To date, most studies have shown that biochar has great potential in carbon sequestration and reduction, as well as soil quality improvement. However, there is limited knowledge of its effect on soil organic carbon (SOC) fractions in tropical farmland. This study aimed to determine the impact of different types and rates of biochar applied in tropical farmlands on so SOC and its active fractions. The SOC, microbial biomass carbon (MBC), dissolved organic carbon (DOC), and soil mineralizable carbon (SMC) in the 0–30 cm soil layers under rice hull (R) and peanut shell (P) biochar treatments were measured. The results showed that the application of R and P biochar increased the contents, stocks, and cumulative stocks of SOC, MBC, and DOC in the 0–10 cm, 10–20 cm, and 20–30 cm soil layers. The contents, stocks, and cumulative stocks increased with increasing biochar application rates. Compared with CK, the ranges of the increased SOC, MBC, and DOC cumulative stocks were 10.76–46.36%, 30.04–195.65%, and 0.02–17.03%, respectively. However, the R60 and P60 had the lowest cumulative stocks of SMC, decreasing by 14.69% and 8.05%, respectively. The biochar treatment of more than 20 t ha⁻¹ reduced the ratio of SMC:SOC and active fractions:SOC. Therefore, it can be inferred that the application of biochar improved the levels of SOC, MBC, and DOC, and the application of more than 20 t ha⁻¹ biochar could decrease soil carbon mineralization, thus improving the stability of SOC in tropical farmlands.

Biochar is widely recognized as a promising amendment for enhancing soil carbon sequestration and microbial functioning, yet how biochar properties and application levels jointly influence potential microbial nutrient limitation and estimated microbial carbon use efficiency (CUE) remains unclear, especially in tropical farmland soils. Here, a 2‐year field experiment was conducted to examine the effects of two types (RB, rice hull biochar and PB, peanut shell biochar) and four application levels (10, 20, 40, and 60 t/ha), with 0 t/ha serving as the control, on labile substrates, microbial biomass, extracellular enzyme activities (EEAs), stoichiometric imbalance, vector‐based indicators of potential nutrient limitation, and estimated microbial CUE. Increasing biochar application levels consistently enhanced substrate availability, with dissolved organic nitrogen (DON) showing a stronger response than dissolved organic carbon (DOC), particularly under PB, thereby decreasing the DOC:DON ratio. Biochar application also increased microbial biomass C, N, and P, and synchronously stimulated C‐, N‐, and P‐acquiring enzyme activities by 15.61%–159.88%, 9.73%–201.21%, and 9.98%–162.96%, respectively; thus, ecoenzymatic stoichiometry remained largely unchanged. Vector angles remained below 45° across all treatments, indicating persistent potential relative N rather than P limitation, but their responses to biochar showed no clear monotonic pattern. Biochar type had a stronger influence than application level on stoichiometric imbalance and estimated microbial CUE. PB increased DON availability and was associated with a lower DOC:DON ratio, reduced C:N imbalance, and higher estimated microbial CUE. Exploratory path analysis associated biochar type mainly with C:N imbalance and estimated microbial CUE, while application level was associated with labile substrates, EEA, and microbial biomass. Overall, our results provide new insights into how biochar quality and application level jointly shape microbial resource stoichiometry, microbial functioning, and soil C cycling in tropical farmlands.

Pineapple is widely cultivated in tropical regions, but intensive management and improper residue disposal can accelerate soil degradation and yield decline. To evaluate under-ridge residue burial (URRB) as a soil improvement practice, parallel field experiments were conducted from 2023 to 2024 at two sites in tropical southern China. A two-factor randomized complete block design was used, with two burial rates (half- and full-rate, i.e., 10 and 20 t ha⁻¹) and three burial depths (10, 20, and 30 cm), plus a residue removal control (CK). Compared with CK, URRB generally increased soil pH and the stocks of soil organic carbon and available nitrogen (N), phosphorus (P), and potassium (K), accompanied by higher leaf area index (LAI), total root surface area (TRSA), photosynthetic pigment contents, fruit vitamin C content, and plant N, P, and K accumulation. Increasing burial rate significantly enhanced plant P and K accumulation and fruit yield, while burial at 30 cm produced greater improvements in soil pH, soil organic carbon (SOC), and available N, P, and K stocks than burial at 10 cm and 20 cm. Compared with other treatments, full-rate residue burial at 30 cm depth under the ridge (FD30) most effectively promoted canopy growth, root development, fruit yield, and chlorophyll a content. On average across the two sites, FD30 increased LAI and TRSA by 20.9% and 34.0%, respectively, compared with CK, with significant increases of 20.1% in fruit yield and 25.3% in fruit vitamin C content. Partial least squares path modeling identified positive indirect associations of soil pH and nutrient availability with yield. Although pineapple yield did not differ significantly between FD30 and FD10, SOC, available N, P, and K stocks were significantly greater under FD30 than under FD10. Thus, FD30 is a more promising strategy for improving both soil fertility and pineapple production under the present experimental conditions. This study also provides new insights into crop residue management in tropical cultivation systems.