Brown manuring and sorghum allelopathy affects weeds, soil health, and paddy yield

This 5-yr study (2012–2017) aimed to evaluate the effects of sesbania (Sesbania rostrata Bremek. & Oberm) brown manure (SBM) and sorghum [Sorghum bicolor (L.) Moench] in rice (Oryza sativa L.)-based rotations on weeds, soil properties, and rice yield. Four rice production systems were used: (a) wheat (Triticum aestivum L.)–rice (control), (b) wheat–rice + SBM, (c) wheat–sorghum–rice, and (d) wheat–sorghum–rice + SBM. Compared with control, allelochemicals accumulated in the rhizospheric soil of all other cropping systems over time and were the highest in the sorghum-based rotations. Allelochemical accumulation in the soil reduced the densities and aboveground biomass of sedges and grassy and broad-leaved weeds. Weed suppression was more evident when SBM in rice was included in the postsorghum field vs. the control. The use of SBM also enhanced soil organic matter (SOM), soil microbial biomass nitrogen (SMBN), and soil microbial biomass carbon (SMBC), and reduced the soil bulk density compared with the control. There was an improvement in SOM by 6.82, 0.57, and 9.09%, SMBC by 3.92, 0.86, and 6.29%, and SMBN by 3.12, 0.97, and 4.43% with wheat–rice + SBM, wheat–sorghum–rice, and wheat–sorghum–rice + SBM rotation, respectively, compared with the control. The highest paddy yield was recorded in the wheat–sorghum–rice–SBM, which was attributed to improved weed suppression in this system vs. wheat–rice rotation. The improved paddy yield in SBM-based rotations was a direct outcome of better soil health and the built up of allelochemicals in the soil profile, which suppressed weeds and improved rice yield.