Effects of Tillage Methods Under Straw Returning on the Labile Organic Carbon Fractions and Carbon Pool Management Index in Black Soil Farmland

Soil-related indicators were measured during the mature period of maize in 2020 and 2021. The purpose of this study was to investigate the effects of tillage methods with straw returning on soil labile organic carbon and carbon pool management index of black soil farmland at the southern foot of the Daxing’an mountains based on four consecutive years of tillage experiment.

Seven treatments were set up in the tillage positioning experiment, including deep tillage with straw returning (DTS), deep loosening and shallow with straw returning (DSS), subsoiling tillage with straw returning (STS), heavy harrowing straw returning (SHS), rotary tillage with straw returning (RTS), and no-tillage straw returning (NTS), with shallow tillage without straw returning (CK). 0-60 cm surface soil was collected. The contents of labile organic carbon (R₃₃₃), medium labile organic carbon (R₁₆₇) and high labile organic carbon(R₃₃) were determined by potassium permanganate solution with different concentrations (0.333, 0.167 and 0.033 mol·L^-1), and the effects of different tillage methods on soil labile organic carbon content and carbon pool management index were studied.

(1) Compared with CK, the R₃₃₃ content in 0-10 cm soil layer treated by DSS, DTS, NTS, RTS and STS significantly increased by 9.0% to 63.7% in two years, respectively. DSS was the highest, followed by DTS and NTS. R₃₃₃ in 10-60 cm soil layer, DTS, DSS and NTS significantly increased by 30.8%-134.5%, 14.1%-97.8% and 18.9%-63.0%, respectively (P<0.05). (2) Compared with CK, the medium labile organic carbon (R₁₆₇) of 0-10 cm soil layer under different tillage methods was significantly increased (P<0.05). The DSS treatment of R₁₆₇ content was the highest, with an increase of 37.3%- 101.0%, and the DTS treatment of 10-60 cm soil layer was the highest, with an increase of 44.8%-72.9%, showing DTS>DSS>NTS treatment. (3) Compared with CK, the R₃₃ content in 0-10 cm soil layer was significantly increased by 13.8%-55.6% under DSS treatment (P<0.05). In 10-20 cm soil layer, R₃₃ content of DTS, DSS, STS, RTS and NTS treatment was significantly increased by 3.6%-29.3% (P<0.05), with DTS being the highest, followed by DSS. In 20-60 cm soil layer, R₃₃ under DTS, DSS and STS treatment was significantly increased (P<0.05), and the proportion was 68.1%-20.0%. (4) The inert organic carbon (IOC) of 0-10 cm soil layer under straw returning was significantly increased by 4.7%-21.8% (P<0.05). In 20-60 cm soil layer, DTS and DSS significantly increased by 5.3%-156.6% and 4.2%-138.8% compared with other treatments (P<0.05). (5) Compared with CK, DTS treatment significantly increased carbon pool activity and carbon pool activity index in 0-20 cm soil layer (P<0.05), and increased by 7.7%-44.8% and 7.7%-45.0%, and significantly increased by DTS, DSS, STS and RTS soil layer carbon pool index (P<0.05). DTS, DSS and NTS treatment significantly increased the carbon pool management index of 0-60 cm soil layer by 21.9%-140.9%, 4.9%-103.7%, 13.3%-62.0% (P<0.05). (6) In 0-60 cm soil layer, R₃₃₃, R₁₆₇, R₃₃ and IOC were positively correlated with organic carbon content, carbon pool activity index, bulk density and geometric mean diameter, but negatively correlated with total soil porosity (P<0.01). Increasing the content of active organic carbon could improve soil physical structure and enhanced aggregate stability. The IOC was positively correlated with maize yield and dry matter mass (P<0.01).

Comprehensive analysis shows that both deep tillage with straw returning (DTS) and deep loosening and shallow with straw returning (DDS) can increase soil labile organic carbon and inert organic carbon to a certain extent, and effectively improve the organic carbon pool of farmland.