Effects of long-term deep tillage and straw return with biochar addition to lime concretion black soil on the aggregates and wheat-maize yield

Lime concretion black soil in the southern region of the Huang-Huai-Hai plain is known for its poor physical structure as a typical low-yield field. Although measures of deep tillage and straw return have been proved to be able to improve soil physical properties and soil fertility, there are still problems such as shallow tillage soil, weak fertility, high bulk density of the deeper soil, poor aggregate structural properties, serious stratification of the deeper soil, low crop yields and other problems, due to the long-term use of rotary tillage of wheat before sowing and no-tillage of maize and straw return tillage. It is important to adopt reasonable tillage practices and organic matter addition to improve the quality of lime concretion black soil. Therefore, in order to investigate the effects of the tillage practices, straw return to the field and biochar addition on the particle size structure distribution of soil aggregates, nutrient characteristics of aggregates, nutrient storage capacity and annual yield of wheat and maize, a long term localization experiment was set up with tillage and straw treatment at the Henan Province Agro ecosystem Field Observation and Research Station since 2014. After the year of 2020, the soils were supplemented with biochar. A three-factor, two-level experimental design was adopted, which consisted of tillage practices (conventional rotary tillage (CT), deep tillage (DT)), straw treatments (straw returned to the field (S), straw not returned to the field (NS)) and biochar addition (biochar addition (B), and no biochar addition (NB)), with a total of eight treatments. The results showed that rotary tillage without biochar addition significantly improved the stability of soil aggregates and soil nutrient stocks in the 0-15 cm soil layer, whereas deep tillage with straw significantly improved the composition of soil aggregates in the >15-30 cm soil layer, which enhanced soil fertility and promoted the increase of crop yield. The correlation analysis showed that the crop yield increase in the lime concretion black soil was more dependent on the improvement of soil physical structure and soil fertility in the deep layer (>15-30 cm). compared with CT-NS-NB (rotary tillage straw no return without biochar) treatment, Biochar addition, such as DT-S-B (deep tillage straw return with biochar addition) especially increased the stability of aggregates in the >15-30 cm layer, and increased the proportion of >2 mm aggregates, MWD and GMD values by 165.88%, 62.37% and 119.81%, respectively. DT-S-B treatment increased the organic carbon and nutrient preservation capacity of >2 mm aggregates and decreased the organic carbon and nutrient preservation capacity of <2 mm aggregates, and significantly increased the organic carbon storage, total nitrogen, total phosphorus and total potassium pools in the >15-30 cm soil layer by 37.41%, 38.99%, 38.99%, 38.99%, and 38.99%, respectively. The treatment of DT-S-B contributed to an average increase of 22.96% in 2-year annual crop yield. Although the increased application of biochar was presented as having the function of improving soil structural properties and promoting soil fertility, it did not show a significant increase in crop yield, probably due to the short application time and small amount of addition. In sum, the application of biochar with deep tillage straw returned to the field significantly improved the particle size distribution and stability of deep soil aggregates in the lime concretion black soil of the southern Huang-Huai-Hai Plain, enhanced the soil fertility and the annual yield of wheat and maize, and ensured the efficient green and sustainable production of farmland.