Excessive N fertilizer application has aggravated soil acidification and loss of N. Although oyster shell powder (OSP) can improve acidic soil, few studies have investigated its ability to retain soil N. Here, the physicochemical properties of latosol after adding OSP and calcined OSP (COSP) and the dynamic leaching patterns of ammonium N (NH₄⁺-N), nitrate N (NO₃⁻-N), and Ca in seepage, were examined through indoor culture and intermittent soil column simulation experiments. Various types of N fertilizer were optimized through the application of 200 mg/kg of N, urea (N 200 mg/kg) was the control treatment (CK), and OSP and COSPs prepared at four calcination temperatures—500, 600, 700, and 800 °C—were added to the latosol for cultivation and leaching experiments. Under various N application conditions, the total leached N from the soil followed ammonium nitrate > ammonium chloride > urea. The OSP and COSPs had a urea adsorption rate of 81.09–91.29%, and the maximum reduction in cumulative soil inorganic N leached was 18.17%. The ability of COSPs to inhibit and control N leaching improved with increasing calcination temperature. Applying OSP and COSPs increased soil pH, soil organic matter, total N, NO₃⁻-N, exchangeable Ca content, and cation exchange capacity. Although all soil enzyme activities related to N transformation decreased, the soil NH₄⁺-N content remained unchanged. The strong adsorption capacities for NH₄⁺-N by OSP and COSPs reduced the inorganic N leaching, mitigating the risk of groundwater contamination.

Intensive agriculture has intensified soil acidification in southern China, threatening crop productivity and ecosystem sustainability. Biochar can neutralize acidity, improve pH buffering, and enhance nutrient retention and microbial habitat in acidic soils. Accordingly, we produced biochars from pruned eucalyptus (ABC), camphora (ZBC), and guava (FBC) branches via pyrolysis at 500 °C. The three biochars were characterized by elemental analysis, Fourier Transform Infrared Spectroscopy (FTIR), and SEM (Scanning Electron Microscopy), and their effects on soil properties, enzyme activities, and bacterial communities were evaluated through a 56-day incubation experiment in an acidified, continuously cropped soil. Physicochemical characterization revealed that ZBC and FBC possessed more oxygen-containing functional groups and greater potential for pH buffering and nutrient release, whereas ABC exhibited higher aromaticity and structural stability. Biochar significantly increased soil pH by 0.62–1.42 units and improved nutrient availability and carbon pools (p < 0.05). Additionally, 4% ZBC increased urease and sucrase activities by 21.54% and 79.34%, respectively, while 2% FBC increased cellulase activity by 25.99%. High-throughput sequencing identified Acidobacteria and Proteobacteria as the dominant phyla; ZBC and FBC at 0.5% and 2% significantly increased Shannon and Chao1 indices. Redundancy analysis indicated that available potassium, pH, soil organic carbon, urease, sucrase, and cellulase were the primary drivers of bacterial community variation and positively associated with carbon-cycling phyla. These findings demonstrate that feedstock-specific biochar properties critically regulate soil biogeochemical processes, offering a sustainable strategy to remediate acidified soils and valorize agroforestry residues.