The Long-Term Effect of Biochar Amendment on Soil Biochemistry and Phosphorus Availability of Calcareous Soils

Biochar soil amendment has proven to be beneficial for soil fertility regulation, while the long-term mechanisms under different soil conditions remain largely unexplored. This study was based on two long-term field experiments in calcareous soils to explore the effect of biochar on fertility and phosphorus availability, as well as potential mechanisms. Experiments were conducted on Fluvo-aquic soil (6 years) and Cinnamon soil (5 years), respectively. Biochar was applied once at 20 t/ha level and mixed with the topsoil. We evaluated soil fertility and phosphorus fractions with and without biochar amendment and analyzed the related extracellular enzyme activity and functional gene abundance. The results indicate a persistent increase in crop yield by 10.8–24.3% in calcareous soils after 5–6 years of treatment. Biochar enhanced soil fertility, including increased SOC, available phosphorus, and potassium content. Moreover, it boosted the activity of extracellular enzymes related to soil carbon, nitrogen, and phosphorus cycling. This effect may be associated with the stability of morphological features and mineral element regulation on biochar surfaces. Biochar soil amendment led to a long-term increase in phosphorus availability, of which available phosphorus increased by 13.1–49.1%. Potential mechanisms included an increase in labile phosphorus forms and biological mechanisms of raising alkaline phosphatase activities and increasing the copy numbers of the phosphatase genes (<i>phoD</i>, <i>phoX</i>, and <i>nifH</i>). In conclusion, biochar soil amendment resulted in sustained improvement in soil fertility in calcareous soils and consistently increased phosphorus availability via biochemical mechanisms.