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Soil aggregate is important to soil quality. Straw return is beneficial for soil amendment, but the effects of straw and straw-derived biochar on aggregate formation and stability in paddy soil are unclear. This study carried out a field experiment for 2 years in Northeast China with four treatments: conventional fertilization (CF), straw (ST, 7.5 t ha−1 year−1), biochar (BC, 2.5 t ha−1 year−1), and biochar-based fertilizer (BCF, 0.75 t ha−1 year−1). Compared with the CF and BCF, BC and ST improved the macroaggregates and significantly increased soil total carbon and aggregate organic carbon, indicating a sustained positive role in promoting the formation and stability of aggregates. BC significantly improved aggregate stability. Correlation analysis showed that macroaggregates (2–0.25 mm) can be increased by increasing the heavy fraction organic carbon (HFOC), BC and ST significantly increased the HFOC, and BC had a greater effect. Scanning electron microscopy (SEM) showed that the combination of biochar and soil particles can be captured with strong water scouring. BCF showed an increasing trend in the formation and stability of soil aggregates. In comparison, biochar had a greater effect on promoting the formation of macroaggregates and the stability of soil aggregates with a significant sustainable effect. [ABSTRACT FROM AUTHOR]

Abstract Biochar is a promising material for soil amendment. However, the sustained effects of biochar vs. straw returning on cultivation in Albic soil are poorly understood. Two-year pot experiments were conducted to explore these effects and to compare the effect of different amounts of biochar on Albic soil. By the end of the 2-year experiment, biochar (10 g·kg−1 each year) and straw (30 g·kg−1 each year) returning had reduced the soil bulk density by 6.62% and 10.30%, respectively, and the higher-level biochar (30 g·kg−1 one-time addition) treatment had significantly reduced the density by 9.93%. The structure of Albic soil also changed: the ratio of the solid soil phase decreased annually, while the gas phase increased. The stability of soil aggregates significantly increased, with straw returning mainly affecting large grains and the biochar affecting grains of all sizes but particularly decreasing the amount of small grains. Biochar treatments altered the maximum field capacity and pH, which increased by 11.85% and 6.70%, respectively, on average. Straw, straw-biochar returning and both biochar amendment levels increased the total and available nitrogen, phosphorus, and potassium contents in Albic soil. Compared to those under straw returning, the contents of total nitrogen and phosphorus under biochar returning were increased by 9.83% and 25.56%, respectively, and the soil organic matter content of the biochar treatment increased significantly. Straw and straw-biochar returning had positive effects on soybean agronomic traits and nutrient (nitrogen, phosphorus, and potassium) absorption. The soybean yield of the biochar treatment was significantly increased by 10% compared to that of the straw returning, and this yield-improvement was maintained for 2 years. Compared to straw returning, biochar returning showed considerable advantages in overcoming the main limitations of cultivating Albic soil, such as increasing the soil pH and fertility, reducing the soil bulk density, and promoting crop nutrient absorption and yield. Overall, the treatment of 30 g·kg−1 biochar was the optimal treatment for Albic soil. Highlights • Both straw and straw-derived biochar returning improved Albic soil physical and chemical properties. • Straw-derived biochar promoted sustainable soybean growth and nutrient uptake. • Straw-derived biochar helped to generate an ideal "new layer" and prevent the physical recovery of Albic soil. • Straw-derived biochar overcame the key limitations to sustainable cultivation of Albic soil. [ABSTRACT FROM AUTHOR]