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.

The partial substitution of chemical nitrogen fertilizers with organic fertilizer and slow-release fertilizer could improve pineapple yield and nitrogen use efficiency (NUE) and decrease greenhouse gas (GHG) emissions. However, the effect of organic and slow-release fertilizer substitution strategies on the carbon footprint (CF), nitrogen footprint (NF) and net ecosystem economic benefits (NEEB) from pineapple fields in the tropics remains largely unclear. Therefore, we conducted a long-term pineapple field trial (2017–2021) for the first time with five fertilization strategies (CK: no fertilizer; F: conventional fertilization(nitrogen (N) 817 kg ha⁻¹, phosphorus pentoxide (P₂O₅) 336 kg ha⁻¹, potassium oxide (K₂O) 945 kg ha⁻¹); RF: reduction of 41.7% N, 72.0% P₂O₅ and 33.1% K₂O on an F basis; RFO: replacement of 20% N input with organic fertilizer on an RF basis; RFOS: replacement of 15% N input with slow-release fertilizer on an RFO basis) to identify the pineapple fruit yield, NUE, CF, NF and NEEB in the tropics. The results showed that in comparison to the F treatment, the RF, RFO and RFOS treatments improved pineapple yield (7.6%, 12.4% and 26.3%, respectively), NUE (66.4%, 75.5% and 87.7%, respectively, p < 0.05) and partial factor productivity of nitrogen (PFP_N) fertilizer (84.8%, 92.8% and 116.7%, respectively, p < 0.05). Additionally, of all the treatments, the RFOS treatment had the highest yield (87.8 t ha⁻¹). N leaching (50.1–69.1%) and ammonia volatilization (21.6–26.2%) were the two primary routes for reactive nitrogen (Nr) loss. The field soils (36.8–45.7%) and N fertilizer production and transportation (21.2–29.5%) dominated the GHG emissions. Compared to the F treatment, the RF, RFO and RFOS treatments showed decreases in Nr losses, NF, GHG emissions and CF of 36.6–41.1%, 43.3–51.9%, 19.0–29.1% and 24.5–41.7%, respectively. Of all the treatments, the RFOS treatment had the lowest CF (191.8 kg CO₂eq ha⁻¹ season) and NF (1.9 kg N t⁻¹ season). Additionally, the NEEB of the RF, RFO and RFOS treatments improved by 13.0–39.9% over that of the F treatment. The RFOS treatment (54,880 USD ha⁻¹) resulted in the highest NEEB of all treatments. Therefore, the substitution of conventional inorganic fertilizers with organic and slow-release fertilizers is an effective method for achieving sustainable pineapple production. However, a process for further reducing GHG emissions from farmland soils and Nr losses from organic fertilizer addition still need attention in terms of pineapple production.

Household biogas, a kind of clean energy, has drawn attention from scholars. To deal with the adjustment of the Chinese agricultural industry and to promote the better development of Chinese household biogas, this article summarizes and analyses the spatial distribution characteristics and influencing factors of the type and number of biogas digesters, biogas production, the biogas fermentation material, and the methods of fermentation residue utilization and ecological agriculture with household biogas in the Chinese household biogas industry by compiling dataset covering 31 provinces. The results show that hydraulic biogas digesters are mainly distributed in northwestern and northeastern China; in addition, continuously stirred biogas digesters and bottom-discharging biogas digesters are mainly distributed in southern and northern China, respectively. Biogas slurry and residue are widely used as fertilizers and feed; furthermore, biogas slurry is also used for seed soaking in northeastern and southern China. The “Three-in-one” and “Four-in-one” biogas ecological models are mostly used in southern and northern China, respectively, and both of them are mainly affected by temperature. Our findings are critical for China's government to formulate appropriate policies and adopt effective measures for promoting the development of cleaner energy and layout of the agricultural industry.