Biochar Impacts on Soil Structure and Carbon Dynamics Vary By Soil Type.

Biochar soil amendments have been regarded as a low cost soil carbon (C) sequestration strategy that can have important beneficial impacts on a range of soil properties and plant production. We investigated the impact of type distinct types of biochar on soil chemical properties, microbial communities, soil aggregation and aggregate associated C within two California agricultural soils in a laboratory incubation study. Soil samples were collected and wet-sieved to obtain four aggregate size classes: large macroaggregates (2000-8000 μm), macroaggregates (250-2000 μm), microaggregates (53–250 μm) and silt and clay fraction (<53 μm). Both biochar types were found to enhance aggregation in the finer textured soil, a with 223% and 137% average increase in mean weight diameter for a low-temperature enhanced biochar and a high-temperature walnut shell biochar, respectively. This increase in aggregate stability was associated with an increase in physically-protected C via incorporation in to macroaggregates. Meanwhile, in a coarser textured soil, biochar had no effect on aggregation and the lack of SOM stabilization in macroaggregates was associated with a significant loss of soil C over the course of the incubation. Biochar was also shown to generally increase soil pH and EC and had distinct impacts on soil microbial communities, all of which may have influenced aggregate stability via diverse mechanisms. Understanding the drivers and soil conditions that control soil-biochar interactions and determine whether C is physically stabilized in aggregates or lost due to priming is an important issue and key consideration in managing agroecosystems C sequestration and adaptation to global change.