Effects of biochar and straw returning on the key cultivation limitations of Albic soil and soybean growth over 2 years

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.