Effects of co-application of straw and biochar on soil nutrients and soybean growth in black soil of cold regions

This field experiment investigated the combined effects of straw and biochar on Black Soil physicochemical properties and soybean growth in cold regions, with four treatments: total straw return (TS), high straw + low biochar (PS), low straw + high biochar (PB), and total biochar (TB). Results showed biochar amendments improved soil physical structure: TB had the lowest bulk density (1.230 g/cm 3 ) and highest porosity (52.72%), with >0.25 mm macroaggregates increased by 28.4% relative to TS. Nutrient supply differed by treatment: TB and PB enhanced nitrogen and phosphorus availability throughout growth, while TS and PS favored potassium and dissolved organic carbon. TB and PB promoted better soybean growth indices (plant height, stem diameter, LAI). PB treatment obtained the highest soybean yield value among all groups, but no statistically significant difference in grain yield was detected among the four straw-biochar combined treatments. Straw–biochar co-application synergistically improved soil structure and regulated nutrients for early promotion and late stability. The PB ratio (13 t/hm 2 straw + 6 t/hm 2 biochar) performed best, effectively improving Black Soil fertility and increasing soybean yield.