Amelioration Effect of Biochar on Nitrogen Transformation and Secondary Salinization of Vegetable Soils in Facilities

The cultivation of vegetables within controlled environments is concomitant with the challenges of soil secondary salinization and diminishing soil fertility, rendering continuous cultivation a barrier to their enduring development. Biochar (BC) has historically served as a soil ameliorant for the mitigation of environmental adversities. In this investigation, soils from greenhouse tomato cultivation with durations of 6 and 20 years were selected to assess the potential of biochar in facilitating soil nutrient transformations (nitrogen conversion) and enhancing the soil milieu via incubation experiments. Quantification of soil inorganic and total nitrogen, alongside enzymes implicated in nitrogen conversion, revealed that biochar supplementation elevated the microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) in the 6-year soil by 19.0–76.4% and 28.3–54.8%, respectively, and in the 20-year soil by 1.64–103.5% and 26.2–73.8%, respectively. Augmentation of urease (UR) activity facilitated urea hydrolysis, restrained nitrate reductase (NR), and nitrite reductase (NiR) activities and effectively curtailed the denitrification process. Biochar utilization amplified the abundance and diversity of soil microorganisms; however, bacterial abundance and diversity in the 20-year soil were considerably inferior to those in the 6-year soil. In the examination of microbial richness and diversity, correlations emerged between soil bacterial richness, diversity indices, and soil physicochemical properties. Redundancy analysis (RDA) at the phylum level further revealed that nitrate-nitrogen NO 3 − -N, NR, and pH constituted the principal environmental factors influencing bacterial community composition. The incorporation of biochar offers novel avenues for nitrogen conversion and soil environmental enhancement in controlled-environment vegetable cultivation.