Pyrogenic carbon controls across a soil catena in the Pacific Northwest

Since turnover times of pyrogenic carbon (PyC) are substantially slower than those of other organic carbon input to soil, it is considered an important constituent of the global C cycle acting as a C sink. In the Pacific Northwest vegetation fires regularly produce PyC, but its accumulation in soils is poorly quantified. Using mid-infrared spectroscopy (MIR) and partial least-squares (PLS) analysis in conjunction with ultraviolet photo-oxidation followed by nuclear magnetic resonance spectroscopy (UV-NMR) techniques, PyC contents were quantified for samples from soil profiles along a vegetation gradient. Sample locations included different forest types as well as sites under agricultural use. While PyC was most prevalent in the first 0.2 m with 7–24% of total soil organic C (SOC), it could be found in the subsoil of all locations. However, PyC concentrations did not change consistently with soil depth. Stock sizes were lowest at the Turkey Farm (0.71 kg m− 2; 10% of SOC) and Organic Growers Farm (1.14 kg m− 2; 8% of SOC) sites, presumably due to the pervasive combustion of grass and cereals. Among the forested sites, lower stocks were observed at sites with higher mean annual temperature (MAT) and lower mean annual precipitation (MAP) such as Metolius (1.71 kg m− 2; 15% of SOC) and Juniper (1.89 kg m− 2; 26% of SOC). In contrast, the highest PyC stocks were found under cooler and moister conditions at Cascade Head dominated by Douglas Fir (Pseudotsuga menziesii (Mirb.)) (5.66 kg m− 2; 16% of SOC) and Soapgrass Mountain (4.80 kg m− 2; 15% of SOC). PyC was only moderately related to non-PyC SOC, which comprises plant residues, their decomposition products and soil biota (r2 = 0.61 and 0.44 for SOC with and without PyC, respectively), suggesting largely independent processes influencing production and disappearance.