Analysis of effect mechanism and risk of biochar on soil fertility and environmental quality

Abstract: Biochar is a solid material that is produced from biomass pyrolysis in oxygen-limited atmosphere at relatively low temperatures. Therefore, the conversion of biosolids (e.g. agricultural waste, invasive species, municipal sewage sludge, etc) into biochar could provide a useful way to achieve resource reuse and environmental management. In China, decreased land resources and soil fertility is of increasing concern presently, which may threat national food security. Therefore, a range of biochars have been tested for their potential in improving farming soil quality considering the high fertility sustained by biochar in Amazonian Dark Earths locally known as Terra Preta de Indio. Some studies have proved that biochar can play a positive role in improving the fertility of agricultural soils and thus increasing crops yield. Moreover, biochar has also been shown to be effective in control and remediation of a range of soil contamination and hence improve soil environmental quality, leading to increased food output and quality. In the present review, the most recent advances in the effect of biochar on agricultural soil fertility and soil environment quality with the underlying mechniasms are reviewed and discussed here in detail. Firstly, most biochars, particularly those produced from wetland plant residues tend to improve water retention capacity of agricultural soil, especially for those with sandy texture. This phenomenon could be caused by the porous structure of biochar, which can increase the water-holding porosity of soils. Secondly, substantial evidence has shown that the bioavailability of key mineral nutrients increase remarkably in response to biochar application, which could be attributed to the sufficient mineral ash in biochar and relatively high caiton exchange capacity (CEC) originated from the rich surface functional groups of biochar. Thirdly, the alkalinity of biochar, which is most likely resulted from the high mineral content, has exhibited potential in mitigation of soil acidity. As a result, the leaching of essential minerals and associated water eutrophication could be controlled to different degrees. However, the long-lasting buffering performance of biochar in the areas of acid rain is still unknown, which needs long-term monitoring with locating tests. Fourthly, with high CEC and mineral content, biochar could be used as fixing agent to sequester heavy metals in agriculture soils through complexation and precipitation, resulting in decreased transfer and accumulation of heavy metals in food chain. On the other hand, a series of negative impacts of biochar on farmland are also highlighted in this review. First of all, biochar may reduce the aggregation of soil particles, and bring ammonia gas gathered in soils which is detrimental to the soil organisms. Simultaneously, the application of biochar into soil could induce the enrichment of polycyclic aromatic hydrocarbon and heavy metals in uncontaminated farmland as well as decreased nitrogen uptake by crops, All of these potential risks can not be ignored. Full realization and systematic investigation on these hazardous effects of biochar is essential to ensure the safe and efficient application of biochar for the improvement of soil fertility and environmental quality. At last, a range of research gaps and future challenges have been characterized and discussed in detail. To achieve better understanding of the effect of biochar on sustainable agriculture with associated mechanisms, long-term experiments in field scales are needed to investigate the persistent output and biogeochemical behaviors of different kinds of biochar. Besides, more field experiments should be carried out to explore the applicability of biochars in differing regions with specific soil types, which could provide more realistic information to improve the local soils. In parallel, ecological and toxicological effects of biochar should be addressed to identify the potential risks. Finally, more efforts should be paid to establish the regulation framework and reference criterion to ensure biochar quality and evaluate its effect on soil fertility and environmental quality.