Soil capacity as a major carbon (C) sink is influenced by land use. Estimates of soil organic carbon (SOC) sequestration have mostly focused on topsoils [0–30 cm official Intergovernmental Panel on Climate Change (IPCC) soil depth]. We investigated SOC stocks and their quality as influenced by land-use changes. Soil samples were collected from five soil depths down to 100 cm of three adjacent fields each representing a different land use—forest, cassava, and rice paddy—in Northeast Thailand. Sequestration of SOC in topsoils under all land uses was higher, as indicated by SOC stocks (59.0–82.0 Mg ha⁻¹) than subsoils (30–100 cm) (27.0–33.0 Mg ha⁻¹). The soil profile (0–100 cm) of the forest had higher stocks of SOC and humic acid (115.0 and 6.8 Mg ha⁻¹, respectively) than those of cultivated land uses [paddy (100.0 and 4.8 Mg ha⁻¹, respectively) and cassava (87.0 and 2.3 Mg ha⁻¹, respectively)], which accounted for an average 30% increase in SOC sequestration over those with only topsoil. Topsoils of the forest had higher humic acid content but narrower E4:E6 ratio [the ratio of absorbances at 465 nm (E4) and at 665 nm (E6)] of humic acids (2.8), indicating a higher degree of humification and stabilization than the cultivated soils (3.2–3.6). Subsoil C was higher quality, as indicated by the lower E4:E6 ratio of humic acids than topsoils in all land uses.

The ability of biochar applications to alter greenhouse gases (GHGs) (CO₂, CH₄, and N₂O) has been attracting research interest. However, inconsistent published results necessitate further exploration of potential influencing factors, including biochar properties, biochar rates, soil textures and mineralogy, and their interactions. Two short‐term laboratory incubations were conducted to evaluate the effects of different biochars: a biochar with low ash (2.4%) and high–volatile matter (VM) (35.8%) contents produced under low‐temperature (350°C) traditional kiln and a biochar with high ash (3.9%) and low‐VM (14.7%) contents produced with a high‐temperature (800°C) Flash Carbonization reactor and different biochar rates (0, 2, and 4% w/w) on the GHG emissions in a loamy‐sand Ultisol and a silty‐clay‐loam Oxisol. In the coarse‐textured, low‐buffer Ultisol, cumulative CO₂ and CH₄ emissions increased with increasing VM content of biochars; however, CO₂ emission sharply decreased at 83 μg VM g⁻¹ soil. In the fine‐textured, high‐buffer Oxisol, there were significant positive effects of VM content on cumulative CO₂ emission without suppression effects. Regarding cumulative N₂O emission, there were significant positive effects in the Mn‐rich Oxisol. Ash‐induced increases in soil pH had negative effects on all studied GHG emissions. Possible mechanisms include the roles biochar VM played as microbial substrates, a source of toxic compounds and complexing agents reducing the toxicity of soil aluminum and manganese, and the role of biochar ash in increasing soil pH affecting GHG emissions in these two contrasting soils.Core Ideas: Biochar's volatile matter and ash constituents affect soil emission of CO₂, CH₄, and N₂O. Biochar volatile matter raised CO₂ emission up to a threshold in coarse‐textured, low‐buffer soil. Biochar volatile matter raised CO₂ emission regardless of its content in fine‐textured, high‐buffer soil. Biochar ash decreased emissions of greenhouse gases regardless of soil texture. Al in Ultisol and Mn in Oxisol interacted with volatile matter and ash in influencing GHG emissions.

While lettuce offers essential human nutrients, it also contains anti-nutrients, particularly nitrate (NO₃⁻). The use of neem leaf extract as a natural nitrification inhibitor has proven itself promising to remediate lettuce tissue NO₃⁻ content. This study evaluated the effects of neem leaf extract on soil properties, soil nitrification, lettuce growth, yield, and NO₃⁻ content. Five nitrification inhibitor treatments were evaluated: (i) no inhibitor (control), (ii) nitrapyrin, and three rates of neem leaf extract based on the dry weight of the raw material: (iii) 1 g kg⁻¹ soil (Neem1), (iv) 2 g kg⁻¹ soil (Neem2), and (v) 4 g kg⁻¹ soil (Neem4). Neem leaf extract generally increased soil concentrations: P (47.6–55.8 mg kg⁻¹), K (45.8–62.7 mg kg⁻¹), Ca (129–164 mg kg⁻¹), and Mg (29.0–35.7 mg kg⁻¹) compared with the control (50.6 mg P kg⁻¹, 35.3 mg K kg⁻¹, 123 mg Ca kg⁻¹, and 24.8 mg Mg kg⁻¹). Neem leaf extracts significantly increased soil NH₄⁺–N concentrations (13.9–30.2 mg kg⁻¹) and nitrification inhibition (12.5–70.5%), but significantly decreased soil NO₃⁻–N concentrations (6.4–13.2 mg kg⁻¹) and net nitrification rates (0.08–0.23 mg N kg⁻¹ day⁻¹) relative to the control (6.6 mg NH₄⁺–N kg⁻¹, 14.7 mg NO₃⁻–N kg⁻¹, 0.26 mg N kg⁻¹ day⁻¹, and 0% nitrification inhibition). The neem leaf extracts significantly decreased shoot fresh weight (13.5–43.1 g plant⁻¹), shoot dry weight (0.84–3.91 g plant⁻¹), and root dry weight (0.14–0.27 g plant⁻¹) compared with the control (52.3 g shoot fresh weight plant⁻¹, 5.36 g shoot dry weight plant⁻¹, and 0.35 g root dry weight plant⁻¹). The significant decreases in the lettuce biomass in the neem extract treatments paralleled the significant decreases in the shoot’s tissue NO₃⁻–N contents and significant increases in tissue NH₄⁺–N content and soil Al concentrations.

A factor that causes inconsistencies in rice yield receiving biochar reported in the literature has been identified as the length of time after biochar incorporation into the soil prior to planting. There is limited information on the effect of the varying lengths of time on soil properties and rice growth. This study aimed to determine the effects of the length of time of incorporation of rice husk biochar (RHB) into an acidic paddy soil before rice transplanting on soil properties and rice yield. A greenhouse experiment was conducted using a highly weathered paddy soil subjected to incorporation periods of RHB at various lengths, including 0, 15, 30, and 60 days before rice transplanting (DBT). The RHB incorporation was under a soil moisture content of 70% of the soil water holding capacity. At harvest time (98 days after incorporation), increases in the length of RHB incorporation led to significantly higher Mg, Mn, and Si concentrations, but lower Ca and Fe concentrations in rice whole shoots. Increasing the length of RHB incorporation to 15, 30, and 60 DBT significantly decreased the total rice grain yield to 61.4 g hill⁻¹, 62.5 g hill⁻¹, and 54.4 g hill⁻¹, respectively, compared to 76.0 g hill⁻¹ found at 0 DBT. The depression of rice grain yield with increasing RHB incorporation periods was due to the antagonistic effects of Mg on Ca and Si on Fe. Immediate rice transplanting without a prior RHB incorporation period is recommended for its use as a soil amendment in acidic paddy soils.