Global meta-analyses showed that biochar application can reduce N₂O emission. However, no relevant review study is available for East Asian countries which are responsible for 70% of gaseous N losses from croplands globally. This review analyzed data of the biochar-induced N₂O mitigation affected by experimental conditions, including experimental types, biochar types and application rates, soil properties, and chemical forms and application rates of N fertilizer for East Asian countries. The magnitude of biochar-induced N₂O mitigation was evaluated by calculating N₂O reduction index (Rᵢₙdₑₓ, percentage reduction of N₂O by biochar relative to control). The Rᵢₙdₑₓ was further standardized against biochar application rate by calculating Rᵢₙdₑₓ per unit of biochar application rate (ton ha⁻¹) (Unit Rᵢₙdₑₓ). The Rᵢₙdₑₓ averaged across different experimental types (n = 196) was −21.1 ± 2.4%. Incubation and pot experiments showed greater Rᵢₙdₑₓ than column and field experiments due to higher biochar application rate and shorter experiment duration. Feedstock type and pyrolysis temperature also affected Rᵢₙdₑₓ; either bamboo feedstock or pyrolysis at > 400 °C resulted in a greater Rᵢₙdₑₓ. The magnitude of Rᵢₙdₑₓ also increased with increasing biochar rate. Soil properties did not affect Rᵢₙdₑₓ when evaluated across all experimental types, but there was an indication that biochar decreased N₂O emission more at a lower soil moisture level in field experiments. The magnitude of Rᵢₙdₑₓ increased with increasing N fertilizer rate up to 500–600 kg N ha⁻¹, but it decreased thereafter. The Unit Rᵢₙdₑₓ averaged across experimental types was −1.2 ± 0.9%, and it was rarely affected by experimental type and conditions but diminished with increasing biochar rate. Our results highlight that since N₂O mitigation by biochar is affected by biochar application rate, Rᵢₙdₑₓ needs to be carefully evaluated by standardizing against biochar application rate to suggest the best conditions for biochar usage in East Asia.

Toxicokinetic studies appertain to the fundamental research of soil bioavailability. However, the research outcomes of aspects influencing uptake and elimination of hydrophobic organic compounds have not been summarized so far. In our review, a recapitulation of available toxicokinetic data (i.e. experimental conditions, if the steady state was reached, uptake and elimination rate constants, and bioaccumulation factors) is presented in well-arranged tables. Further, toxicokinetic models are overviewed in the schematic form. In the review, the required information could be quickly found and/or the experimental gaps easily identified. Generally a little is known about the effects of soil properties other than soil organic matter. Limited or no data are available about soil treatment, food supply during laboratory exposure, and metabolization in oligochaeta. The impact of these factors might be important especially for arable soils with typically low organic matter content but high consequences on humans. Besides these circumstances, other uncertainties between published studies have been found. Firstly, the scientific results are provided in heterogenous units: bioaccumulation factors as well as the rate constants are reported in dry or wet weight of soil and earthworms. The steady state is another critical factor because the time to reach the equilibrium is influenced not only by soil and compound characteristics but for example also by aging. Nevertheless, toxicokinetic studies bring irreplaceable information about the real situation in soil and our review help to define missing knowledge and estimate the scientific priorities.

The retention of organic pollutant (OP) in soils is commonly attributed to interactions with soil organic matter (SOM), perhaps overlooking substantial involvement of soil minerals. In this study, 36 soil samples with far-ranging ratios of clay to organic carbon were used to examine contribution of minerals on soil sorption of pentachlorophenol (PCP) and phenanthrene (PHE). Sorption isotherms (n = 216) were fit individually using three typical sorption models, with the most fitted Kd values screened out for quantification of the net mineral contribution to total sorption via development of mathematical model accounting for associations between minerals and SOM. Two mineral-relevant parameters [adsorption distribution coefficient (Kmin) and mineral contribution index (MCI)] were simultaneously defined. Previously reported soil sorption data of PCP, PHE and butachlor (13, 12 and 46, respectively) were also extracted and included to improve the credibility of mathematic model. The average MCI values were calculated as 0.421, 0.405 and 0.512 in PCP, PHE and butachlor treated soils, respectively, very close to or even over than the minerals dominant critical value (0.5). This suggested the significant, or even predominant, contribution of minerals – as compared to SOM. Significant dependence of MCI with four conventional parameters of soil property further offered the possibility to roughly evaluate mineral contributions based on estimated threshold values of soil property parameters (especially TOC). This study provides an accessible approach for predicting the contribution of minerals in soil OP retention, especially highlighting their predominant roles vs. SOM in regulating OP removal in most of subsurface soil or contaminated brownfields where organic carbon content of soil was very low, that was not like what previously believed.

Fencing for grazing exclusion and grazing are common land-use methods in the semi-arid areas of the Loess Plateau in China, which have been widely found to change grassland soil organic carbon (SOC); however empirical studies that evaluated driving factors of soil carbon (C) stocks under the different land use are still weak. In this study, we investigated soil physicochemical and soil respiration (Rs) in the fenced and grazed grassland, to study the soil C stock variations and the main driving mechanism of soil C accumulation. The results showed that bulk density (BD), soil moisture content (SMC), and soil porosity (SP) had no significant difference between fenced and grazed grassland. Fencing increased the SOC, total nitrogen (TN), and C/N ratio, and significantly increased the aboveground biomass (AGB), belowground biomass (BGB), and the amount of soil large macro-aggregates in the topsoil layer (0-10 cm), and the soil stability was improved. Meanwhile, grazing increased soil temperature (ST) and Rs. The soil C stock in the topsoil layer (0-10 cm) of fenced grassland was significantly higher than that of grazed grassland. The soil C/N ratio, BD, and MWD explained large proportions of the variations in soil C stocks. Our results indicate that fencing can improve the stability of soil structure, and reduce Rs, then increase soil C stocks, which is an effective way to improve soil C stocks of grassland ecological in semi-arid areas of northwest China.

The burning of crop residues (traditionally called ‘Parali’) has recently become a hot topic in India because it is presumed to be one of the reasons for abnormally high levels of air pollution in New Delhi, the capital city of India, after harvesting of Kharif crops during winter months. During the process of finding out a feasible solution for quick disposal of agricultural waste in a safer way, a novel method has been developed by the authors in which crop residue is converted into a useful product, biochar, which can be applied back to the fields for amendment of soil. It not only reduces the introduction of harmful gases into the environment but also improves the physical and chemical properties of the soil. This method is very simple and can be adopted by an individual farmer without much investment and technical skills. Many studies have been conducted on the factors involved in the production and use of biochar as a soil amendment; but in India, not much work has been carried out yet, as it is relatively a new concept in terms of using biomass for biochar production and application. Although biochar is not a new product, it has drawn the attention of researchers and other stakeholders in the near past because of its usefulness in improving the physical and chemical properties of the soil and at the same time reducing greenhouse gas emission, which is one of the biggest challenges for the modern world to protect the environment.