Straw and straw biochar differently affect phosphorus availability, enzyme activity and microbial functional genes in an Ultisol

Crop straw is commonly returned back to agricultural fields to improve soil nutrient status. In order to compare the effects of straw returning modes (direct and carbonization returning) on the phosphorus (P) availability in acidic soils and explore possible chemical and microbial mechanisms, a pot experiment was conducted. The rice straw, canola stalk at the rate of 1% (w/w) and their corresponding biochar produced by the same amount of straw at 350 °C and 550 °C were used, and two-season crops (rice and soybean) were planted. Results indicated that the content of available P in biochar-treated soils was significantly higher than in the straw-treated soils owing to the biochar soluble P and increased pH. Straw returning increased the activities of urease, sucrase and catalase more than biochar. Biochar mode significantly increased the activity of alkaline phosphatase (ALP), while decreased the acid phosphatase (ACP) relative to the straw mode. Likewise, there were a significant rise in the copy number of phoD gene and a drop of phoC in the biochar mode. The P functional genes (phoD, gcd and pqqC) had the higher copy numbers in soils with biochar made at 350 °C. Similarly, biochar made at 350 °C improved the yields of rice and soybean more effectively. Therefore, straw returning modes affected the availability of P differently via chemical and microbial pathways and the ALP regulated by phoD played a crucial role in the conversion of P. Results demonstrated that biochar returning had a larger impact on the availability of P and developed the effectiveness of crop production than the straw returning directly.