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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) (CO 2 , CH 4 , and N 2 O) 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 CO 2 and CH 4 emissions increased with increasing VM content of biochars; however, CO 2 emission sharply decreased at 83 mg VM g -1 soil. In the fine-textured, high-buffer Oxisol, there were significant positive effects of VM content on cumulative CO 2 emission without suppression effects. Regarding cumulative N 2 O emission, there were significant positive effects in the Mn-rich Oxisol. Ashinduced 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.

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A greenhouse experiment was conducted to estimate the influence of various application rates of eucalyptus-derived biochar combined with cricket frass on the soil properties and soil N transformation, and, in turn, affecting both shoot biomass yield and nitrate (NO3-) contents of Chinese kale (Brassica oleracea). Two consecutive kale crops were grown to investigate the temporal effect of the combined amendments of cricket frass and biochar. Six rates of biochar, 0%, 0.125%, 0.25%, 0.5%, 1%, and 2% w/w in combination with 0.55% w/w of cricket frass, were applied only once at the start of the experiment in sandy loam soil. Shoot biomass significantly increased under treatments of 0.125% to 0.5% w/w in the first kale crop and 0.125% to 0.25% w/w in the second crop compared to the cricket frass alone. However, the higher rates of 0.25% and 0.5% w/w within the first and second crops decreased shoot biomass relative to their lower rates in each crop. Tissue NO3- concentrations of the first kale crop significantly decreased under all biochar rates, whereas the opposite effect was observed in the second crop. These contrasting effects of biochar on tissue NO3- concentrations were attributed to nitrification inhibition in the first crop and nitrification stimulation in the second crop. The 0.125% w/w rate of eucalyptus-derived biochar was, therefore, recommended to be combined with cricket frass to improve yield and reduce tissue NO3- content in the production of Chinese kale.

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.

Soil conservation practices, such as reduced and no tillage, have been found to enhance soil nitrogen (N) sequestration through decreasing the rate of N mineralisation of added organic materials. Nitrogen mineralisation is not only affected by tillage, but also by the quality (chemical composition) of the organic residues. This study evaluated the interaction of residue quality and soil disturbance on N mineralisation in a sandy soil. A 112-day incubation experiment was conducted with two levels of soil disturbance (undisturbed and disturbed conditions) and five plant residue amendments of contrasting quality. The contrasting quality (N, lignin (L), and polyphenols (Pp)) (in g kg–1) amendments follow: (i) unamended; (ii) Sesbania grandiflora (N 44, L 173, Pp 9.2); (iii) Indigofera hirsuta (N 41, L 177, Pp 30); (iv) Dipterocarpus tuberculatus (N 8.2, L 203, Pp 71); and (v) Eucalyptus camaldulensis (N 9.7, L 126, Pp 110). Residues (ii) and (iii) were fresh legume leaves, while (iv) and (v) were non-legume leaf litter. Disturbance only significantly increased N mineralisation rates in the legume-residue treated soils (increases of 18.8% for S. grandiflora and 27.1% for I. hirsuta) during the early stage of decomposition (first 14 days). In the legume treatment, disturbance significantly increased the ammonification, but decreased nitrification in soil relative to undisturbed soils. The difference in patterns of ammonification and nitrification was more pronounced in the early than in the later period of decomposition. This indicated an inhibitory effect of soil disturbance on nitrification, which was particularly pronounced in the legume-treated soils. The Pp content of residues was the major quality parameter regulating the soil ammonium-N and nitrate-N concentrations. Minimum soil disturbance should be adopted under legume soil organic amendment so that both ammonification and nitrification components of N mineralisation process can occur normally, and nitrate-loving crops can take up N in the form of nitrate-N which will enhance their yields. Moreover, undisturbed conditions under legume organic amendments reduced N mineralisation, resulting in enhancing soil N sequestration.