Effects of wheat straw and its biochar application on soil physiochemical properties and organic carbon fractions in flue-cured tobacco field

Objectives To understand the effects of wheat straw and its biochar on soil physiochemical properties and organic carbon fractions to provide basis for improvement of flue-cured tobacco planting soil.

Methods A two-year field experiment was carried on the meadow-cinnamon soil in 2016 and 2017 in Zhucheng, Shandong Province. Four treatments were designed, namely, chemical fertilizer alone (CK), chemical fertilizer plus wheat straw (FS), chemical fertilizer plus biochar of 2.25 t/hm² (FB1), and 4.50 t/hm² (FB2). At tobacco harvest, soil samples were collected in the plough layer (0−20 cm), and soil physiochemical properties and contents of microbial biomass C (MBC), hot-water extractable C (HWC), labile organic C (LOC), light fraction organic C (LFOC), and carbon pool management index (CPMI) were investigated.

Results The TOC contents in FB1 and FB2 was 74.9% and 116.0% significantly higher than that in CK, whereas no significant difference was found between FS and CK. The variation trend of LFOC in the four treatments was similar to that of TOC, and LFOC content in FB1 and FB2 was 154% and 326% significantly higher than that in CK. No significant difference in HWC content was observed between FS and FB2, while HWC content in FS treatment was significantly higher than that in FB1 and CK. In comparison with CK, HWC content in FS was increased by 107%. MBC content in FS and FB2 was significantly higher by 252% and 144% than that in CK, but no significant difference was found between FB1 and CK. LOC content in FS was significantly increased by 68.9% compared to that in CK, while no significant difference was found among FB1, FB2 and CK treatments. In addition, wheat straw returning (FS) significantly decreased soil bulk density and increased soil water content and soil available P content, which had a better effect on some soil physical and chemical properties than those of biochar treatments. The FS treatment also had the highest value of CPMI, which was 73.5% significantly higher than that of CK. However, the biochar treatments (FB1 and FB2) decreased or changed a little CPMI in comparison with CK.

Conclusions The continuous incorporation of wheat straw could increase the contents of soil labile organic carbon fractions of MBC, HWC and LOC, improve soil physiochemical properties, whereas wheat straw biochar could increase the stability of soil organic carbon, which is beneficial for the long-term stable fixation of soil organic carbon.