Biochars are charcoals used as soil amendments, and they have many beneficial effects on soil health. However, one negative effect is biochars often reduce concentrations of soil nitrogen that are available to plants. This is believed to be due to the high carbon and low nitrogen contents of biochars, which deprive soil microbes of nitrogen as they decompose the biochar, and cause microbes to tie up nitrogen from soil. We tested whether we could predict biochar impacts on soil nitrogen from the quantities of carbon and nitrogen in biochar that can be consumed soil microbes. Because biochars are mostly composed of carbon in molecules that can not be consumed by microbes, the microbially-available portion is generally small. We measured the microbially-available carbon and nitrogen in ten biochars, and measured how they impacted nitrogen concentrations in two soils from Oregon.

This dataset includes characteristics of ten biochars and two soils, and measurements from two incubation experiments. In the first experiment we incubated 13C-labeled biochars with two soil for 101 days, and measured production of biochar- and soil-respired CO2 and soil dissolved inorganic nitrogen. In the second experiment we expanded to study ten biochar types, including seven biochars that were not isotopically-labeled. We measured how much dissolved inorganic nitrogen was produced by amended soils over 28 days.

Surprisingly, we found all ten biochars increased rather than decreased soil nitrogen concentrations one month after application. We also found that biochars produced at high temperatures, which were more difficult for soil microbes to consume than low-temperature biochars, stimulated more soil decomposition and released more soil nitrogen. It appeared that microbes increased soil decomposition in response to additions of biochar, and this then increased plant-available nitrogen at least temporarily. These unexpected results show that biochar can sometimes have beneficial impacts on soil nitrogen, and that biochar impacts cannot be readily predicted from the qualities of the biochars themselves. These results are relevant to biochar users, and to biochar producers interested in how to make biochars more beneficial for plant growth. These results indicate that biochar users cannot predict nitrogen impacts, and should therefore monitor soil nitrogen concentrations to ensure levels are sufficient for plant growth.