Towards predicting biochar impacts on plant-available soil nitrogen content
Claire L. Phillips, Kylie M. Meyer, Manuel Garcia-Jaramillo, Clara S. Weidman, Catherine E. Stewart, Thomas Wanzek, Michael A. Grusak, Donald W. Watts, Jeff Novak, Kristin M. Trippe
Biochar ›› 2022, Vol. 4 ›› Issue (1) : 0.
Towards predicting biochar impacts on plant-available soil nitrogen content
| • | A diverse set of biochar types all increased net soil N-mineralization in a 4-week aerobic incubation. |
| • | For individual feedstocks, soil N-mineralization increased with pyrolysis temperature. |
| • | Biochar mineralizable-C and -N did not predict soil N-mineralization across feedstocks. |
Biochars can improve soil health but have been widely shown to reduce plant-available nitrogen (N) owing to their high carbon (C) content, which stimulates microbial N-immobilization. However, because biochars contain large amounts of C that are not microbially available, their total elemental C:N ratio does not correspond well with impacts on soil N. We hypothesized that impacts on soil plant-available N 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. Contrary to expectation, all the biochars increased soil N-mineralization relative to unamended soils. Also unexpected, higher temperature (500 and 700 °C) barley biochars with less Cmin stimulated more soil decomposition and more soil N-mineralization than a 350 °C barley biochar. However, across diverse biochar feedstocks and production methods, none of the biochar characteristics correlated with soil N-mineralization. The finding of improved soil N-mineralization adds complexity to the range of soil N responses that can be expected in response to biochar amendment. Because of the limited ability to predict soil N responses from biochar properties, users should monitor soil N to manage soil fertility.
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