Biochar is widely used as a soil amendment. Enzyme activity is an important factor that reflects soil metabolic activity, and is involved in biochemical processes such as organic matter decomposition and nutrient cycling in soils. However, the effects of biochar prepared for different straw materials on soil enzyme activity and soil nutrients are rarely studied. Through pot experiments, the effects of different straw (wheat, rice, maize) biochars (obtained by pyrolysis at 500 °C) on soil organic carbon, nitrogen, available phosphorus, and enzyme activity were studied in paddy soil. The results showed that the addition of biochar increased the soil organic carbon content, which gradually decreased with the extension of the rice growth period. The soil ammonium nitrogen content gradually decreased as the rice growth period continued; however, the soil nitrate nitrogen content first decreased and then increased over the rice growth period. Soil invertase, phosphatase, and urease activity first increased and then decreased, and the enzyme activity was the highest at the heading stage of rice. At this time, there were also significant correlations between enzyme activity and carbon, nitrogen, and phosphorus levels, except in the case of soil urease activity. The geometric mean of the investigated enzyme activities was the highest after amendment with rice straw biochar. These results indicate that the response of enzyme activity to biochar depends on the biochar feedstock and the rice growth stage.

As a carbon-rich material produced by pyrolysis of biomass, biochar features low cost, large specific surface area, and widely available feedstocks based on the functional diversity and environmental-friendly properties, it has received increasing attention in the fields of pollutant removal due to three win–win effects of water remediation, carbon sequestration and reutilization of wastes. To design excellent biochar-based catalysts for environmental applications, one must understand recent advances in the catalysts for contaminant removal. This review focuses on the current application of biochar-based catalysts in redox systems, Fenton-like systems, sonocatalytic systems and photocatalytic systems. Besides in-depth discussion in effects of preparation conditions on physicochemical characteristics of biochars, the review supplements new preparation technologies of biochar and biochar-based catalysts. Most importantly, the advantages/shortcomings, catalysis mechanisms, as well as the pollutant removal ability of different types of biochar-based catalysts are discussed. The environmental risks of the catalyst applications are also elaborated on. Future research on biochar-based catalyst production and its environmental applications is discussed. The review provides a good overview of the current advances of biochar-based catalysts in pollutant control and the futureresearch directions. Graphical Abstract

Soil moisture is a key factor affecting crop growth and crop yield. Information on soil moisture is critical for crop growth monitoring and yield estimation. This study evaluated the OPtical TRApezoid Model (OPTRAM) on soil moisture estimates in the Sanjiang Plain, China, using Moderate Resolution Imaging Spectroradiometer (MODIS) data and in situ soil moisture data acquired from May to September in 2016 and 2017. This model was empirically calculated by parameterizing the relationship between the normalized difference vegetation index (NDVI) and shortwave infrared transformed reflectance (STR). In addition, the precipitation data were collected and used for verification of the results. According to the scatterplots of STR and NDVI, OPTRAM can estimate soil moisture from July to September, while it's inapplicable in May and June. Evaluation results indicated that OPTRAM‐based soil moisture estimates provide overall RMSE from 0.05 to 0.13 cm³/cm³, bias from −0.11 to 0.06 cm³/cm³, and R² from 0.10 to 0.50, respectively, for all investigated sites. The performance of OPTRAM compared with the precipitation data showed good agreement. In conclusion, OPTRAM could be used to estimate soil moisture with reasonable accuracy in most areas from July to September in Sanjiang Plain.