Organic carbon and nutrient release from a range of laboratory-produced biochars and biochar–soil mixtures

Biochar has shown promise as a soil amendment that increases carbon sequestration and fertility, but its effects on dissolved organic carbon (DOC), nitrogen (N) and phosphorus (P) cycling and loss is not well understood. Here, nutrient release from a variety of new and aged biochars, pure and mixed with soils, is examined using batch extraction and column leaching. In successive batch extractions of biochar, cumulative losses were about 0.1–2, 0.5–8 and 5–100% of the total C, N and P initially present, respectively, with greater releases from biochars made at lower temperature and from grass. Ammonium was usually the most abundant N form in leachates but nitrate was also abundant in some biochars, while organic N and P represented as much as 61% and 93% of the total N and P lost, respectively. Release of DOC, N and P into water was correlated with biochar volatile matter content and acid functional group density. However, P release via Mehlich-1 extraction was more strongly related to ash content, suggesting a mineral-associated P fraction. Columns with soil/biochar mixtures showed evidence of both soil nutrient sorption by biochar and biochar nutrient sorption by soil, depending upon biochar and soil type. This study demonstrates that biochars contain a range of nutrient forms with different release rates, explaining biochar's variable effect on soil fertility with soil and crop type and over time.

► Biochar releases DOC, N and P into aqueous solution, much of it in organic form.
► Biochars made at lower temperature and from grass release relatively more nutrients.
► Leached DOC & N correlate with volatile matter and P release with ash content.
► Biochar sorbs soil nutrients and soil sorbs nutrients released by biochar.