Deep tillage combined with straw biochar return increases rice yield by improving nitrogen availability and root distribution in the subsoil.

After over 20 years of practicing shallow rotary tillage combined with straw direct return in the middle and lower Yangtze regions of China, the yield potential has gradually disappeared, and new alternative tillage practices are required to continuously increase rice yield. This paper investigates the yield effect and mechanism of deep tillage combined with straw return through two consecutive years of a field experiment. Three tillage depths (shallow tillage 6–7 cm, ST; medium tillage 12–14 cm, MT; and deep tillage 25–28 cm, DT) plus three straw management methods (no straw return, NR; straw direct return, SR; and straw biochar return based on the same amount of carbon input as SR, BR) were set up as experiment treatments. The aboveground and belowground growth traits of rice and soil nitrogen (N) status were determined. Compared with ST and MT, the DT enlarged rice root distribution and increased soil N content in the middle and lower soil layers (7–28 cm) and decreased the root–shoot ratio at the flowering stage, which was conducive to absorbing more soil nutrients from the subsoil and allocating more photosynthetic products to the aboveground parts and grain. The MT and DT increased spike number and rice yield compared with ST. Compared with NR, both SR and BR increased soil total and available N contents, and the remaining soil available N content at the harvest stage of BR was lower than that of SR. Straw return (SR and BR) improved rice yield by increasing spike number and kernels per spike, and the effect of BR on increasing rice yield was greater than that of SR. The DT combined with BR increased root distribution and N content in the subsoil, promoted the utilization of subsoil's nutrients, and increased rice yield. It is therefore a promising tillage practice for achieving high rice yields. • DT increased root distribution and N content in the subsoil. • DT had the lowest root-shoot ratio, allocating more biomass to aboveground growth. • DT increased spike number and straw return increased spike number and kernels per spike. • DT combined with BR is a promising approach for increasing rice yield. [ABSTRACT FROM AUTHOR]