Fresh and Field Aged Biochar Have Different Impacts on Soil Bulk Density, Saturated Hydraulic Conductivity and Nitrate Leaching.

Little is known about how soil properties and processes change as biochar ages in soil environments.  Here we collected soils from field plots receiving 0, 22.4, 44.8, 67.2, 89.6 and 112.1 Mg ha^-1 biochar amendments incorporated to a depth of 30 cm in Fall 2010 (old biochar).  Soil containing no biochar was collected from a buffer strip adjacent to the plots and mixed with the same biochar at the same rates (new biochar).  The biochar used in this study was produced from mixed wood feedstock in a low-temperature auger bed gassifier operated between 500 - 575°C.  The soil (1209 g) was repacked to an initial bulk density of 1.1 g cm^-3 into 77 mm diameter PVC columns.  The columns were fertilized, leached 6 times, and then spring wheat was grown during a 197 day greenhouse study.  Bulk density increased during the incubation period but was generally higher in columns amended with new biochar relative to columns amended with old biochar.  Bulk density was lowest for columns amended with 44.8 Mg ha^-1 old biochar.   Saturated hydraulic conductivity was significantly higher in no-biochar control soils than soils amended with old biochar, but there was no effect of new biochar on saturated hydraulic conductivity.  Cumulative nitrate leaching was lower for soil fertilized with NH₄ than for soils fertilized with NO₃.   Cumulative nitrate leaching decreased slightly with increasing biochar rates for soils amended with new biochar, however there was no apparent effect of old biochar on NO₃ leaching.  The results demonstrate that soil response to biochar amendments can change as biochar ages in soil environments over the course of three years and the importance of considering biochar aging.