Towards predicting biochar impacts on plant-available soil nitrogen content

Authors: Phillips, Claire; MEYER, KYLIE - Oregon State University; GARCIA-JARAMILLO, MANUEL - Oregon State University; WEIDMAN, CLARA - Oregon State University; Stewart, Catherine; Wanzek, Thomas; Grusak, Michael; Watts, Donald - Don; Novak, Jeffrey; Trippe, Kristin

Submitted to: Biochar
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2021
Publication Date: 2/17/2022
Citation: Phillips, C.L., Meyer, K.M., Garcia-Jaramillo, M., Weidman, C., Stewart, C.E., Wanzek, T.A., Grusak, M.A., Watts, D.W., Novak, J.M., Trippe, K.M. 2022. Towards predicting biochar impacts on plant-available soil nitrogen content. Biochar. 4. Article 9. https://doi.org/10.1007/s42773-022-00137-2.
DOI: https://doi.org/10.1007/s42773-022-00137-2

Interpretive Summary: 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 causes 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 by 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. Surprisingly, we found all of the 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.

Technical Abstract: Biochars can improve soil health but have been widely shown to reduce plant-available nitrogen (N) owing to their high carbon (C) content. Because biochars contain large amounts of C that are chemically resistant to decomposition, their total elemental C:N ratio does not correspond well with impacts on soil N. We hypothesized that impacts on soil plant-available nitrogen would relate to biochar mineralizable-C (Cmin) content, and that C:N ratios of the mineralizable biochar component could provide a means for predicting conditions of net soil N-mineralization or -immobilization. We conducted two laboratory experiments, the first measuring biochar Cmin from respiration of isotopically labeled barley biochars manufactured at 300°, 500°, and 750°C, and the second characterizing Cmin by proxy measurements for ten biochars from six feedstocks at several temperatures. For both experiments, soils were incubated with 2% biochar by mass to determine impacts to soil N-mineralization. As expected, as production temperature increased, biochars had less Cmin, as determined by respiration, H:C ratio, and KMnO4-oxidizable carbon content. However, contrary to some previous studies, high temperature biochars with less Cmin stimulated more soil decomposition (i.e. priming) and more soil N-mineralization than low temperature biochars. Also unexpected, all the biochars increased soil N-mineralization relative to unamended soils, and C:N ratios of biochar water extracts did not correlate with soil N-mineralization. None of the biochar characteristics correlated with soil N-mineralization across diverse biochar feedstocks and production methods. Because of the complexity of soil-N responses to biochar amendment, users should monitor soil nitrogen to manage soil fertility.