Soil Biochar in Rocky Mountain Foothills - Forest and Shrublands.

More than 80% of the terrestrial organic carbon (C) pool is in soils. We measured an under-evaluated form of soil C that has been sequestered for millennia in temperate forest ecosystems through additions to soil of biomass-derived black C (a.k.a. charcoal or wood-char, hereafter called bio-char). Our objective is to quantify, and eventually model, this slow pool of soil C in temperate forest soils between short-steppe grasslands and alpine tundra along the Colorado Front Range, just west of Denver, CO.  Using a digital elevation map (NRCS), a vegetation layer (CO Division of Wildlife) and a roads layer (NRCS) we used GIS (ArcGIS 9, ESRI 2007) to define 0.25 ha sample polygons between lower (2200 m) and upper treeline (3500 m) in each of the five dominant forest types. Sample polygons were stratified such that each was located on a southeast aspect, 10-30% slope, and within a 200 m band central to each forest type. We collected composite samples from mineral soils (0-10 cm) in four of the five forest types using ten randomly generated sampling points per 0.25 ha polygon. Soil splits were analyzed for biochar C (heated, weak nitric acid digestion method) and for total C on a Carlo Erba 1108 CHN analyzer.  Soil biochar is most abundant in ponderosa pine (Pinus ponderosa) surface mineral soils (4.43 +/- 4.27 Mg C ha^-1) and spruce-fir (Picea englemannii – Abies lasiocarpa) soils (4.3 +/- 2.77 Mg C ha^-1).  Less than half as much biochar is found Douglas fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta) forest soils with 1.9 +/- 0.9 and 1.58 +/- 0.55 Mg C ha^-1 respectively.  We have not yet analyzed quaking aspen (Populous tremuloides) surface mineral soils for soil biochar.  The soil biochar proportion of total soil carbon for surface soils in these forests, is 14% in ponderosa, 13% in spruce fir, 11% in Douglas fir and 9% in lodgepole forest soils.  Soil biochar variability is surprisingly low in lodgepole pine forests where we have sampled 22 soils from three forest polygons at 2800 m – 3000  m elevation;  within polygon and between polygon lodgepole samples have the lowest variability of all forest types.  Historically, high intensity crown fires have scoured these sites resulting in low biochar sequestration and low variability across this landscape.  Ponderosa pine and spruce-fir forests are more likely to endure under frequent, low-intensity, ground fires which result in more soil biochar and greater biochar variability.