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Abstract
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.
Highlights
Biochar incorporation into agronomic systems has become an increasingly popular management practice in the past decade
In the Willamette soil, all the biochars caused significantly more soil respiration than unamended soil (p ≤ 0.02 for each pairwise comparison), and the strength of the priming effect increased with biochar pyrolysis temperature
This study provided initial steps towards evaluating biochar characteristics that may be useful for predicting negative impacts on plant-available N
Summary
Biochar incorporation into agronomic systems has become an increasingly popular management practice in the past decade. The addition of biochar has been shown to boost crop yields (Spokas et al 2012) by altering nutrient cycling (Mia et al 2017), increasing soil pH (Clough and Condron 2010), and in some cases altering soil water properties (Liu et al 2017; Phillips et al 2020a), while sequestering biomass carbon in long-lived forms (Baldock and Smernik 2002; Kuzyakov et al 2009; Woolf et al 2010). High C materials produced by pyrolysis, gasification, and other low-oxygen heating processes. Many biochars are made from low-value biomass such as wood or straw feedstocks with wide C:N ratios, which confers wide C:N ratios to the resulting biochar. Depending on the feedstock and pyrolysis conditions, biochar can have a total C:N ratio ranging from 6.5 to 640 (Bonanomi et al 2017)
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