Pyrogenic carbon in native grassland soils along a climosequence in North America

Abstract

Elucidating the role of pyrogenic carbon (Cpyr) as a global pool for CO2 sequestration in temperate ecosystems requires information on the contribution of Cpyr to soil organic carbon (SOC) across different climatic regions. We investigated the effect of climate and basic soil properties on the accumulation of Cpyr in surface soils across the native North American prairies. Topsoil samples (0-10 cm) of 18 native grassland sites along temperature and precipitation transects from central Saskatoon, Canada, to south Texas, USA, were analyzed for benzenecarboxylic acids as molecular markets for Cpyr after nitric acid oxidation in the bulk soil (<2 mm), clay- (<2 μm) and silt-sized (2-20 μm) separates. Cpyr contributed between 4 and 18% to SOC, the major proportions of Cpyr being mostly found in the clay and silt fractions. Cpyr in soil did not correlate with basic inorganic soil properties such as pH or clay content, but it increased significantly as SOC contents increased (r2 = 0.78; P < 0.001). Obviously, Cpyr co-accumulated with dead biomass. Since this accumulation depends on climate, also Cpyr contents in soil depend on climate and may be predicted by the logarithm of the ratio of mean annual temperature (MAT) to mean annual precipitation (MAP) (P < 0.05). We conclude that the natural potential of soils to sequester Cpyr reflects both, (1) higher Cpyr production at sites with high plant biomass in moist climates, and (2) lower Cpyr residence time at soils acting as a CO2 source in warm climates.