Increasing pressure on farming systems due to rapid urbanization and population growth has severely affected soil health and fertility. The need to meet the growing food demands has also led to unsustainable farming practices with the intensive application of chemical fertilizers and pesticides, resulting in significant greenhouse gas emissions. Biochar, a multifunctional carbon material, is being actively explored globally for simultaneously addressing the concerns related to improving soil fertility and mitigating climate change. Reviews on biochar, however, mainly confined to lab-scale studies analyze biochar production and its characteristics, its effects on soil fertility, and carbon sequestration. The present review addresses this gap by focusing on biochar field trials to enhance the current understanding of its actual impact on the field, w.r.t. agriculture and climate change. The review presents an overview of the effects of biochar application as observed in field studies on soil health (soil’s physical, chemical, and biological properties), crop productivity, and its potential role in carbon sequestration. General trends from this review indicate that biochar application provides higher benefits in soil properties and crop yield in degraded tropical soils vis-a-vis the temperate regions. The results also reveal diverse observations in soil health properties and crop yields with biochar amendment as different studies consider different crops, biochar feedstocks, and local climatic and soil conditions. Furthermore, it has been observed that the effects of biochar application in lab-scale studies with controlled environments are not always distinctly witnessed in corresponding field-based studies and the effects are not always synchronous across different regions. Hence, there is a need for more data, especially from well-designed long-term field trials, to converge and validate the results on the effectiveness of biochar on diverse soil types and agro-climatic zones to improve crop productivity and mitigate climate change.

Biochar is black carbon produced from pyrolysis of biomass and may be added to soil to sequester carbon and improve soil water retention. To date models to predict changes in soil water retention with biochar amendment are still missing and therefore direct measurements are required for every biochar/soil combination, which can be time-consuming. Here, a predictive model for biochar's effect on soil water retention was developed and tested that includes water retained in biochar intrapores and biochar's impact on interpores between particles. The independently measured parameters needed for the model are the particle size distributions (PSDs) and particle densities for biochar and soil, water retention data for biochar-free soil, biochar intrapore volume distribution from mercury porosimetry, amount of biochar added, bulk density of the biochar/soil mixture, and dew point potentiometer measurements of biochar. The model was tested using poultry litter and wood biochars amended to two soils (sand and sandy loam) at 2 and 7% mass fraction. The model predicted changes in the soil water characteristic well for the biochar amendment, with RMSE decreasing by ~50% when the full model was used. Model predictions of the change in available water capacity with biochar amendment for eight biochar/soil combinations had an average absolute error of 0.017 ± 0.006 and an average relative error of 10¯0 ± 40%. The model correctly predicted the increase in available water content when sandy loam was amended with wood biochar, and the decrease if amended with poultry litter biochar. The model provides an improved understanding of the mechanisms by which biochar alters water retention, and a means to estimate the initial change in available water capacity for a particular biochar/soil combination if necessary biochar and soil properties are measured.