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. Resources in this dataset:Resource Title: Biochar Properties. File Name: Biochar Properties.csvResource Description: Describes production conditions and provides chemical characteristics of ten biochars used to evaluate impacts of biochar amendment on plant-available nitrogen in soil. Data were collected at the at USDA-ARS National Forage Seed Production Research Unit in Corvallis, OR in 2019-2020.Resource Title: Experiment 1 Timeseries. File Name: Experiment 1 Timeseries.csvResource Description: Three 13C-labeled barley biochars produced at 350, 500, and 700 degrees Celsius were incubated in two Oregon soils with contrasting levels of organic matter for 101 days to determine impacts of biochar on soil C and N-mineralization. Dataset provides timeseries of CO2 measured with an LGR Ultraportable Greenhouse Gas Analyzer, and nitrate and ammonium measured from 2M KCl extracts using colorimetric methods. Data were collected at the USDA-ARS National Forage Seed Production Research Unit in Corvallis, Oregon in 2019. Isotopic values of respired CO2 were measured from Exetainer samples that were send to the USDA-ARS in Ft.Collins and analyzed using a gas chromatograph-isotope ratio mass spectrometer (Isoprime Inc, UK). Quantities of CO2 respired from biochar and soil were calculated from a 2-member isotopic mixing model. Resource Title: Experiment 2 Timeseries. File Name: Experiment 2 Timeseries.csvResource Description: Ten biochars were incubated in an Oregon clay-loam soil to test impacts on plant-available soil nitrogen concentration. Biochars were added to a soil at a rate of 2% by mass, wetted to 60% water-filled pore space, and incubated at 23 degrees C under aerobic conditions for 28 days. Soil nitrate and ammonium concentrations were measured on days 0, 14, and 28, using 2M KCl extraction solutions and colorimetric methods. Net N-mineralized on days 14 and 28 was determined by subtracting total inorganic nitrogen on day 0 from values on days 14 and 28, respectively. The impact of biochar amendment was assessed by subtracting net N-mineralized in unamended soils from net N-mineralized in amended soils. Data were collected at the USDA-ARS National Forage Seed Production Research Center in 2020.Resource Title: Data dictionary for: Towards predicting biochar impacts on plant-available soil nitrogen content. File Name: DataDictionary_PAN Experiment.xlsxResource Description: Provides data descriptions for all resources in the dataset.Resource Title: Soil Properties. File Name: Soil Properties.csvResource Description: Provides soil properties for two Oregon soils used to evaluate impacts on biochar on plant-available soil nitrogen concentrations. Soils were collected near Corvallis, Oregon and were analyzed at the USDA-ARS National Forage Seed Production Center in 2019.