 > C5 > C3 > C2 > C1 > CK.

Conclusions The physico-chemical properties and enzymatic activities of the continuous cucumber cropping soil were significantly affected by biochar application. The optimal application rate of biochar was 40 t/hm² and 30 t/hm², respectively for soil physico-chemical properties and nutrient contents. The SQI can be used to clearly evaluate the effect of biochar application on continuous cucumber cropping soil. Biochar at 30 t/hm² was optimal in improving the properties of continuous cucumber cropping soil.